C ₂ -Symmetric Rhenacyclobutadienes: Key Intermediates in Re(V) Alkylidyne-Catalyzed Alkyne Metathesis Reactions

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.