Photoenolization reactions

Photolysis of 5-methyl-1,4-naphthoquinone (24) gives 4-hydroxy-5-methyli-dene naphthalen-1 (5//)-one (25) in the ground state within 2 ps of excitation, with a quantum yield of 1.0.59 Formally, this is a photoenolization reaction however, the product of the reaction also has quinone methide functionality,... 

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. 

The synthetic utility of such photoenolization reactions continues to be of interest. Thus, the photochemistry of the aldehydes (26) has been examined using... 

The photoreduction efficiency of ortho-zSkyl benzophenone derivatives is greatly reduced by intramolecular enolization process, known as photoenolization reaction. For example, orf/io-ethyl benzophenone 2 on photoirradiation in deuterated hydroxylic solvents gives deuterated ethyl benzophenone 3 by photoenolization without reduction [3]. 

CM-Crotonaldehyde (111) has been used as a model for the theoretical investigation of photoenolization reactions (Sevin et al., 1979 Dannenberg and Rayez, 1983). 

The photo-Fries rearrangement has been used in various synthetic reactions. A recent application concerned the synthesis of the mitomycin precursor (148) in high yield from the lactone (147). Finally, reference is made to the efficient photochemical deconjugation of piperidine esters (149) to the endocyclic isomers (150), presumably by a photoenolization reaction. ... 

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 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).

The mechanism of photorelease from o-nitrotoluene derivatives is different from and more complicated than the release mechanism of the photoenols from o-alkyl arylketones (Scheme 12). The photorelease mechanism from o-nitrotoluene derivatives is strongly affected by the reaction medium and its pH value. 

In 2003, Banerjee et al. designed an efficient photoremovable protecting group for the release of carboxylic acids based on similar p-elimination from photoenols (Scheme 14). They showed that o-alkyl acetophenone derivatives with various ester groups in the p-position release their ester moiety in high chemical yields. The authors proposed that the photorelease took place as shown in Scheme 14 but did not support the mechanism with transient spectroscopy. Formation of 21, which is expected to be the major product in the reaction, was not confirmed, and thus, the authors speculated that 21 undergoes polymerization to yield oligomers. 

The enhanced reaction in ethanol is attributed to weakening of the tight, intramolecular hydrogen bond which is believed to facilitate photoenolization of 2-hydroxybenzophenone. t>-Hydroxybutyrophenone undergoes photoenolization in preference to Type II cleavage.25 The enol of o-benzylbenzo-phenone (2) forms a Diels-Alder adduct with dimethyl acetylenedicarboxylate, indicating that reaction with a reactive substrate is possible.30... 

The mechanism of photoenolization in 1-methylanthraquinone has been studied in detail (Gritsan, N. P. Khmelinski, I. V. Usov, . M., J. Am. Chem. Soc., 1991, 113, 9615). The reaction was found to occur in both the singlet and triplet (nn ) states. Develop a Dauben-Salem-Turro state correlation diagram for the photochemical step and, on the basis of your diagram, discuss the efficiency of the reaction... 

The reaction can occur from the excited singlet and the triplet state as evidenced from the quenching of reaction in 2-pentanone and 2-hexa-none with piperylene. Piperylene can quench the reaction only partially, suggesting singlet state reaction mechanism for the unquenched fraction. Photoenolization resembles Type II process in that a y-hydrogen migrates to the carbonyl oxygen. 

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. 

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. 

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. 

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