Abstract
Hydroboration reactions of 1-octene and 1-hexyne with H 2BBr·SMe 2 in CH 2Cl 2 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, k 2, 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 k 1, whilst for k 2 these were large and negative, with small values for enthalpies. This is indicative of a limiting dissociative ( D) for the dissociation of Me 2S and associative mechanism ( A) for the hydroboration process. The overall activation parameters, Δ H ≠ and Δ S ≠, 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 H 2BBr·SMe 2 and HBBr 2·SMe 2, the results showed that the rate of dissociation of Me 2S from H 2BBr·SMe 2 is on average 34 times faster than it is in the case of HBBr 2·SMe 2. Similarly, the rate of hydroboration with H 2BBr·SMe 2 was found to be on average 11 times faster than it is with HBBr 2·SMe 2. It is also clear that by replacing a hydrogen substituent with a bromine atom in the case of H 2BBr·SMe 2 the mechanism for the overall process changes from limiting dissociative ( D) to interchange associative ( I a).
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