ORIGIN OF PI-FACIAL STEREOSELECTIVITY OF HYDRIDE REDUCTION OF CYCLOHEXANONES - A NEW INTERPRETATION BASED ON QUANTITATIVE-ANALYSIS OF EXTERIOR FRONTIER ORBITAL EXTENSION

A new theoretical interpretation for pi-facial stereoselectivity of hy dride reduction of cyclohexanones has been proposed based on the simpl e assumption that the lowest unoccupied molecular orbital (LUMO) of th e substrates should play an essential role in the reaction: pi-Facial stereoselectivity is described by relative magnitude of exterior front ier orbital extension (EFOE) of the LUMO above or below the carbonyl p lane, which is defined by integrated ''hole density'' of the LUMO at t he carbonyl carbon over three-dimensional spatial points outside repul sive molecular surface, where the absolute total value of the gaussian functions belonging to the carbonyl carbon makes the maximum contribu tion to the total value of the LUMO wave function so that the driving force vector on hydride is maximally directed toward the carbonyl carb on. A good linear correlation is observed for ten alkyl-substituted cy clohexanones (NaBH4 in 2-propanol) between the difference in the squar e of EFOE densities for the axial (EFOE(ax)) and equatorial (EFOE(eq)) areas (lambda = EFOE(ax)(2) - EFOE(eq)(2)) and the difference in acti vation enthalpy for equatorial (Delta H-eq(double dagger)) and axial ( Delta H-ax(double dagger)) attack (Delta Delta H double dagger = Delta H double dagger(eq) - Delta H-ax(double dagger)). (C) 1997 Elsevier S cience Ltd.