Transition-state modelling for the aldol reaction of chiral Z and E enol 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 borinates 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 on boron makes it a prostereogenic centre, and two diastereomeric ate complexes (29 and 30) can be formed on aldehyde complexation. These ate complexes are calculated to display different re : si face selectivities. The experimental results are similar to the ones predicted if the aldol reaction proceeds via the less selective ate complex 29.
COMPUTER-ASSISTED DESIGN OF CHIRAL BORON ENOLATES - THE ROLE OF ATE COMPLEXES IN DETERMINING ALDOL STEREOSELECTIVITY / A. BERNARDI, A. COMOTTI, C.M.A. GENNARI, C. HEWKIN, J. GOODMAN, A. SCHLAPBACH, I. PATERSON. - In: TETRAHEDRON. - ISSN 0040-4020. - 50:4(1994), pp. 1227-1242.
COMPUTER-ASSISTED DESIGN OF CHIRAL BORON ENOLATES - THE ROLE OF ATE COMPLEXES IN DETERMINING ALDOL STEREOSELECTIVITY
A. BERNARDIPrimo
;C.M.A. GENNARI;
1994
Abstract
Transition-state modelling for the aldol reaction of chiral Z and E enol 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 borinates 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 on boron makes it a prostereogenic centre, and two diastereomeric ate complexes (29 and 30) can be formed on aldehyde complexation. These ate complexes are calculated to display different re : si face selectivities. The experimental results are similar to the ones predicted if the aldol reaction proceeds via the less selective ate complex 29.
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