Change in mechanistic pathway of hydroboration: A detailed kinetic study of H2BBr·SMe2 and HBBr2·SMe2

Hydroboration reactions of 1-octene and 1-hexyne with H2BBr·SMe2 in CH2Cl2 were studied as a function of concentration and temperature, using 11B NMR spectroscopy. The reactions exhibited saturation kinetics. The rate of dissociation of dimethyl sulfide from boron at 25 °C was found to be (7.36 ± 0.59 and 7.32 ± 0.90) × 10−3 s−1 for 1-octene and 1-hexyne, respectively. The second order rate constants, k2, for hydroboration worked out to be 7.00 ± 0.81 M s−1 and 7.03 ± 0.70 M s−1, while the overall composite second order rate constants, k K, were (3.30 ± 0.43 and 3.10 ± 0.37) × 10−2 M s−1, respectively at 25 °C. The entropy and enthalpy values were found to be large and positive for k1, whilst for k2 these were large and negative, with small values for enthalpies. This is indicative of a limiting dissociative (D) for the dissociation of Me2S and associative mechanism (A) for the hydroboration process. The overall activation parameters, ΔH≠ and ΔS≠, were found to be 98 ± 2 kJ mol−1 and +56 ± 7 J K−1 mol−1 for 1-octene whilst, in the case of 1-hexyne these were found out to be 117 ± 7 kJ mol−1 and +119 ± 24 J K−1 mol−1, respectively. When comparing the kinetic data between H2BBr·SMe2 and HBBr2·SMe2, the results showed that the rate of dissociation of Me2S from H2BBr·SMe2 is on average 34 times faster than it is in the case of HBBr2·SMe2. Similarly, the rate of hydroboration with H2BBr·SMe2 was found to be on average 11 times faster than it is with HBBr2·SMe2. It is also clear that by replacing a hydrogen substituent with a bromine atom in the case of H2BBr·SMe2 the mechanism for the overall process changes from limiting dissociative (D) to interchange associative (Ia).