Nilchi, A. (1988) Selective hydrogenation catalysed by transition metal complexes. (PhD thesis), Kingston Polytechnic, .

Abstract

This work is an investigation of the mechanism by which norbornadiene, methyl oleate and methyl linoleate are hydrogenated in acetone at 30[sup]oC, 1 atmosphere or 3 atmospheres pressure, using rhodium complexes of the type [Rh(diene)L[sub]n][sup]+A[sup]-(where diene = norbornadiene , L = tertiary phosphine, phosphite, A = CIO[sup]-[sub]4 or PF[sup]-[sub]6). The results were interpreted assuming three active catalyst species, (Rh(diene)L[sub]2)[sup]+, (RhH[sub]2L[sub]2)[sup]+ and RhHL[sub]2. Also investigated were the effects of adding acid (HClO[sub]4) or base (Net[sub]3) and how this altered the equilibrium (RhH[sub]2L[sub]2)[sup]+<--> H[sup]+ + RhHL[sub]2. At atmospheric pressure, the rate of hydrogenation of norbornadiene and norbornene varies with ligand in the order PPh[sub]3 < PPh[sub]2Me < PPhMe[sub]2, suggesting that oxidative addition of hydrogen is an important first stage of the catalysis. The addition of acid, slowed the rate of hydrogenation for catalysts containing more electron donating ligands (relative to triphenyl phosphine), indicating that the monohydride was a more active specie than the dihydride. With triphenylphosphine or less electron donating ligands in the catalyst, the rate remains constant or increase slightly, indicating that an "unsaturate route" emanating from a diene complex is probably important. The catalyst containing cyclohexylphosphine ligands which are strongly electrondonating but sterically crowded is ineffective in hydrogenation, suggesting that steric crowding may cause an alternative route to operate. Higher pressure (3 atm.) causes faster hydrogenation and provided other mechanistic insights. For methyl oleate at atmospheric pressure, the rate of hydrogenation varies with ligand in the order PPh[sub]3 > PPh[sub]2Me > PPhMe[sub]2, but this order is reversed at 3 atm. pressure. The rate of isomerisation of methyl oleate varies with ligand in the order PPh[sub]3 < PPh[sub]2Me < PPhMe[sub]2, at both pressures. The rate of isomerisation of methyl oIeate and methyl linoleate is lowest, when there is no or a slight excess of acid present but is highest in presence of a base (especially for the catalyst containing diphenylmethylphosphine ligand). Mechanistic interpretations were made.

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