HB(OH)(2) AND H2CO AS PROBES FOR A STUDY ON BINDING OF DIALKOXYBORANES AND KETONES TO OXAZABOROLIDINES CAPABLE OF CATALYZING THE ENANTIOSELECTIVE REDUCTION OF KETONES

Dihydroxyborane - formaldehyde adducts of oxazaborolidine were used as models of alkoxyborane - ketone adducts of chiral oxazaborolidines (e .g. 1) being potentially involved as intermediates in the enantioselec tive reductions (catalyzed by 1). The models were studied by means of ab initio MO methods. Energies (MP2/6-31G//MP2/6-31G*) of the formati on of HB(OH)(2) - H2CO adducts were clearly negative (e.g. - 94 kJ mol (-1)) but only about 70% of that of the corresponding H2BOH adduct. In all these adducts both ends of H2CO were bound (H2CO as a bidentate l igand). Isomeric monodentate adducts were predicted to be unstable. Op timization (MP2/6-31G) of one conformer of the monodentate adducts le d to the corresponding bidentate adducts whereas in the case of anothe r conformer a hydride transfer [from the HB(OH)(2) moiety to the H2C=O one] followed by opening of the oxazaborolidine ring took place (Delt a E=-246 kJ mol(-1)). Copyright (C) 1996 Elsevier Science Ltd