Silicon Effects. II. Structure and Stability of 1-Phenyl-2-(trimethylsilyl)ethyl Cation in Solution

Article abstract of DOI:10.1246/bcsj.64.2249

Solvolysis rates have been measured in various solvents at 25 deg C for 1-(substituted phenyl)-2-(trimethylsilyl)ethyl trifluoroacetates (1a - 1g; R=H, 4-Me, 4-Cl, 4-Br, 3-Cl, 3,4-Cl2, and 3,5-Cl2, respectively) and structurally related compounds, 1-phenylethyl-, 3,3-dimethyl-1-phenylbutyl-, and 1-(4-methylphenyl)ethyl trifluoroacetates (3a, 4, and 5).In dioxane/water mixtures 1g solvolyzes with the same sensitivity to the change in solvent ionizing power as that for a k_c substrate 5.The solvolyses of 1e and 5 exhibit almost identical α-deuterium kinetic isotope effects (k_H/K_D) of 1.18 - 1.19 in aq dioxane.Substituent effect on the solvolysis of 1 in 90 percent aq dioxane is expressed by an LArSR equation: log k^X/k^H = -3.05 (?⁰ + 1.05 Δ?(mean)⁺_R) (R = 0.9997).These findings are consistent with k_c mechanism for the solvolysis of 1.Relative rates for the solvolysis of 1a, 3a, and 4 in 30 percent aq dioxane are 2.99x10⁵:2.84:1.0 indicating solvolytic generation of α-(trimethylsilylmethyl)benzyl cation to be about 7 kcal mol^-1 energetically more favorable than that of the corresponding α-alkylbenzyl cations.