Hydrolysis of hydro(pyrrolyl-1)borates

Abstract

Abstract The hydrolysis of hydro(pyrrolyl-l)borates ([BHn(NC4H4)4-n]−, n = 1,2,3) can be treated as a kinetically one-step reaction outside of the mildly acidic region. In strongly acidic medium the hydrolysis takes place in a stepwise manner; the intermediates (boranes and the cationic boron compounds) being hydrolyzed more slowly than the borate anion. In the first step of the hydrolysis of [BH3(NC4H4)]− the BH bond, while in case of [BH2(NC4H4)2]− and [BH(NC4H4)3]− the BN bond is breaking. In neutral and mildly alkaline medium, the hydrolysis is a general acid catalyzed reaction (ASE2 mechanism). It becomes to a special H+-ion catalyzed reaction (A-1 mechanism) in strongly alkaline region since the protonated intermediate can be reversed to the original borate upon reaction with the OH− ion. The hydrolysis presumably takes place through an intermediate which is protonated on the pyrrolyl nitrogen. Concomitant to the hydrolysis an isotopic exchange reaction was observed on the Cα and Cβ atoms of the pyrrolyl group in heavy water. In the hydrolysis of the [BH3(NC4H4)]−-anion the N-protonated intermediate is assumed to be able to reverse to the original borate even in acidic or neutral region, at least in part.