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 TiCl 4 or GaCl 3 and of 1a - Cl with GaCl 3 in CH 2 Cl 2 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 × 10 8 to 5 × 10 8 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.