PHOTOCYCLOADDITION OF THIOCARBONYL COMPOUNDS TO OLEFINS . REACTION OF THIOBENZOPHENONE WITH STYRENE AND SUBSTITUTED STYRENES

Thiobenzophenone reacts with styrene, on irradiation with ultraviolet or visible light to give 2,2,3,3,5-pentaphenyl-l,4-dithiane. The mechanism of this reaction has been studied kinetically. It is shown that the reaction proceeds through the addition of the n → π* triplet state of thiobenzophenone, which behaves like a thiyl radical, to styrene. The characteristic difference between the photocycloaddition of thiobenzophenone affording 1,4-dithianes and that of benzophenone to give oxetanes is discussed in terms of the reactivities of carbon radicals toward a thiocarbonyl sulfur and a carbonyl oxygen. The high reactivity of the intermediate composite radical toward the thioketone leads to 1,4-dithianes from thiobenzophenone. It is suggested that the use of photoexcited thiobenzophenone as a model of thiyl radicals simplifies the study of the addition reactions of thiyl radicals to olefins. From this viewpoint, the relative rates of the addition reactions of thiobenzophenone with unsubstituted and various para- and meta-substituted styrenes have been examined. Based on the absence of a substituent effect, it is proposed that the sulfur atom β to the radical center stabilizes it by forming a sulfur-bridged radical. The stabilizing effect of this type is strong enough to overshadow the effects of substituents in styrenes. © 1968, American Chemical Society. All rights reserved.