Acid controlled alkyne dimerisation initiated by a Ru–carbene precursor

The Grubbʹs catalyst Cl2(PR3)2RuCHPh (1) is an excellent precursor for the dimerisation of terminal alkynes. Thermal treatment and addition of two equivalents of phenylacetylene to complex 1 generates a new Ru–vinylidene (3). Complex 3 catalyses a selective product formation for trans-tail-to-tail enynes. Addition of acetic acid enhances the yield and the reaction rate dramatically and a reversed stereoselectivity for the formation of Z-isomers is obtained. The stereoselectivity can be easily tuned by addition of acetic acid. The influence of the acid on the reaction pathway has been revealed. A simple one-pot preparation of tail-to-tail enynes is established.