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

The stereo- and chemoselectivity in the additions of four model α-substituted allylboronates to benzaldehyde was examined under the standard thermal (uncatalyzed) conditions and the novel Lewis and Brønsted acid-catalyzed conditions. With either of Sc(OTf)3 or triflic acid as catalysts, an α-ethyl allylboronate, 1a, led to a surprising inversion of stereoselectivity 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 α-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.