Kinetics of Reductions of Substituted Benzaldehydes with B-Alkyl-9-borabicyclononane (9-BBN)

Article abstract of DOI:10.1021/ja00366a025

Second-order rate constants for the reaction of B-n-octyl-9-BBN with para-substituted benzaldehydes were obtained.Electron-withdrawing groups on the benzaldehyde increase the rate of reduction.The rate constants correlate with ?⁺ (ρ +1.03).Relative rates for reduction of para-substituted benzaldehydes with B-3-pinanyl-9-BBN gave a ρ of +0.49.The relative rates are consistent with a hydride addition to the carbonyl carbon in the rate-determining step.Activation parameters were obtained for the reaction of three benzaldehydes with B-n-octyl-9-BBN.The major barrier to the reaction is entropy.The large negative entropies of activation (-43 to -49 eu) indicate a highly ordered transition state.It is postulated that an organoborane-carbonyl oxygen complex is an intermediate in the reduction.