Five-coordinate olefin reactivity

D. Reactivity of Five-Coordinate [M (Ai-ligand)(olefin)] Complexes (M = Rh, Ir)... 

Evidence indicates that the stereodetermining step in the catalytic cycle is metallacyclobutane formation and that the stereochemistry of the metallacycle is transferred to the product (Fig. 4.28). Consequently, trans metallacyclobutanes rearrange to yield trans olefins, and cis metallacyclobutanes yield cis olefins. The synthesis of five-coordinate adducts of the general formula (NAr)(OR)(L)M= CH(BuO (e.g., L = PMes), which are not metathesis active, demonstrated that binding of a donor ligand (in a substrate, for example) was detrimental to catalytic activity, and also confirmed the reactivity patterns of different alkylidene conformations. 

Mimoun has given an alternative possibility, suggesting the formation of peroxo metallacyclic adducts following complexafion of the olefin to the metal [171,475]. The reaction sequence (Fig. 1.13) involves a reactive species denoted as Ln-Mo, a Mo(VI) ion with a set of alkoxy ligands. The reaction starts by formation of an alkylperoxo Mo complex by ligand exchange of an alkylperoxide with an alcohol. This is followed by complexafion of the olefin by a coordination bond. A subsequent peroxy metallation of the olefin produces a five-membered... 

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