Homogeneous catalysis. Mechanisms of the catalytic mukaiyama aldol and sakurai allylation reactions

Article abstract of DOI:10.1021/ja00121a015

The mechanisms of a number of Mukaiyama aldol and Sakurai allylation reactions catalyzed by the Lewis acids [Ti(Cp)₂(OTf)₂], Ph₃COTf, and Ph₃CClO₄ have been investigated. It is found that hydrolysis of 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 Me₃SiOTf or Me₃SiClO₄, both of which are powerful catalysts for these reactions. Dehydration of the solvent or addition of a hindered base to quench the acid does not necessarily prevent the formation of these silyl catalysts. In the case of the [Ti(Cp)₂(OTf)₂] Lewis acid, Me₃SiOTf is generated as a consequence of the mechanism, and it is shown that all of the catalysis proceeds by the Me₃SiOTf species and that [Ti(Cp)₂(OTf)₂] acts only as an initiator for the production of Me₃SiOTf. For the case of the presumed Ph₃COTf catalyst, the Mukaiyama aldol reaction proceeds exclusively by the Me₃SiOTf catalyst and Ph₃COTf is neither an initiator nor a catalyst. It generates Me₃SiOTf by hydrolysis. It is shown that the rate and stereoselectivity of Me₃SiOTf catalysis depend on the concentration of triflate ions. The rate slows and the selectivity switches with an increase in triflate concentration. The rate of Sakurai catalysis of allylsilane with an acetal using Ph₃CClO₄ can be quantitatively accounted for by invoking only Me₃SiClO₄ catalysis. The Me₃SiClO₄ can be generated by hydrolysis, but when this is suppressed, Me₃SiClO₄ can be formed by allylation of Ph₃CClO₄. It is suggested that many of the reported catalysts may only be agents for the production of the real catalysts, namely, Me₃SiX species. The characteristics of Lewis acids which may obviate the intrusion of Me₃SiX catalysis are outlined.