Td corrigé Theoretical part - IS MU pdf

Theoretical part - IS MU

Recall neutralization reaction as the most basic experiment or acid-base ..... Aliphatic ketones possess n, * lowest singlets that undergo intersystem ...... The TD DFT calculation showed the nearest n, * state to lie 14.3 kcal mol?1 above.




part of the document




Masaryk University
Faculty of Science

Department of Chemistry










Photoinduced Hydrogen Atom Transfer: Application and Theoretical Aspects


Diploma Thesis







Brno 2009 TomᚠŠolomek
















...Those sciences are vain and full of errors which
are not born from experiment, the mother of certainty...

-- LEONARDO DA VINCI, 1452-1519























Acknowledgement
I would like to thank prof. Petr Klán Ph.D. for a thorough guidance during my work on the thesis, for his patience and the freedom to participate in so many interesting research projects which he gave me. I would like to thank prof. Thomas Bally Ph.D. who taught me much during my great time at the University of Fribourg and who supervised and approved the part of the thesis concerning the nitrobenzyl moiety. I thank prof. Christian Bochet Ph.D. for proposal of this project. I would also like to thank all the organic photochemistry group members, especially Jaromír Literák, for help and fruitfull discussions. I thank my parents and Hanka for supporting me on the studies. Finally, I would like to thank the Rector’s Programme to Support Masaryk University Students‘ Creative Work for financial support.


Table of Contents
 TOC \o "1-2" \t "Nadpis 3;3;Nadpis 4;4" Table of Contents  PAGEREF _Toc228816510 \h 3
Introductory Notes  PAGEREF _Toc228816511 \h 3
Literature Part  PAGEREF _Toc228816512 \h 5
2.1. Introduction  PAGEREF _Toc228816513 \h 5
2.2. Setting Up the Scene  PAGEREF _Toc228816514 \h 5
2.2.1. Making the Connection Arguments  PAGEREF _Toc228816515 \h 6
2.3. Photochemistry of Ketones in Context of Hydrogen Atom Abstraction  PAGEREF _Toc228816516 \h 7
2.3.1. ³-Hydrogen Atom Abstraction and 1,4-Biradicals  PAGEREF _Toc228816517 \h 8
2.3.2. More Remote Hydrogen Atom Abstractions  PAGEREF _Toc228816518 \h 19
2.4. The Nitroaromatic Chromophore  PAGEREF _Toc228816519 \h 19
2.4.1. Solvent-Mediated Photoredox Reactions  PAGEREF _Toc228816520 \h 20
2.4.2. Intramolecular Hydrogen Atom Abstractions  PAGEREF _Toc228816521 \h 21
Experimental Section  PAGEREF _Toc228816522 \h 24
3.1. Instrumentation  PAGEREF _Toc228816523 \h 24
3.2. Quantum Chemical Calculations  PAGEREF _Toc228816524 \h 24
3.3. Preparation of Compounds  PAGEREF _Toc228816525 \h 25
Part I: Discussion on the Photochemistry of Dimethylphenacyl Epoxides  PAGEREF _Toc228816526 \h 37
4.1. Preparation of Model Compounds  PAGEREF _Toc228816527 \h 37
4.2. Photochemistry of Model Compounds  PAGEREF _Toc228816528 \h 38
4.3. Mechanistic and Computational Studies  PAGEREF _Toc228816529 \h 42
4.4. The Strategy of Indanocine Synthesis Using a Photochemical Key Step  PAGEREF _Toc228816530 \h 51
4.5 Conclusion I  PAGEREF _Toc228816531 \h 57
Part II: A Contribution to the o-Nitrobenzyl Photochemistry  PAGEREF _Toc228816532 \h 59
5.1. Introduction  PAGEREF _Toc228816533 \h 59
5.2. Comments on the Methods Used  PAGEREF _Toc228816534 \h 60
5.3. Structure of Triplet o-Nitrotoluene  PAGEREF _Toc228816535 \h 63
5.4. Differences of Aci-nitro Intermediate and its Triplet State  PAGEREF _Toc228816536 \h 66
5.5. The Impact of the Benzylic Substitution on the Geometries  PAGEREF _Toc228816537 \h 67
5.6. Thermodynamic and Kinetic Parameters and Their Correlations  PAGEREF _Toc228816538 \h 70
5.7. Conclusion II  PAGEREF _Toc228816539 \h 77
List of Abbreviations  PAGEREF _Toc228816540 \h 78
References  PAGEREF _Toc228816541 \h 79
Introductory Notes

The present work concerns with the photoinduced hydrogen atom transfer reactions. This process has been used for the synthesis of indan-1-one derivatives via photochemical enolization and epoxide ring opening with subsequent cyclization of the substituted 1(omethylphenyl)-2,3-epoxypropan-1-one moiety. Mechanistic studies and quantum chemical calculations are provided to explain the scope of this reaction. The approach used is discussed in regard to the synthesis of biologically interesting compounds like Pterosines and Indanocine.
The triplet state hydrogen atom transfer reaction of the substituted o-nitrobenzyl moiety has been explored by quantum chemical calculations to reveal the triplet state reactivity of this kind of chromophore. Valuable information of the geometry, thermodynamic and kinetic parameters are presented and are believed to stimulate further experimental and theoretical research in the photoremovable protecting groups development.

Chapter 2 describes intramolecular hydrogen atom transfer described in the literature part putting great emphasis on the photoinduced fashion of this process in aromatic ketones and the onitrobenzyl chromophore.

Chapter 3 describes theoretical and experimental procedures used in this work. All synthetic products have been characterized by 1H and 13C or 2D NMR spectroscopy and mass spectroscopy where necessary.

Finally, the next two chapters summarize and discuss the experimental and theoretical observations.








Literature Part
2.1. Introduction
Hydrogen transfer is the reaction of fundamental importance in all branches of chemistry. Recall neutralization reaction as the most basic experiment or acid-base catalysis concept in organic chemistry. These examples can be denoted as proton transfer. However, depending upon the degree of charge transfer they are often referred to as hydrogen atom transfer (HAT). It plays a central role in various oxidation reactions like combustion of fuels ADDIN EN.CITE Hayes20094002473-2482<Go to ISI>://000264111000028Hayes, C. J.Burgess, D. R.Kinetic Barriers of H-Atom Transfer Reactions in Alkyl, Allylic, and Oxoallylic Radicals as Calculated by Composite Ab Initio MethodsJournal of Physical Chemistry AMar 19200911311ISI:0002641110000281, atmospheric chemistry ADDIN EN.CITE Kuwata20071905032-5042<Go to ISI>://000247034000019Kuwata, K. T.Dibble, T. S.Sliz, E.Petersen, E. B.Computational studies of intramolecular hydrogen atom transfers in the beta-hydroxyethylperoxy and beta-hydroxyethoxy radicalsJournal of Physical Chemistry Atransition-state theory; quadratic configuration-interaction;density-functional thermochemistry; classical mechanical theory;zero-point energies; initiated oxidation; alkoxy radicals; barrierheights; moller-plesset; basis-setsMacalester Coll, Dept Chem, St Paul, MN 55105 USA. SUNY Coll Environm Sci & Forestry, Dept Chem, Syracuse, NY 13210 USA. Kuwata, KT, Macalester Coll, Dept Chem, St Paul, MN 55105 USA. kuwata@macalester.eduJ. Phys. Chem. AJun 14200711123ISI:0002470340000192, synthetic chemistry, biochemistry or chemical biology. ADDIN EN.CITE Corrie20034208546-8554<Go to ISI>://000184137900031Corrie, J. E. T.Barth, A.Munasinghe, V. R. N.Trentham, D. R.Hutter, M. C.Photolytic cleavage of 1-(2-nitrophenyl)ethyl ethers involves two parallel pathways and product release is rate-limited by decomposition of a common hemiacetal intermediateJournal of the American Chemical SocietyJul 16200312528ISI:000184137900031Il'ichev20044104581-4595<Go to ISI>://000220752300045Il'ichev, Y. V.Schworer, M. A.Wirz, J.Photochemical reaction mechanisms of 2-nitrobenzyl compounds: Methyl ethers and caged ATPJournal of the American Chemical SocietyApr 14200412614ISI:0002207523000453,4 Understanding the mechanisms in the most simple cases allows us to propose new kinetic models which can be used to predict the behaviour of even more complex systems. Nowadays, this approach is crucial in developing new types of fuels or combustion technologies due to the world’s rapidly changing energy requirements. Rapid climate change threat became more open to public, thus further accelerating the atmospheric chemistry research. Photoinduced hydrogen atom transfer is often used in chemistry of photoremovable protecting groups. ADDIN EN.CITE Kessler20034501179-1181<Go to ISI>://000182265500006Kessler, M.Glatthar, R.Giese, B.Bochet, C. G.Sequentially photocleavable protecting groups in solid-phase synthesisOrganic LettersApr 17200358ISI:000182265500006Robles20054403545-3547<Go to ISI>://000230858500039Robles, J. L.Bochet, C. G.Photochemical release of aldehydes from alpha-acetoxy nitroveratryl ethersOrganic LettersAug 42005716ISI:000230858500039Riguet20074305453-5456<Go to ISI>://000251614900031Riguet, E.Bochet, C. G.New safety-catch photolabile protecting groupOrganic LettersDec 202007926ISI:000251614900031Bochet20004606341-6346<Go to ISI>://000088985600019Bochet, C. G.Wavelength-selective cleavage of photolabile protecting groupsTetrahedron LettersAug 1220004133ISI:000088985600019Klan20004701569-1571<Go to ISI>://000087292600019Klan, P.Zabadal, M.Heger, D.2,5-dimethylphenacyl as a new photoreleasable protecting group for carboxylic acidsOrganic LettersJun 12000211ISI:000087292600019Zabadal2001510694-699<Go to ISI>://000172627300001Zabadal, M.Klan, P.Photoremovable protecting groupsChemicke Listy20019511ISI:000172627300001Klan2002500920-923<Go to ISI>://000179535400014Klan, P.Pelliccioli, A. P.Pospisil, T.Wirz, J.2,5-Dimethylphenacyl esters: A photoremovable protecting group for phosphates and sulfonic acidsPhotochemical & Photobiological SciencesNov2002111ISI:000179535400014Literak200549043-46<Go to ISI>://000225889800005Literak, J.Wirz, J.Klan, P.2,5-Dimethylphenacyl carbonates: A photoremovable protecting group for alcohols and phenolsPhotochemical & Photobiological Sciences200541ISI:000225889800005Kammari200748050-56<Go to ISI>://000243204700005Kammari, L.Plistil, L.Wirz, J.Klan, P.2,5-Dimethylphenacyl carbamate: a photoremovable protecting group for amines and amino acidsPhotochemical & Photobiological Sciences200761ISI:0002432047000055-13 It is worth to mention photoactivatable molecules, also called caged compounds, which upon irradiation irreversibly release a species possessing desirable physical, chemical, or biological qualities. ADDIN EN.CITE Corrie20034208546-8554<Go to ISI>://000184137900031Corrie, J. E. T.Barth, A.Munasinghe, V. R. N.Trentham, D. R.Hutter, M. C.Photolytic cleavage of 1-(2-nitrophenyl)ethyl ethers involves two parallel pathways and product release is rate-limited by decomposition of a common hemiacetal intermediateJournal of the American Chemical SocietyJul 16200312528ISI:000184137900031Il'ichev20044104581-4595<Go to ISI>://000220752300045Il'ichev, Y. V.Schworer, M. A.Wirz, J.Photochemical reaction mechanisms of 2-nitrobenzyl compounds: Methyl ethers and caged ATPJournal of the American Chemical SocietyApr 14200412614ISI:0002207523000453,4 All these examples often include hydrogen atom transfer as a key step.
The next few chapters will concern with the intramolecular HAT, primarily focusing on the photoinduced intramolecular hydrogen atom abstraction in aromatic ketones. Substituted nitroaromatic compounds will be considered too.

2.2. Setting Up the Scene
There is a wide variety of hydrogen atom transfer reactions. However, the most important ones share a major feature which is when the oxygen abstracts hydrogen. Let us note that one can encounter several of them in different fields of chemistry (Scheme 2.1): (1) the famous Barton reaction ADDIN EN.CITE Barton19605202640-2641<Go to ISI>://A1960WB33700061Barton, D. H. R.Beaton, J. M.Geller, L. E.Pechet, M. M.A New Photochemical ReactionJournal of the American Chemical Society19608210ISI:A1960WB3370006114, (2) the Norrish type II cleavage ADDIN EN.CITE Wagner1971530Wagner, P. J.1971Type-II Photoelimination and Photocyclization of KetonesAccounts of Chemical Research45168-&ISI:A1971J326700002<Go to ISI>://A1971J32670000215, (3) the McLafferty rearrangement ADDIN EN.CITE McLafferty195954082-87<Go to ISI>://A1959WC85200017McLafferty, F. W.Mass Spectrometric Analysis - Molecular RearrangementsAnalytical Chemistry1959311ISI:A1959WC8520001716, (4) related alcohol or ether radical cation reactions ADDIN EN.CITE Kingston1974550215-242<Go to ISI>://A1974S476600004Kingston, D. G.Bursey, J. T.Bursey, M. M.Intramolecular Hydrogen Transfer in Mass-Spectra .2. Mclafferty Rearrangement and Related ReactionsChemical Reviews1974742ISI:A1974S47660000417 and (5) retro-ene reaction of carbonyl compounds. ADDIN EN.CITE Loncharich19875606947-6952<Go to ISI>://A1987K887100008Loncharich, R. J.Houk, K. N.Transition Structures of Ene Reactions of Ethylene and Formaldehyde with PropeneJournal of the American Chemical SocietyNov 11198710923ISI:A1987K88710000818
Although all the reactions mentioned above are essentially the same fundamental type of process, significant differences can be found for their activation energies which are related to their open-, closed-shell (the very last example) or charged open-shell character. ADDIN EN.CITE Dorigo1990107508-7514<Go to ISI>://A1990ED19100009Dorigo, A. E.McCarrick, M. A.Loncharich, R. J.Houk, K. N.Transition Structures for Hydrogen-Atom Transfers to Oxygen - Comparisons of Intermolecular and Intramolecular Processes and Open-Shell and Closed-Shell SystemsJournal of the American Chemical SocietyUNIV CALIF LOS ANGELES, DEPT CHEM & BIOCHEM, LOS ANGELES, CA 90024 USA.J. Am. Chem. Soc.Oct 10199011221ISI:A1990ED1910000919 There are also structural similarities in cyclic transition states for the first four. Based upon ab initio calculations of Houk et al. ADDIN EN.CITE Dorigo1990107508-7514<Go to ISI>://A1990ED19100009Dorigo, A. E.McCarrick, M. A.Loncharich, R. J.Houk, K. N.Transition Structures for Hydrogen-Atom Transfers to Oxygen - Comparisons of Intermolecular and Intramolecular Processes and Open-Shell and Closed-Shell SystemsJournal of the American Chemical SocietyUNIV CALIF LOS ANGELES, DEPT CHEM & BIOCHEM, LOS ANGELES, CA 90024 USA.J. Am. Chem. Soc.Oct 10199011221ISI:A1990ED1910000919, three distinct groups can be made according to the character of the oxygen atom. We will very briefly discuss the ‘open-shell’ group consisting of the ground state alkoxy radicals and the triplet aromatic ketones putting great emphasis on the latter. Finally, similarities of the nitrobenzyl moiety photochemistry will be discussed.

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Scheme 2.1.

2.2.1. Making the Connection Arguments
It is surprising at first glance that one can explain some reaction features for excited state compounds simply by looking at relatively well studied ground state alkoxy radicals. In order to understand these features in HAT in such two different species, several arguments need to be introduced. Firstly, it has long been known the triplet formaldehyde is sp3 hybridized at carbon. ADDIN EN.CITE Wagner1972270Wagner, P. J.Zepp, R. G.Kelso, P. A.Kemppain.Ae,1972Type-II Photoprocesses of Phenyl Ketones - Competitive Delta-Hydrogen Abstraction and Geometry of Intramolecular Hydrogen-Atom TransfersJournal of the American Chemical Society94217500-&J. Am. Chem. Soc.ISI:A1972N757500039<Go to ISI>://A1972N75750003920 Additionally, the C–O bond length in triplet aldehydes has been shown to be 0.10 Å longer than that in their ground states. ADDIN EN.CITE Lin1971570280-&<Go to ISI>://A1971I478600007Lin, C. T.Moule, D. C.Band System of Propynal at 4140 AJournal of Molecular Spectroscopy1971372ISI:A1971I47860000721 The triplet ketones behave the same way. This can be supported by comparing the dipole moments of benzophenone and formaldehyde triplets ADDIN EN.CITE Hochstra.Rm19685804929-&<Go to ISI>://A1968C420100031Hochstra.Rm,Lin, T. S.Magnetic and Electric Field Spectra of Organic Crystals - Optical Measurements of Zero-Field SplittingsJournal of Chemical Physics19684911ISI:A1968C42010003122 which are of equal value (1.7 D) characteristic for a C–O single bond suggesting that ketones undergo similar geometrical changes upon excitation. ADDIN EN.CITE Wagner1972270Wagner, P. J.Zepp, R. G.Kelso, P. A.Kemppain.Ae,1972Type-II Photoprocesses of Phenyl Ketones - Competitive Delta-Hydrogen Abstraction and Geometry of Intramolecular Hydrogen-Atom TransfersJournal of the American Chemical Society94217500-&J. Am. Chem. Soc.ISI:A1972N757500039<Go to ISI>://A1972N75750003920 All these features suppose that triplet ketones could possibly demonstrate the behaviour of alkoxy radicals.
The experimental activation energies for the Barton reaction and the Norish type II process fall within 5–10 kcal mol–1. ADDIN EN.CITE Barton19605202640-2641<Go to ISI>://A1960WB33700061Barton, D. H. R.Beaton, J. M.Geller, L. E.Pechet, M. M.A New Photochemical ReactionJournal of the American Chemical Society19608210ISI:A1960WB33700061Wagner1971530Wagner, P. J.1971Type-II Photoelimination and Photocyclization of KetonesAccounts of Chemical Research45168-&ISI:A1971J326700002<Go to ISI>://A1971J32670000214,15 A higher barriers of 14.9 kcal mol–1 and 14.0 kcal mol–1 were found at MP2/6-31G*//HF/3-21G level of theory for the intramolecular hydrogen atom abstraction via a six-membered transition state for the Barton and the Norrish type II reactions, respectively. ADDIN EN.CITE Dorigo1990107508-7514<Go to ISI>://A1990ED19100009Dorigo, A. E.McCarrick, M. A.Loncharich, R. J.Houk, K. N.Transition Structures for Hydrogen-Atom Transfers to Oxygen - Comparisons of Intermolecular and Intramolecular Processes and Open-Shell and Closed-Shell SystemsJournal of the American Chemical SocietyUNIV CALIF LOS ANGELES, DEPT CHEM & BIOCHEM, LOS ANGELES, CA 90024 USA.J. Am. Chem. Soc.Oct 10199011221ISI:A1990ED1910000919 Both reactions appeared to be almost thermoneutral computationally. This kind of HAT regioselectivity is observed in general. 1,5- and 1,6-Hatom transfers are significantly lower in energy than those for shorter ‘migrations’ by 7–25 kcal mol–1. ADDIN EN.CITE Hayes20094002473-2482<Go to ISI>://000264111000028Hayes, C. J.Burgess, D. R.Kinetic Barriers of H-Atom Transfer Reactions in Alkyl, Allylic, and Oxoallylic Radicals as Calculated by Composite Ab Initio MethodsJournal of Physical Chemistry AMar 19200911311ISI:0002641110000281 However, the reason for the 1,5-H-atom preference in these two reactions slightly differs. The chair-like, six-membered transition state is suggested to reflect the strain-free character of cyclohexane which has been shown by quantum chemical calculations to be the case for the triplet ketones indeed. The seven-membered transition state for ´-hydrogen atom abstraction was found to be 0.9 kcal mol 1 higher at MP2/6-31G*//HF/3-21G than that of the ³-hydrogen abstraction. ADDIN EN.CITE Dorigo1990107508-7514<Go to ISI>://A1990ED19100009Dorigo, A. E.McCarrick, M. A.Loncharich, R. J.Houk, K. N.Transition Structures for Hydrogen-Atom Transfers to Oxygen - Comparisons of Intermolecular and Intramolecular Processes and Open-Shell and Closed-Shell SystemsJournal of the American Chemical SocietyUNIV CALIF LOS ANGELES, DEPT CHEM & BIOCHEM, LOS ANGELES, CA 90024 USA.J. Am. Chem. Soc.Oct 10199011221ISI:A1990ED1910000919 On the other hand, the µ-hydrogen atom abstraction in alkoxy radicals (note the different nomenclature, the ³-hydrogen in carbonyls corresponds to ´-hydrogen in alcohols) through a seven-membered transition state is favored enthalpically by approximately 0.8 kcal mol 1 and entropy factors have to be considered in this case. ADDIN EN.CITE Dorigo1987302195-2197<Go to ISI>://A1987G695000056Dorigo, A. E.Houk, K. N.Transition Structures for Intramolecular Hydrogen-Atom Transfers - the Energetic Advantage of 7-Membered over 6-Membered Transition StructuresJournal of the American Chemical SocietyUNIV CALIF LOS ANGELES,DEPT CHEM & BIOCHEM,LOS ANGELES,CA 90024.J. Am. Chem. Soc.Apr 119871097ISI:A1987G69500005623 The activation Gibbs free energies are truly in the correct order reflecting the experimentally observed regioselectivity.

2.3. Photochemistry of Ketones in Context of Hydrogen Atom Abstraction
So far we have seen a couple of examples of HAT reactions and have found some common features among alkoxy radicals and triplet ketones to point out one can look at two completely different systems in a similar manner. However, in order to fully understand the experimental work which we will discuss in Chapter 4, we need to deepen our knowledge of aromatic ketones photochemistry in regard of HAT. To do this we will explore this topic in more detail in the next few sections.

2.3.1. ³-Hydrogen Atom Abstraction and 1,4-Biradicals
The best known example of intramolecular hydrogen abstraction in excited ketones is the type II photoelimination originally discovered by Norrish. ADDIN EN.CITE Wagner2004597Wagner, P. J.;Klan, P.;2004Norrish Tpye II Photoelimination of Ketones:Cleavage of 1,4-Biradicals Formed by ?-Hydrogen AbstractionHorspool, W.;Lenci, F.;CRC Handbook of Photochemistry and PhotobiologyNew YorkCRC Press2nd24 He found that ketones with ³-hydrogens cleaved upon excitation to methyl ketones and alkenes which was in contrast to the previously discovered type I cleavage producing acyl and alkyl radicals. ADDIN EN.CITE Wirz2009601Wirz, J.;Klan, P.;2009Photochemistry of Organic Compounds: From Concepts to PracticePostgraduate Chemistry SeriesWiley-Blackwell5821st978-1-4051-6173-225 Later on it was found the quantum yields of type II reaction were relatively low and other processes must compete. This was answered by the pioneering work of Yang and Yang who discovered that cyclobutanol forms upon irradiation and proposed a common reactive intermediate for both reactions. ADDIN EN.CITE Yang19586102913-2914<Go to ISI>://A1958WB38600092Yang, N. C.Yang, D. D. H.Photochemical Reactions of Ketones in SolutionJournal of the American Chemical Society19588011ISI:A1958WB3860009226 They suggested a 1,4-biradical intermediate which was formed after the ³-hydrogen atom was abstracted by the excited carbonyl oxygen. The whole sequence of processes can be referred to as the Norrish/Yang reaction (Scheme 2.2). Thanks to the ease of experimental following of the type II cleavage, valuable information have been gained about the biradicals and the hydrogen atom transfer reaction itself.

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Scheme 2.2. The Norrish/Yang reaction

2.3.1.1. Behaviour of the Biradical Intermediate
We stated that the low quantum yields of type II cleavage products formation could be explained by considering the cyclobutanol formation from the common biradical intermediate. However, we would find the sum of the quantum yields of both types of reactions does not equal unity. To solve for this little conundrum we have to look at the 1,4-biradicals more closely.
One of the first indirect experimental evidence for this reactive intermediate was based on observation of (+)-4-methyl-1-phenyl-1-hexanone racemization by Wagner et al.. ADDIN EN.CITE Wagner1972250Wagner, P. J.Zepp, R. G.Kelso, P. A.1972Type-II Photoprocesses of Phenyl Ketones - Evidence for a Biradical IntermediateJournal of the American Chemical Society94217480-&J. Am. Chem. Soc.ISI:A1972N757500036<Go to ISI>://A1972N75750003627 The total quantum yield of the racemization, elimination and cyclization was found to be close to unity. After alcohol addition, the quantum yield of products formation significantly increased while that of the racemization decreased. The primary kinetic isotope effect with kH/kD value of 4.8 for the triplet reaction was observed. All these experiments indicated the formation of 1,4-biradical intermediate which can proceed to products or revert back to the starting ketone thus non-productively loosing the excitation (which is, however, not the case for the racemization). Moreover, it was revealed that alkyl thiols can efficiently trap the biradicals derived from valerophenone and ³-methoxyvalerophenone, suggesting a solvated biradical lifetime of ~ 10 6 s. ADDIN EN.CITE Wagner1972620287-&<Go to ISI>://A1972L324300057Wagner, P. J.Zepp, R. G.Trapping by Mercaptans of Biradical Intermediates in Type Ii PhotoeliminationJournal of the American Chemical Society1972941ISI:A1972L32430005728 Further evidence was given by the fact that only the triplet, but not the singlet, component of 5-methyl-2-heptanone produced racemization at the ³-carbon. ADDIN EN.CITE Wagner1972620287-&<Go to ISI>://A1972L324300057Wagner, P. J.Zepp, R. G.Trapping by Mercaptans of Biradical Intermediates in Type Ii PhotoeliminationJournal of the American Chemical Society1972941ISI:A1972L32430005728
The first direct observation of the biradical intermediate was given by Scaiano et al. ADDIN EN.CITE Small1977630431-434<Go to ISI>://A1977DW91300018Small, R. D.Scaiano, J. C.Direct Detection of Biradicals Generated in Norrish Type-Ii ReactionChemical Physics Letters1977503ISI:A1977DW9130001829 They employed laser flash photolysis (LFP) to detect the triplet 1,4-biradical from ³methylvalerophenone. Later they used paraquat (N,N'-dimethyl-4,4'-bipyridinium dichloride) to trap the biradical intermediate by oxidation of the hydroxy radical site, thus reducing the paraquat to radical cation species kinetics of which can be followed easily spectroscopically. ADDIN EN.CITE Small1977640453-456<Go to ISI>://A1977CY58200006Small, R. D.Scaiano, J. C.One Electron Reduction of Paraquat Dication by Photogenerated BiradicalsJournal of Photochemistry197766ISI:A1977CY5820000630 With these powerful methods in hand several important conclusions could be made. The ns lifetimes have been found which are lengthened by adding Lewis bases or alcohols. ADDIN EN.CITE Small1978650246-248<Go to ISI>://A1978FZ09500016Small, R. D.Scaiano, J. C.Solvent Effects on Lifetimes of Photogenerated BiradicalsChemical Physics Letters1978592ISI:A1978FZ0950001631 This nicely explains the observations of Wagner discussed above. The biradicals are almost temeprature independent, having decay activation energies within 1 kcal mol–1. ADDIN EN.CITE Small19776602126-2131<Go to ISI>://A1977DZ66400018Small, R. D.Scaiano, J. C.Photochemistry of Phenyl Alkyl Ketones - Lifetime of Intermediate BiradicalsJournal of Physical Chemistry19778122ISI:A1977DZ6640001832 Their lifetimes are shortened by adding paramagnetic species such as oxygen which is known to speed up the intersystem crossing. ADDIN EN.CITE Turro2009671Turro, N. J.;Ramamurthy, V.;Scaiano, J. C.;2009Principles of Molecular Photochemistry: An IntroductionUniversity Science Books5201st978-1-891389-57-333 Whether the physical or chemical rate-determining step for their decay is operative is not completely resolved yet.
The most modern femtosecond spectroscopy techniques allowed the observation of the singlet 1,4-biradicals formed from excited singlet alkanones in the gas phase. The lifetimes have been reported in the range of 400 to 700 fs. ADDIN EN.CITE De Feyter2000680260-+<Go to ISI>://000084700400049De Feyter, S.Diau, E. W. G.Zewail, A. H.Femtosecond dynamics of Norrish type-II reactions: Nonconcerted hydrogen-transfer and diradical intermediacyAngewandte Chemie-International Edition2000391ISI:00008470040004934

2.3.1.2. The Nature of Excited State
Two consecutive intermediates can be found by looking at the mechanism of the type II reaction (see Scheme 2.2). We have already described the biradicals above. However, for the hydrogen atom abstraction itself one needs to understand the character of the excitation in the ketone and its geometry as well. We will start with the former.
The carbonyls are excited to the corresponding state upon irradiation. After rapid internal conversion (IC) the first singlet excited state is populated where we find only two choices which are productive (except the radiative deactivation or IC to the ground state). Aliphatic ketones possess n,pð* lowest singlets that undergo intersystem crossing (ISC) to their n,pð* lowest triplets with rate constant of ~ 108 s 1. ADDIN EN.CITE Wirz2009601Wirz, J.;Klan, P.;2009Photochemistry of Organic Compounds: From Concepts to PracticePostgraduate Chemistry SeriesWiley-Blackwell5821st978-1-4051-6173-225 This process is slow enough to compete efficiently with the hydrogen atom abstraction reaction which is believed to occur with rate constants of ~ 109 s–1. ADDIN EN.CITE Wagner1971530Wagner, P. J.1971Type-II Photoelimination and Photocyclization of KetonesAccounts of Chemical Research45168-&ISI:A1971J326700002<Go to ISI>://A1971J32670000215 The ISC in aromatic ketones is even faster (> 1010 s–1), thus the excited state chemistry could be derived solely from their triplet states. ADDIN EN.CITE Wagner19666901245-&<Go to ISI>://A19667462500030Wagner, P. J.Hammond, G. S.Mechanisms of Photochemical Reactions in Solution .38. Quenching of Type 2 Photoelimination ReactionJournal of the American Chemical Society1966886ISI:A1966746250003035 This looks like a bit of simplification since we do not have to consider any excited singlet state reactions (although HAT has been shown to involve excited singlets at least partially ADDIN EN.CITE Wagner19717007328-&<Go to ISI>://A1971L244900042Wagner, P. J.Jellinek, T.Intramolecular Quenching of Excited Singlets of Phenyl Omega-Dialkylaminoalkyl Ketones - Singlet State Type-Ii Photoelimination of Alpha-DimethylaminoacetophenoneJournal of the American Chemical Society19719326ISI:A1971L24490004236). However, introducing the benzene ring allows an energetically close pð,ðpð* excitation to occur. They are far less reactive compared to their n,pð* counterparts. The reason can be viewed in the way the unpaired spin is distributed in the molecule. The n,pð* state possesses an electron in the half-filled p orbital on oxygen, thus manifesting the chemical reactivity of alkoxy radicals. There is only little spin density on oxygen in the pð,ðpð* state and the electrons are mainly delocalized in the benzene ring. Their energy order can be drastically influenced by substitution of the benzene ring or varying the solvent.
Experiments with naphthyl ketones which have the pð,ðpð* state lower in energy by ~ 9 kcal mol 1 demonstrated that the HAT can occur even from the pð,ðpð* state, but its efficiency is only 0.01 0.001 % that of the lowest n,pð* triplets. ADDIN EN.CITE Hammond1962710207-&<Go to ISI>://A19623083B00012Hammond, G. S.Leermakers, P. A.Mechanisms of Photoreactions in Solution .6. Reduction of 1-Naphthaldehyde and 2-AcetonaphthoneJournal of the American Chemical Society1962842ISI:A19623083B00012Deboer19737203963-3969<Go to ISI>://A1973P845500022Deboer, C. D.Herkstro.Wg,Marchett.Ap,Schultz, A. G.Schlessi.Rh,Norrish Type-Ii Rearrangement from Pi Pi Triplet-StatesJournal of the American Chemical Society19739512ISI:A1973P84550002237,38 When the energy gap is reasonably small ( 99200p-(NH+)330> 99330o-(N)19> 9919m-(N)31> 9931p-(N)68> 9968H139913p-OCF3139514o- CF313> 9913m- CF332> 9932p- CF328> 9928o-alkyl3.02512m-alkyl3.93512p-alkyl1.81810o-OMe0.30310m-OMe0.020.1515p-OMe0.0616p-OAc4.42518o-F149914m-F189918p-F159915m-Cl169920p-Cl3.01618m-CO2Me289928p-CO2Me124030o-CN239923m-CN309930p-CN6.82132m-COR145028p-COR2.7930p-SMe< 0.001< 0.0114p-SCF33.01032 Source: Adapted from Wagner and Park ADDIN EN.CITE Wagner1991740Wagner, P. J.;Park, B. S.;1991Photoinduced hydrogen atom abstraction by carbonyl compoundsOrg. Photochemistry1122740

According to spectroscopic measurements of dipole moment changes the pð,ðpð* state was assigned to be more polar and the n,ðpð* state less polar than the ground state. ADDIN EN.CITE Wagner1973750Wagner, P. J.Kemppain.Ae,Schott, H. N.1973Effects of Ring Substituents on Type-II Photoreactions of Phenyl Ketones - How Interactions between Nearby Excited Triplets Affect Chemical ReactivityJournal of the American Chemical Society95175604-5614ISI:A1973Q502700027<Go to ISI>://A1973Q50270002741 Thus electron-donating groups are believed to stabilize the pð,ðpð* states. This can be clearly seen in Table 2.1. Electron-donating group in the para position like p-OMe, p-SMe decreases the population of the reactive state. In the case of the latter, the energy gap becomes so pronounced that the reaction is almost completely suppressed. Similar trend can be found by considering the conjugative effect with electron-withdrawing groups in the para position. These groups obviously stabilize pð,ðpð* triplets more than they stabilize n,ðpð* triplets inductively when compared with meta substituents where only the inductive effect applies. ADDIN EN.CITE Wagner19817607329-7335<Go to ISI>://A1981MS23700048Wagner, P. J.Siebert, E. J.Deactivation of Triplet Phenyl Alkyl Ketones by Conjugatively Electron-Withdrawing SubstituentsJournal of the American Chemical Society198110324ISI:A1981MS2370004842 The ortho substituents have the same type of inductive effect magnifying the electron deficiency of the carbonyl oxygen thus increasing the reactivity in HAT. Besides this effect steric hindrance must be considered, too. For instance, o-CN group enhances the valerophenone triplet reactivity whereas that of o-CO2Me valerophenone (not included in Table 2.1) is deactivated. ADDIN EN.CITE Wagner19817607329-7335<Go to ISI>://A1981MS23700048Wagner, P. J.Siebert, E. J.Deactivation of Triplet Phenyl Alkyl Ketones by Conjugatively Electron-Withdrawing SubstituentsJournal of the American Chemical Society198110324ISI:A1981MS2370004842
Even a correlation with well known linear free energy relationships was attempted. It has been found that the triplet excitation energies of benzophenones correlate with the common Hammett sð ðvalues of substituents. It was assumed that the n,ðpð* triplet levels of phenyl alkyl ketones are affected the same way. However, n,ðpð* triplet of benzophenone is energetically far enough below the pð,ðpð* triplet hence all the substituents we have considered do not invert the triplet levels. ADDIN EN.CITE Wagner1973750Wagner, P. J.Kemppain.Ae,Schott, H. N.1973Effects of Ring Substituents on Type-II Photoreactions of Phenyl Ketones - How Interactions between Nearby Excited Triplets Affect Chemical ReactivityJournal of the American Chemical Society95175604-5614ISI:A1973Q502700027<Go to ISI>://A1973Q50270002741 Inductively electron-withdrawing substituents such as CF3 do correlate very well with substituent sð ðvalues as the n,ðpð* state is the lowest here. ADDIN EN.CITE Loutfy19737702251-2252<Go to ISI>://A1973Q326500003Loutfy, R. O.Correlation between N,Pi Triplet Energy of Some Acetophenones and Corresponding Electroreduction PotentialsTetrahedron19732915ISI:A1973Q32650000343 But the Hammett constants do not adequately describe the effect of substituents on pð,ðpð* transition energies and the energy gap between the states of interest does not correlate at all. It can be concluded the simple Hammett relation is inappropriate for description of the reactivity when all possible substituents are considered.
The polarity of both electronic states differs as we noted above, so the environment can influence the electronic states of alkyl aryl ketones, too. Polar solvents such as alcohols are expected and have been observed to produce the same triplet states inversion as the electron-donating groups do. ADDIN EN.CITE Rauh19687802246-&<Go to ISI>://A1968A986800007Rauh, R. D.Leermake.Pa,Solvent Effects Upon Phosphorescence Lifetimes and Photoreactivity of ButyrophenoneJournal of the American Chemical Society1968909ISI:A1968A986800007Li197079015-18<Go to ISI>://000203002900005Li, Y. H.Lim, E. C.Vibronic perturbation of the lowest triplet state and phosphorescence of aromatic ketonesChemical Physics LettersOct 1197071ISI:00020300290000544,45 The very same behaviour has been observed in photochemistry on polar silica-gel surface. ADDIN EN.CITE Hasegawa2000800437-442<Go to ISI>://000088520000001Hasegawa, T.Kajiyama, M.Yamazaki, Y.Surface photochemistry of alkyl aryl ketones: energy transfer and the effect of a silica-gel surface on electronic states of excited moleculesJournal of Physical Organic ChemistryAug2000138ISI:00008852000000146 This is because the polar solvents destabilize the n,ðpð* state by lowering the energy level of the n orbitals on oxygen and stabilize the pð,ðpð* state by lowering the energy of the pð* orbital. Hence the quantum yields rapidly decrease in polar solvents like methanol. A note must be made here. We mentioned in the previous chapter (2.3.1.1.) that the lifetimes of triplet biradical intermediates have been found to prolong by adding Lewis bases into the solution. This change in the lifetimes is attributed to hydrogen bonding by the relatively acidic OH group of the biradical to the Lewis base thus preventing the disproportionation back to the starting ketone. As a result the quantum yield of the whole type II process can reach almost unity. This is observed for alcohols, too. However, at alcohol concentrations exceeding ~ 0.5 M, a rapid decrease suddenly appears due to energy levels inversion. The ability of different additives to solvate biradicals closely matches their basicities. ADDIN EN.CITE Wagner1972810Wagner, P. J.Kemppain.Ae,Kochevar, I. E.1972Type-II Photoprocesses of Phenyl Ketones - Procedures for Determining Meaningful Quantum Yields and Triplet LifetimesJournal of the American Chemical Society94217489-&ISI:A1972N757500037<Go to ISI>://A1972N75750003747 The rate constant of the ³-hydrogen atom abstraction itself (kHn,pð in Eq. 2.1) is largely solvent independent. ADDIN EN.CITE Wagner19678205898-&<Go to ISI>://A1967A156000029Wagner, P. J.Solvent Effects on Type 2 Photoelimination of Phenyl KetonesJournal of the American Chemical Society19678923ISI:A1967A15600002948

2.3.1.3 The Nature of the CH Bond
In the previous section we have listed several factors which have an impact on the photoreactivity of the carbonyl oxygen. However, the rate of the HAT reaction is influenced by the second reactive site of the molecule which is the ³-CH bond.
A biradical intermediate is formed after the hydrogen atom was abstracted. It is reasonable to suppose the radical at ³-carbon will be stabilized by neighbouring substituents (affecting the CH bond energy and electron density) and, according to Bell-Evans-Polanyi principle, the rate constant of the process will depend on this substitution. The following table summarizes the kinetic parameters of the ³-substituted butyrophenones photochemistry (Table 2.2. ADDIN EN.CITE Wagner1991740Wagner, P. J.;Park, B. S.;1991Photoinduced hydrogen atom abstraction by carbonyl compoundsOrg. Photochemistry1122740
The relative values in the table should hold for any type of hydrogen abstraction by n,ðpð* triplets. ADDIN EN.CITE Wagner2004597Wagner, P. J.;Klan, P.;2004Norrish Tpye II Photoelimination of Ketones:Cleavage of 1,4-Biradicals Formed by ?-Hydrogen AbstractionHorspool, W.;Lenci, F.;CRC Handbook of Photochemistry and PhotobiologyNew YorkCRC Press2nd24 This is indeed observed and will be discussed in the experimental section in the case of o-nitrobenzyl moiety (Chapter 5).
Since the radicals are electron deficient species, negative inductive effect is expected to decrease the rate of the HAT. This can be clearly seen from the very last two rows of the table. However, for most substituents there is an interplay of inductive and resonance effects. Resonance stabilizes the radicals thus an increase in the rate of the reaction is expected. But the opposite is found for the cyano group since the inductive effect strongly counteracts. ðNote the rate constant for dialkyl amino group and its protonated form. This suggests the process in the case of amino substituents should be strongly pH dependent. There is absolutely no correlation between the quantum yields ðFðII and the triplet state reactivities. This indicates the whole process is much more complex which is indeed the case.


Table 2.2. Rate constants in benzene of triplet ³-hydrogen atom abstraction for ³-substituted butyrophenones
³-substituentQuantum yield of type II process (FðII)kobs
(107 s–1)H0.350.7Alkyl0.3314-20Dimethyl0.2550Phenyl0.5040Vinyl0.2650RS0.2764PhS0.3245RSO0.031.2R2N0.02580MeO0.2362HO0.3140PhO0.3222OC(=O)Me0.481.2C(=O)OMe0.501.0Chloro0.111.0N30.0060.5Fluoro0.410.8CN0.320.4RSO20.200.04NHR2+0.009a0.01NR3+0.001a0.001 a In methanol
Source: Adapted from Wagner and Park ADDIN EN.CITE Wagner1991740Wagner, P. J.;Park, B. S.;1991Photoinduced hydrogen atom abstraction by carbonyl compoundsOrg. Photochemistry1122740

2.3.1.4. The Geometry Aspects of ³-Hydrogen Atom Abstraction
Two major factors must be obeyed for an efficient bifunctional intramolecular reaction to occur. The first one is the distance of the two distinct functional groups which undergo the reaction. The second one is their relative orientation in space. The same principles apply in HAT and the rate of the process is strongly dependent on these two factors. There is a vast amount of possible conformers in solution, the population and equilibria of which are dictated by their energy according to Boltzmann distribution and only a small portion of them can be reactive in HAT. This is very important to realize since even the regioselectivity of HAT can be drastically influenced by conformations. Note the selective ´-hydrogen abstraction in Paquette`s synthesis of dodecahedrane. ADDIN EN.CITE Paquette1982830774-783<Go to ISI>://A1982NA88300021Paquette, L. A.Balogh, D. W.An Expedient Synthesis of 1,16-DimethyldodecahedraneJournal of the American Chemical Society19821043ISI:A1982NA8830002149
In order to discuss the proximity and orientation we need to set up some definitions. Four relevant parameters can be introduced (Figure 2.1). The first is d, the distance between the carbonyl oxygen and the ³-hydrogen atom. It seems reasonable for this parameter to equal the sum of the van der Waals radii for oxygen and hydrogen, which is 2.72 Å. ADDIN EN.CITE Bondi1964840441-&<Go to ISI>://A19646650B00003Bondi, A.Van Der Waals Volumes + RadiiJournal of Physical Chemistry1964683ISI:A19646650B0000350 The second parameter ( is defined as the dihedral angle of the carbonyl function and the ³-H atom. Thus ( ðstands for the angle by which the ³-hydrogen lies outside the plane of the carbonyl group. Since the n,ðpð* state has a half-filled p orbital on oxygen in the plane of the carbonyl group, very small values (near 0Ú) are expected. The other two parameters define the C=O···H (") and C-H···O (¸) angles. Since an n-orbital on oxygen is 2p or sp2 hybridized, values ranging from 90Ú to 120Ú should apply for the former. The latter one should have the optimum value of 180Ú theoretically. ADDIN EN.CITE Dorigo1987302195-2197<Go to ISI>://A1987G695000056Dorigo, A. E.Houk, K. N.Transition Structures for Intramolecular Hydrogen-Atom Transfers - the Energetic Advantage of 7-Membered over 6-Membered Transition StructuresJournal of the American Chemical SocietyUNIV CALIF LOS ANGELES,DEPT CHEM & BIOCHEM,LOS ANGELES,CA 90024.J. Am. Chem. Soc.Apr 119871097ISI:A1987G69500005623 It is understandable that measurements of these parameters are almost impossible in solution. Nevertheless, their common values were obtained by measuring the X-ray diffraction patterns of molecules that can undergo the HAT in the solid state. This approach of comparing ground state geometry parameters with triplet reactivity is valid here because the n,ðpð* excitation is so highly localized on the carbonyl group that geometric changes in the rest of the molecule are negligible.

Figure 2.1. Geometry parameters relevant to HAT reaction depiction

A table from various research efforts was compiled by Scheffer et al. ADDIN EN.CITE Scheffer2004857Scheffer, J.R.;Scott, C.;2004Crystal Strucuture-Solid-State Reactivity Relationships:Toward a Greater Understanding of Norrish/Yang Type II PhotochemistryHorspool, W.;Lenci, F.;CRC Handbook of Photochemistry and PhotobiologyNew YorkCRC Press51 A deeper analysis of the collected data relatively confirms some of our ideal values defined above. However, vast deviations are observed, too. The distance d ranges from 2.17 Å to 3.15 Å. Nevertheless, most of the values fall within the sum of van der Waals radii 2.72 ± 0.2 Å. The observed values of ( ðclearly contradict the ideal value of 0Ú. This discrepancy has been explained and some evidence for a cos2( dependence for the rate of ³-hydrogen abstraction has been provided by Wagner. ADDIN EN.CITE Wagner2004597Wagner, P. J.;Klan, P.;2004Norrish Tpye II Photoelimination of Ketones:Cleavage of 1,4-Biradicals Formed by ?-Hydrogen AbstractionHorspool, W.;Lenci, F.;CRC Handbook of Photochemistry and PhotobiologyNew YorkCRC Press2ndWagner19898605389-5392<Go to ISI>://A1989AW70800001Wagner, P. J.Zhou, B. L.Efficient Solid-State Photocyclization of Sterically Congested Alpha-Ortho-Tolyl Ketones Despite Poor Geometries for Hydrogen AbstractionTetrahedron Letters19893040ISI:A1989AW7080000124,52 Such a relationship reflects the angle dependence of the wavefunction describing the p orbital. With respect to the angle ", the experimental data are in reasonable agreement with the hypothetical values in between 90 and 120Ú and on average much closer to the lower boundary. It is thought this lower value preference to be an evidence for the non-bonding orbital of almost pure 2p-like in character or more likely due to the geometric constraints inherent in a six-membered transition state process. The same reasoning is applied with the last of the used parameters. It is simply impossible to attain linear arrangement of C-H···O angle in a six-membered transition state. With this type of argument in mind one can be satisfied with the average value of 115 ± 10Ú found for ¸.

2.3.1.5. Photoenolization
Another very nice application of HAT reaction with ³-regioselectivity is the intramolecular photoenolization of o-alkylphenyl ketones. ADDIN EN.CITE Zwicker19638702671-&<Go to ISI>://A19633104B00050Zwicker, E. F.Yang, N. C.Grossweiner, L. I.Role of - Pi' Triplet in Photochemical Enolization of Omicron-BenzylbenzophenoneJournal of the American Chemical Society19638517ISI:A19633104B00050Huffman19658805417-&<Go to ISI>://A19657059300030Huffman, K. R.Loy, M.Ullman, E. F.Photoenolization of Some Photochromic Ketones . Scope and Mechanism of ReactionJournal of the American Chemical Society19658723ISI:A1965705930003053,54 The first stages of the reaction are the same as those for the classical type II process (Scheme 2.3). There is, however, difference for the biradical decay. Two isomeric xylylenols of Z- and E-configuration are generated via the triplet biradical intermediate, 3E. The triplet enol (3E) was reported by experiments with paraquat and LFP of o-methylacetophenone (Scheme 2.3) with lifetime of 900 ns in degassed MeOH. ADDIN EN.CITE Small19778907713-7714<Go to ISI>://A1977EA14800055Small, R. D.Scaiano, J. C.Role of Biradical Intermediates in Photochemistry of Ortho-MethylacetophenoneJournal of the American Chemical Society19779923ISI:A1977EA14800055Das19799006965-6970<Go to ISI>://A1979HT16800030Das, P. K.Encinas, M. V.Small, R. D.Scaiano, J. C.Photoenolization of Ortho-Alkyl-Substituted Carbonyl-Compounds - Use of Electron-Transfer Processes to Characterize Transient IntermediatesJournal of the American Chemical Society197910123ISI:A1979HT1680003055,56 Surprisingly, piperylene did not affect its lifetime which was assigned to its low triplet energy ADDIN EN.CITE Kumar19839105143-5144<Go to ISI>://A1983RA11300054Kumar, C. V.Chattopadhyay, S. K.Das, P. K.Triplet Excitation Transfer to Carotenoids from Biradical Intermediates in Norrish Type-Ii Photoreactions of Ortho-Alkyl-Substituted Aromatic Carbonyl-CompoundsJournal of the American Chemical Society198310515ISI:A1983RA1130005457, but decreased its yield by quenching the triplet ketone precursor.

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Scheme 2.3. Photoenolization of o-methylacetophenone ADDIN EN.CITE Pelliccioli20019307931-7932<Go to ISI>://000170506400030Pelliccioli, A. P.Klan, P.Zabadal, M.Wirz, J.Photorelease of HCl from o-methylphenacyl chloride proceeds through the Z-xylylenolJournal of the American Chemical SocietyAug 15200112332ISI:00017050640003058

There is, however, evidence the reaction is not exclusively of triplet origin. ADDIN EN.CITE Wagner1976920239-240<Go to ISI>://A1976BB52200042Wagner, P. J.Chen, C. P.Rotation-Controlled Excited-State Reaction - Photoenolization of Ortho-Alkyl Phenyl KetonesJournal of the American Chemical Society1976981ISI:A1976BB5220004259 Excited singlet ketone can compete efficiently with ISC and yield only the Z-enol. The difference in the diastereoselectivity is a consequence of CAr CO bond rotation absence in the excited singlet state. The Z-enol decays rapidly with tð = 730 ns in MeOH or tð ~ 20 ns in nonpolar solvents like cyclohexane by intramolecular reketonization. The lifetime of E-enol of various ketones range from mðs to hours and requires proton transfer through solvent. ADDIN EN.CITE Zwicker19638702671-&<Go to ISI>://A19633104B00050Zwicker, E. F.Yang, N. C.Grossweiner, L. I.Role of - Pi' Triplet in Photochemical Enolization of Omicron-BenzylbenzophenoneJournal of the American Chemical Society19638517ISI:A19633104B00050Huffman19658805417-&<Go to ISI>://A19657059300030Huffman, K. R.Loy, M.Ullman, E. F.Photoenolization of Some Photochromic Ketones . Scope and Mechanism of ReactionJournal of the American Chemical Society19658723ISI:A19657059300030Small19778907713-7714<Go to ISI>://A1977EA14800055Small, R. D.Scaiano, J. C.Role of Biradical Intermediates in Photochemistry of Ortho-MethylacetophenoneJournal of the American Chemical Society19779923ISI:A1977EA14800055Das19799006965-6970<Go to ISI>://A1979HT16800030Das, P. K.Encinas, M. V.Small, R. D.Scaiano, J. C.Photoenolization of Ortho-Alkyl-Substituted Carbonyl-Compounds - Use of Electron-Transfer Processes to Characterize Transient IntermediatesJournal of the American Chemical Society197910123ISI:A1979HT1680003053-56
Photoenolization of o-alkyl ketones was the first reaction recognized to show rotational control of triplet reactivity in which an irreversible bond rotation produces a reactive conformer of the excited state. ADDIN EN.CITE Wagner1976280239-240<Go to ISI>://A1976BB52200042Wagner, P. J.Chen, C. P.Rotation-Controlled Excited-State Reaction - Photoenolization of Ortho-Alkyl Phenyl KetonesJournal of the American Chemical SocietyMICHIGAN STATE UNIV,DEPT CHEM,E LANSING,MI 48824.J. Am. Chem. Soc.1976981ISI:A1976BB5220004259 This phenomenon arises from the coexistence of the syn and anti conformers of the parent ketone in the ground state which upon irradiation yield two different triplet excited ketones. Comparing with the known parameters of competing type II process, the rate constant of the CAr–CO bond rotation was estimated and considered as the rate-determining step for the through long lived triplet photoenolization (Scheme 2.4).

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Scheme 2.4. Rotational control of o-alkyl ketones photoenolization in triplet state

The photoenolization can be extended by introduction of a leaving group to að-position of the carbonyl group. LPF was exerted to decipher the photochemical mechanism of such a system. The model compound was o-methylphenacyl chloride (1). ADDIN EN.CITE Pelliccioli20019307931-7932<Go to ISI>://000170506400030Pelliccioli, A. P.Klan, P.Zabadal, M.Wirz, J.Photorelease of HCl from o-methylphenacyl chloride proceeds through the Z-xylylenolJournal of the American Chemical SocietyAug 15200112332ISI:00017050640003058 After irradiation of 1 in benzene, 3E and both Z- (tð = 225 ns) and E-enols (tð = 10 ms) were observed. Addition of piperylene reduced only the amount of 3E and both Z- and E-isomers of the xylylenol of 1. The identity of Z- and E-enols of 1 were assigned according to the Stern-Volmer relation analysis. The longer lived species gave linear relation since it was exclusively triplet derived whereas the faster one reached a plateau at higher concentrations of the quencher. This indicated both singlet and triplet reaction pathways and the Z-configuration assignment for the faster transient. Similar results were obtained in acetonitrile whereas the triplet pathway was absent in MeOH. A mechanism for the formation of observed final products was proposed by considering all the arguments above (Scheme 2.5). Only the indanone product (2) is formed solely from E-enol formed via triplet in benzene. Although no cation species was observed during the experiments, it is believed that 2 as well as the 2-methoxymethylacetophenone (3) are formed via this cationic intermediate in methanol.

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Scheme 2.5. The mechanism proposal for photolysis of 1; only the ground state processes are depicted

There is one more mechanism by which HAT can proceed which deserves our attention. The phosphorescence measurements of o-methyl aryl ketones at very low temperatures (15 - 80 K) suggest quantum mechanical tunnelling to occur. ADDIN EN.CITE Garciagaribay199498012095-12096<Go to ISI>://A1994QA28800077Garciagaribay, M. A.Gamarnik, A.Pang, L.Jenks, W. S.Excited-State Intramolecular Hydrogen-Atom Transfer at Ultralow Temperatures - Evidence for Tunneling and Activated Mechanisms in 1,4-DimethylanthroneJournal of the American Chemical SocietyDec 28199411626ISI:A1994QA28800077Garciagaribay199597010264-10275<Go to ISI>://A1995TB35300011Garciagaribay, M. A.Gamarnik, A.Bise, R.Pang, L.Jenks, W. S.Primary Isotope Effects on Excited-State Hydrogen-Atom Transfer-Reactions - Activated and Tunneling Mechanisms in an Ortho-MethylanthroneJournal of the American Chemical SocietyOct 18199511741ISI:A1995TB35300011Johnson19969904697-4700<Go to ISI>://A1996UB16200004Johnson, B. A.Gamarnik, A.GarciaGaribay, M. A.Deuterium tunneling in triplet 5,8-dimethyl-1-tetralone by phosphorescence detection between 80 and 15 KJournal of Physical ChemistryMar 21199610012ISI:A1996UB16200004Johnson19999608114-8115<Go to ISI>://000082706500020Johnson, B. A.Garcia-Garibay, M. A.Rate acceleration below 20 K in the H-atom tunneling of triplet ortho-methyltetralonesJournal of the American Chemical SocietySep 8199912135ISI:000082706500020Johnson20019506941-6942<Go to ISI>://000169803000031Johnson, B. A.Hu, Y. F.Houk, K. N.Garcia-Garibay, M. A.Vibrationally assisted tunneling in a hydrogen atom transfer reactionJournal of the American Chemical SocietyJul 18200112328ISI:000169803000031Campos200594010178-10179<Go to ISI>://000230700700028Campos, L. M.Warrier, M. V.Peterfy, K.Houk, K. N.Garcia-Garibay, M. A.Secondary alpha isotope effects on deuterium tunneling in triplet o-methylanthrones: Extraordinary sensitivity to barrier widthJournal of the American Chemical SocietyJul 27200512729ISI:00023070070002860-65 The use of photoenolization for this purpose possesses several advantages. The major one is the possibility of fluorescence measurements of the produced photoenols which reketonize at such low temperatures only slowly. Moreover rigid molecules can be used, thus various competing deactivation processes do not need to be considered. Very low temperatures are neccessary for reaction rates of thermally activated reactions to reach that of other deactivation processes. For instance, clear phosphorescence signal could be detected for deutero-substituted o-methylanthrone whereas no signal was observed for the protio-substituted one. The results of Jenks, Garcia-Garibay et al. confirmed the existence of a very large primary isotope effect and demonstrated a triplet state reaction with contributions from both thermally activated and quantum mechanical tunnelling mechanism.
2.3.2. More Remote Hydrogen Atom Abstractions
We have discussed only the possibility of ³-hydrogen abstractions until now. This regioselectivity comes from both the favorable enthalpically and entropically factors for the six-membered transition state for excited ketones (see Section 2.2.1). Although this type of hydrogen abstractions occur frequently, there are many examples where more remote hydrogen abstraction are observed. For these reactions to take place, six-membered transition state must be made unfavourable making the ³-hydrogen less reactive or completely impossible. Various substitutions are possible for the former case by introducing steric hindrance into the cyclohexane-like transition state or by influencing the reactivity through electronic effects. Another possibility is to control the regioselectivity by prohibiting the reactive conformations to emerge, thus lowering the probability of the two functions to meet or distorting the necessary proximity of the C=O group and the hydrogen. The complete absence of any ³-hydrogens is the solution for the latter. Since the more and more remote abstractions are energetically and conformationally more demanding, the previous arguments must be considered very carefully for designing any new experiments. ´-Hydrogen atom transfers in excited ketones can be often met in literature ADDIN EN.CITE Paquette1982830774-783<Go to ISI>://A1982NA88300021Paquette, L. A.Balogh, D. W.An Expedient Synthesis of 1,16-DimethyldodecahedraneJournal of the American Chemical Society19821043ISI:A1982NA88300021Wagner197110004958-&<Go to ISI>://A1971K343500077Wagner, P. J.Zepp, R. G.Gamma-Hydrogen Vs Delta-Hydrogen Abstraction in Photochemistry of Beta-Alkoxy Ketones - Overlooked Reaction of Hydroxy BiradicalsJournal of the American Chemical Society19719319ISI:A1971K34350007749,66 but even very remote hydrogen abstractions have been reported by Breslow ADDIN EN.CITE Breslow19801010170-177<Go to ISI>://A1980JV83400002Breslow, R.Biomimetic Control of Chemical SelectivityAccounts of Chemical Research1980136ISI:A1980JV8340000267 and Winnick et al. ADDIN EN.CITE Winnik197410206182-6184<Go to ISI>://A1974U143000039Winnik, M. A.Lee, C. K.Basu, S.Saunders, D. S.Conformational-Analysis of Hydrocarbon Chains in Solution - Carbon-TetrachlorideJournal of the American Chemical Society19749619ISI:A1974U14300003968 with their study of n-alkyl p-benzoylbenzoates with alkyl groups containing from 14 to 20 carbon atoms. The ketones were shown to photocyclize after hydrogen was abstracted from methylene groups from carbons between C9 and C17. For such a large cyclic transition states almost no strain penalty is paid to enthalpy of activation. It is interesting to compare the rate constants for these very remote HAT with kH ~ 5 × 105 s–1 and increasing rate by roughly 1 × 105 s–1 with each additional methylene group beyond C9 ADDIN EN.CITE Winnik197410206182-6184<Go to ISI>://A1974U143000039Winnik, M. A.Lee, C. K.Basu, S.Saunders, D. S.Conformational-Analysis of Hydrocarbon Chains in Solution - Carbon-TetrachlorideJournal of the American Chemical Society19749619ISI:A1974U14300003968, kH ~ 5 × 106 s 1 for o-alkoxyphenyl ketones (´-abstraction) ADDIN EN.CITE Wagner199010305199-5211<Go to ISI>://A1990DK85500031Wagner, P. J.Meador, M. A.Park, B. S.The Photocyclization of Ortho-Alkoxy Phenyl KetonesJournal of the American Chemical SocietyJun 20199011213ISI:A1990DK8550003169 and kH ~ 2 × 105 s–1 for bð-(o-tolylphenyl)-propiophenones (µ-abstraction). ADDIN EN.CITE Zhou198910406796-6799<Go to ISI>://A1989AM21600045Zhou, B.Wagner, P. J.Long-Range Triplet Hydrogen Abstraction - Photochemical Formation of 2-Tetralols from Beta-ArylpropiophenonesJournal of the American Chemical SocietyAug 16198911117ISI:A1989AM2160004570 The rates for ³ and ´- HAT can be, however, of the same order of magnitude. ADDIN EN.CITE Wagner1972270Wagner, P. J.Zepp, R. G.Kelso, P. A.Kemppain.Ae,1972Type-II Photoprocesses of Phenyl Ketones - Competitive Delta-Hydrogen Abstraction and Geometry of Intramolecular Hydrogen-Atom TransfersJournal of the American Chemical Society94217500-&J. Am. Chem. Soc.ISI:A1972N757500039<Go to ISI>://A1972N75750003920

2.4. The Nitroaromatic Chromophore
Now we will skip to completely different type of chromophore which is expected to follow different photophysical and photochemical routes. This is indeed the case for many examples, however, a simple analogy from basic organic chemistry could suggest a similar outlook at both the nitroaromatic and aromatic ketones can be done. It is well known that both nitro and carbonyl compounds belong to electron-withdrawing groups and both undergo reductions under properly chosen conditions. We have discussed HAT reactions of carbonyl compounds which are in fact the reduction processes and are fast and efficient whenever n,ðpð* excitation occurs. For example both the lowest excited singlet and triplet state of o-nitrophenol or o-nitrobenzaldehyde are n,ðpð* in origin. ADDIN EN.CITE Slavíèek20091054Slavíèek, P.;Onèák, M.;2009Laimgruber200810603872-3882<Go to ISI>://000257145900005Laimgruber, S.Schmierer, T.Gilch, P.Kiewisch, K.Neugebauer, J.The ketene intermediate in the photochemistry of ortho-nitrobenzaldehydePhysical Chemistry Chemical Physics20081026ISI:00025714590000571,72 Thus similar behaviour is expected from this point of view. We will see and discuss very briefly some exprimental findings supporting this surmise that both systems can be viewed in the same way in regards. Yet, caution must be paid when connecting their photophysics and photochemistry. For example introducing the nitro group on the benzene ring of an aromatic ketone causes the excitation to be concentrated on the nitro group exclusively. ADDIN EN.CITE Pei20071070153-158<Go to ISI>://000245401200030Pei, K. M.Ma, Y. F.Zheng, X. M.Li, H. Y.Resonance Raman spectroscopic and density functional theory study of p-nitroacetophenone (PNAP)Chemical Physics LettersMar 2220074371-3ISI:00024540120003073 Ultrafast ISCs within hundreds of femtoseconds were also identified for nitroaromatic compounds ADDIN EN.CITE Mohammed200810803823-3830<Go to ISI>://000255292200004Mohammed, O. F.Vauthey, E.Excited-state dynamics of nitroperylene in solution: Solvent and excitation wavelength dependenceJournal of Physical Chemistry AMay 1200811217ISI:000255292200004Zugazagoitia20081090358-365<Go to ISI>://000252483300003Zugazagoitia, J. S.Almora-Diaz, C. X.Peon, J.Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational studyJournal of Physical Chemistry AJan 2420081123ISI:00025248330000374,75, and more importantly, the gap between the lowest triplets of nitroaromatic compounds is higher ADDIN EN.CITE Zugazagoitia20081090358-365<Go to ISI>://000252483300003Zugazagoitia, J. S.Almora-Diaz, C. X.Peon, J.Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational studyJournal of Physical Chemistry AJan 2420081123ISI:00025248330000375 (with n,ðpð* as the lowest triplet) than for aromatic ketones. Therefore the substitution does not have such fatal impact on triplet reactivity as will be seen in Chapters 4 and 5 where one can compare the triplet state excitation character.

2.4.1. Solvent-Mediated Photoredox Reactions
The irradiation of 4-nitrobenzaldehyde results in isomerization to 4-nitrosobenzoic acid. ADDIN EN.CITE Gorner1998380155-158<Go to ISI>://000072306000010Gorner, H.Photoisomerization of p-nitrobenzaldehyde to p-nitrosobenzoic acid in aqueous solutionJournal of Photochemistry and Photobiology a-ChemistryJan 3119981122-3ISI:00007230600001076 The reaction is dependent on the concentration of water and independent on concentration of the compound itself. No catalysis is observed in the pH range 0 to 10 and the reaction is unaffected by radical trapping agents such as dioxane, methanol or oxygen. In aqueous ammonia, 4-nitrosobenzamide is formed which suggests a ketene intermediate (Scheme 2.6). LPF study point to a deprotonation of the triplet state of parent compound instead of HAT.

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Scheme 2.6. Photoisomerization of 4-nitrobenzaldehyde ADDIN EN.CITE Gorner1998380155-158<Go to ISI>://000072306000010Gorner, H.Photoisomerization of p-nitrobenzaldehyde to p-nitrosobenzoic acid in aqueous solutionJournal of Photochemistry and Photobiology a-ChemistryJan 3119981122-3ISI:00007230600001076

The ketene intermediate is also observed in the photochemistry of 4-nitrophenylglyoxylic acid which decarboxylates upon irradiation presumably in the triplet state. ADDIN EN.CITE Gorner19913905518-5523<Go to ISI>://A1991FW47700029Gorner, H.Currell, L. J.Kuhn, H. J.Photoreaction of (Para-Nitrophenyl)Glyoxylic Acid to Para-Nitrosobenzoic Acid in Aqueous-SolutionJournal of Physical ChemistryJul 1119919514ISI:A1991FW4770002977 The transient obsorption at 350 nm was assigned to the ketene intermediate on the basis of its long lifetime (~ 1.5 mðs in water).
The same kind of oxidation at the benzylic position and reduction of the nitro group to the nitroso one is also observed in the case of 4-nitrobenzyl alcohol and the m-substituted systems. ADDIN EN.CITE Wan19831100136-138<Go to ISI>://A1983PY52600032Wan, P.Yates, K.Photoredox Chemistry of Meta-Nitrobenzyl and Para-Nitrobenzyl Alcohols in Aqueous-Solution - Observation of Novel Catalysis by the Hydronium and Hydroxide Ions in These PhotoreactionsJournal of Organic Chemistry1983481ISI:A1983PY52600032Wan19861110Wan, P.;Yates, K.;1986Photoredox chemistry of nitrobenzyl alcohols in aqueous solution.Acid and base catalysis of reactionsCan. J. Chem.64207678,79 Water is essential for the process in any case. The transformation of the p-nitrobenzylalcohol can be triplet sensitized and quenched by 3,5-cyclohexadiene-1,2-dicarboxylic acid.

2.4.2. Intramolecular Hydrogen Atom Abstractions
We stated in the introduction to this chapter that we would try to see the analogy between the photochemistry of ketones and nitro compounds. However, until now we have seen some photoreductions but the resemblance was only feeble. We now turn to true HAT from the o-benzylic position to the excited nitro group.
As in the previous examples, the nitro group after excitation abstracts the hydrogen atom reducing itself after several steps to nitroso group while the benzylic position is oxidized. This way o-nitrosobenzyl alcohol, o-nitrosobenzaldehyde or o-nitrosobenzoic acid is formed from o-nitrotoluene, o-nitrobenzyl alcohol or o-nitrobenzaldehyde, respectively. The intramolecular abstraction of hydrogen from the o-methyl group is usually followed by relaxation to the aci-nitro form of the parent compound which can be deprotonated depending on the pH to the aci-nitro anion (the pKa is 1.1-3.7). ADDIN EN.CITE Schworer200111201441-1458<Go to ISI>://000170156300014Schworer, M.Wirz, J.Photochemical reaction mechanisms of 2-nitrobenzyl compounds in solution I. 2-nitrotoluene: Thermodynamic and kinetic parameters of the aci-nitro tautomerHelvetica Chimica Acta2001846ISI:00017015630001480 The similar ketene structure is proposed when nitrobenzaldehyde is irradiated. ADDIN EN.CITE George19801130492-495<Go to ISI>://A1980JJ01400007George, M. V.Scaiano, J. C.Photochemistry of Ortho-Nitrobenzaldehyde and Related StudiesJournal of Physical Chemistry1980845ISI:A1980JJ01400007Yip19891140109-116<Go to ISI>://A1989U410100001Yip, R. W.Sharma, D. K.The Reactive State in the Photo-Rearrangement of Ortho-NitrobenzaldehydeResearch on Chemical IntermediatesMar1989112ISI:A1989U410100001Laimgruber200511807901-7904<Go to ISI>://000234007700013Laimgruber, S.Schreier, W. J.Schrader, T.Koller, F.Zinth, W.Gilch, P.The photochemistry of o-nitrobenzaldehyde as seen by femtosecond vibrational spectroscopyAngewandte Chemie-International Edition20054448ISI:000234007700013Leyva200811705046-5053<Go to ISI>://000256492200005Leyva, V.Corral, I.Schmierer, T.Heinz, B.Feixas, F.Migani, A.Blancafort, L.Gilch, P.Gonzalez, L.Electronic states of o-nitrobenzaldehyde: A combined experimental and theoretical studyJournal of Physical Chemistry AJun 12200811223ISI:000256492200005Laimgruber200811603872-3882<Go to ISI>://000257145900005Laimgruber, S.Schmierer, T.Gilch, P.Kiewisch, K.Neugebauer, J.The ketene intermediate in the photochemistry of ortho-nitrobenzaldehydePhysical Chemistry Chemical Physics20081026ISI:000257145900005Heinz20081150274-281<Go to ISI>://000259834000022Heinz, B.Schmierer, T.Laimgruber, S.Gilch, P.Excited state processes of nitrobenzaldehydes probed by ultrafast fluorescence and absorption spectroscopyJournal of Photochemistry and Photobiology a-ChemistrySep 2520081992-3ISI:00025983400002272,81-85 Both intermediates have been successfully isolated by matrix isolation techniques. ADDIN EN.CITE Dunkin200111901414-1425<Go to ISI>://000170171000023Dunkin, I. R.Gebicki, J.Kiszka, M.Sanin-Leira, D.Phototautomerism of o-nitrobenzyl compounds: o-quinonoid aci-nitro species studied by matrix isolation and DFT calculationsJournal of the Chemical Society-Perkin Transactions 220018ISI:000170171000023Kuberski19921200105-110<Go to ISI>://A1992KB89800010Kuberski, S.Gebicki, J.Evidence for a Ketene Intermediate in the Photochemical Transformation of Matrix-Isolated O-NitrobenzaldehydeJournal of Molecular StructureDec 11992275ISI:A1992KB8980001086,87 The aci-nitro form (or ketene) can rearrange back to the starting material or cyclize to give benzisoxazoline intermediate (in the case of nitrotoluene rearrangements) which further opens to the final product (Scheme 2.7). These ground state processes are now well understood. ADDIN EN.CITE Il'ichev200012307856-7870<Go to ISI>://000088947900017Il'ichev, Y. V.Wirz, J.Rearrangements of 2-nitrobenzyl compounds. 1. Potential energy surface of 2-nitrotoluene and its isomers explored with ab initio and density functional theory methodsJournal of Physical Chemistry AAug 24200010433ISI:000088947900017Schworer200112201441-1458<Go to ISI>://000170156300014Schworer, M.Wirz, J.Photochemical reaction mechanisms of 2-nitrobenzyl compounds in solution I. 2-nitrotoluene: Thermodynamic and kinetic parameters of the aci-nitro tautomerHelvetica Chimica Acta2001846ISI:000170156300014Gaplovsky2005121033-42<Go to ISI>://000225889800004Gaplovsky, M.Il'ichev, Y. V.Kamdzhilov, Y.Kombarova, S. V.Mac, M.Schworer, M. A.Wirz, J.Photochemical reaction mechanisms of 2-nitrobenzyl compounds: 2-Nitrobenzyl alcohols form 2-nitroso hydrates by dual proton transferPhotochemical & Photobiological Sciences200541ISI:00022588980000480,88,89 When alcohol or aldehyde as the parent compounds are irradiated, benzisoxazolinol or benzisoxazolinone are formed, respectively.

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Scheme 2.7. The ground state chemistry after o-nitrotoluene irradiation

Nanosecond and picosecond laser excitations of various o-nitrobenzyl moiety derivatives have been done to understand its photochemical behaviour. ADDIN EN.CITE George19801130492-495<Go to ISI>://A1980JJ01400007George, M. V.Scaiano, J. C.Photochemistry of Ortho-Nitrobenzaldehyde and Related StudiesJournal of Physical Chemistry1980845ISI:A1980JJ01400007Gorner2005370822-828<Go to ISI>://000232777500007Gorner, H.Effects of 4,5-dimethoxy groups on the time-resolved photoconversion of 2-nitrobenzyl alcohols and 2-nitrobenzaldehyde into nitroso derivativesPhotochemical & Photobiological SciencesOct2005410ISI:000232777500007Heinz20081150274-281<Go to ISI>://000259834000022Heinz, B.Schmierer, T.Laimgruber, S.Gilch, P.Excited state processes of nitrobenzaldehydes probed by ultrafast fluorescence and absorption spectroscopyJournal of Photochemistry and Photobiology a-ChemistrySep 2520081992-3ISI:000259834000022Yip198413405770-5772<Go to ISI>://A1984TU20800004Yip, R. W.Sharma, D. K.Giasson, R.Gravel, D.Picosecond Excited-State Absorption of Alkyl Nitrobenzenes in SolutionJournal of Physical ChemistryCONCORDIA UNIV,CANADIAN PICOSECOND LASER FLASH PHOTOLYSIS CTR,MONTREAL H3G 1M8,QUEBEC,CANADA. UNIV MONTREAL,MONTREAL H3C 3V1,QUEBEC,CANADA. YIP, RW, NATL RES COUNCIL CANADA,DIV CHEM,OTTAWA K1A 0R6,ONTARIO,CANADA.J. Phys. Chem.19848824ISI:A1984TU20800004Yip198513505328-5330<Go to ISI>://A1985AVP2500002Yip, R. W.Sharma, D. K.Giasson, R.Gravel, D.Photochemistry of the Ortho-Nitrobenzyl System in Solution - Evidence for Singlet-State Intramolecular Hydrogen AbstractionJournal of Physical ChemistryCONCORDIA UNIV,CANADIAN PICOSECOND LASER FLASH PHOTOLYSIS CTR,MONTREAL H3G 1M8,QUEBEC,CANADA. UNIV MONTREAL,DEPT CHIM,MONTREAL H3C 3V1,QUEBEC,CANADA. YIP, RW, NATL RES COUNCIL CANADA,DIV CHEM,OTTAWA K1A 0R6,ONTARIO,CANADA.J. Phys. Chem.19858925ISI:A1985AVP250000281,85,90-92 This is very complicated since the process is faster than that of ketones. It is, however, now well established that the reaction can proceed through both singlet and triplet state. This conclusion is based on the rapid formation (after several picoseconds) of the aci-nitro intermediate with absorption maximum at ~ 440–450 nm and no signal between 625 and 650 nm which is believed to belong to the triplet state. A decrease in the yield but not the lifetime of such intermediates by trans-piperylene was reported by some research groups. This can be interpreted in terms of a triplet precursor. ADDIN EN.CITE George19801130492-495<Go to ISI>://A1980JJ01400007George, M. V.Scaiano, J. C.Photochemistry of Ortho-Nitrobenzaldehyde and Related StudiesJournal of Physical Chemistry1980845ISI:A1980JJ0140000781 Nevertheless, there are investigators who did not find any influence of added cis-piperylene on the yield of the transient. ADDIN EN.CITE Yip19891140109-116<Go to ISI>://A1989U410100001Yip, R. W.Sharma, D. K.The Reactive State in the Photo-Rearrangement of Ortho-NitrobenzaldehydeResearch on Chemical IntermediatesMar1989112ISI:A1989U41010000182 To my knowledge, the ratio of the singlet and triplet pathway has not been reported properly. The investigations of geometry of HAT in nitroaromatic compounds are rare. One very nice example of HAT in rigid systems with bridgehead o-benzylic hydrogen has been demonstrated. ADDIN EN.CITE Yip19911260Yip, R. W.;Sharma, D. K.;Blanchet, D.;Giasson, R.;Gravel, D.;1991Photochemistry of the o-nitrobenzyl system in solution:effects of O···H distance and geometrical constraint on the hydrogen transfer mechanism in the excited stateCan. J. Chem.691193-120093 No aci-nitro intermediate is possible in this case but the expected product was formed. LFP showed the triplet state involvement and the formation of 1,4-biradical had been proposed. However, there is no direct evidence for such a biradical to exist in photochemistry of the o-nitrobenzyl moiety.
It is well-established that the rate determining step of the whole process is one of the ground state reactions. Nevertheless, primary kinetic isotope effect (KIE) was found for the HAT indicating an additional rate-determining or slow step in the excited state. ADDIN EN.CITE Blanc200412507174-7175<Go to ISI>://000221963600010Blanc, A.Bochet, C. G.Isotope effects in photochemistry. 1. o-Nitrobenzyl alcohol derivativesJournal of the American Chemical SocietyJun 16200412623ISI:00022196360001094 This primary KIE showed an excitation wavelength dependence suggesting a non-Kasha type photochemistry to be operative. The explanation could be given by reaching different states with different excitation wavelength. The rate and efficiency of the ISC can depend on the energy of incident light as was shown recently for excited-state dynamics of nitroperylene. ADDIN EN.CITE Mohammed200810803823-3830<Go to ISI>://000255292200004Mohammed, O. F.Vauthey, E.Excited-state dynamics of nitroperylene in solution: Solvent and excitation wavelength dependenceJournal of Physical Chemistry AMay 1200811217ISI:00025529220000474 Consequently it seems the multiplicity of the process depends on what singlet excited state is populated directly after the absorption of a photon.
Similar behaviour is observed for o-nitrophenol. The gas phase irradiation produced HONO and an aci-nitro isomer formation was proposed as an intermediate. ADDIN EN.CITE Bejan200612402028-2035<Go to ISI>://000236970300006Bejan, I.Abd El Aal, Y.Barnes, I.Benter, T.Bohn, B.Wiesen, P.Kleffmann, J.The photolysis of ortho-nitrophenols: a new gas phase source of HONOPhysical Chemistry Chemical Physics2006817ISI:00023697030000695 Matrix isolation proved this assumption. ADDIN EN.CITE Nagaya2006127067-71<Go to ISI>://000239753300014Nagaya, M.Kudoh, S.Nakata, M.Infrared spectrum and structure of the aci-nitro form of 2-nitrophenol in a low-temperature argon matrixChemical Physics LettersAug 1820064271-3ISI:00023975330001496 This is however in contrast to aromatic ketones chemistry where hydroxy group causes pð,ðpð* character of both the lowest excited singlet and triplet state and proton transfer in excited singlet is found to be the major deactivation channel. ADDIN EN.CITE Catalan199712807914-7921<Go to ISI>://A1997YB79500033Catalan, J.Palomar, J.dePaz, J. L. G.Intramolecular proton or hydrogen-atom transfer in the ground and excited states of 2-hydroxybenzoyl compoundsJournal of Physical Chemistry AOct 16199710142ISI:A1997YB79500033Yi20051290297-302<Go to ISI>://000231534100009Yi, P. G.Liang, Y. H.Cao, C. Z.Intramolecular proton or hydrogen-atom transfer in the ground- and excited-states of 2-hydroxybenzophenone: A theoretical studyChemical PhysicsAug 2920053153ISI:00023153410000997,98 The lowest excited singlet state in 2-nitrophenol is, however, n,pð* in origin. ADDIN EN.CITE Slavíèek20091054Slavíèek, P.;Onèák, M.;200971 Nevertheless, there is only limited information on the nitrophenol chemistry in the literature.
If no benzylic hydrogens (að-hydrogens) are available free bð-hydrogen atoms will be involved. ADDIN EN.CITE Kitaura197113001583-&<Go to ISI>://A1971J441200004Kitaura, Y.Matsuura, T.Photoinduced Reactions .46. Photochemistry of Hindered Nitrobenzene DerivativesTetrahedron1971278ISI:A1971J44120000499
We will finish with one nice example of intramolecular hydrogen atom transfer in irradiation of N-[wð-(4-nitrophenoxy)alkyl]-anilines in benzene. ADDIN EN.CITE Nakagaki19851310262-266<Go to ISI>://A1985AUC6400018Nakagaki, R.Hiramatsu, M.Mutai, K.Nagakura, S.Photochemistry of Bichromophoric Chain Molecules Containing Electron-Donor and Acceptor Moieties - Dependence of Reaction Pathways on the Chain-Length and Mechanism of Photoredox Reaction of N- Omega-(Para-Nitrophenoxy)Alkyl AnilinesChemical Physics Letters19851213ISI:A1985AUC6400018Nakagaki19871320171-176<Go to ISI>://A1987G360000014Nakagaki, R.Hiramatsu, M.Mutai, K.Tanimoto, Y.Nagakura, S.Photochemistry of Bichromophoric Chain Molecules Containing Electron-Donor and Acceptor Moieties - External Magnetic-Field Effects Upon the Photochemistry of N- Omega-(Para-Nitrophenoxy)Alkyl AnilinesChemical Physics LettersFeb 2019871342ISI:A1987G360000014Nakagaki19881330Nakagaki, R.Mutai, K.Hiramatsu, M.Tukada, H.Nagakura, S.1988Magnetic field effects upon photochemistry of bichromophoric chain molecules containing nitroaromatic and arylamino moieties:Elucidation of reaction mechanism and control of reaction yieldsCan. J. Chem.661989100-102 A light induced cleavage to give aniline and the nitroso aldehyde or the so-called photo-Smiles rearrangement was found to depend on the length of the alkyl chain (Scheme 2.8). This photoredox process, but not the Smile rearrangement, can be markedly influenced by a magnetic field which indicates a biradical intermediate involvement. Notice the similarity with remote hydrogen atom abstractions in the photochemistry of ketones we discussed above where HAT from carbons C9 to C17 was observed.

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Scheme 2.8.














Experimental Section
3.1. Instrumentation

Nuclear magnetic resonance spectra were recorded on a Bruker AVANCE 300 NMR spectrometer. Chemical shifts are reported in ppm (´) relative to an internal standard, tetramethylsilane (TMS) at 0.00 ppm.
Gas chromatography was performed on a Shimadzu GC 2010 gas chromatograph equipped with a 58 m DB-5MS capillary column (0.25 mm internal diameter, coated with 0.25 µm thin layer of phenyl arylene polymer virtually equivalent to a (5%-phenyl)-methylpolysiloxane), He as a carrier gas and a flame (H2 + air) ionization detector.
GC/MS was performed by Mgr. Ing. Lubomír Prokeš on a Shimadzu GC 2010 gas chromatograph equipped with a 30 m DB-XLB capillary column (0.25 mm in diameter, coated with 0.25 µm thin layer of 14 % diphenylpolysiloxane), helium as a carrier gas and a mass spectrometer Shimadzu MS 250 detector in positive mode with EI or on a Hawlett-Packard GCMS 5890/5971 chromatogram with 59 m J&W DB-5MS column (0.25 mm internal diameter, 0.25 µm stationary phase coating), He as a carrier gas and quadrupole mass detector HP 5971 with EI (70 eV).
HPLC analyses were done on a SHIMADZU LC-20AD with RP-HPLC glass column SGC C-18 or C-8 (7 mm; 150 nm). A SHIMADZU SPD-10A was used as a UV detector.
UV absorption spectra and absorption coefficients were obtained on a SHIMADZU UV 1601 UV-VIS spectrophotometer with matched 1.0 cm quartz cells.
Thin layer chromatography was performed using precoated silica gel plates Silica gel 60 F254 (0.20 mm thickness, Merck) and visualized under a UV lamp.
Column chromatography was carried out with silica gel 63-100 µm (Merck).

3.2. Quantum Chemical Calculations
All the calculations were performed with the GAUSSIAN 03 ADDIN EN.CITE Frisch20041376Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A.;2004Gaussian 03, Revision C.02,Wallingford CTGaussian, Inc.,103 package of programs. A different approach was used for Chapter 4 and 6.
In Chapter 4, the geometries were optimized at the B3LYP ADDIN EN.CITE Becke19931380Becke, A. D.;1993J. Chem. Phys.985648104 level of theory with the standard 6-31G(d) basis set. In the case of flat potential energy surface near the minimum, where the optimization procedure using the approximate Hessian matrix was terminated prior to full convergence, force constants were calculated analytically. For all stationary points harmonic vibrational frequencies were computed at B3LYP/6-31G(d) and scaled by 0.9613 ADDIN EN.CITE Scott19961390Scott, A. P.;Radom, L. J.1996J. Phys. Chem.10016502Merrick2007162011683-11700<Go to ISI>://000250809400036Merrick, J. P.Moran, D.Radom, L.An evaluation of harmonic vibrational frequency scale factorsJournal of Physical Chemistry ANov 15200711145ISI:000250809400036105,106 to provide thermal correction to Gibbs free energy at 298 K and 1 atm. Finally, single point energies for the B3LYP geometries were computed at the B3LYP and BMK ADDIN EN.CITE Boese20041400Boese, A. D.;Martin, J. M. L.2004J. Chem. Phys.1213405107 levels of theory with the 6-311+G(3df,2p) basis set. Transition structures (TS) on the potential energy surface were located by using the facility of GAUSSIAN for the synchronous transit-guided quasi-Newton method. ADDIN EN.CITE Peng19961410Peng, C.;Ayala, P. Y.;Schlegel, H. B.;Frisch, M. J.;1996J. Comp. Chem.1749108 The reaction pathway for each TS to verify its connection to local minima was computed by using intrinsic reaction coordinate ADDIN EN.CITE Gonzales19891420Gonzales, C;Schlegel, H. B.;1989J. Chem. Phys.902154109 (IRC) approach as implemented in GAUSSIAN 03. For all stationary points, the wavefunction stability was tested.
In Chapter 5, single point energies for closed-shell molecules were carried out on B3LYP/6-31G* optimized geometries at B3LYP/6-311+G(3df,2p) level of theory. Harmonic vibrational frequencies were calculated at the B3LYP/6-31G* level and used after scaling (0.9806 ADDIN EN.CITE Scott19961390Scott, A. P.;Radom, L. J.1996J. Phys. Chem.10016502Merrick2007162011683-11700<Go to ISI>://000250809400036Merrick, J. P.Moran, D.Radom, L.An evaluation of harmonic vibrational frequency scale factorsJournal of Physical Chemistry ANov 15200711145ISI:000250809400036105,106) to provide zero-point vibrational energies (ZPVEs). The final energies are provided at 0 K (sum of the single point energy and the scaled ZPVE). The transition state structures were treated the same way as described above.
The radical stabilization energies relative to 2-nitrobenzyl radical (RSEs) using the isodesmic reaction approach were computed on ROB3LYP /6-31G* optimized geometries using the restricted and unrestricted versions of the double-hybrid DFT methods B2PLYP ADDIN EN.CITE Grimme20061430Grimme, S.2006J. Chem. Phys.124034108110 and MPW2-PLYP ADDIN EN.CITE Schwabe20061440Schwabe, T.;Grimme, S.2006Phys. Chem. Chem. Phys.84398111 with 6-31+G(2d,p) basis set. Harmonic vibrational frequencies were calculated at the ROB3LYP/6-31G* level and used after scaling (0.9806) to provide zero-point vibrational energies (ZPVEs). The final energies are provided at 0 K (sum of the single point energy and the scaled ZPVE).
Orbitals and pictures of molecules are obtained from MOPLOT freeware program.

3.3. Preparation of Compounds
The following section summarizes experimental procedures used for preparation of the model compounds and their irradiation. Deuteration experiments are described, too.

3.3.1. 1-(2,5-Dimethylphenyl)propan-1-one (4)
The mixture of aluminium chloride (32.1 g, 240.5 mmol) and p-xylene (19.6 g, 185 mmol) in 250 ml of CS2 under N2 atmosphere was cooled down to 0 ÚC in an ice-bath. Then propionyl chloride (17.8 ml, 203 mmol) was added dropwise during 1.5 hour keeping the temperature at 1 3 ÚC. The reaction mixture was stirred for 1 hour at 0 ÚC then warmed up to the ambient temperature and stirred for additional 2 hours. The reaction mixture was worked-up by pouring on ice (400 ml). After ice melted the aqueous layer was extracted 3 times with 100 ml of CH2Cl2. Combined organic extracts were washed with brine and dried with magnesium sulfate. The solvent evaporation afforded crude product mixture which was purified by vacuum distillation to obtain 28.8 g (96 %) of colourless liquid.

Yield: 28.8 g (96 %), colourless liquid
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.08 (t, J = 7.3 Hz, 3H), 2.22 (s, 3H), 2.34 (s, 3H), 2.75 (q, J = 7.3 Hz, 2H), 6.96 (d, J = 7.8 Hz, 1H), 7.01 (d, J = 7.8 Hz, 1H), 7.31 (s, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 8.0 (CH3), 20.3 (CH3), 20.4 (CH3), 34.1 (CH2), 128.5 (CH), 131.3 (CH), 131.4 (CH), 134.2 (Cq), 134.7 (Cq), 137.7 (Cq), 204.1 (Cq)
MS (EI, 70 ev): m/z 162 (M+), 133, 105, 91, 77, 65, 51, 41

3.3.2. 1-(2,5-Dimethylphenyl)-2-phenylethanone (5)
The mixture of aluminum chloride (17.3 g, 130 mmol) and p-xylene (10.6 g, 100 mmol) in 170 ml of CS2 under N2 atmosphere was cooled down to -10 ÚC in an ice-bath. Then phenylacetyl chloride (14.0 ml, 105 mmol) was added dropwise keeping the temperature below -5 ÚC. The reaction mixture was stirred for 1 hour at this temperature then warmed up to ambient temperature and stirred for additional 3 hours. The reaction mixture was worked-up by pouring on ice (400 ml). After ice melted the aqueous layer was extracted 4 times with 70 ml of CH2Cl2. Combined organic extracts were washed with brine and dried with magnesium sulfate. The solvent evaporation afforded crude product mixture which was purified by vacuum distillation (135140 ÚC, 1 mm Hg) to obtain 21.3 g (95 %) of colourless liquid which crystallized in fridge.

Yield: 21.3 g (95 %), colourless crystals
1H NMR (300 MHz, CDCl3): ´ (ppm) 2.42 (s, 3H), 2.46 (s, 3H), 4.25 (s, 2H), 7.16 (d, J = 7.8 Hz, 1H), 7.22 (dd, J1 = 7.8 Hz, J2 = 1.5 Hz, 1H), 7.31 (m, J = 7.0 Hz, 3H), 7.36 (d, J = 7.0 Hz, 2H), 7.58 (d, J = 1.5 Hz, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 20.8 (CH3), 21.0 (CH3), 48.4 (CH2), 126.9 (CH), 128.6 (CH), 129.2 (CH), 129.6 (CH), 131.9 (CH), 132.1 (CH), 134.7 (Cq), 135.1 (Cq), 135.3 (Cq), 137.8 (Cq), 201.6 (Cq)
MS (EI, 70 ev): m/z 224 (M+), 133, 105, 91, 77, 65, 51, 39

3.3.3. 1-(2,5-Dimethylphenyl)-2-methylprop-2-en-1-one (6)
Acetic acid (10ml) was added to a magnetically stirred solution of 1-(2,5-dimethylphenyl)propan-1-one (4) (1.62 g, 10 mmol) with aqueous solution of formaldehyde (2.4 ml, 37 % solution, 30 mmol). Then catalytic amount (few drops) of morpholine was added and the resulting mixture was refluxed for 5 days. After cooling the reaction mixture to ambient temperature this was neutralized with 20 % aqueous NaOH and extracted three times with 50 ml of CH2Cl2. Combined organic extracts were washed with brine and dried with magnesium sulfate. 1.95 g of crude slightly yellow product mixture was obtained. The flash column chromatography of the mixture (silica, petroleum ether : ethyl acetate, 8:1) furnished colourless oily compound. Vacuum distillation causes the product mixture to polymerize.

Yield: 1.65 g (95 %), colourless liquid
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.99 (s, 3H), 2.19 (s, 3H), 2.25 (s, 3H), 5.53 (s, 1H), 5.89 (s, 1H), 6.99 (s, 1H), 7.02 (d, J = 7.9 Hz, 1H), 7.06 (d, J = 7.9 Hz, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 17.0 (CH3), 19.0 (CH3), 20.6 (CH3), 128.1 (CH), 129.5 (CH2), 130.3 (CH), 130.5 (CH), 132.6 (Cq), 134.3 (Cq), 138.9 (Cq), 145.0 (Cq), 200.5 (Cq)
MS (EI, 70 ev): m/z 174 (M+), 159, 145, 133, 115, 105, 91, 77, 65, 51, 41

3.3.4. 1-(2,5-Dimethylphenyl)-2-phenylprop-2-en-1-one (7)
Piperidine (0.13 ml, 1.3 mmol), acetic acid (0.12 ml, 2.1 mmol) and formaline (4 ml, 37 % aqueous solution, 50 mmol) were successively added to a magnetically stirred solution of 1-(2,5-dimethylphenyl)-2-phenylethanone (5) (2.92 g, 13.0 mmol) in 50 ml of MeOH and the resulting mixture was refluxed for 4 hours followed by concentration in vacuo. Water (50 ml) was added to the residue and the mixture was extrated three times with 30 ml of CH2Cl2. Combined organic extracts were washed twice with 50 ml of water and with brine and dried with magnesium sulfate. 2.8 g of crude product mixture was obtained. Purification was done by the flash column chromatography (silica, petroleum ether : ethyl acetate, 8:1).

Yield: 2.5 g (81 %), white solid
1H NMR (300 MHz, CDCl3): ´ (ppm) 2.33 (s, 3H), 2.42 (s, 3H), 5.75 (s, 1H), 6.17 (s, 1H), 7.14 (d, J = 7.8 Hz, 1H), 7.19 (dd, J1 = 7.8 Hz, J2 = 1.3 Hz, 1H), 7.29 (d, J = 1.3 Hz, 1H), 7.38 (m, 3H), 7.47 (dd, J1 = 7.9 Hz, J2 = 1.7 Hz, 1H),
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 19.9 (CH3), 21.0 (CH3), 126.9 (CH2), 128.2 (CH), 128.46 (CH), 128.5 (CH), 129.9 (CH), 131.2 (CH), 131.6 (CH), 134.4 (Cq), 135.0 (Cq), 137.0 (Cq), 149.8 (Cq), 199.9 (Cq)
MS (EI, 70 ev): m/z 236 (M+), 159, 145, 133, 105, 77, 63, 51

3.3.5. 1-(2,5-Dimethylphenyl)-2,3-epoxy-3-phenylpropan-1-one (8a) and 1-(2,5-Dimethylphenyl)-2,3-epoxybutan-1-one (8b)
1-(2,5-Dimethylphenyl)-2,3-epoxy-3-phenylpropan-1-one (8a) and 1-(2,5-Dimethylphenyl)-2,3-epoxybutan-1-one (8b) were prepared as described previously. ADDIN EN.CITE Šolomek20071452Šolomek, T.2007Photochemical synthesis of indan-1-ones via dimethylphenacyl epoxidesDepartment of ChemistryBrnoMasaryk University58Bachelor work112


8a:
Yield: 94 % (1.00 g), white solid
1H NMR (300 MHz, CDCl3): ´ (ppm) 2.34 (s, 3H), 2.49 (s, 3H), 4.05 (d, J = 1.8 Hz, 1H), 4.08 (d, J = 1.8 Hz, 1H), 7.17 (d, J = 7.8 Hz, 1H), 7.23 (d, J = 7.8 Hz, 1H), 7.33-7.42 (m, 5H), 7.47 (s, 1H)
13C NMR (75.5 MHz, DMSO): ´ (ppm) 19.9 (CH3), 20.3 (CH3), 58.5 (CH), 61.3 (CH), 126.2 (CH), 128.5 (CH), 128.8 (CH), 129.3 (CH), 131.5 (CH), 132.7 (CH), 134.5 (Cq), 135.1 (Cq), 135.4 (Cq), 135.6 (Cq), 196.7 (Cq)
MS (EI, 70 ev): m/z 252 (M+), 237, 222, 207, 178, 148, 133, 119, 105, 91, 77, 51, 44


8b:
Yield: 35 % (180 mg) over two steps, colourless oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.44 (d, J = 5.0, 3H), 2.32 (s, 3H), 2.40 (s, 3H), 3.12 (dq, J1 = 5.0, J2 = 1.8 Hz, 1H), 3.71 (d, J = 1.8 Hz, 1H), 7.10 (d, J = 7.6 Hz, 1H), 7.17 (d, J = 7.6 Hz, 1H), 7.41 (s, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 17.2 (CH3), 20.0 (CH3), 20.6 (CH3), 55.6 (CH), 59.3 (CH), 128.9 (CH), 131.5 (CH), 132.4 (CH), 134.9 (Cq), 135.0 (Cq), 135.5 (Cq), 198.4 (Cq)
MS (EI, 30 eV): m/z (M+) 190 not observed, 175, 133, 117,115, 105, 103, 79, 77, 58, 43


3.3.6. (2,5-Dimethylphenyl)(2-methyloxiran-2-yl)methanone (8c)
Stirred solution of 1-(2,5-dimethylphenyl)-2-methylprop-2-en-1-one (6) (3.95 g, 22.7 mol) in 100 ml of MeOH was cooled in ice bath. Then H2O2 (5.8 ml, 30 % aqueous solution, 56.75 mmol) was added dropwise followed by addition of cooled KOH (0.64 g, 11.35 mmol) in 20 ml of MeOH. The temperature was kept below 0 ÚC and the mixture was stirred for 1.5 hour. The progress of the reaction was monitored by TLC. After completion, the reaction was quenched with 150 ml of water and extracted three times with 30 ml of CH2Cl2. The organic extracts were washed with brine (50 ml) and dried with magnesium sulfate. The crude product mixture was purified by flash column chromatography (silica, petroleum ether : ethyl acetate, 20:1 to 10:1) to obtain 3.45 g (80 %) of pure product.

Yield: 3.45 g (80 %), colourless oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.53 (s, 3H), 2.20 (s, 3H), 2.22 (s, 3H), 2.66 (d, J = 5.4 Hz, 1H), 2.71 (d, J = 5.4 Hz, 1H), 6.95 (d, J = 7.8 Hz, 1H), 7.01 (d, J = 7.8 Hz, 1H), 7.15 (s, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 17.0 (CH3), 19.0 (CH3), 20.3 (CH3), 51.7 (CH2), 59.2 (Cq), 127.8 (CH), 130.3 (CH), 131.0 (CH), 133.4 (Cq), 134.1 (Cq), 135.4 (Cq), 203.8 (Cq)
MS (EI, 30 eV): m/z 190 (M+), 174, 159, 145, 133, 105, 91, 77, 65, 51, 41

3.3.7. (2,5-Dimethylphenyl)(2-phenyloxiran-2-yl)methanone (8d)
MCPBA (1.25 g, 70 % purity, 5.1 mmol) was added to a magnetically stirred solution of 1-(2,5-dimethylphenyl)-2-phenylprop-2-en-1-one (7) (1.0 g, 4.2 mmol) in 20 ml of CH2Cl2 and the resulting mixture was refluxed for 16 hours. After the mixture was cooled to ambient temperature this was washed three times with 30 ml of aqueous solution of NaHCO3, once with 30 ml of brine and dried with magnesium sulfate. 880 mg of crude mixture was obtained. Flash column chromatography (silica, petroleum ether : ethyl acetate, 20:1) furnished 460 mg of pure compound.

Yield: 460 mg (43 %), white solid
1H NMR (300 MHz, CDCl3): ´ (ppm) 2.29 (s, 3H), 2.49 (s, 3H), 3.07 (d, J = 5.6 Hz), 3.3 (d, J = 5.6 Hz), 7.12 (d, J = 7.8 Hz), 7.17 (d, J = 7.8 Hz), 7.37 (m, 3H), 7.52 (m, 3H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 20.7 (CH3), 21.0 (CH3), 54.8 (CH2), 63.5 (Cq), 126.2 (CH), 128.6 (CH), 128.7 (CH), 130.9 (CH), 131.7 (CH), 132.8 (CH), 134.6 (Cq), 135.0 (Cq), 135.7 (Cq), 134.4 (Cq), 199.4 (Cq)
MS (EI, 30 eV): m/z 252 (M+), 237, 193, 178, 133, 119, 105, 91, 77, 65

3.3.8. Irradiation of prepared epoxy ketones 8a-d

Irradiation of 8a-d was carried out as described previously. ADDIN EN.CITE Šolomek20071452Šolomek, T.2007Photochemical synthesis of indan-1-ones via dimethylphenacyl epoxidesDepartment of ChemistryBrnoMasaryk University58Bachelor work112 HPLC was used to monitor the progress of the reaction. The conversion was kept below 95 % in order to avoid secondary photoreactions. Product mixture was separated by flash column chromatography (silica, petroleum ether : ethyl acetate, 10:1 to 2:1).

Products isolated from irradiation of 8a:
9a: a 1:1 mixture of diastereomers
Yield: 67 %, colourless viscous oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 2.40 (s, 3H), 2.43 (s, 3H), 2.65 (dd, J1 = 17.2, J2 = 4.4 Hz, 1H), 2.80-2.94 (m, 2H), 2.99-3.13 (m, 2H), 3.22 (dd, J1 = 16.7, J2 = 4.1 Hz, 1H), 4.80 (d, J = 9.6 Hz, 1H), 4.93 (s, broad), 5.59 (d, J = 2.1 Hz, 1H), 7.22-7.49 (m, 14H), 7.58 (s, 1H), 7.61 (s, 1H)
13C NMR (75,5 MHz, CDCl3): ´ (ppm) 21.0 (CH3), 21.04 (CH3), 26.3 (CH2), 29.5 (CH2), 53.5 (CH), 55.1 (CH), 72.0 (CH), 75.8 (CH), 123.7 (CH), 123.95 (CH), 125.5 (CH), 126.1 (CH), 126.2 (CH), 127.0 (CH), 127.3 (CH), 128.1 (CH), 128.4 (CH), 128.5 (CH), 136.2 (CH), 136.4 (Cq), 136.7 (CH), 137.1 (Cq), 137.2 (Cq), 137.7 (Cq), 141.4 (Cq), 142.7 (Cq), 151.4 (Cq), 152.1 (Cq), 207.3 (Cq), 209.7 (Cq)

Products isolated from irradiation of 8b:
9b: a 1:1 mixture of diastereomers
Yield: 64 %, colourless oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.22 (d, J = 6.50 Hz, 3H), 1.24 (d, J = 6.80 Hz, 3H), 2.31 (s, 3H), 2.35 (s, 3H), 2.58-2.78 (m, 3H), 3.03-3.23 (m, 3H), 3.89-4.01 (m, 1H), 4.42 (m, 1H), 7.29-7.41 (m, 4H), 7.44 (s, 1H), 7.49 (s, 1H)
13C NMR (75,5 MHz, CDCl3): ´ (ppm) 20.9 (CH3), 21.0 (CH3), 26.9 (CH2), 29.1 (CH2), 53.6 (CH), 54.2 (CH), 66.8 (CH), 68.8 (CH), 123.4 (CH), 123.7 (CH), 126.0 (CH), 126.1 (CH), 136.0 (CH), 136.4 (CH), 136.5 (Cq), 137.0 (Cq), 137.2 (Cq), 137.4 (Cq), 151.2 (Cq), 151.9 (Cq), 208.2 (Cq), 209.5 (Cq)

10b: 
Yield: 12 %, colourless oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.33 (d, J = 6.3 Hz, 2H), 2.38 (s, 3H), 2.85 (dd, J1 = 13.5 Hz, J2 = 5.7 Hz, 1H), 3.16 (dd, J1 = 13.5 Hz, J2 = 5.4 Hz, 1H), 4.24 (m, 1H), 4.84 (d, J = 15.2 Hz, 1H), 4.93 (d, J = 15.2 Hz, 1H), 7.12 (d, J = 7.7 Hz, 1H), 7.26 (d, J1 = 7.7 Hz, J2 = 0.8 Hz, 1H), 7.67 (d, J = 0.8 Hz, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 20.9 (CH3), 21.0 (CH3), 49.7 (CH2), 69.1 (CH2), 71.2 (CH), 127.6 (CH), 128.9 (CH), 132.6 (CH), 137.6 (Cq), 139.4 (Cq), 200.3 (Cq)
The missing quarternary carbon in 13C NMR spectrum was proved by HMBC experiment
MS (EI, 30 eV): m/z 190 (M+), 149, 119, 103, 91, 77, 65, 51, 39

11b: 
Yield: 7 %, colourless oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 2.17 (s, 3H), 2.34 (s, 3H), 2.45 (s, 3H), 5.84 (s, 1H), 7.11 (d, J = 7.8 Hz, 1H), 7.15 (d, J = 7.8 Hz, 1H), 7.28 (s, 1H), 15.96 (s, broad, 1H)
13C NMR (75,5 MHz, CDCl3): ´ (ppm) 20.1 (CH3), 20.8 (CH3), 25.6 (CH3), 100.8 (CH), 128.8 (CH), 131.3 (CH), 131.35 (CH), 133.8 (Cq), 135.3 (Cq), 135.8 (Cq), 188.1 (Cq), 192.9 (Cq)

Products isolated from irradiation of 8c:
9c: 
Yield: 65 %, colourless oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.23 (s, 3H), 2.39 (s, 3H), 2.83 (d, J = 17 Hz, 1H), 3.17 (d, J = 17 Hz, 1H), 3.61 (d, J = 10.5 Hz, 1H), 3.80 (d, J = 10.5 Hz, 1H), 7.33 (d, J = 7.8 Hz, 1H), 7.42 (d, J = 7.8 Hz, 1H), 7.53 (s, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 21.2 (CH3), 22.8 (CH3), 37.9 (CH2), 51.4 (Cq), 68.2 (CH2), 124.3 (CH), 126.6 (CH), 136.2 (Cq), 136.7 (CH), 137.7 (Cq), 150.9 (Cq), 211.4 (Cq)





10c: 
Yield: 15 %, colourless viscous oil
1H NMR (300 MHz, CDCl3): ´ (ppm) 1.15 (d, J = 6.6 Hz, 3H), 2.37 (s, 3H), 3.32 (m, 1H), 3.67 (m, J1 = 11.2 Hz, 1H), 4.00 (dd, J1 = 11.2 Hz, J2 = 6.8 Hz, 1H), 4.83 (d, J1 = 15.2 Hz, 1H), 4.94 (d, J1 = 15.2 Hz, 1H), 7.10 (d, J = 7.7 Hz, 1H), 7.24 (d, J = 7.7 Hz, 1H), 7.58 (s, 1H)
13C NMR (75.5 MHz, CDCl3): ´ (ppm) 11.6 (CH3), 21.1 (CH3), 46.6 (CH), 71.9 (CH2), 72.8 (CH2), 127.3 (CH), 129.3 (CH), 132.3 (CH), 137.7 (Cq), 138.5 (Cq), 138.6 (Cq), 204.0 (Cq)

11c: 
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