Steric control of a ruthenium-catalysed alkyne hydrogenation reaction

Article abstract of DOI:10.1016/0022-328X(90)80260-7

The cations ⁺ and ⁺ have been used in a study of the effect of variation of group 15 donor ligands (L", L') on the hydrogenation reaction of alkynes. ⁺ was found to catalyse hydrogenation of both alkynes and alkenes, the course of the reaction being dependent on the size of the ligand L'.The rate of hydrogenation of alkynes catalysed by ⁺ was found to increase with the cone angle, Θ, until ca 120 deg C and then decrease as Θ is raised still further.When the cone angle of L' < 120 deg C no alkene hydrogenation occurs.Addition of excess PMe3 (10 equiv.) to reaction solutions containing ⁺ was found to increase the selectivity of the alkyne hydrogenation reaction with concomitant decrease in the hydrogenation rate.Addition of a range of other ligands L' (2 - 3 equiv.) to solutions of ⁺ (L" = PMe3, PMe2Ph) also resulted in a decrease in the rate of the alkyne hydrogenation, but the decrease was dependent on the size of L'.The ⁺ complexes were found to catalytically hydrogenate alkynes exclusively when the sum of the cone angles for the L' and L" ligands was 5Θ = 610 +/- 20 deg C.No correlation between electronic parameters and the reaction rate and product selectivity is apparent from our results.Mechanistic features of the catalysed hydrogenation of alkenes and alkynes in the presence of these ruthenium complexes are discussed.