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 re actions and the borane reaction order of 1.6. In contrast, purified borane- dimethyl sulfide reacts distinctly slower and yields very different kinetic s. The main reaction, the reduction of the ketone by the borane-dimethyl su lfide complex forming the monoalkoxyborane, is first order in both reactant s similar to the reduction of pinacolone by the borane-tetrahydrofuran comp lex. The overall reaction with catecholborane proceeds much slower and appe ars to be much more complex. The reaction order is two in catecholborane an d zero in ketone. Obviously, the reactive intermediates are derived from di meric species of catecholborane. The kinetics of both different types of re action can successfully be simulated by numerical integration. The results obtained from the evaluation of the kinetics are complemented by semi-empir ical calculations in order to characterize the possible intermediates.