Diastereocontrol in open-chain nucleophilic attack on a double bond adjacent to a stereogenic centre carrying a silyl group.
The bis[dimethyl(phenyl)silyl]cuprate reagent introduces a silyl group to the beta-position of three alpha,beta-unsaturated esters: methyl Z-4-dimethyl(phenyl)silylpent-2-enoate 11, and methyl Z- and E-(1'-dimethylphenylsilylbenzyl)but-2-enoates 14 and 15, diastereoselectively in the unexpected sense, syn to the silyl group in the conformation in which the hydrogen atom is 'inside'. The selectivity is low (58:42) in the first case 11, where the nucleophilic attack is adjacent to the stereogenic centre carrying the silyl group, and moderate (72:28) for both Z- and E-alpha,beta-unsaturated esters 14 and 15, where the nucleophilic attack is at the other end of the double bond from the stereogenic centre. It is conceivable that nucleophilic attack actually takes place in a conformation in which the donor substituent, the silicon-carbon bond, is out of conjugation with the double bond.
Betson
Mark S
MS
Department of Chemistry, Lensfield Road, Cambridge, UK CB2 1EW.
Fleming
Ian
I
Ouzman
Jacqueline V A
JV