The σ-CAM mechanism:: σ complexes as the basis of σ-bond metathesis at late-transition-metal centers

Complexes in which a σ-H-E bond (E = H, B, Si, C) acts as a two-electron donor to the metal center are called σ complexes. Clues that it is possible to interconvert σ ligands without a change in oxidation state derive from C-H activation reactions effecting isotope exchange and from dynamic rearrangements of σ complexes (see Frontispiece). Through these pathways, metathesis of M-E bonds can occur at late transition metals. We call this process σ-complex-assisted metathesis, or σ-CAM, which is distinct from the familiar σ-bond metathesis (typical for d0 metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving σ complexes). There are examples of σ-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange ofalkanes. It may also occur in silylation and alkene hydrogenation. © 2007 Wiley-VCH Verlag GmbH &. Co. KGaA.