Kinetics and mechanism of hydroboration reactions of HBBr2 · SMe2 and HBCl2 · SMe2 – Application of 11B NMR spectroscopy

The kinetics and mechanism of the hydroboration reactions of 1-octene with HBBr2 · SMe2 and HBCl2 · SMe2, in CH2Cl2 as a solvent, were studied. Rates of hydroboration were monitored using 11B NMR spectroscopy. The reactions exhibited simple second-order kinetics of the form kobs=k2′[Nu]. The HBCl2 · SMe2 was found to be 20 times more reactive than the HBBr2 · SMe2. The overall activation parameters (ΔH≠, ΔS≠) for the reaction of HBBr2 · SMe2 with 1-octene were found to be 82 ± 1 kJ mol−1, −18 ± 4 J K−1 mol−1 and with 1-hexyne were 78 ± 4 kJ mol−1 −34 ± 12 J K−1 mol−1. For the reaction of HBCl2 · SMe2 with 1-octene, ΔH≠ and ΔS≠ were 104 ± 5 kJ mol−1 and 43 ± 16 J K−1 mol−1, respectively. The activation parameters (ΔH≠, ΔS≠) for the dissociation of Me2S from HBBr2 · SMe2 were found to be 104 ± 2 kJ mol−1, +33 ± 8 J K−1 mol−1, respectively. Based on the activation parameters, it was concluded that the detaching of Me2S from the boron centre follows a dissociative mechanism, while the hydroboration process follows an associative pathway. It was also concluded that the dissociation of Me2S from the boron centre is the rate determining step.

A detailed kinetic and mechanistic study of HBBr2 · SMe2 and HBCl2 · SMe2 with 1-octene and 1-hexyne has been undertaken, including the role of the Lewis base (Me2S) on the hydroboration of these haloboranes. The kinetic data were obtained by exploiting the applicability of 11B NMR spectroscopy as a tool for hydroboration kinetics.Figure optionsDownload as PowerPoint slide