Hydrogenation of pentynes catalyzed by substituted derivatives of H4Ru4(CO)12 with phosphorus-donor ligands

The catalytic hydrogenation under mild conditions of 1-pentyne and 2-pentyne in the presence of phosphine and phosphite substituted derivatives of H4Ru4(CO)12 has been investigated. The initial rates of hydrogenation of both 1- and 2-pentyne with the monosubstituted derivatives were found to follow the trend: P(OPh)3 ≅ P(OEt)3 < PPh3 ≅ P(n-Bu)3. The initial rates of hydrogenation of 2-pentyne decrease with increasing substitution of the catalyst, while the opposite trend is observed for 1-pentyne. The specificity of all the catalysts investigated, towards the formation of cis-2-pentene and 1-pentene from 2-pentyne and 1-pentyne respectively, is similar and is improved as compared with that observed in the presence of H4Ru4(CO)12. The experimental results are interpreted on the basis of the stability of the alkyne carbonylmetal intermediates and of the influence which increasing substitution on H4Ru4(CO)12 plays on the formation of active sites. The complexity of this multistep process, however, does not allow a comprehensive rationalisation of the experimental data.