Super-acid catalysed addition of allylsilanes to carbonyl compounds; synthetic and mechanistic aspects

Abstract

The addition of allyltrimethylsilane 8 to a variety of aldehydes and one ketone (cyclohexanone) was found to be induced by the super-acid TfOH₂⁺B(OTf)₄^–(Tf CF₃SO₂). In contrast to the analogous Lewis acid catalysed reaction, only catalytic amounts (0.5–3 mol%) of the super-acid were required. Modest stereoselectivities were observed with the ‘α-chiral’ aldehydes 15, 21 and 24; in the last two cases, the sense of the selectivity was the opposite of that predicted by ‘chelation control’. Application of the method to the addition of but-2-enyltrimethylsilane 29 to benzaldehyde demonstrated that, unlike the fluoride-catalysed analogue, it is regioselective with respect to allylic inversion. Control experiments employing Me₃SiOTf, B(OTf)₃ or TfOH as catalysts established that none is sufficiently active to account for the results. Two possible mechanisms are postulated, one in which the activating species is a superacidic proton and one in which it is an ‘Me₃Si⁺’ unit. On the basis of the stereochemical results, it is argued that the latter is marginally more likely.