Photochemistry of ketones in solution. XXXIX. 3,4-Dimethyl-4-trichloromethyl-2,5-cyclohexadien-1-one

Abstract

The photochemistry of the title compound 5 in a number of solvents is described and compared with that of the parent dienone 1 originally studied by Schuster and Patel. The effects on the photochemistry produced by introduction of the additional methyl group are varied and pronounced. In tert-butyl alcohol, both epimeric 6-methyl-6-trichloromethylbicyclo[3.1.0]hexenones (lumiketones) 7 and 8 are formed from 5. The stereochemistry, assigned on the basis of spectral data, hydrogenation experiments, and an X-ray crystal analysis, shows the major product has the CC13 group in the exo orientation. It is concluded that the sole lumiketone formed from the parent dienone 1 also has an exo CC13 group. These results are rationalized mechanistically on the basis of a competition of steric effects, favoring an endo orientation in the intermediate and derived lumiketone of the bulkier dienone C-4 substituent (CCI3 in these systems), and coulombic effects favoring an exo orientation of CC13 and other electron-rich groups. The electronic effects clearly dominate the reaction course in these systems. Other photoproducts include 2-chloro-3,4-dimethyl-4-trichloromethylcyclohex-2-en-l-one (10), a bicyclo[3.2.Ojhexenone (13), 3,4-dimethylphenol ( 14), 3,5-dimethylphenol (15), and hydrogen chloride. Compounds 13 and 15 are formed by secondary photolyses of the initially formed lumiketones. Mechanisms for formation of all products are suggested. Noteworthy is the novel formation of 10, which is proposed to involve trapping of zwitterion 37, derived from 5, by HC1 generated in situ, a new mode of zwitterion trapping. The surprising formation of phenol 14 in good yield in benzene, r-BuOH, and other poor H-donor solvents, in contrast to the lack of phenol formation from 1 in such solvents, was shown in benzene to involve H abstraction not from the solvent but from ground state dienone, presumably from the allylic methyl group. Quantum yield data are presented. The triplet yield for 5 in benzene was found to be close to unity. Sensitization and quenching experiments in several solvents confirm that all products are triplet derived. As in earlier studies, quenching by 1,3-cyclohexadiene is somewhat more efficient than by //•tm-piperylene. The Stern-Volmer quenching slopes in 2-propanol for formation of lumiketone 7 decreased as dienone concentration was increased, while the slopes for formation of phenol 14 simultaneously increased. These results require two different triplets as precursors to these products, at least in 2-propanol. The triplet leading to lumiketones is suggested to be a , * triplet, showing a typical self-quenching effect. The triplet which abstracts hydrogen from the solvent, leading to phenol 14, is suggested to be the lowest , * triplet. The increased quenching slopes in the latter case are tentatively attributed to trapping by ground state dienone of radicals, produced in the reaction, which could act as triplet quenchers. The "two triplet" mechanism required by the results is compared to previously proposed mechanism...

Chapter 8. Photochemistry

“…In the triplet reaction this product is accompanied by the usual 1,2-acyl-migration p r o d ~c t . ~' A further example of sensitized 1,3-acylmigration has been detected in a reinve~tigation~~ of the photochemistry of the norbornenone (66), which is converted into (67).63 This product is also photolabile and is converted by a 1,2-acyl shift into product (68). The reaction of the enone (66) can be sensitized by acetone, acetophenone, or benzophenone.…”

Stereochemical assignment by X-ray crystallography to the product of trapping by solvent of the intermediate zwitterion on irradiation of a 2,5-cyclohexadienone

“…Fractional coordinates, bond distances, and bond angles are shown in Tables 1, 3, and 4, while thermal factors and structure factors are reported in Tables 2 and 5 which are published as supplemental material. 6 An ORTEP (8) drawing of the molecule is shown in Fig. 1.…”