Methylidene complex, Schrock

Methylidene-rare earth complexes (27) have been synthesized and fully characterized.32 In these complexes, the methylene should be seen as a doubly charged negative ligand. Nevertheless, these methylidene complexes react as Schrock-type nucleophilic carbenes, (27) therefore being analogous to the Tebbe reagent. 

One of the most famous Ti carbene complexes is methylidene 44. Although 44 probably occurs fleetingly as an intermediate in Schrock carbene reactions, it is too reactive to be isolated or observed spectroscopically. Two related compounds, however, have been synthesized 34 Cp2TiCH2(PEt3), in which the phosphine donates electrons to Ti to stabilize the complex, and the immediate precursor to 44, known as Tebbe s reagent (45, equation 10.1535). 

In mechanistic studies, the molybdacyclobutane of a MAP catalyst was found to break up to ethylene/methylidene intermediates 4500 times faster than the corresponding tungstacycle (at 40°C) [19]. Syn and anti proton exchange were also found to be significantly faster (up to lOOx s) for molybdacycles than for tungsta-cycles. Methylidene rotation about the M=C bond was determined to be comparatively slower for molybdenum complexes (<0.2 s ) than for tungsten complexes (3.6-230 s ). Schrock and coworkers proposed that many of these properties contribute to the superior efficiency of the tungsten MAP catalysts relative to their molybdenum counterparts. 

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