Iridium-Catalysed Enantioselective Hydrogenation

There are very few examples of highly enantioselective hydrogenations of olefins lacking a polar group. Pfaltz and co-workers have recently found a new class of catalysts that are chiral analogues of Crabtree's catalyst, bearing chelating chiral phosphino-oxazoline (PHOX) ligands. Twelve new PHOX ligands with stereogenic phosphorus atoms were prepared by an efficient route. For six of the corresponding iridium complexes the X-ray structures were determined, providing a direct assignment of configuration. The catalytic activity of these new iridium complexes was tested in the enantioselective hydrogenation of various alkenes. In many cases better results, compared to the benchmark catalysts with the same backbone, were obtained. The oxazoline moiety controls the product configuration in most cases, with the phosphine part tuning the enantioselectivities. In many cases a strong matched/mismatched effect was observed, mostly favouring the (SP,SC)-configuration of the ligand. Another subject was the preparation of new, readily available phosphinite ligands, and the catalytic application of their iridium complexes to asymmetric hydrogenation. Gratifyingly, both complexes show excellent selectivities in enantioselective hydrogenation reactions of trisubstituted alkenes, often higher than those obtained with much more complex ligands. We found a strong counterion influence in the kinetic profiles of asymmetric hydrogenation with cationic iridium-PHOX catalysts. The maximum reaction rates were found to be as follows: [Al{OC(CF3)3}4]- > BArF- > [B(C6F5)4]- > PF6- >> BF4- > CF3SO3- The study of iridium hydrido species led to the isolation of trimeric cluster species that are believed to be important deactivation products of our catalytic systems. These compounds were fully characterised and proved to be inactive in hydrogenation reactions.