Hydroboration Kinetics. 7. Kinetics and Mechanism of the Reduction of Aldehydes and Ketones with 9-Borabicyclononane Dimer

Article abstract of DOI:10.1021/ja00346a038

The kinetics of the reduction of a number of aldehydes and ketones with 9-borabicyclo<3.3.1>nonane dimer, (9-BBN)2, was studied at 25.0 deg C.The reduction of aldehydes and reactive ketones followed first-order kinetic behavior; with less reactive ketones, intermediate or three-halves-order kinetics was observed.Thus the mechanism is very similar to that of the hydroboration of alkenes and alkynes by (9-BBN)2, involving 9-BBN monomer as the intermediate.The relative rates of reduction of representative aldehydes and ketones were determined by the competitive method since the kinetic study could not reveal the effect of the structure upon the reactivity.A comparison with NaBH4 shows that (9-BBN)2 is less susceptible to steric effects, though the same trend is observed with both reagents.Increasing the steric hindrance on one side of the carbonyl function in ketones leads to a modest rate decrease while that on both sides leads to a considerable rate decrease.Electron-withdrawing substituents decrease and electron-releasing ones increase the rate of reduction of aldehydes and ketones.These facts strongly suggest that the boron atom of the reducing species, 9-BBN monomer, is coordinated with the carbonyl oxygen during the reduction process.