Ketones photoenolization

The photoreactivity of o-methyl acetophenone 11 has been studied exten-sively it is somewhat different from 1 because the singlet excited ketone (Sik) in 11 intersystem crosses to its triplet state in less than quantitative yields, as observed for 1 (Scheme 8). Thus, Sik in 11 decays by both intramolecular H-atom abstraction to form exclusively photoenol Z-13 and intersystem crossing to Tik of 11. Haag et al. estimated that Tik of 11 has a lifetime of 10 ns in benzene and decays by intramolecular H-atom abstraction to form biradical 12. The maximum... 

It is well known that o-hydroxyphenyl ketones phosphoresce very weakly and do not undergo photoreduction363,368 or photoelimination,366 presumably because of rapid enolization in the excited state. Lamola has performed quenching studies which indicate lifetimes of o-hydroxybenzophenone triplets of less than 10"8 sec. However, these lifetimes, as well as phosphorescence efficiencies, are markedly enhanced by the addition of alcoholic solvents,383 presumably because hydrogen bonding slows down photoenolization. 

Buffer catalysis has been applied to induce chiral induction by enantio-selective protonation remarkable enantiomeric excess was achieved in the photodeconjugation of a,/3-unsaturated ketones and esters by using chiral catalysts for the ketonization of photoenols in aprotic solvents.29... 

For some time the synthetic potential of this reaction as a source of Diels-Alder adducts underwent considerable study. One outcome of these studies was the realization that of the four possible o-xylylenol isomers, only ones with the enolic OH group pointed out (the F-photoenol ) reacted with dienophiles [23]. Mechanistic studies picked up in the 1970s, after Matsuura and Kitaura reported that, in the absence of dienophiles, benzocyclobutenols are formed from 2,6-dialkylphenyl ketones but not from simple o-alkylphenyl ketones [24]. Previously the absence of cyclobutenol products had been quite puzzling and led to suggestions that they were formed from the initial biradical but underwent rapid electrocyclic opening to the o-xylylenols. 

We have recently discovered at Michigan State University that the well-known photoenolization of ori/zo-alkyl ketones 87) involves two distinct rotamers 88>. The syw-triplet enolizes very rapidly (>109 sec-1), the anti-triplet so slowly ( 107 sec-1) that y-hydrogen abstraction can compete. The rate of enolization of the tfwfr -triplet is apparently determined by the necessary rotation into the syn conformation, since it is independent of substituents on the benzene ring and of the nature of the oriAo-alkyl group. 

In addition to the reactions discussed in Section 7.4.5, a,/3-unsaturated ketones undergo isomerization to give the deconjugated )3,y-unsaturated compound. According to Scheme 48, this reaction may be analyzed as a Norrish type II reaction with cleavage of a r bond instead of a a bond. However, it may also be interpreted as a photoenolization. (Cf. Section... 

A detailed study of the photoenolization of the ketones (143) and their reketonization has been reported. The enone (144) undergoes deconjugation when subjected to u. v. irradiation. White et al. have observed that the photochemical decoiyugation reaction of the triene (145) yields (146) as a mixture of E)- and i isomers. Deconjugation,... 

Gebicki et al. have reported that lH-3H-exchange in o-methyl-substituted phenyl ketones is a valuable tool for probing the formation of transient photoenols. 

A case in point is the photoenolization reaction of o-alkylphenyl ketones (Section 6.3.6) and related compounds. Detailed studies of the temperature dependence of both the primary photoreaction386 3X8 and of the back transfer reaction in the ground state389 have provided convincing evidence for tunnelling contributions in these reactions. 

Sengupta, D., Chandra, A. K., Role of Tunneling of Hydrogen in Photoenolization of a Ketone,... 

Connolly, T. J., Durst, T., Photochemically Generated Bicyclic o Quinodimethanes Photoenolization of Bicyclic Aldehydes and Ketones, Tetrahedron 1997, 53, 15969 15982. 

Giese has reported [51, 52] that the triplet state of an excited phenacyl ketone can undergo intramolecular hydrogen atom abstraction from an attached peptide, resulting in photocyclization of the phenacyl group with peptides, and cyclization with peptide backbone. One of the attractions of the DMP linker is that their fast photoenolization will probably help to minimize unwanted hydrogen atom abstraction from an attached peptide. 

Laser flash photolysis of o-tolyl vinyl ketone-methyl vinyl ketone copolymers [e,g., (17)] has produced some interesting observations. The triplet state of the aromatic carbonyl decays to give 1,4-biradicals that are capable of transferring an electron to an acceptor such as paraquat. In the absence of an acceptor the macroradicals decay to yield photoenols, which then tautomerize to regenerate the original polymer (Scheme 30). 

Hydrogen transfer, of which the foregoing are examples, is also important in the photoenolization of suitably substituted phenyl aldehydes and ketones. Thus Horii et a/.ea have reported formation of the enols (84), which could be... 

Scheme 61 expresses, suitably briefly, current mechanistic understanding of photoenolization for cases in which all of the transient species between the conformations of the ketone educt (113s+113a) and those of the photoenol product (114s+114a) are identifiable [120]. 

Both Norrish Type II photoproducts remained incarcerated. Furthermore, the surrounding host prevented ketonization of 105, which requires acid or base catalysis. In fact, the resistance of incarcerated photoenol to ketonize is remarkable. Incarcerated 105 required... 

Irradiation of 2,5-dimethylbenzoyl oxiranes results in formation of p-hydroxy functionalized indanones (74) that structurally resemble biologically active pterosines. Nanosecond laser flash photolysis and quantum-chemical calculations based on density functional theory support a photoenolization mechanism.Reversible photochemical transformations of cis- and tra v-2,3-dimethyloxirane radical cations in freonic matrices at 77 K has been observed and explained by the interconversion between the open (C2-C3 broken) and the cyclic forms. Photocatalytic ring opening of a-epoxy ketones in cyclopentanone, using 2,4,6-triphenylpyrylium tetra-fluoroborate as electron acceptor, results in the diastereoselective formation of 2,5-dioxaspiro[4,4]nonanes through Ca-O bond cleavage and subsequent nucleophilic attack by cyclopentanone to the radical cation intermediates. ... 

---