Photolysis acetophenone-sensitized

Acetophenone-sensitized photolysis of l-methyl-3-phenyl-2-cyclohexene-carbaldehyde oxime acetate (56) A solution of 56 (298 mg, 1.16 mmol)... 

Swenton found a somewhat analogous behaviour in the photodecomposition of 2-azidobiphenyl. Direct photolysis resulted in ring closure to carbazole (71%), acetophenone sensitized photolysis produced 43% of the azo compound and less than 8% carbazole. The presence of piperylene, a triplet quencher, during photolysis reduced the formation of azo compound to 4% and enhanced the carbazole yield to 89%. 

While direct photolysis of (46) gave (48), the sensitized photolysis of (46) with acetophenone gave only the cyclopropane product (>93%). These authors favor a stepwise oxa-di-w-methane reaction mechanism (8.69) inasmuch as a concerted reaction should produce the product without a change in multiplicity. That is, the product would have to be formed in the triplet state and not enough energy is available for the product to be formed in an excited state ... 

As with 2-cyclopentenone, the ratio (70) (71) varies with the molar concentration of the enone, the head-to-head dimer (71) becoming increasingly important at higher concentrations/133 This reaction is efficiently sensitized by acetophenone, benzophenone, thioxanthone, and naphthalene. The same enone concentration effect was observed in the sensitized photo-dimerization as in the direct photolysis. Similarly, quenching of the dimerization by piperylene was not accompanied by a change in dimer ratio. Systematic... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

The quantum yield for isomerization in the direct photolysis was found to be d> = 0.94. The reaction could also be sensitized with acetophenone ( = 1.02) and quenched with piperylene, indicating a reactive triplet species with a rate constant kr of 3 x 10l°sec-1. With a 3-(p-methoxyphenyl) derivative two products were obtained<81) ... 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

The photodimerization of coumarin has been studied in several solvents and their nature has an effect on this rather complex reaction. In a polar medium such as methanol, the only product is the cis head-to-head isomer (244) but in acetonitrile, this is accompanied by the trans head-to-head dimer (245), which becomes the main product of the reaction in non-polar solvents like benzene or dioxan. Small amounts of the head-to-tail isomers (246) are also formed in non-polar solvents (66JA5415). In some solvents, the presence of benzophenone as sensitizer is essential and it also alters the relative proportion of dimeric isomers (64JA3103, 66CB625). Photolysis of osthole (247) in the presence of acetophenone gives a mixture of two quite different dimers (248) and (249) (80MI22302), representing addition at the exocyclic and endocyclic double bond respectively. 

The addition of aliphatic aldehydes and ketones to alkenes is less successful as a preparative procedure for oxetanes. An essential requirement for addition is that the triplet energy of the alkene must be considerably greater than that of the carbonyl. If this condition is not fulfilled, energy transfer to the alkene can occur,279 sensitizing, for example, dimerization of the alkene. This is clearly illustrated 280, 281 for norbornene (264) which on irradiation in the presence of benzophenone (ET 68.5 kcal/mole) forms the adduct 265 photolysis in acetone (ET 75 kcal/mole) affords only norbornene dimers (266 and 267), whereas acetophenone, which has intermediate triplet energy (Et 73.6 kcal/mole) forms both oxetanes and norbornene dimers. 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

Photolysis of j -methoxy and /i-phenyl phenyl zizides afforded the corresponding azo derivatives in 94% and 81% respectively. A mixture of these two azides yielded the three possible azo products in about equal amounts The tendency towards dimerization on photolysis is strengthened by addition of sensitizers. 2-Azidobiphenyl yielded in the presence of acetophenone mainly the azo derivative (112), with only minor amounts of carbazole . Azidoformates can also form... 

According to101, LXXIII (R=H) is formed in the photolysis of diluted alcoholic solutios of quinone methide LXXII sensitized by acetophenone however, LXXIII (where R is the residue of alcohol used CH2OH or (C(CH3)2OH) originates according to102. ... 

The main classes of initiators based on photofragmentation (scission of covalent bonds) are limited to UV applications, and they include peroxides, azo compounds, benzoin derivatives, acetophenone derivatives, ketoxime esters of benzoin, tri-azines, etc. On photolysis, these classes of initiators, depending on their wavelength of sensitivity, are able to initiate the polymerization of monomers such as methyl methacrylate in negative-tone lithographic imaging. 

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