Catalysis of Organic..

Musolino, Apa, Donato and Pietropaolo 249Figure 1 shows a typical product composition-time plot of hydrogenation andisomerization reactions at the reported experimental parameters.Composition (%)10080604020(Z)-2-Penten-1-olValeraldehyden-Pentanol(E)-3-Penten-1-ol(Z)-3-Penten-1-ol(E)-2-Penten-1-ol00 100 200 300 400 500Time (min)Figure 1. Composition - time profile for the hydrogenation and isomerization reactionof (Z)-2-penten-1ol in ethanol at 303 K and at 0.1 MPa H 2 pressure, in the presence ofRhCl(PPh 3 ) 3 and NEt 3 .DiscussionData in table 1 allow a direct comparison of both activity values and selectivity,chosen at 80% conversion of the substrate, and clearly demonstrate that the reaction,in the presence of NEt 3 , is faster than that carried out in its absence. Indeed, whereasthe dihydride species mainly behaves as a hydrogenation catalyst, the mono hydridecomplex behaves both as an isomerization and hydrogenation species (2). All ourresults may be interpreted on the basis of these fundamental concepts. In fact, when aZ substrate reacts with H 2 , in the presence of RhCl(PPh 3 ) 3 and NEt 3 in ethanol, thegeometric E derivatives are formed with a good selectivity. Conversely, (E)-2-buten-1-ol is mainly converted to the corresponding hydrogenated compound.As we expect, the reactivity of E isomers, both in the presence and in the absenceof NEt 3 , is slower than that of the analogous Z derivatives. Such behavior is mainlydue to an enhanced steric hindrance of the E derivatives in comparison with theanalogous Z isomers and the reactivity slows down as the chain length increases, atleast when we consider the reactivity of (E)-2-penten-1-ol and (E)-2-hexen-1-ol, forwhich no selectivity values were detected owing to the very low reaction rateobtained.With analogous substrates, the heterogeneous hydrogenation vs. isomerizationreactions show behavior that is worth considering. Comparison of the results reportedhere, with those previously obtained for analogous organic molecules hydrogenatedon Pd/TiO 2 systems (3), reveals a reduced reactivity variation between Z and Egeometric isomers in the heterogeneous catalysis. A possible explanation is that a