Additions of functionalized α-substituted allylboronates to aldehydes under the novel Lewis and Bronsted acid catalyzed manifolds

The stereo- and chemo selectivity in the additions of four model alpha-substituted allylboronates to benzaldehyde was examined under the standard thermal (uncatalyzed) conditions and the novel Lewis and Bronsted acid-catalyzed conditions. With either of Sc(OTf)(3) or triflic acid as catalysts, an alpha-ethyl allylboronate, 1a, led to a surprising inversion of stereo selectivity that can be tentatively rationalized through subtle differences in the geometry of the allylboration transition state between uncatalyzed and catalyzed pathways. It was also found that the chemoselectivity of alpha-silyl reagents (1c and 1d) can be reversed upon use of a catalyst, providing allylsilation products instead of the allylboration product obtained from the thermal uncatalyzed reaction. (c) 2005 Elsevier Ltd. All rights reserved.