SUPER-ACID CATALYZED ADDITION OF ALLYLSILANES TO CARBONYL-COMPOUNDS - SYNTHETIC AND MECHANISTIC ASPECTS

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 'alpha-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.