Parts-per-million Level Loading Cyclometalated Ru(II)-NHC Catalyzed Selective Oxidation of Olefins to Carbonyls.

Abstract

Oxidative cleavage of olefins is a useful reaction in organic synthesis. The most well-known catalytic system is the osmium based Lemieux-Johnson catalyst, which generally requires high catalyst loading and tends to suffer from rapid overoxidation to produce the acid predominantly. Hence, the development of a mild, general, and selective method toward the oxidative cleavage of alkenes to carbonyl compounds is highly desired. In this work, a highly efficient ruthenium-based catalyst for olefin oxidation has been demonstrated by employing a fused π-conjugated imidazo[1,5-a]quinoxaline (ImQx) based NHC ligand with bidentate C(carbanion)^CNHC motif. Strong C-donor ligands, paired with a rigid backbone and ruthenium redox activity, provided exceptionally high catalytic activity and a long lifetime for olefin oxidation. Complex showed high catalytic activity and a long lifetime, TONs are several million. The catalyst tolerates numerous functional groups and can be applicable to challenging biomass, natural products, sugar, amino acids, and fatty acid-derived substrates. Based on kinetic studies, thermodynamic activation parameters, and DFT study, the mechanistic finding demonstrated that [3+2] cycloaddition reaction is the key step in the oxidation process. The use of the by-product NaIO3 in the catalytic efficiency has been disclosed for the first time.