Base-Free Catalytic Transfer Hydrogenation of Ketones Using Cp*Ir(III)Cl Pyridinesulfonamide Complexes

Abstract

Although examples of iridium complexes containing diamine ligands that catalyze transfer hydrogenation are known, few examples exist of iridium complexes that contain pyridinesulfonamide ligands, and the catalytic activity of such complexes is limited. Therefore, the O'Connor group is interested in exploring new catalysts for transfer hydrogenation using air and moisture tolerant complexes possessing pyridinesulfonamide ligand scaffolds. The synthesis, characterization, and catalysis of Cp*Ir(III)Cl complexes (Cp* = pentamethylcyclopentadienyl) containing pyridinesulfonamide ligands have been achieved. These complexes are active in hydrogen transfer catalysis, as aryl ketones with electron withdrawing and electron donating substituents are effectively reduced to an alcohol at 85oC (80-90% conversion) with 1 mol% of the iridium precatalyst. The electronic nature of the catalyst plays a role in product yield. This portion of the project focuses on an expanded substrate scope to include aliphatic ketones and diones, activated and unactivated alkenes, levulinic acid, and b-ketoesters. Blank trials indicate that basic and halide abstracting additives are not necessary for high catalytic activity. Positional studies show that ortho and meta isomers are reduced the faster than para. Mercury poisoning support homogeneous catalysis. Lastly, preliminary results demonstrate the use of tandem catalysis to form deoxygenated products from electron-rich aryl ketones.