Origin of pi-facial stereoselectivity of nucleophilic addition to carbonylcompounds. Application of the exterior frontier orbital extension model tocyclohexanones with polar substituent

The application of the exterior frontier orbital extension model (EFOE mode l) to pi-facial stereoselection of cyclohexanones with a polar substituent at C3 or C4 have been described. The theoretical prediction that a linear c orrelation should exist between the pi-facial (axial and equatorial) differ ence in the square of the EFOE densities (lambda = EFOE(ar)(2) - EFOE(eq)(2 )) and experimentally obtained facial selectivity (In(ax/eq)) has been demo nstrated for 5-substituted 2-decalones(l), 4-substituted cyclohexanones (2) and 3-substituted cyclohexanones (3). In addition, facial stereoselection of 3, where the substituent is located closer to the reaction center compar ed with those of 1 or 2, showed remarkable dependence on steric effects eva luated by the pi-plane-divided accessible space (PDAS), another important q uantity of the EFOE model, which represents simple summation of the pi-plan e-divided exterior three-dimensional space nearest to the reaction center o utside the van der Waals surface of the substrate. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.