Designing new free-radical reducing agents: an ab initio study of hydrogen atom transfers from some silacyclopentadienes to methyl radical
Abstract
Ab initio molecular orbital calculations using the triple-ζ 6-311G** basis set, with (MP2, QCISD) and without (UHF) the inclusion of electron correlation calculate that methyl radicals abstract hydrogen atom from silacyclopentadiene (3, R = H), 1-methylsilacyclopentadiene (3, R = CH3) and 1-silylsilacyclopentadiene (3, R = SiH3) via transition states (5) in which the attacking and leaving radicals adopt collinear (or nearly so) arrangements. Transition states (5) which have (overall) Si–C separations of about 3.17–3.19 Å are calculated at MP2/6-311G**; structures (5) appear to be sensitive to the nature of the substituent on silicon, with an earlier transition state calculated for the reaction involving the silyl-substituted silane (3, R = SiH3), while the methyl-substituted system (3, R = CH3) is calculated to proceed with a slightly later transition state at the MP2/6-311G** level of theory. Energy barriers (ΔE1‡) of 35–42 kJ mol–1 are calculated for the forward reactions, while the reverse reactions (ΔE2‡) are calculated to require about 119–124 kJ mol–1 at MP2/6-311G**. Inclusion of higher-order correlation correction (QCISD/6-311G**//MP2/6-311G**) has only a marginal effect on the calculated energy barriers.