On the Mechanism of the Reaction of alpha-Substituted Ketones with Allyltributylstannane.

The mechanisms for the reaction of allyltributylstannane with a number of fragmentation probes, alpha-substituted acetophenones, were studied. All reactions were shown to proceed through free radical chain sequences since they could be initiated by AIBN and inhibited by m-dinitrobenzene (DNB). alpha-Halo- and alpha-(benzoyloxy)acetophenones (I and II, PhCOCR(1)R(2)X; X = F, Cl, Br, OCOPh; R(1), R(2) = H, Me) yielded the allylation products, PhCOCR(1)R(2)CH(2)CH=CH(2)), through a chain sequence involving as the propagation step: an electron transfer from Bu(3)Sn(*) to I and II, fragmentation of the ketyl anion PhCOCR(1)R(2)X(*)(-), and addition of PhCOCR(1)R(2)(*) to allyltributylstannane. The reactions of alpha-(arylsulfonyl)acetophenones (IIIa-c, PhCOCR(1)R(2)Y, Y = SO(2)Tol-p), however, gave a nearly 1:1 mixture of allyl tosyl sulfone and the corresponding ketone, PhCOCHR(1)R(2). The (1)H and (13)C NMR of the reaction mixture between allyltributylstannane and alpha-(p-methylbenzenesulfonyl)isobutyrophenone substantiated the intermediacy of the tin enolate PhC(OSnBu(3))=CMe(2). These results suggested that a radical addition elimination mechanism was involved in the reactions of IIIa-c with allylstannane. The reaction of alpha-phenylthioacetophenone (IV, PhCOCH(2)SPh) gave both the electron transfer and the addition elimination products (PhCOCH(2)CH(2)CH=CH(2), PhCOCH(3)), indicating that both pathways were involved in the formation of the products.