Platinum Metals Review - Volume 54, Issue 1, 2010

Asymmetric transfer hydrogenation (ATH) is the reduction of prochiral compounds with a hydrogen donor other than hydrogen gas in the presence of a chiral catalyst. The asymmetric reduction of a wide variety of ketone and aldehyde substrates has been carried out in water using catalysts based on complexes of ruthenium(II), rhodium(III) and iridium(III), affording fast reaction rates and good enantioselectivities without the use of organic solvents and with easy separation of catalyst and product. For ATH of ketones, the Rh(III) complexes appear to perform better than the Ru(II) and Ir(III) complexes in terms of activity, enantioselectivity and substrate scope. However, their performance varies with the choice of ligands, and simple Ir(III)-diamine complexes were found to be excellent catalysts for the reduction of aldehydes.

The largest and most significant type of geological deposit of platinum group elements (PGEs) is that associated with magmatic base metal sulfide minerals in layered mafic or ultramafic igneous intrusions. The common association of PGEs with sulfide minerals is a result of processes of magmatic and sulfide liquid segregation and fractionation. The mineralogical nature of the ores is dependent on a number of factors during sulfide liquid fractionation. The most significant of these with regard to the mineralogy of the two most important metals, platinum and palladium, is the presence and concentration of semimetals such as bismuth and tellurium within the mineralising sulfide liquid. Whereas rhodium, iridium, osmium and ruthenium are almost always present in solid solution within the resultant base metal sulfide minerals; should sufficient semimetals be present, Pd and especially Pt will form discrete minerals (such as platinum bismuthides) around the margins of, and possibly away from, the sulfides.