PHOTODECARBOXYLATION OF 2-PHENYLPROPIONIC ACID IN SOLUTION AND INCLUDED WITHIN BETA-CYCLODEXTRIN

Photolysis of 2-phenylpropionic acid (1) in acetonitrile, methanol or benzene leads to ethylbenzene (2), 2,3-diphenylbutane (3d,l and meso), 1-(2-ethylphenyl)-1-phenylethane (4), 1-(4-ethylphenyl)-1-phenylethane (5) and acetophenone (6). In cyclohexane or carbon tetrachloride, solvent derived products are formed. These results involve homolytic cleavage of the C-C bond a to the carboxy group, which affords 1-phenylethyl radical (PER) as key intermediate, The alpha,alpha coupling of PER in solution to give 3 is nonstereoselective; by contrast, formation of the meso isomer is preferred upon inclusion of 1 within beta-cyclodextrin, This is attributed to the coupling of two long-lived PER-CD units.