Molybdenum imido alkylidene catalysts

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. 

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Schrock, R. R. Olefin metathesis by molybdenum imido alkylidene catalysts. Tetrahedron 1999, 55, 8141-8153. 

Molybdenum imido alkylidene catalysts supported by sihca gel remain stable and highly active for alkene metathesis. ... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

Connon SJ, Blechert S. Recent developments in olefin crossmetathesis. Angew. Chem., Int. Ed. 2003 42 1900-1923. Schrock RR, Hoveyda AH. Molybdenum and mngsten imido alkylidene complexes as efficient olefin-metathesis catalysts. Angew. Chem. Int. Ed. 2003 42 4592-4633. 

Asymmetric syntheses of natural products cyclization reactions using molybdenum and tungsten imido alkylidene complexes as efficient olefin-metathesis catalysts 03AG(E)4592. 

R.R. Schrock, A.H. Hoveyda - Molybdenum and Tungsten Imido Alkylidene Complexes as Efficient Olefin-Metathesis Catalysts,... 

DFT calculations on the reactivity of d catalysts, carried out on simplified models of bisalkoxy imido molybdenum alkylidene complexes, suggested that the separated reactants achieve metathesis via a Chauvin-type mechanism, with a single step for the metallacyclobutane formation and one for the cycloreversion [5, 36]. Although the two metallacyclobutanes (TBP and SP) are minima of the potential energy surface, only the TBP intermediate is on the reaction pathway. Moreover, calculations showed that the preferential attack of ethene occurs trans to one of... 

FIGURE 20.17 Well-defined imido molybdenum alkylidene ROMP catalysts. 

In recent years, catalysts and protocols for the Z-selective formation of alkenes via alkene metathesis have been reported. Most progress has been made in the successful optimization of molecular molybdenum-alkylidene complexes. The substituents at their imido ligand fragment and at the electron-poor alcoholate/... 

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