Well-Defined Tungsten and Molybdenum Catalysts

This general type of metal carbenes is termed Schrock carbenes . These carbenes differ from Fischer carbene complexes (those with o-hetero-substituents) in a number of ways. Schrock carbenes are viewed as X2 ligands with -i-2 charge, whereas Fischer carbenes are considered neutral L ligands. Fischer carbenes are electrophilic at the a-carbon and Schrock carbenes are nucleophilic. Thus complexes 1 and 2 are formally W(VI) and Mo(VI) complexes [66]. 

There are a number of differences between the tungsten and molybdenum complexes. The main generality drawn from the data is that the tungsten complexes promote olefin metathesis quite a bit faster than the molybdenum complexes, but the tungsten complexes are less tolerant of functionality. The tungstacyclobutane is more stable than the molybdacyclobutane. Several tungstacyclobutanes have 

The molybdenum catalyst 2 has been used extensively for ADMET polymerization. This complex is easier to handle than the tungsten analog and is more tolerant of functionality. This complex has allowed the synthesis of polymers containing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosilanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45]. Aldehydes, ketones, and protic functionahty are not tolerated. The molybdenum alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fashion [64]. 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

The high trans content witnessed in ADMET polymers produced by these catalysts can be explained by the preference of the all-pseudoequatorial configuration in the metallacyclobutane intermediates, which is suggested by NMR and X-ray data [23] (Fig. 6.2). The all-equatorial conformation of the metallacyclobutane leads to the production of trans olefins. 

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