Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh(xantphos)]+†
The [Rh(xantphos)]+ fragment acts as an effective catalyst for the hydroboration of the alkene TBE (tert-butyl ethene) using the amine–borane H3B·NMe3 at low (0.5 mol%) catalyst loadings to give the linear product. Investigations into the mechanism using the initial rate method and labelling studies show that reductive elimination of the linear hydroboration product is likely the rate-limiting step at the early stages of catalysis, and that alkene and borane activation (insertion into a Rh–H bond and B–H oxidative addition) are reversible. The resting state of the system has also been probed using electrospray ionization mass spectrometry (ESI-MS) using the pressurised sample infusion (PSI) technique. This system is not as effective for hydroboration of other alkenes such as 1-hexene, or using phosphine borane H3B·PCy3, with decomposition or P–B bond cleavage occurring respectively.