[2+2+2] Cyclotrimerization of Alkynes and Isocyanates/Isothiocyanates Catalyzed by Ruthenium-Alkylidene Complexes

Ruthenium carbene catalysts are able to catalyze crossed [2+2+2] cyclotrimerizations of alpha,omega-diynes with isocyanates, isothiocyanates, and carbon disulfide. Both aliphatic and aromatic isocyanates can be used to produce fused 2-pyridones, although aliphatic isocyanates were more reactive. Aromatic isocyanates give better results when they bear electron donating substituents. The reaction of unsymmetrical alpha,beta-diynes gave a product only with the substituent adjacent to the 2-pyridone nitrogen. Isothiocyanates gave thiopyranimines upon reaction with the C=S bond, whereas CS2 reacted efficiently to give a thioxothiopyrane.