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 View the MathML sourcekobs=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.