Lewis acid-aldehyde-solvent interactions. A computational approach for determining the critical amount of THF molecules necessary for achieving a high enantioselection in chiral oxazaborolidinone-promoted asymmetric aldol reactions

The effect of THF as a solvent on the en antioselectivity of oxazaborolidinone-promoted asymmetric aldol reactions of aldehydes with silylketene acetals was investigated. The use of 4-5 M equiv of THF, relative to the chiral borane, was required to achieve a high enantioselectivity. A solvent-modeling study, based on ab initio calculations of intermolecular interactions, revealed the existence of an extended hydrogen bonding network in the resulting assembly, which was composed of THF molecules, the aldehyde, and the oxazaborolidinone. The model rationally provides a spatially suitable active site for controlling the stereochemical outcome of the reaction. (C) 2004 Elsevier Ltd. All rights reserved.