R. Uggla et al., ON THE ROLE OF PI-STACKING IN ALDEHYDE COMPLEXES OF N-SULPHONYLATED OXAZABOROLIDINONES USED AS CHIRAL CATALYSTS, Tetrahedron : asymmetry, 7(9), 1996, pp. 2725-2732
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