Tungsten carbene complexes propagating

A variety of isolated pentacoordinate tungsten-carbene complexes are known to be active metathesis catalysts [53]. At least one of these systems has been proposed to be living based primarily on 1H NMR identification of the propagating alkylidene [53 e]. To date, verification of the living nature of these catalysts through GPC determination of polydispersities are still pending. The solution NMR studies do confirm the mechanism of the metathesis reaction, but do not insure that all of the requisite factors for a living system are met. 

The tungsten analogue of 149a adds to 139 to form a transoid square-pyramidal metallacyclobutane eomplex, which can be isolated and characterized at low temperature. Its rearrangement to the propagating metal carbene complex can also... 

Synthetic procedures are in place today that allow the polymer end-functionalization for all commonly used carbene complexes based on ruthenium, tungsten, and molybdenum. From a practical and applications point of view, both ends of a polymer chain are equally useful. From a mechanistic point of view, the functionalization of the polymer chain end using the reactivity of the propagating carbene complex is much more readily achieved than functionalization of the focal group. Nonetheless, functional initiation is an attractive way to ensure complete end (start) functionalization, which has received comparatively little attention so far. Reliable methods for the functional derivatization of commercially available carbene catalysts will allow not only the synthesis of mono-telechelic polymers with high degrees of end-functionalization but also polymerization from surfaces or solid supports and open up more synthetic pathways to hetero-telechelic polymers. 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

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