Theoretical Study of the Ruthenium-Catalyzed Cyclocotrimerization of Alkynes with Isocyanates and Isothiocyanates:  Chemoselective Formation of Pyridine-2-one and Thiopyrane-2-imine

The cyclocotrimerization of acetylene with isocyanate HNCO and isothiocyanate HNCS, mediated by CpRuCl, is theoretically investigated on the basis of DFT/B3LYP calculations. By these means, the experimental result can be rationalized as to why, with HNCO, a nitrogen-heterocycle is formed, but with HNCS a sulfur-heterocycle is formed. According to the proposed mechanism, the key reaction step is the addition of a double bond to a metallacyclopentatriene formed by oxidative coupling of two acetylene ligands coordinated to CpRuCl, giving a bicyclic carbene intermediate. This double-bond-addition is initiated by η¹ attack at the ruthenium center, and it is just the attacking atom that is going to be incorporated into the cycle. Thus, the chemoselectivity originates from the fact that, for HNCO, N attack is preferred over O, but for HNCS, S attack is preferred over N. The onward reaction is a reductive elimination to give a coordinatively unsaturated metallaheteronorbornene intermediate finally rearranging to a ligated heterocycle. Completion of the cycles is achieved by an exothermic displacement of the respective heterocyclic product by two acetylene molecules which regenerates the bisacetylene complex.