Coupling of terminal alkynes by RuHXL2 (X = Cl or N(SiMe3)2, L = PiPr3)

The compounds RuL2HX, where L = PiPr3 and X = Cl or N(SiMe3)2, are catalyst precursors for dimerization of terminal alkynes to enynes and also to cumulenes at 23 °C; selectivity among these products is X-dependent, but not high. Conversion of Ru species onto the catalytic cycle was undetectably small, so alternative approaches to understanding the catalytic mechanism were employed: stoichiometric reactions, independent synthesis of candidate intermediates, and trapping with CO. These show the intermediacy of vinylidenes and vinyl compounds, and reveal conversion of cumulenes to the thermodynamically more stable enynes.