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

A kinetic study was carried out for the reaction of benzaldehyde and borane (BH3) in tetrahydrofuran. The effect of BH3 concentration on the rate constant showed that the reaction order with respect to BH3 was 1.6. Substituent effects gave a linear Hammett plot with a ρ value of −0.51. It was concluded that the reaction proceeds through a rate‐determining hydride‐transfer transition state with two BH3 molecules, in which one molecule of BH3 acts as a reducing agent and the other serves as a catalyst.