Synthesis of NHC-Oxazoline Pincer Complexes of Rh and Ru and Their Catalytic Activity for Hydrogenation and Conjugate Reduction

We describe the preparation and catalytic reactions of new CCN pincer Rh and Ru complexes containing NCH-oxazoline hybrid ligands. Oxazolinyl-phenyl-imidazolium derivatives (3) were suitable ligand precursors for the CCN pincer scaffold. C–H bond activation of 3 with RhCl3·3H2O in the presence of NEt3 yielded the desired CCN pincer Rh complexes 5 in 13–27% yields. The related CCN pincer Ru complexes 8–10 were synthesized in good yields by C–H bond activation of p-cymene Ru complexes 7 in the presence of NaOAc in DMF. The chiral complexes 8 and 9 had two diastereomers according to the coordination of CO and OAc ligands. The CCN Rh complexes showed catalytic activity for conjugate reduction of ethyl β-methylcinnamate with hydrosilane, with moderate enantioselectivity. The CCN Ru complexes were found to be active in the hydrogenation of aromatic ketones. In particular, hydrogenation of 9-acetylanthracene took place at not only the CO bond but also the anthracene ring. The Ru complexes were also used as catalysts in the transfer hydrogenation of 9-acetylanthracene with 2-propanol; again, both the CO bond and the anthracene ring were hydrogenated.