Ruthenium(II)-Catalyzed Selective Intramolecular [2 + 2 + 2] Alkyne Cyclotrimerizations

In the presence of a catalytic amount of Cp*RuCl(cod), 1,6-diynes chemoselectively reacted
with monoalkynes at ambient temperature to afford the desired bicyclic benzene derivatives in good yields.
A wide variety of diynes and monoynes containing functional groups such as ester, ketone, nitrile, amine,
alcohol, sulfide, etc. can be used for the present ruthenium catalysis. The most significant advantage of
this protocol is that the cycloaddition of unsymmetrical 1,6-diynes with one internal alkyne moiety
regioselectively gave rise to meta-substituted products with excellent regioselectivity. Completely intramolecular alkyne cyclotrimerization was also accomplished using triyne substrates to obtain tricyclic aromatic
compounds fused with 5−7-membered rings. A ruthenabicycle complex relevant to these cyclotrimerizations
was synthesized from Cp*RuCl(cod) and a 1,6-diyne possessing phenyl terminal groups, and its structure
was unambiguously determined by X-ray analysis. The intermediary of such a ruthenacycle intermediate
was further confirmed by its reaction with acetylene, giving rise to the expected cycloadduct. The density
functional study on the cyclotrimerization mechanism elucidated that the cyclotrimerization proceeds via
oxidative cyclization, producing a ruthenacycle intermediate and subsequent alkyne insertion initiated by
the formal [2 + 2] cycloaddition of the resultant ruthenacycle with an alkyne.