CARBONIUM ION-SILANE HYDRIDE TRANSFER REACTIONS .4. STRUCTURE AND REACTIVITY AT SILICON

Rates of hydride transfer from substituted silanes to the tris(2,6-dimethoxyphenyl)methyl cation were measured spectrophotometrically in acetic acid. For a series of triarylsilanes the value of p was -1.84 and for aryldimethylsilanes p was -1.01. For both series a better correlation was obtained for σ than for σ+. The primary kinetic-isotope effect kH/kD was measured for transfer from triphenylsilane and triphenyldeuteriosilane to the tris(2,6-dimethoxyphenyl)methyl cation, the 9-p-anisylxanthyl cation and the phenyl-p-tolyldeuteriomethyl cation and found to be between 1.51 and 1.89 for the three carbonium ions. The results indicate that the transition state for hydride transfer from silicon involves a four center array (trigonal bipyramid at silicon) in which nucleophilic participation by solvent aids in displacement of hydride from silicon. © 1969.