HOMOGENEOUS CATALYSIS - MECHANISMS OF THE CATALYTIC MUKAIYAMA ALDOL AND SAKURAI ALLYLATION REACTIONS

The mechanisms of a number of Mukaiyama aldol and Sakurai allylation r eactions catalyzed by the Lewis acids [Ti(Cp)(2)(OTf)(2)], Ph(3)COTf, and Ph(3)CClO(4) have been investigated. It is found that hydrolysis o f the Lewis acid by trace amounts of water in the solvent can lead to the formation of acid. The acid then reacts with the silyl enol ether or allylic silane to generate Me(3)SiOTf or Me(3)SiClO(4), both of whi ch are powerful catalysts for these reactions. Dehydration of the solv ent or addition of a hindered base to quench the acid does not necessa rily prevent the formation of these silyl catalysts. In the case of th e [Ti(Cp)(2)(OTf)(2)] Lewis acid, Me(3)SiOTf is generated as a consequ ence of the mechanism, and it is shown that all of the catalysis proce eds by the Me(3)SiOTf species and that [Ti(Cp)(2)(OTf)(2)] acts only a s an initiator for the production of Me(3)SiOTf. For the case of the p resumed Ph(3)COTf catalyst, the Mukaiyama aldol reaction proceeds excl usively by the Me(3)SiOTf catalyst and Ph(3)COTf is neither an initiat or nor a catalyst. It generates Me(3)SiOTf by hydrolysis. It is shown that the rate and stereoselectivity of Me(3)SiOTf catalysis depend on the concentration of triflate ions. The rate slows and the selectivity switches with an increase in triflate concentration. The rate of Saku rai catalysis of allylsilane with an acetal using Ph(3)CClO(4) can be quantitatively accounted for by invoking only Me(3)SiClO(4) catalysis. The Me(3)SiClO(4) can be generated by hydrolysis, but when this is su ppressed, Me(3)SiClO(4) can be formed by allylation of Ph(3)CClO(4). I t is suggested that many of the reported catalysts may only be agents for the production of the real catalysts, namely, Me(3)SiX species. Th e characteristics of Lewis acids which may obviate the intrusion of Me (3)SiX catalysis are outlined.