Journal of the American Chemical Society p. 4570 - 4581 (1995)
Update date:2022-08-10
Topics:
Hollis, T. Keith
Bosnich
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.
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