Metallacyclobutadienes, rearrangement

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]. 

Another possible explanation for the skeletal rearrangement is that metallacyclobutadiene A isomerizes to an -cyclopropei l metal complex upon addition of the alkyne and that the alkyne inserts into a M cyclopropenyl o-bond. The resulting yS cyclo propenylvinyl complex then rearranges to the observed cycle pentadienyl product. 

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