Kinetics and mechanism of hydrolysis of some amine–cyanoborances in acid perchlorate media

In acid media ([H+]= 0.02–0.2 mol dm–3), isoproplyamine– and diisopropylamine–cyanoborane are hydrolysed completely to boric acid, hydrogen, hydrogen cyanide and the salt of the corresponding amine. The reaction occurs in steps, viz. (1) rapid loss of CN– as HCN and one H– as H2, (2) rate-determining B–H bond breaking and loss of the second H– as H2 and (3) very slow B–N bond breaking. The B–H bond breaking is rate determining also in the acid hydrolysis of BH3(CN)– ion. Step (2) is first order both in [substrate] and [H+]. Rate constant and activation parameters for step(2) have been determined. Cyanoborane adducts of tertiary amines, Me3N and Pri3N, Do not hydrolyse and it appears that the steric bullk of the non-leaving amine group has the dominant effect on the rate of hydrolysis of the adducts.