Selective CC Bond Formation between Alkynes Mediated by the [RuCp(PR 3 )] + Fragment Leading to Allyl, Butadienyl, and Allenyl Carbene Complexes—An Experimental and Theoretical Study

The reaction of alkynes with [RuCp(PR 3 )(CH 3 CN) 2 ]PF 6 (R=Me, Ph, Cy) affords, depending on the structure of the alkyne and the substituent of the phosphine ligand, allyl carbene or butadienyl carbene complexes. These reactions involve the migration of the phosphine ligand or a facile 1,2 hydrogen shift. Both reactions proceed via a metallacyclopentatriene complex. If no α CH bonds are accessible, allyl carbenes are formed, while in the presence of α CH bonds butadienyl carbenes are typically obtained. With diphenylacetylene, on the other hand, a cyclobutadiene complex is formed. A different reaction pathway is encountered with HCCSiMe 3 , ethynylferrocene (HCCFc), and ethynylruthenocene (HCCRc). Whereas the reaction of [RuCp(PR 3 )(CH 3 CN) 2 ]PF 6 (R=Ph and Cy) with HCCSiMe 3 affords a vinylidene complex, with HCCFc and HCCRc this reaction does not stop at the vinylidene stage but subsequent cycloaddition yields allenyl carbene complexes. This latter CC bond formation is effected by strong electronic coupling of the metallocene moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the RuC bond. The vinylidene intermediate appears only in the presence of bulky substituents of the phosphine coligand. For the small R=Me, head‐to‐tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual allyl carbene complexes. X‐ray structures of representative complexes are presented. A reasonable mechanism for the formation of both allyl and allenyl carbenes has been established by means of DFT calculations. During the formation of allyl and allenyl carbenes, metallacyclopentatriene and vinylidene complexes, respectively, are crucial intermediates.