Kinetics of Hydride Abstractions from 2‐Arylbenzimidazolines

Abstract

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.

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

“…34,35 Scheme 5 Reactions catalysed by acetohydroxy acid isomeroreductase (AHIR) and a simplified depiction of the hydride transfer step within the enzyme active site. 39,40 All have the ability to (at least formally) transfer hydride to a suitable substrate. Typically, this requires a catalyst of some sort (see next section).…”

“…The kinetics of hydride transfer from organo-hydrides is also complicated. 39,[55][56][57][58][59][60] A high thermodynamic hydricity is no guarantee of a high reaction rate! Mayr, Zhu and co-workers recently measured the kinetics of hydride transfer from a number of benzimidazolines and dihydropyridines to cationic acceptors of established electrophilicity.…”

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…Since the latitude for increasing the Lewis basicity of the silane is rather narrow, 39 we have explored the latter strategy. To this end, we have utilized antiaromaticity in combination with perfluorination to prepare even more strongly Lewis acidic boranes than B(C6F5)3.…”

Benzhydrylium and tritylium ions: complementary probes for examining ambident nucleophiles

“…By using the benzhydrylium method, it was possible to determine the nucleophilicity parameter N (and s N ) of various classes of hydride donors, including hydrocarbons, silanes, stannanes, borohydrides and dihydropyridines [19,[55][56][57][58][59].…”