Aerobic oxidation of primary alcohols catalyzed by copper complexes of 1,10-phenanthroline-derived ligands

Abstract

Five copper complexes [(L¹)₂Cu(H₂O)](ClO₄)₂ (1), [(L¹)Cu(H₂O)₃](ClO₄)₂ (1a), [(L³)₂Cu(H₂O)](ClO₄)₂ (2), [(L⁵)₂Cu(H₂O)](ClO₄)₂ (3) and [(L⁶)₂Cu](ClO₄) (4) (where L¹ = 1,10-phenanthroline, L³ = 1,10-phenanthroline-5,6-dione, L⁵ = 1,10-phenanthrolinefuroxan and L⁶ = 2,9-dimethyl-1,10-phenanthrolinefuroxan), and in situ prepared copper complexes of 2,9-dimethyl-1,10-phenanthroline (L²) or 2,9-dimethyl-1,10-phenanthrolinedione (L⁴) were used for aerial oxidation of primary alcohols to the corresponding aldehydes under ambient conditions. The copper catalysts have been found to catalyze a series of primary alcohols including one secondary alcohol with moderate turnover numbers and selectivity towards primary alcohols. Copper(II) complexes 1 (or 1a) and 2 were found to be the better catalysts among all other systems explored in this study. A copper(II)-superoxo species is implicated to initiate the oxidation reaction. Structural and electronic factors of 1,10-phenanthroline-based ligands affecting the catalytic results for aerial oxidation of alcohols are discussed.