Kinetics and mechanism of hydroboration reactions of HBBr 2 · SMe 2 and HBCl 2 · SMe 2 – Application of 11B NMR spectroscopy

Abstract

The kinetics and mechanism of the hydroboration reactions of 1-octene with HBBr 2 · SMe 2 and HBCl 2 · SMe 2, in CH 2Cl 2 as a solvent, were studied. Rates of hydroboration were monitored using 11B NMR spectroscopy. The reactions exhibited simple second-order kinetics of the form k obs = k 2 ′ [ Nu ] . The HBCl 2 · SMe 2 was found to be 20 times more reactive than the HBBr 2 · SMe 2. The overall activation parameters (Δ H ≠, Δ S ≠) for the reaction of HBBr 2 · SMe 2 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 HBCl 2 · SMe 2 with 1-octene, Δ H ≠ and Δ S ≠ were 104 ± 5 kJ mol −1 and 43 ± 16 J K −1 mol −1, respectively. The activation parameters (Δ H ≠, Δ S ≠) for the dissociation of Me 2S from HBBr 2 · SMe 2 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 Me 2S from the boron centre follows a dissociative mechanism, while the hydroboration process follows an associative pathway. It was also concluded that the dissociation of Me 2S from the boron centre is the rate determining step.