Abstract
AbstractIn 2007 two of us defined the σ‐Complex Assisted Metathesis mechanism (Perutz and Sabo‐Etienne,Angew. Chem. Int. Ed.2007,46, 2578–2592), that is, the σ‐CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal–element bonds through partner‐interchange of σ‐bond complexes. The key concept that defines a σ‐CAM process is asingletransition state for metathesis that is connected by two intermediates that are σ‐bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ‐bond complexes have been isolated or computed as well‐defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ‐bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ‐CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ‐CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional σ‐bond ligands, agostic complexes, sp2‐carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal–element bonds.
Highlights
The key concept that defines a s-CAM process is a single transition state for metathesis that is connected by two intermediates that are s-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product
The principle behind the proposed s-CAM (s-Complex Assisted Metathesis) mechanism is that s-bond complexes can interchange the partners that form the s-bond(s) donating to the metal
This interchange could lead to metathesis at constant oxidation state (Scheme 1 a)
Summary
An alkyl ligand is replaced by a silyl ligand or vice versa. The entering reagent in this case is a silane and the co-product is an alkane alongside the required metal silyl complex. In 2007 two of us published a review entitled “The s-CAM mechanism: s-complexes as the basis of s-bond metathesis at late-transition-metal centers”.[1] The principle behind the proposed s-CAM (s-Complex Assisted Metathesis) mechanism is that s-bond complexes can interchange the partners that form the s-bond(s) donating to the metal This interchange could lead to metathesis at constant oxidation state (Scheme 1 a). The overall reaction can be productive (E 1⁄46 E’) or degenerate (E = E’) Common to both situations are three key features of the mechanism: (1) that a vacant site is required for the initial coordination of E’ÀH, (2) that two successive s-bond complexes are formed as reaction intermediates or isolable species, (3) the oxidation state of the metal center in the precursor, intermediates and product remains constant throughout the process. We outline advances in s-bond complex synthesis and characterization since the 2007 review, to provide context for the discussion of mechanism
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