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.