Reactions of carbocations with unsaturated hydrocarbons: electrophilic alkylation or hydride abstraction?

Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.