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 tetrakis3,5‐bis(trifluoromethyl)phenyl]borate (BAr_F) 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 tetrakis3,5‐bis(trifluoromethyl)phenyl]borate. In one case, in the hydrogenation of a tetraalkyl‐substituted CC bond, Ir(Py)(PCy₃)(COD)]BAr_F gave higher conversion than catalyst 11 . However, with several other substrates complex 11 proved to be superior.