Ruthenium-Catalyzed Hydrative Cyclization of 1,5-Enynes

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Ruthenium-catalyzed 1,1-difunctionalization of alkynes can be achieved through ruthenium vinylidene intermediates. In this context, Lee s group reported the 

The hydrative cyclization involves the formation of a ruthenium vinylidene, an anti-Markovnikov addition of vater, and cyclization ofan acylmetal species onto the alkene. Although the cyclization may occur through a hydroacylation [32] (path A) or Michael addition [33] (path B), the requirement for an electron- vithdra ving substituent on the alkene and lack of aldehyde formation indicate the latter path vay to be the more likely mechanism. Notably, acylruthenium complex under vent no decarbonylation in this instance. 

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