Benzimidazole–triazole ligands with pendent triazole functionality: unexpected formation and effects on copper-catalyzed aerobic alcohol oxidation

Abstract

A series of benzimidazole–triazole ligands (NN′) having a pendent triazole arm with different triazole substituents including CH₂Ph (3a), cyclo-C₆H₁₁ (3b), and CH₂SiMe₃ (3c) were obtained in moderate yields from Cu-catalyzed oxidative C–N cyclization of the respective amine-triazole compounds N,N′-bis((1-R-1,2,3-triazol-4-yl)methyl)benzene-1,2-diamine (2a–2c). Treatment of CuCl₂ with one equiv. of the benzimidazole–triazole ligands afforded the corresponding Cu^II complexes with the empirical formula of Cu(NN′)Cl₂ (4a–4c). Crystal structures of 4b and 4c reveal mononuclear and dinuclear Cu^II complexes, respectively. Despite the differences in triazole substituents and their solid state structures, ESR spectra indicate the same molecular structures in CH₃CN solution whereas CV data suggest similar redox potentials for 4a–4c. Catalytic activities for aerobic oxidation of benzyl alcohol to benzaldehyde follow this trend: 4c > 4a > 4b. In addition, the catalytic system 4c/TEMPO/Cu⁰/NMI (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyloxyl, NMI = N-methylimidazole) exhibited high activities for oxidation of activated alcohols (i.e., benzyl alcohol derivatives and allylic alcohol) in CH₃CN at room temperature.