Kinetics of hydride abstractions from 2-arylbenzimidazolines.

The rates of the hydride abstractions from the 2-aryl-1,3-dimethyl-benzimidazolines 1a-f by the benzhydrylium tetrafluoroborates 3a-e were determined photometrically by the stopped-flow method in acetonitrile at 20 degrees C. The reactions follow second-order kinetics, and the corresponding rate constants k2 obey the linear free energy relationship log k2(20 degrees C) = s(N+E), from which the nucleophile-specific parameters N and s of the 2-arylbenzimidazolines 1a-c have been derived. With nucleophilicity parameters N around 10, they are among the most reactive neutral C--H hydride donors which have so far been parameterized. The poor correlation between the rates of the hydride transfer reactions and the corresponding hydricities (DeltaH0) indicates variable intrinsic barriers.