Homogeneous hydrogenation of tri- and tetrasubstituted olefins: Comparison of iridium-phospinooxazoline [Ir-PHOX] complexes and Crabtree catalysts with hexafluorophosphate (PF6) and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) as counterions

Four iridium complexes with achiral phosphino-oxazoline (PHOX) ligands were readily prepared in two steps starting from commercially available phenyloxazolines. The air-stable complexes with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) as counterion showed high reactivity in the hydrogenation of a range of tri- and tetrasubstituted olefins. The best results were obtained with an iridium complex (11) derived from a dicyclohexylphosphino-oxazoline ligand bearing no additional substituents in the oxazoline ring. With several substrates, which gave only low conversion with the Crabtree catalyst, [Ir(PY)(PCY3)(COD)]PF6, full conversion was observed. The productivity of the Crabtree catalyst could be strongly increased by replacing the hexafluorophosphate anion with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. In one case, in the hydrogenation of a tetraalkyl-substituted C = C bond, [Ir(Py)(PCY3)(COD)]BArF gave higher conversion than catalyst 11. However, with several other substrates complex 11 proved to be superior.