Photoenolization

Direct evidence for dependence of the rate of photoenolization on structure has been obtained in a study of the flash spectroscopic behavior of compounds 28 and 29.513 The concentrations of quenchers required to suppress the enolization reaction are considerably greater for 29 than for 28, indicating the greater reactivity of tertiary C—H bonds as well as steric interference to energy transfer. 

Irradiation of o-methylbenzophenone produces a long-lived species which is not the enol. Recent evidence points to the cyclization 

Since acetonaphthones have 7r,7r configurations in their lowest triplet states, it seems probable that l-benzoyl-2-methylnaphthalene, 30, would also. As suspected, 30 does not undergo photoenolization.362 However, 6-benzoylbenzanthrone, 31, which would also be expected to possess a low-lying triplet, does form a photoenol, as determined by formation of yellow coloration at low temperatures and deuterium exchange at the benzylic position. It is entirely possible that the greater reactivity of the benzyhydryl C—H bond in 31 allows reaction even in a relatively unreactive state, or in an n,tt singlet. 

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. 

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Gamarnik A, Johnson B A and Garcia-Garibay M A 1998 Effect of solvents on the photoenolization of omicron-methylanthrone at low temperatures. Evidence for H-atom tunneling from nonequilibrating triplets J. Rhys. Chem. A102 5491... 

Chromone, 2-amino-3-chloro-synthesis, 3, 713 diacetate, 3, 714 Chromone, 3-aroyl-photochemistry, 3, 695 Chromone, 2-benzhydryl-3-benzoyl-photoenolization, 3, 695 Chromone, 3-benzoyl-2-benzyl-photoenolization, 3, 695 Chromone, 3-benzoyl-2-methyl-synthesis, 3, 823 Chromone, 2-benzyl-in photochromic processes, 1, 387 Chromone, 3-benzyl-photolysis, 3, 695 Chromone, 3-bromo-synthesis, 3, 828 Chromone, 3-bromoacetyl-ring opening, 3, 713 Chromone, 3-bromo-2-methyl-reactions... 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

Table 4.14 Diels-Alder reactions of photoenol of 66 with 67...

Entries 10 and 11 are examples of reactions involving thermal generation of quinodimethanes. In Entry 12 a quinodimethane is generated by photoenolization and used in conjunction with an IMDA reaction to create the carbon skeleton found in the hamigerans, which are marine natural products having antiviral activity. 

Another common hydrogen transfer reaction of carbonyl triplet is the photoenolization of the c-methylbenzoyl chromophore, illustrated in reaction 3 for the syn conformer of c-methylaceto-phenone (j+). Reaction 3 can act as a very efficient energy sink, and a number of properties of this group led us to believe that this process could be used to reduce photodegradation i.e. the excellent absorption characteristics of the chromophore, the short triplet lifetime and the fact that the disappearance of the carbonyl triplet does not take place at the expense of the formation of another excited state. 

Scheme 5.19. Photoenolization/Diels-Alder reaction sequence is the synthesis of podophyllotoxin (5-103).

Photoenolization is a frequently encountered process for aromatic and a,/)-unsaturated carbonyl compounds5. Typically, an allylic or benzylic /-hydrogen is abstracted by the... 

Photoelimination from 3-substituted o-alkyl arylketones 48 Photoelimination from a-substituted o-alkyl acetophenone derivatives 54 Photorelease via photoenolization followed by intramolecular lactonization 57 Photorelease from nitrotoluene derivatives 66... 

Several researchers have investigated the photoenolization of various o-methyl acetophenone and o-methyl benzophenone derivatives. The mechanism of photoenolization of o-methyl benzophenone, 1, has been studied with laser flash photolysis and can be described as follows (Scheme 2) Irradiating 1 forms its first... 

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... 

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