Quantum chemical modeling of chiral catalysis. Part 7. On the effects controlling the coordination of borane to chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones

Abstract

Oxazaborolidine derivatives ( 1'a–c) of 2-aminoethanol, N-methyl-2-aminoethanol, 2-aminopropanol, 2-hydroxymethylpyrrolidine and 2-azabicyclo[3.3.0]octan-8-ol and their borane adducts ( 2'a–e) were investigated by means of ab initio molecular orbital methods. Energies (6-31G//6-31G) of the formation of adducts ( 2'd–e) of fused / bridged oxazaborolidines in which there was a partial BN double bond at the ring fusion were about 20 kJ mol −1 more negative than those ( 2'a–c) of simple oxazaborolidines. Formation of the cis-adduct of borane to the oxazaborolidine derivative of 2-aminopropanol was only 5 kJ mol −1 (6-31G//6-31G) favoured over that of the corresponding trans-adduct. Formation of the cis-fused borane adduct of the oxazaborolidine derivative of 2-azabicyclo[3.3.0]octan-8-ol was found to be about 50 kJ mol −1 (6-31G//6-31G) more advantageous than that of the corresponding trans-fused adduct.