The origin of Felkin-Anh control from an electropositive substituent adjacent to the carbonyl group

Ab initio calculations on the gas phase reaction of lithium hydride with 2- silylacetaldehyde, 2-silylpropionaldehyde, 2-trimethylsilylacetaldehyde, 2- trimethylsilylpropionaldehyde and 3-trimethylsilylbutan-2-one have been per formed. For each of the substrates containing an SiH3 group, the hydride ap proaches syn to the silyl substituent in the lowest-energy transition struc ture. In contrast, for each of the substrates containing the larger SiMe3 g roup, the more conventional Felkin-Anh transition structure, in which the h ydride attacks anti to this substituent, is lowest in energy. These results suggest that the diastereoselectivity of nucleophilic attack on a carbonyl group, adjacent to a stereogenic centre to which a trimethylsilyl group is attached, is largely controlled by the size of this substituent, rather th an by the electronic effect of the electropositive silicon it contains. Oth er influences on the preferred conformations of the reactants and transitio n structures are discussed.