Mechanistic studies on transition metal complex catalysed homogeneous hydrogenation
The development of systems capable of catalysing the reduction of unsaturated bonds with very high selectivities is one of the greatest successes of asymmetric catalysis. The mechanism by which rhodium complexes catalyse alkene hydrogenation has been effectively established through a combination of kinetic studies and isolation and characterisation of the intermediates in solution. It was hoped to elucidate the corresponding ruthenium catalysed mechanism using similar techniques. Following the synthesis of a selection of ruthenium catalysts, their activity towards the reduction of a selection of dehydroamino acid derivatives at ambient temperature and pressure was investigated. Having successfully tested out the activity of these catalysts, NMR studies were initiated in order to observe and characterise intermediates in the catalytic cycle. Considerable effort was put into the NMR studies, the systems were looked at under both argon and hydrogen, but although some substrate containing species were observed, it was found to be effectively impossible to break into the catalytic cycle and observe intermediate species using NMR techniques. Electrospray mass spectrometry investigations yielded more successful results, though still no hydrogen containing intermediates were observed. However, it was possible to make a detailed kinetic study of several ruthenium catalyst / substrate systems by looking at the dependence of reaction rate on classic variables such as hydrogen pressure, catalyst concentration and substrate concentration. The combined results of electrospray experiments, kinetic analyses and kinetic modelling using computer packages enabled a possible mechanistic pathway to be proposed.