Studies on the Mechanism of Transition-Metal- Assisted Sodium Borohydride and Lithium Aluminum Hydride Reductions

Abstract

Studies on the course of transition-metal-assisted NaBH, and LiA1H4 reductions of (1) nitriles, (2) alkenes, and (3) alkyl halides are described. (1) The kinetics of benzonitrile (PhCN) reduction indicate that at high nitri1e:catalyst ratios, the rate of reduction is independent of PhCN concentration. Furthermore, the rate of PhCN reduction exhibits a first-order dependence on NaBH, concentration, as measured over a fourfold range of BH; concentrations and several half-lives of PhCN. Moreover, when PhCN was reduced with equimolar mixtures of NaBH, and NaBD4, a significant primary kinetic isotope effect was observed. Rapid complexation of PhCN to the boride surface apparently activates the nitrile group toward rate-determining hydride addition from dissolved, uncoordinated NaBH,. (2) The selective reduction of alkenes by NaBH,-CoCI2 is attributed to adventitious heterogeneous catalytic hydrogenation. (3) The combination of LiAIH, with CoCI, forms a black precipitate of cobalt aluminide (CoAI) which was isolated, analyzed by atomic absorption spectroscopy, and shown to play an essential catalytic role in the reduction of alkyl halides. Labeling experiments demonstrate stereochemical randomization by a heterogeneous process involving solvent as the hydrogen donor and LiAIH, as a co-reductant. A radical mechanism involving halide atom transfer or oxidative addition to the aluminide is proposed.