Imido molybdenum alkylidene complexes

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). 

Molybdenum imido alkylidene complexes also have been employed for a variety of other catalytic reactions of interest to the organic or polymer chemist, among them selective cross-couplings of olefins, [99] polymerization of terminal alkynes, [100-102] step-growth polymerization of dienes, [103,104] and cyclopolymerization of 1,6-heptadiynes. [105-107]... 

Molybdenum imido alkylidene complexes that contain bulky carboxylate ligands have been prepared and evaluated for the cyclopolymerizations of... 

Schrock, R.R., Muidzek, J.S., Bazan, G.C., Robbins, J., Dimare, M., Oregan, M., Synthesis of molybdenum imido alkylidene complexes and some reactions involving acyclic olefins, J. Am. Chem. Soc. 1990, 112 3875-3886. 

With molybdenum imido alkylidene complexes, three of four possible stereoregular ROMP polymers can be synthesized [26]. 

Synthesis of the main family of Schrock s molybdenum-imido-alkylidene complexes... 

Anders, U Nuyken, O., Buchmeiser, M.R., and Worst, K. (2002) Fine-tuning of molybdenum imido alkylidene complexes for the cyclopolymerization of 1,6-heptadiynes to give polyenes containing exclusively five-membered rings. Macromolecules, 35,9029. 

Schrock, R. R., Murdzek, J. S., Bazan, G. C., Robbins, J., DiMare, M and O Regan, M B. Synthesis of Molybdenum Imido Alkylidene Complexes and Some Reactions Involving Acyclic Olefins. JAm Chem Soc 112, 3875-3886 (1990). 

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. 

Schrock, R. R. Recent advances in olefin metathesis by molybdenum and tungsten imido alkylidene complexes. J. Mol. Catal. A Chemical... 

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,... 

Reactivity of Molybdenum-imido-alkoxy-alkylidene Complexes [92]... 

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... 

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