The kinetics and mechanism of trimethylamine-haloborane hydrolysis

Article abstract of DOI:10.1021/ic50088a026

The rates of solvolysis of trimethylamine-monohaloboranes in aqueous dioxane at 25° increase in the series (CH₃)₃NBH₂Cl < (CH₃)₃NBH₂Br < (CH₃)₃NBH₂I. The diiodoborane adduct reacts more slowly than the monoiodo compound but faster than trimethylamine-borane. Only a very slight retardation in rate is observed on substitution of deuterium for hydrogen on boron in the diiodo derivative (k_H/k_D = 1.1); however, a noticeable solvent isotope effect (k_H2O/k_D2O = 1.8) is observed for hydrolysis of both the mono- and diiodoborane adducts in 67% aqueous dioxane at 25°. Rates increase with increasing water content of the aqueous dioxane solvent system. No significant effect on rate of acidity is seen for hydrogen ion concentrations as high as 0.3 M or hydroxide ion concentrations up to 0.1 M, nor is the rate appreciably affected by the addition of potassium chloride or potassium iodide up to 0.3 M. The results indicate a mechanism for hydrolysis of the halo compounds quite different from that postulated for amine-BH₃ adducts. It is proposed that a rate-determining cleavage of a boron-halogen bond is followed by rapid collapse of an incipient boron(1+) ion. An analogy to nucleophilic substitution reactions in haloborane-amines leading to kinetically stable boronium ions is suggested.