The Halogen-scrambling Mechanism Leading to Formation of the Mixed Boron Trihalide Adducts of Trimethylamine

Trimethylamine adducts of the boron trihalides, Me3N•BX3 are shown to react in solution with free boron trihalides BY3 to yield mixed halide adducts such as Me3N•BX2Y. Isotope labelling shows that the B—N bond remains intact and that the reaction proceeds in either direction, i.e. either X or Y may be the heavier halogen. A bridged transition state involving five-coordinate boron is postulated. Only in the case of Me3N•BI3 is an ionic predissociation of a B—X bond indicated by reaction with ionic species. Two adducts do not react in solution without the presence of excess Lewis acid, again indicative of a lack of predissociation. In the gas phase two adducts will scramble their halogens even in the absence of excess acid but these reactions occur via B—N bond cleavage.