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

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 (K-11 = 2.5 x 10(4) M-1 and K-21 = 6.3 x 10(3) 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.

Automated summary

In this study, amphiphilic tris(triazolyl)amines functionalized with poly(ethylene glycol) were investigated as bi-functional surfactants for copper-catalyzed aerobic alcohol oxidation in water. The results showed that NBBT550 exhibited the highest activity among the tested surfactants. However, there was significant leaching of copper ions from the catalyst system, leading to the need for additional copper catalyst after every four catalytic cycles.