Lemieux-Johnson oxidation with N,N-bidentate bisPYA ruthenium complexes: insights into reaction conditions and catalyst design

The carbonyl group is arguably the most versatile functional group for synthetic purposes. Here, a series of N,N’-bis(pyridylideneamide) (bisPYA) complexes of ruthenium(II), [Ru(cym)(bisPYA)Cl]⁺ (cym=cymene), were synthesized and applied as catalysts for Lemieux-Johnson oxidation of alkenes to access carbonyl functionalities. Complexes bearing para- and ortho-bisPYA ligands induce higher activity and selectivity than their mesoionic meta-bisPYA analogues. The non-coordinating counterion of the cationic complexes affects the catalytic performance markedly, with PF₆⁻ enhancing the activity compared to trifluoromethanesulfonate, OTf⁻. Likewise, replacing the anionic ancillary chloride ligand by a neutral and more labile MeCN ligand led to increased activity. Electrochemical and solubility measurements revealed no direct correlation of ligand donor strength to catalytic performance. Variation of the solvent mixture and the ratio of its components provided access to high-performance catalysts. In H₂O/MeCN/CH₂Cl₂ at 3 : 1:0.5 (v/v) ratio, turnover frequencies up to 10,000 h⁻¹ were obtained for the Lemieux-Johnson oxidation of styrene with these Ru(bisPYA) complexes, allowing quantitative conversions within 30 min at 0.1 mol % catalyst loadings and corresponding to a 5 times faster reaction than the best catalyst known to date for this transformation.