Enolsilanes and Stannanes

Corey examined and documented another class of amino-acid-derived N-sulfonamide oxazaborolidines, characterized by their convenient synthesis, for the catalytic asymmetric Mukaiyama aldol reaction. Oxazaborolidine 256, for example, can be assembled from the condensation of L-N-tosyl-tryp-tophan and n-butylboronic acid. It was shown to be an effective catalyst for the Mukaiyama addition of enoxysilanes with a variety of aldehydes (Equation 25) [127]. 

Based on NMR spectroscopic investigations, Corey proposed structure 254 for the activated aldehyde complex [128]. In this structure, one face of the aldehyde is effectively blocked from attack by the tryptophan ring [127]. The observation that the saturated system gave rise to products displaying considerably diminished selectivity is consistent with the unique electronic interaction between the electron-rich indole and the electron-deficient, polarized C=0 group coordinated to the chiral Lewis acid center. 

Chiral hydrogen bond donors in enantioselective aldol reactions  

Addition reactions of dienolates and aldehydes can also be effected through the use of bisphosphine copper(I) complexes [145, 146]. A mixture of p-Tol-BINAP, Cu(0Tf)2, and (Bu4N)Ph3Sip2 leads to a complex that efficiently promotes rapid aldol addition reactions of acetoacetate-derived dienolates to give adducts with high yields and selectivities. The addition to crotonaldehyde served as an important launch point in Carreira s total syn- 

An important mechanistic aspect of the Cu-bisphosphine-catalyzed addition reactions of dienolates to aldehydes is worth highlighting. Infra-red spectroscopic studies have collectively provided insight into the details of this process. In this respect, these processes have been suggested to proceed through metaloenolate intermediates [146]. As such, the process complements the traditional Mukaiyama aldol addition reaction, which typically proceeds through Lewis activation of the electrophilic aldehyde partner. 

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