The role of water in the stereoselective hydrogenation of 1,2-diphenylacetylene catalyzed by the water-soluble [{RuCl<inf>2</inf>(mtppms) <inf>2</inf>}<inf>2</inf>]

Density functional theory with dielectric continuum models was applied to describe the reaction mechanism of the stereoselective hydrogenation of phenyl-substituted alkynes with the water-soluble ruthenium complex [{RuCl 2-(mtppms)2}2] mtppms [(meta-sulfonatophenyl) diphenylphosphane] in acidic aqueous solutions. The water solvent is modelled by the inclusion of a small cluster of three water molecules in addition to a continuum model. The possible reaction mechanisms leading to the formation of the different (Z and E) stereoisomers are computationally evaluated. It was shown that the reaction takes place in two steps; one hydrogen comes from the hydrido complex, whereas the second hydrogen is transferred from the hydroxonium ions present in the solution. The formation of the different stereoisomers is determined by the order of the two hydrogenation steps, and the analysis suggests that the formation of the (Z)-isomer is more favourable, which is consistent with the experimental results. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.