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) the inclusion of electron correlation predict that the transition states (12-14) involved in homolytic (1,2)-translocation reactions of silyl (SiH3), germyl (GeH3) and stannyl (SnH3) groups between carbon centres, between carbon and nitrogen, and between carbon and oxygen proceed via homolytic substitution mechanisms involving front-side attack at the group (IV) heteroatom. While migrations between carbons are predicted to be unlikely with calculated activation barriers of 71-137 kJ mol(-1), depending on the level of theory migrations from carbon to nitrogen 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 barrier of 50.8-63.2 kJ mol(-t) when electron correlation is included, in excellent agreement with experimental data. In addition, the analogous 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 unexpectedly, MP2/DZP calculations predict that the stannylmethoxyl radical (H3SnCH2O.) rearranges to the stannyloxymethyl radical (H3SnOCH2.) without barrier.