The kinetics and mechanism of hydrolysis of nitrogen base adducts of cyano- and fluroboranes

Abstract

The preparation of amine-cyanoboranes was carried out by the addition of amine to acidified solutions of sodium cyanoborohydride. The hydrolysis of morpholine-cyanoborane is second-order overall, first-order each in substrate and hydroxide ion. Specific base-catalysis is assumed in the absence of evidence for general base-catalysis and on the fact that an inverse solvent isotope effect was observed. A conjugate base mechanism is proposed which involves proton removal from nitrogen by base in a rapid pre-equilibrium step, followed by hydrolytic decomposition of the conjugate base of substrate. That the nitrogen-hydrogen bond is a structurally important feature of the proposed mechanism is evidenced by the fact that tertiary amine-cyanoboranes were found to hydrolyze very slowly in comparison to the secondary amine adducts and at rates independent of the concentration of hydroxide ion. The addition of hydrogen peroxide to the alkaline hydrolysate significantly retards the rate. A transition state model involving the rate-limiting loss of cyanide ion is more satisfying than one involving a solvent assisted dissociation but such details are only conjectural. Trimethylamine-dibromocyanoborane was synthesized from trimethylamine- cyanoborane and bromine. The formation of this compound suggests that interesting derivative chemistry may be found via nucleophilic displacement of halide ion from such substrates. The rates of the acid-catalyzed hydrolysis of trimethylaminefluoroborane were correlated with the Hammett acidity function. A mechanism is postulated which involves protonation of fluoride in a rapid pre-equilibrium step, followed by loss of HF in the rate-limiting step. In buffered solutions (pH= 1-2) hydrolysis rates were enhanced in D20, however, at 0.75-3.0 M H+(D+) the opposite effect was observed. A possible explanation is that boron bonded hydrogens are rapidly exchanged with deuterium from solvent and, thereby, impose a secondary isotope effect which offsets the expected rate increase. Preliminary investigation of the exchange reaction seem to support this idea.