BORANE AS A MODEL OF LEWIS ACIDIC ACTIVATORS OF CHIRAL N-SULPHONYLATED OXAZABOROLIDINONES USED TO CATALYZE ASYMMETRIC DIELS-ALDER REACTIONS

Lewis acids as activators of chiral N-sulphonylated oxazaborolidinones (e.g. 1) were studied by means of ab initio MO methods at the MP2/6-3 1G level. Small models [N-sulphonylated 1,3,2-oxazaborolidin-5-one 2 as a model of oxazaborolidinones and borane as a model of an activatin g Lewis acids] were used to probe preferred coordination sites of the Lewis acids. Acidity of the ring boron (of 2) was predicted to increas e as borane coordinates on any of the oxygens of 2. Formation of the m ost stable one (i.e. 3) of the O-C=O-adducts was favoured by about 15 +/- 2 kJ mol(-1) (MP2/6-31G//MP2/6-31G*) over that of the related O-S O2-adducts. The diborane type of hydride-bridged borane N-adduct 8 was the most stable [46 kJ mol(-1) (MP2/6-31G//MP2/6-31G*)below the leve l of 3] one of all the adducts inspected. Copyright (C) 1996 Elsevier Science Ltd