Hyperconjugation Effect in Substituted Methyl Boranes: An Orbital Deletion Procedure Analysis

The hyperconjugation effect in the substituted methyl boranes, XCH BH (X = H, CH, NH, PH , OH, SH, F, Cl, Br), has been quantitatively evaluated by using the orbital deletion procedure (ODP), where the p orbital on boron is deactivated. Except for the case of X = NH, which forms a three-membered ring, the magnitude of the hyperconjugative stabilization in all other substituted methylborane ranges from 6.8 to 3.4 kcal/mol. Significant structural changes are observed, particularly the shortening of the central B-C bond distance and the reducing of the corresponding XCB and HCB bond angles. In general, the strength of the hyperconjugative interaction between the occupied σC-X bond and the vacant p orbital on boron is correlated to the electronegativity of X, and the competition between the donation ability of the sigma; and the σ bonds determines the preference of the staggered or eclipsed structure as the energy minimum state. When the donation abilities of the C-X and C-H bonds are comparable, other factors such as electron correlation and steric effect may play elaborate roles in the geometrical propensity of the most stable structures.