On the radical Brook and related reactions: an ab initio study of some (1,2)-silyl, germyl and stannyl translocations

Abstract

Ab initio molecular orbital calculations using a (valence) double-ζ 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^–1 when electron correlation is included, in excellent agreement with experimental data. In addition, the analogous translocation to oxygen in the silylmethoxyl 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.