Chauvin mechanism for olefin metathesis

A recent report by Mayr of slow polymerization of PhC=CH by (PMe3)2Cl2(PhC=CPh)W=CHPh fulfills expectations based on the classic Chauvin mechanism for olefin metathesis (78). The presence of a carbene and a vacant coordination site are prerequisites for metallocyclo-butene formation with free alkyne. Mayr has both the carbene and the alkyne initially present in the catalyst, but there is no evidence for direct involvement of the cis alkyne in the actual polymerization mechanism. 

Fig. 4.17. The pairwise [2 + 2] mechanism and the Chauvin mechanism for olefin metathesis.

By analogy to olefin metathesis, alkyne metathesis occurs between a complex containing a metal-carbon triple bond - a metal carbyne (or alkylidyne) - and an alkyne substrate [66]. As illustrated in Fig. 4.21, this mechanism parallels the Chauvin mechanism for olefin metathesis after alkyne coordination to the metal center, [2 - - 2] cycloaddition between the metal carbyne and the alkyne yields a metallacyclobutadiene, which rearranges and fragments productively to afford a new carbyne and a new alkyne (Fig. 4.21) [54]. 

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