Controlling sequential hydroboration and mechanistic pathway by means of Lewis base/solvent effects: A detailed kinetic and mechanistic study

Abstract

11B NMR spectroscopy was utilized to study hydroboration reactions of 1-octene and 4-octene with H 2BCl·THF in THF. The dependence of k obs on the nucleophile concentration and temperature was investigated. The reactions exhibited simple second-order kinetics of the form k obs = k 2 ′ [ Nu ] . The activation parameters (Δ H ≠, Δ S ≠) for the disappearance of H 2BCl·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 (Δ H ≠, Δ S ≠) for the formation of R 2BCl·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 H 2BCl·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.