In situ H-1-PHIP-NMR studies of the stereoselective hydrogenation of alkynes to (E)-alkenes catalyzed by a homogeneous [Cp*Ru](+) catalyst

The hydrogenation of internal alkynes using a [Cp*Ru(alkene)](+) complex le ads to the formation of (E)-alkenes. This ruthenium complex represents one of the few homogeneous catalysts that trans-hydrogenate internal alkynes di rectly and stereoselectively. We have studied its stereoselectivity by in s itu PHIP-NMR spectroscopy (PHIP=para-hydrogen induced polarization). With t his method the initially formed products can be identified and characterize d even at very low concentrations and low conversions. Furthermore, their s ubsequent fate can be evaluated with high sensitivity and with time resolut ion. Different alkyne substrates were used to demonstrate the universal app licability of this catalyst. The catalyst is not active in combination with terminal alkynes, however, possibly due to the formation of a rather stabl e vinylidene complex. A mechanism proceeding via a binuclear complex is pro posed to explain the formation of the (E)-alkenes.