Kinetics of the reduction of pinacolone by borane-dimethyl sulfide and catecholborane in THF

The kinetics of the reduction of the ketone, pinacolone by borane-dimethyl sulfide and catecholborane have been investigated in tetrahydrofuran. Both overall reactions are composed of several subsequent and in part competing reactions. The use of commercial borane-dimethyl sulfide results in fast reactions and the borane reaction order of 1.6. In contrast, purified borane-dimethyl sulfide reacts distinctly slower and yields very different kinetics. The main reaction, the reduction of the ketone by the borane-dimethyl sulfide complex forming the monoalkoxyborane, is first order in both reactants similar to the reduction of pinacolone by the borane-tetrahydrofuran complex. The overall reaction with catecholborane proceeds much slower and appears to be much more complex. The reaction order is two in catecholborane and zero in ketone. Obviously, the reactive intermediates are derived from dimeric species of catecholborane. The kinetics of both different types of reaction can successfully be simulated by numerical integration. The results obtained from the evaluation of the kinetics are complemented by semi-empirical calculations in order to characterize the possible intermediates.