E/Z selectivity in ruthenium-mediated cross metathesis

The influence of ancillary ligand on the activity of diverse NHC-ruthenium indenylidene complexes in cross metathesis (CM) have been examined. The phospine/NHC ancillary ligands tested lead to varied E/Z selectivities as a function of catalyst ancillary ligands. CM reaction between olefins leads to good initial Z-selectivity. However, secondary metathesis is responsible for the isomerisation of the product mixture leading to the thermodynamically favored E-isomer. Very active Ru catalysts (that activate easily under mild conditions) achieved very good conversions in short reaction times but isomerisation of the initially formed product was rapidly achieved as well. When less labile leaving groups (NHC) were attached to the Ru center, initiation time proved much longer but up to 1.3/1 E/Z ratio was achieved even at long reaction times. The effects of ancillary ligand on initiation and E/Z selectivity are discussed.