QUANTUM-CHEMICAL MODELING OF CHIRAL CATALYSIS .19. STRAIN AND STABILITY - OXAZADIBORETANES POTENTIALLY INVOLVED IN THE ENANTIOSELECTIVE REDUCTION OF KETONES PROMOTED BY CHIRAL OXAZABOROLIDINES

The relative stabilities of 2,7-dioxa-5-aza-1,6-diborabicyclo [3.2.0(1 ,5)]heptane (e.g. 4'a) and -8-aza-1,9-diboratricyclo[6.2.0(1,8),0(4,8) ]decane derivatives (e.g. 4'b) were studied by means of ah initio MO ( RHF) methods. The stabilities were assessed in the light of energies o f reactions leading to the opening of the oxazadiboretane ring system of the derivatives. Opening reactions of the tricycle derivatives givi ng rise to the regeneration of the catalyst were found to require appr oximate to 20 kJ mol(-1) (MP2/6-31G//6-31G) more energy than those of the corresponding bicycle derivatives. Cleavages of the tricycle syste ms leading to the rupture of both the oxazaborolidine and oxazadiboret ane rings were found to require approximate to 30 W mol(-1) (MP2/6-31G //6-31G) more energy than those of the bicycle ones. The stability of oxazadiboretane adducts increases with the increasing angle strain rel ated to the partial B=N pi-bond of the parent oxazaborolidines. The ro le of electron correlation in the description of the stability of oxaz adiboretanes (relative to the parent amino- and alkoxyboranes) was fou nd to be more significant than that of polarization functions.