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

Abstract

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.

Enantioselectivity Induced by Oxazaborolidine Supported on Mesoporous Silica or by Its Analog in Homogeneous Phase

“…The final solid is functionalized by several species, and in order to check the influence of these residual species on the silica, the reduction of acetophenone by BH 3 has been realized in the presence of various samples presenting a precise surface environment (Table 2). The reaction involves 0.7 equivalents of borane in THF with acetophenone to achieve a complete conversion since the borane provides two hydrides and thus can reduce two equivalents of ketone [15,16] (entry 1). Borane (BH 3 Lewis acid) can coordinate to an OH function (Lewis base), resulting in its loss of activity, as previously observed by Asefa et al [17].…”

Reaction Profiles and Substituent Effects for the Reduction of Carbonyl Compounds with Monomer and Dimer Simple Metal Hydride Reagents

“…The simple metal hydrides of B and Al are known to form dimers. In the presence of a coordinating species, on the other hand, these reagents, especially BH 3 , are known to form stable complexes. 1b-f, Thus, BH 3 in THF was reported to exist as a 1:1 complex with THF and to react as a monomer 1e 1e, To see the effect of aggregation for the reactions in the model reactions in the gas phase, we examined computationally the reactions of dimer reagents and compared the results with those of monomer reagents using the B reagents as examples.…”

“…The reduction of aldehydes and ketones is one of the most fundamental reactions of organic chemistry. Metal hydrides are common reducing reagents for such a reaction in an organic laboratory, and the reaction mechanism has been studied extensively . Simple metal hydrides such as BH 3 and AlH 3 are interesting chemical species in that they are a Lewis acid and therefore electrophilic at the metal center while the hydrogens are nucleophilic, and therefore the reactions could be either electrophilic or nucleophilic.…”

Kinetic study of BH3 reduction of benzaldehydes: identification of effective reducing species

“…The reduction of aldehydes and ketones is one of the most fundamental reactions in organic chemistry. Metal hydrides are common reducing reagents for such a reaction in an organic laboratory, and the reaction mechanism has been studied extensively . Simple metal hydrides such as borane (BH 3 ) and aluminum hydride have interesting characteristics in that they are Lewis acids and thus electrophilic at the metal center, whereas the hydrogens are nucleophilic and therefore the reactions could be either electrophilic or nucleophilic.…”