Towards a Comprehensive Hydride Donor Ability Scale

Towards a Comprehensive Hydride Donor Ability Scale:

Abstract

Rates of hydride transfer from several hydride donors to benzhydrylium ions have been measured at 20 °C and used for the determination of empirical nucleophilicity parameters N and s_N according to the linear free energy relationship log k_20 °C=s_N(N+E). Comparison of the rate constants of hydride abstraction by tritylium ions with those calculated from the reactivity parameters s_N, N, and E showed fair agreement. Therefore, it was possible to convert the large number of literature data on hydride abstraction by tritylium ions into N and s_N parameters for the corresponding hydride donors, and construct a reactivity scale for hydride donors covering more than 20 orders of magnitude.

The lineup for hydride donors spans more than 20 orders of magnitude in reactivity and is based on the kinetics of hydride-transfer reactions towards benzhydrylium (Ar₂CH⁺) and tritylium ions (Ar₃C⁺). Linear free energy relationships according to the correlation equation log k=s_N(N+E) showed that the electrophilicities E of these carbocations can be used for the determination of the hydrides’ nucleophilicity parameters N and s_N (see figure), the knowledge of which is expected to facilitate the selection of efficient reductants in organic syntheses.