Dihydrogen and methane elimination from adducts formed by the interaction of carbenium and silylium cations with nucleophiles.
Stationary points for reactions R'R' 'HX(+) + YH --> [R'R' 'X-Y](+) + H(2) (I) and R'(CH(3))HX(+) + YH -->[R'HX-Y](+) + CH(4) (II) (R', R' ' = CH(3), H; X = C, Si; Y = CH(3)O, (CH(3))(2)N, and C(6)H(5)) are located and optimized by the B3LYP/aug-cc-pVDZ method. A similar mechanism was found to be operative for both types of reactions with X = C and X = Si. Formation of the intermediate (adduct) results in the transfer of electron density from the electron-rich bases to the X atoms and in the growth of a positive charge on a hydrogen atom attached to Y. This mobile proton may shift from Y to X, and the relative energies of transition states for elimination reactions (Delta) depend on the ability of the X atom to retain this proton. Therefore, Deltagrows on going from Si to C and with increasing numbers of methyl substituents. For X = C, the Deltavalue for both reactions correlates well with the population of the valence orbitals of X in a wide range from -44 kcal/mol (methyl cation/benzene) to 31 kcal/mol (isopropyl cation/methanol). For X = Si this range is more narrow (from -19 to -5.0 kcal/mol), but all Delta values are negative with the exclusion of silylium ion/benzene systems, adducts of which are pi- rather than sigma-complexes. The energy minima for product complexes for H(2) elimination are very shallow, and several are dissociative. However, complexes with methane which exhibit bonding between X and the methane hydrogen are substantially stronger, especially for systems with X = Si. The latter association energy may reach 8 kcal/mol (Si...H distance is 2 A).
Ignatyev
Igor S
IS
Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, USA.
Schaefer
Henry F
HF