Key rhenacyclobutadiene intermediates were isolated from the reactions of alkynes with a d2 Re(V) alkylidyne complex (5a) bearing bidentate phosphinoanilide ligands. Single crystal X-ray diffraction analysis revealed a pseudo-C2-symmetric structure wherein two phosphines are trans-oriented. These rhenacyclobutadienes can undergo 2 + 2 cycloreversion when treated with PMePh2 or another alkyne and are able to effect internal alkyne metathesis. This work represents the first observation of reversible 2 + 2 cycloaddition reactions of alkynes with a non-d0 alkylidyne complex, elucidates the mechanism of Re(V)-alkylidyne-catalyzed alkyne metathesis, and provides valuable insight for the rational design of new alkyne metathesis catalysts with improved activity based on non-d0 alkylidynes.
Tsang et al. (Mon,) studied this question.