ON THE ROLE OF PI-STACKING IN ALDEHYDE COMPLEXES OF N-SULPHONYLATED OXAZABOROLIDINONES USED AS CHIRAL CATALYSTS

Aldehyde complexes of Lewis acidic chiral N-sulphonylated oxazaborolid inones (e.g. 1) were studied by means of density functional methods at the JMW/DNP level. As a model of such complexes was chosen the acrole in N-phenylsulphonyl-1,3,2-oxazaborolidin-5-one adduct 2a. Compared wi th the related adducts of the parent oxazaborolidinone (2b; the phenyl of 2a replaced by hydrogen) the bidentate bonding of acrolein was fou nd to be significantly dependent on the presence of the phenyl group. As the group was replaced by hydrogen the O-so2 - C-c=o bond lengthene d substantially (by 0.225 Angstrom) upon optimization. Distances betwe en the vinyl and phenyl moieties were in the range of 3.7 - 4.0 Angstr om in the complex 2a suggest pi-stacking. Almost equal energies of the formation of 2a and 2b imply the stacking to be passive in nature. Re sults of the Natural Bond Order analysis of 2a support passive pi-stac king; significant attractive interactions between the vinyl and phenyl groups of 2a were not found. Copyright (C) 1996 Elsevier Science Ltd