KINETICS OF THE REACTIONS OF LASER-FLASH PHOTOLYTICALLY GENERATED CARBENIUM IONS WITH ALKYL AND SILYL ENOL ETHERS - COMPARISON WITH THE REACTIVITY TOWARD ALKENES, ALLYLSILANES AND ALCOHOLS

Diarylcarbenium ions (benzhydryl cations) are generated from diarylmethyl chlorides by 20-ns laser pulses (248 nm) in acetonitrile solution at 20-degrees-C. The second-order rate constants for their reactions with n- and pi-nucleophiles (anions, alcohols, water, alkenes, allylsilanes, alkyl and silyl enol ethers) are determined by monitoring the decay of the UV-vis transients at variable nucleophile concentrations. Only reactive nucleophiles (k2 > 10(6)-10(7) L mol-1 s-1) can be investigated by this method because of the concomitant reactions of the carbenium ions with the solvent acetonitrile and the chloride ions produced in the photoheterolysis. The largest observed values of k2 are approximately 2 X 10(10) for reactions with anions and (2-4) X 10(9) L mol-1 s-1 for reactions with neutral nucleophiles. Alkoxy-substituted ethylenes are 300-10(5) times more reactive than the corresponding alkyl-substituted ethylenes. The reactivities of structurally analogous alkyl and silyl enol ethers differ by less than 1 order of magnitude. In sharp contrast to the situation previously observed for the reactions of benzhydryl cations with alkenes, the nucleophilic reactivities of the enol ethers correlate with their ionization potentials and not with the stabilities of the carbenium ions produced in the rate-determining step. The rate constants measured for the reactions of the flash photolytically generated benzhydryl cations with alkenes and allylsilanes agree well with those extrapolated from the reactivities of these nucleophiles toward less electrophilic benzhydryl cations, which have previously been determined by conventional techniques. Combination of the two sets of data yields a nucleophilicity scale with respect to the reference electrophile (p-H3CC6H4)2CH+.