Chauvin-type mechanism

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... 

Some schemes and reactions of simple and multiple metal-carbon bonds involving metallosquares (Chauvin-type) mechanisms are recalled and represented on the next page. - ... 

Katz has proposed a carbenic Chauvin-type metathesis mechanism for the polymerization of alkynes. This mechanism involves two alkynes coordinated to in the catalytically active species. The ETC catalyzed ligand exchange process is adequate to rapidly introduce two or three alkyne ligands on in such a way that the Katz mechanism is then operating as shown on the following scheme. 

IV.20. For Chauvin-type catalytic mechanisms involving metallosquares, see ... 

Alkyne metathesis, which is the redistribution of triple bonds between two alkynes, has been proven to hold great synthetic promise. The now-accepted mechanism for the alkyne metathesis, which was proposed by Katz and McGinnis in 1975, involves a Chauvin-type series of metal carbyne and metallacyclobutadiene intermediates as the... 

More than half a century ago it was observed that Re207 and Mo or W carbonyls immobilized on alumina or silica could catalyze the metathesis of propylene into ethylene and 2-butene, an equilibrium reaction. The reaction can be driven either way and it is 100% atom efficient. The introduction of metathesis-based industrial processes was considerably faster than the elucidation of the mechanistic fundamentals [103, 104]. Indeed the first process, the Phillips triolefin process (Scheme 5.55) that was used to convert excess propylene into ethylene and 2-butene, was shut down in 1972, one year after Chauvin proposed the mechanism (Scheme 5.54) that earned him the Nobel prize [105]. Starting with a metal carbene species as active catalyst a metallocyclobutane has to be formed. The Fischer-type metal carbenes known at the time did not catalyze the metathesis reaction but further evidence supporting the Chauvin mechanism was published. Once the Schrock-type metal carbenes became known this changed. In 1980 Schrock and coworkers reported tungsten carbene complexes... 

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. 

While the basic Chauvin mechanism of olefin metathesis has been appreciated and acknowledged for some time, the studies on both Schrock and Grubbs type catalysts show that the knowledge of the mechanistic steps necessary to access Ghauvin-metathesis intermediates, and details concerning the structures and geometries of these intermediates, can lead to ever-improving and selective catalysts for this important polymerization reaction. 

Different types of olefin metathesis, all proceeding according to the Chauvin mechanism and catalyzed by Schrock-type or Grubbs-type metathesis catalysts. Tandem, domino and cascade metathesis reactions couple several of these reactions (in particular ROMP + ROM). 

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