Structure and mechanism in aerobic alkene epoxidations promoted by ruthenium complexes of bis(dihydrooxazole) ligands

Abstract

Reaction of [RuCl₂(NCMe)₂(cod)] with bis(dihydrooxazoles) gave [RuCl₂(cod){(S,S)-R¹₂C-([graphic omitted])₂}](cod = cycloocta-1,5-diene; R¹= H, R²= CH₂Ph or Prⁱ; R¹= Me, R²= Prⁱ). The benzyl complex has been crystallographically characterised. The IR spectra of the complex of the hexamethyl-substituted compound suggest that the ligand is relatively distorted and this is borne out by crystallographic comparison of the model complexes cis-[W(CO)₄{(S,S)-R₂C([graphic omitted])₂}](R = H or Me). Mechanistic studies of the epoxidation of styrene and stilbenes in the presence of isobutyraldehyde and molecular oxygen using the ruthenium complexes as catalysts in the presence and absence of 4-tert-butylcatechol as a radical trap revealed that the metals act as promoters for the production of PrⁱCO₃H and that this carries out the epoxidation, either directly or by formation of oxo-ruthenium species.