Reactions

As with the mechanism for alkene metathesis, the reaction begins with the coordination of a first alkyne to the alkylidine reagent (0). The resulting complex then undergoes ligand dissociation (1), prior to a [2+2] cycloaddition step, which affords a first metallacyclobutadiene intermediate (2). After an isomerization step (3), the complex undergoes a reverse [2+2] cycloaddition to generate the active species, an alkylidyne complex bearing the group R₁ (4). At this stage, the active species is obtained.

This latter undergoes a [2+2] cycloaddition with a new alkyne, leading to a new metallacyclobutadiene intermediate (A). After a subsequent isomerization (B) and reverse [2+2] cycloaddition (C), the desired disubstituted alkyne product is afforded. To regenerate the active species, another alkyne bearing the R₁ group undergoes cycloaddition (D) and isomerization (E) followed by reverse cycloaddition (F), releasing a molecule of ethyne as by-product. It’s important to note that, as in alkene metathesis, homo-metathesis can occur as a competing pathway, leading in this case to the symmetrical alkyne bearing two R₁ substituents, in our example.