Stannanes as free-radical reducing agents: an ab initio study of hydrogen atom transfer from some trialkyltin hydrides to alkyl radicals

Ab initio molecular orbital calculations using a (valence) double-xi pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation, predict that hydrogen atoms, methyl, ethyl, isopropyl and tert-butyl radicals abstract hydrogen atoms from stannane and trimethyltin hydride via transition states in which the attacking and leaving radicals adopt a colinear arrangement. Transition states in which (overall) Sn-C separations of 3.50 Angstrom have been calculated; these distances appear to be independent of the nature of the attacking radical and alkyl substitution at tin. At the highest level of theory (QCISD/DZP//MP2/DZP), energy barriers (Delta E-1(++)) of 18-34 kJ mol(-1) are predicted for the forward reactions, while the reverse reactions (Delta E-2(++)) are calculated to require 140-170 kJ mol(-1). These values are marginally affected by the inclusion of zero-point vibrational energy correction. Importantly, QCISD and MP2 calculations predict correctly the relative order of radical:reactivity toward reduction by stannanes: tert-butyl> isopropyl> ethyl, By comparison, SCF/DZP, AM1 and AM1(CI = 2) calculations perform somewhat more poorly in their prediction of relative radical reactivity.