Olefin also Chauvin mechanism

The early development of Mechanism 3 was bold for its day because Fischer carbene complexes had just been discovered a few years earlier, and alkylidenes were not yet known. The carbene complexes prepared before 1971 also did not catalyze olefin metathesis. With the discovery of Schrock carbene complexes and the demonstration that some alkylidenes could promote metathesis, the non-pairwise mechanism became more plausible (Section 11-1-2). It was, however, the elegant work of Katz and co-workers that provided early substantial support for the Herisson-Chauvin mechanism. 

The reaction pathway shown is the widely accepted Chauvin mechanism " which involves a metalacycle formed by C—C bond formation within an Ti -olefin-carbene complex. It should be noted that the metal complex is not really a catalyst, but rather a promoter, because it is transformed into a new carbene. Theory suggests"" that the carbene fragment and the olefin should be parallel, as shown in the Scheme, for the C—C bond to form. It also suggests that a great deal of the added reactivity of the C systems is derived from the NHC enforcing this orientation on the carbene through steric interactions. 

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