RE-EXAMINATION OF THE MECHANISM OF ACID-CATALYZED AMINE-BORANE HYDROLYSIS - HYDROLYSIS OF NH3-BH3

Studies of the acid-catalyzed hydrolysis of ammonia-borane confirm the trend [formula ommited]Unlike the behavior of Me3N-BH3, hydrolysis of NH3•BH3 (at pH 4-5) is faster than boron-bonded hydrogen exchange. A comparison of rates in acetate and deuterated acetate buffer solutions shows a normal solution isotope effect [fromula ommited] This is approximately the same as that observed for Me3N•BH3 (Me3N-BD3) in 0.29 M H2SO4 (D2SO4). Results are interpreted to be consistent with a previously suggested mechanism involving cis displacement of •BH3 via electrophilic attack of the proton of a general acid at amino nitrogen (the nitrogen-boron electron pair). An alternative mechanism involving reversible protonation of substrate to a five-coordinate boron species with subsequent loss of H2 cannot be precluded, but difficulties persist in efforts to reconcile such a model with kinetic effects, particularly those of N-alkyl substitution and relative rates of hydrolysis and substrate-solvent hydrogen exchange. © 1979, American Chemical Society. All rights reserved.