Browsing by Author "Xaba, N."

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    Change in mechanistic pathway of hydroboration: A detailed kinetic study of H2BBr·SMe2 and HBBr2·SMe2
    (Journal of Organometallic Chemistry, 2007-09-01) Xaba, N.; Jaganyi, D.
    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).
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    Controlling sequential hydroboration and mechanistic pathway by means of Lewis base/solvent effects: A detailed kinetic and mechanistic study
    (Polyhedron, 2009-12-08) Xaba, N.; Jaganyi, D.
    11B NMR spectroscopy was utilized to study hydroboration reactions of 1-octene and 4-octene with H2BCl·THF in THF. The dependence of kobs on the nucleophile concentration and temperature was investigated. The reactions exhibited simple second-order kinetics of the form . The activation parameters (ΔH≠, ΔS≠) for the disappearance of H2BCl·THF were respectively found to be 51 ± 2 kJ mol−1, −115 ± 8 J K−1 mol−1 in the case of 1-octene and 44 ± 1 kJ mol−1, −146 ± 4 J K−1 mol−1 in the case of 4-octene. Similarly, the activation parameters (ΔH≠, ΔS≠) for the formation of R2BCl·THF were found to be 80 ± 3 kJ mol−1, −36 ± 9 J K−1 mol−1 in the case of 1-octene and 71 ± 2 kJ mol−1, −63 ± 6 J K−1 mol−1 in the case of 4-octene. Based on the activation parameters, it was concluded that hydroboration reactions involving H2BCl·THF in THF proceed through a direct attack mechanism, with the magnitude of the negative values of activation entropy signifying a compact transition state i.e. a limiting associative pathway (A). The increase in the steric strain due to the compact nature of the transition state resulted in the rate of the second step of hydroboration of 4-octene being slightly higher than that of 1-octene, by a factor of 1.5. This could be accounted for by postulating a forced partial dissociation of THF from the complex due to steric crowding and reduction of electrophilicity of the boron atom because of attachment of the first alkyl chain. This could also account for small negative entropy values for this step. Alternatively, a parallel reaction most likely a rearrangement process through formation of a π-complex occurred as a result of steric strain that was created around the boron atom on attachment of two alkyl chains at the central position.
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    The role of the double bond position in hydroboration using HBBr2 · SMe2, HBCl2 · SMe2 and H2BBr · SMe2: A detailed kinetic and mechanistic study
    (polyhedron, 2009-04-23) Xaba, N.; Jaganyi, D.
    Hydroboration reactions of 4-octene with HBBr2 · SMe2, HBCl2 · SMe2 and H2BBr · SMe2 in CH2Cl2 were studied as function of concentration and temperature and compared with those of 1-octene. On average, hydroboration with dihaloborane proceeded 16 times slower for 4-octene than for 1-octene. In the case of the reactions with the monohaloborane, this factor is halved. This can be explained by the difference in the relative rates of dissociates of Me2S from the dihaloborane and a monohaloborane complex, respectively. The reactions involving H2BBr · SMe2 also exhibited a k−2 value, an indication of the presence of a parallel reaction, most likely a rearrangement process facilitating isomerization by way of a π-complex. The moderate ΔH≠ values accompanied by small ΔS≠ values (94 ± 4 kJ mol−1, −3 ± 13 J K−1 mol−1 for HBBr2 · SMe2; 93 ± 1 kJ mol−1, −17 ± 4 J K−1 mol−1 for HBCl2 · SMe2 and in the case of H2BBr · SMe2, 90 ± 13 kJ mol−1, +12 ± 44 J K−1 mol−1 and 83 ± 13 kJ mol−1, −24 ± 45 J K−1 mol−1, respectively, for the k2 and k−2 processes) imply a process that is dissociatively dominated, with the overall mode of activation being interchange dissociative (Id).