20) The g factor observed is larger than those of simple alkyl radicals. It is due to the large spin-orbital coupling constant of sulfur atom (£p = 382 cm-1) as compared with that of carbon atom (¿jp = 28 cm-1).

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Irradiation of 3,5-diphenyl-S-aryl-2(5H)-furanones in benzene results in a novel rearrangement and affords a mixture of 3,4-diphenyI-S-aryl-and 3,S-diphenyl-4-aryl-2( 5H)-furanones. Phosphorescence, sensitization, and quenching studies confirm the involvement of a T-T* triplet state in these reactions. The rate constants of aryl group migration were determined from quenching studies using piperylene as a quencher. These were found to be 0.18 X lolo, 1.3 X 1O'O. and 0.9 X 10'O s-' for migration of phenyl, p-anisyl, and p-cyanophenyl. respectively. Thus, p-An and p -C N P h are more reactive migrating groups than phenyl by factors of 7.2 and 5.0 in benzene. The preferential migration ofp-anisyl over phenyl (16/ 1) and p-cyanophenyl ovcr phenyl (3.5/ I ) suggests that the excited state involved in benzene has significant odd-electron character on the :j carbon of the tuJ-unsaturated lactone moiety. A striking feature of these rearrangements is that a substantial crobsover in migratory aptitude occurs upon changing the solvent from ben7ene to inethanol. A iionniigratingp-anisyl group W;IS found to significant-I! accelerate the rate of phenyl migration in methanol. Whereas the migration ofp-anisyl in preference to phenxl in benzene is attributable to stabilimtion of the radical like free valence by the migrating group, the slight preference for migration of phenyl over p-anisyl in methanol may be attributed to stabilization of the electron deficicncy which develops at C5 by the nonmigrating p-anisql group. The above results serve to emphasize the important role that electron distribution plays in photochemical migratory processcs Light-induced transformations of five-membered enol lactones have been the subject of recent intensive These compounds undergo a facile decarbonylation when subjected to ultraviolet excitation and produce a,@unsaturated ketones as primary photoproducts. Chapman and McIntosh have previously noted that a critical requirement for clean photochemical cleavage of the acyl-oxygen bond is the presence of a double bond adjacent to the ether ~x y g e n .~ Stabilization of the incipient oxy radical was considered to be a determining factor in the photocleavage of this bond. In sharp contrast to the extensive studies concerned with the photochemistry of enol lactones. only scattered reports have appeared concerning the photochemical behavior of the related a,P-unsaturated y-lactone As part of a continuing program dealing with the photochemical transformations of unsaturated lactones,I0 we found that 5,5-diaryl-2( 5H)-furanones undergo aryl group migration when subjected to ultraviolet excitation." During the course of our studies with this system, we uncovered a striking crossover in the competitive migratory aptitudes of aryl groups which arises from a solvent alteration.12 As there are relatively few examples of excited state migratory aptitudes being controlled by solvent variation,I3 we decided to examine the photochemistry of this system in greater detail. We report here the results of this study w...

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