The development of novel hetrogeneous catalysts for the hydrogenation of prochiral imines
Catalysts have been studied for the enantioselective hydrogenation of prochiral imines. Work has centred on the support of homogeneous catalysts, with the intention of establishing a heterogeneous catalyst without loss of activity. Initially, established heterogeneous systems were investigated. As the methyl pyruvate system of cinchona modified catalysts was to be used, this system was first investigated with a series of platinum catalysts. Although these results provided some interesting results they maintained that the 5% Pt/alumina catalyst was the best for this hydrogenation. Investigations using this system for prochiral imine hydrogenation resulted in the production of chiral amines. Hydrogenation of N-(1 -methylbenzylidene)-2,3-dimethylbutylamine gave encouraging percentage conversions, but unfortunately, enantiomeric excesses were particularly low. These results led to investigations on supporting a suitable homogeneous catalyst. Initial work involving Wilkinson's catalyst investigated different preparations, solvents and supports. Results achieved in this section of the study showed an increase in activity of clay and zeolite supported catalysts when run in 1,1,1-trichloroethane and ethanol, but a drop in activity when benzene was the reaction solvent. Work with imines centred around two homogeneous catalysts. BINAP and DIOP ligands were used to create rhodium and iridium centred catalysts which were used to homogeneously hydrogenate two imines (N-(1,2-dimethylpropylidene)aniline and N-(1 methylbenzylidene)-2,3-dimethylbutylamine). These catalysts were then tethered to a Montmorillonite clay support and NaY zeolite, before repeated hydrogenations of the above imines. Results achieved were very encouraging, with an increase in activity being noted for some of the supported catalysts.