The mechanisms of alkane eliminations from the intermediates produced by reactions of the hydroxide and methoxide negative ions with tetramethylsilane in the gas phase

Abstract

ab initio Calculations (6-21G level, GAUSSIAN 82) and both ion cyclotron resonance (i.c.r.) and flowing afterglow (F.A.) experiments have been used to study the reactions of HO^– and MeO^– with Me₄Si (and related systems). ab initio Calculations suggest that interconversion of apical and equatorial methyl groups in trigonal bipyramidal [Me₂H₃Si^–](formation energy –200 kJ mol^–1) occurs through a square pyramidal transition state (barrier 12 kJ mol^–1). Deuterium labelling (F.A.) studies show that for the fast reaction (1) the elimination of CH₄ is statistical: no deuterium isotope effect is observed. HO^–+ Me₄Si → Me₃SiO^–+ CH₄(1) The MeO^– ion reacts with Me₄Si in i.c.r. experiments by two slow processes: MeO^–+ Me₄Si → Me₃SiO^–+ C₂H₆(2), MeO^–+ Me₄Si → Me₂(MeO)SiCH₂^–+ CH₄(3) The latter process (3) is not observed in F.A. experiments and is likely to be formed from excited MeO^– species. ab initio Calculations for the model system MeO^–-MeSiH₃ suggest that reactions (1)–(3) are stepwise: the key step is cleavage of the Si-C bond in the reactive intermediate to produce a reacting solvated methyl anion. There are high internal barriers for reactions (2) and (3), and both secondary and primary (where appropriate) deuterium isotope effects are noted for these two reactions in i.c.r. experiments.