Influence of Chain Length on the Electrophilic Reactivity of Carbocations

The electrophilic reactivities of the bis(p-methylphenyl)carbenium ion 1b+ and its macromolecular analogue 1a+ have been compared in slow reactions with allylsilanes and in fast reactions with silyl enol ethers. Treatment of 1b-Cl with TiCl4 or GaCl3 and of 1a-Cl with GaCl3 in CH2Cl2 at −70 °C gave solutions of the carbocations 1b+ and 1a+, the concentration of which was measured photometrically. Addition of allylsilanes led to the exponential decay of the benzhydryl cation concentrations, from which the second-order rate constants have been derived. The rate constants determined for the reactions of 1a+ or 1b+ with allyltriphenylsilane and with allylchlorodimethylsilane, respectively, agree within experimental error. Laser flash photolyses of 1a-Cl and 1b-Cl in acetonitrile/dichloromethane mixtures yielded the corresponding benzhydryl cations, which showed pseudo-first-order decay in the presence of variable concentrations of 1-trimethylsiloxycyclohexene or 1-trimethylsiloxycyclopentene. The second-order rate constants ranged from 1 × 108 to 5 × 108 L mol-1 s-1 but differed for 1a+ and 1b+ by less than 30%. It is concluded, therefore, that macromolecular carbocations and their low molecular weight analogues have the same reactivity