Development and Application of Carbene Ligands for Sustainable Catalysis

Abstract

N-Heterocyclic carbenes (NHCs), recognized for their potent σ-donor capabilities, have garnered considerable attention in the homogeneous catalysis since the successful isolation of the stable free carbene by Arduengo in 1991. Various structural modifications to the fundamental imidazolylidene framework has been employed to fine-tune the electronic and steric characteristics of NHC ligands. Aligned with our commitment to developing advanced NHC-based catalysts, we propose a range of ligands featuring diverse electronic and steric attributes. These ligands find application in chemical reactions, particularly in olefin metathesis with ethylene and synthesis of acrylate with ethylene and CO2. Chapter 2 describes the applications of indol-2-ylidene ruthenium (IdY-Ru) catalysts and aminooxycarbene ruthenium (AAOC-Ru) catalysts for olefin metathesis with ethylene. IdY-Ru exhibited excellent catalytic activity for ethenolysis (TON 61200 and selectivity for methyl oleate 95%; TON 53200 and selectivity for cis-cyclooctene 70%). AAOC-Ru showed TON 286000 and selectivity for methyl oleate 98% with tricyclohexylphosphine copper chloride (Cy3P-CuCl). Chapter 3 explores the preparation of nickel catalysts containing diphenylphosphine-chelated imidazo[1,5-a]pyridinylidene carbene ligands (PPh2-ImPy-Ni). PPh2-ImPy-nickel(0) catalysts were conveniently generated in situ from the corresponding imidazolium salts and exhibited excellent reactivity in the coupling of ethylene and CO2 to yield acrylates, surpassing the previously reported catalysts including pyridine-chelated imidazo[1,5-a]pyridylidene nickel catalysts. Remarkably, the in-situ generated PPh2-ImPy-Ni(0) catalysts afforded the sodium acrylate product with the highest reported turnover numbers (up to 570), demonstrating outstanding yield (up to 82 % based on base equivalent).