ON THE RADICAL BROOK AND RELATED REACTIONS - AN AB-INITIO STUDY OF SOME (1,2)-SILYL, GERMYL AND STANNYL TRANSLOCATIONS

Ab initio molecular orbital calculations using a (valence) double-zeta pseudopotential basis set (DZP) with (MP2, QCISD) and without (SCF) t he inclusion of electron correlation predict that the transition state s (12-14) involved in homolytic (1,2)-translocation reactions of silyl (SiH3), germyl (GeH3) and stannyl (SnH3) groups between carbon centre s, between carbon and nitrogen, and between carbon and oxygen proceed via homolytic substitution mechanisms involving front-side attack at t he group (IV) heteroatom. While migrations between carbons are predict ed to be unlikely with calculated activation barriers of 71-137 kJ mol (-1), depending on the level of theory migrations from carbon to nitro gen and from carbon to oxygen are predicted to be facile, For example, rearrangement of the (silylmethyl)aminyl radical (H3SiCH2NH.) to the silylaminomethyl species (H3SiNHCH2.) is predicted to proceed with a b arrier of 50.8-63.2 kJ mol(-t) when electron correlation is included, in excellent agreement with experimental data. In addition, the analog ous translocation to oxygen in the sylylmethoxyl radical (H3SiCH2O.), the prototypical radical Brook rearrangement, is calculated to require only 19.9 kJ mol(-1) at the MP2/DZP + ZPVE level. Somewhat unexpected ly, MP2/DZP calculations predict that the stannylmethoxyl radical (H3S nCH2O.) rearranges to the stannyloxymethyl radical (H3SnOCH2.) without barrier.