S(N)2 AND AD(N)-E MECHANISMS IN BIMOLECULAR NUCLEOPHILIC SUBSTITUTIONS AT VINYL CARBON - THE RELEVANCE OF THE LUMO SYMMETRY OF THE ELECTROPHILE

The lowest vacant orbitals of a series of model vinyl systems with substituents of different electron-withdrawing power have been computed at the 3-21G(*)//3-21G(*), 6-31G(*)//3-21G(*), or 6-311G(**)//3-21G(*) levels. In situations where the first vacant orbitals with sigma and pi symmetry at the carbon centers are separated by >0.01 hartree, an almost complete correspondence is found between the symmetry of the lowest orbital and the stereochemical outcome of nucleophilic substitution on the corresponding real substrates. This finding is in line with the assumption that the interaction of the approaching nucleophile with the orbital of pi symmetry directs the attack orthogonally to the molecular plane (Ad(N)-E mechanism, leading to retention of configuration or stereoconvergence), while that with an orbital with sigma symmetry determines the attack in the molecular plane (S(N)2-Vin mechanism, leading to inversion of configuration). The situations where this correspondence fails, or the energy gap is smaller, are related to substrates for which no or other mechanisms (nonconcerted or ligand-coupling) have been proposed.