Dimerization of terminal alkynes catalyzed by indenyl ruthenium(II) complexes

The indenyl ruthenium complexes [Ru(η5-C9H7)X(dppm)] (X=H, 1; CCPh, 2; (E)-CHCHPh, 3), [Ru(η5-C9H7)X(PPh3)2] (X=H, 4; CCPh, 5), [Ru(η5-C9H7)X(dppe)] (X=H, 6; CCPh, 7), [Ru(η5-C9H7)Cl(COD)] (8), (dppm=bis(diphenylphosphino)methane; dppe=1,2-bis(diphenylphosphino)ethane; COD=1,5-cyclooctadiene) catalyze the dimerization of phenylacetylene to (E)- and (Z)-1,4-diphenylbut-1-en-3-yne. The cyclopentadienyl complex [Ru(η5-C5H5)H(dppm)] (9) is inactive. The activity of the complexes depending on phosphine follows the order dppm>bis-PPh3>dppe, after 13 h. The catalysis is less efficient for the aliphatic 1-octyne than for phenylacetylene. Addition of PPh3 to complexes 5 and 8 enhances the conversion to the dimerization products. The isomeric distribution of E and Z enynes is dependent on different factors, such as temperature, reaction time, substrate to catalyst molar ratio, nature of σ-ligand. The isomers (E) and (Z)-1,4-diphenylbut-1-en-3-yne are synthesized on a semipreparative scale by catalysis of either complex 4 or 5, and can be separated from the isomeric mixture by standard methods. The procedure represents a one pot preparation of the two isomers starting from one terminal alkyne by CH activation.