A. Bernardi et al., COMPUTER-ASSISTED DESIGN OF CHIRAL BORON ENOLATES - THE ROLE OF ATE COMPLEXES IN DETERMINING ALDOL STEREOSELECTIVITY, Tetrahedron, 50(4), 1994, pp. 1227-1242
Transition-state modelling for the aldol reaction of chiral Z and E en
ol borinates (1 and 2, Scheme 1) bearing mixed ligands (L(1) = Ipc, L(
2) = 9) predicted higher enantioselectivities man those calculated and
experimentally tested with CI symmetric systems L(1)= L(2) = Ipc, L(1
) = L(2) = 9). Reagent 8 was prepared and used to generate E enol bori
nates 24, which reacted with aldehydes to give the anti aldol products
25-28 with substantially lower enantiomeric excesses than predicted.
This unexpected result suggested that ate complex formation may be an
important factor in controlling the selectivity of the boron-mediated
aldol reaction. In particular, the presence of two different ligands o
n boron makes it a prostereogenic centre, and two diastereomeric ate c
omplexes (29 and 30) can be formed on aldehyde complexation. These ate
complexes are calculated to display different re : si face selectivit
ies. The experimental results are similar to the ones predicted if the
aldol reaction proceeds via the less selective ate complex 29.