Mechanistic Insights into Catalytic Transfer Hydrogenation Using Cp*Ir(III)Cl Pyridinesulfonamide Complexes

Abstract

Previous work in the O’Connor lab focused on the development of (Cp*)Ir pyridinesulfonamide chloride complexes (Cp* = pentamethylcyclopentadienyl) to catalyze transfer hydrogenation. This project has two focuses: 1). Complete the substrate screen for the reduction of aldehydes and 2). The elucidation of the reaction mechanism via the synthesis and characterization of catalytically competent intermediates. Selective reduction of aldehydes in the presence of ketones is observed. Linear and aromatic aldehydes are reduced in good yield (95%-100%). Cp*Ir(pyridinesulfonamide)(OTf) complexes, where OTf- = triflate, were synthesized in 87% yield. The complex was characterized by 1H and 19F NMR spectroscopy. Attempts to use this precursor to prepare hydride analogs have been conducted using a wide variety of hydride sources, including H2, HBF4, and Na[BH3(CN)]. Additionally, a Cp*Ir bridging hydride dimer was synthesized and evaluated for transfer hydrogenation catalysis. This complex exhibits similar TOFs (TOF = turnover frequency) to the pyridinesulfonamide precatalysts for the reduction of acetophenone and benzaldehyde. Other mechanistic studies to evaluate the need for ligand flexibility were conducted by synthesizing complexes with more structurally rigid ligands to study the impact on catalytic conversion. Specifically, precatalysts containing a methylene linker with varying groups on the sulfonamide were synthesized in 66%-88% yield. These precatalysts were found to have slightly lower conversions than precatalysts containing an ethylene linker (76% vs. 89% for the reduction of acetophenone). A quionlinesulfonamide precatalyst was also evaluated and exhibited lower activity than ethylene and methylene linker precatalysts. The identity of counteranion has little effect on catalysis as the evidenced by similar TOFs. A final study was conducted to evaluate the identity of the metal center using LC-MS (liquid chromatography-mass spectrometry).