Unsymmetrical PEG-substituted tris(triazolyl)amines as bi-functional surfactants for copper-catalyzed aerobic oxidation of alcohols in water

Abstract

Amphiphilic tris(triazolyl)amines functionalized with poly(ethylene glycol) 1-(1-R-1H-1,2,3-triazol-4-yl)-N,N-bis((1-benzy-1H-1,2,3-triazol-4-yl)methyl)methanamine [R = PEG200 (NBBT200-OH), mPEG550 (NBBT550), and mPEG2000 (NBBT2000)] were investigated as bi-functional surfactants serving as N donor ligands and surfactants for copper-catalyzed aerobic alcohol oxidation in water. The catalyst system CuBr/NMI/TEMPO/NBBTn (NMI = N-methylimidazole; TEMPO = 2,2,6,6-tetramethylpiperidine 1-oxyl, n = 200-OH, 550, 2000) shows oxidation activity toward 4-methoxybenzyl alcohol following the trend: NBBT550 > NBBT2000 > NBBT200-OH. Although UV-Vis spectrophotometric titrations suggest large binding constants between Cu2+ and NBBT550 in water (K11 = 2.5 × 104 M−1 and K21 = 6.3 × 103 M−1), flame atomic absorption spectroscopy (FAAS) reveals ca. 35 ppm of Cu ions from the catalyst system leaching into the EtOAc extract. As a result, in the reusability study, additional 5 mol% CuBr was added to the reaction mixture before the fourth catalytic run to achieve high conversions to the oxidized product for at least six catalytic cycles.