Aerobic oxidative coupling of alcohols and amines over Au–Pd/resin in water: Au/Pd molar ratios switch the reaction pathways to amides or imines

Abstract

A facile switch of the reaction pathways of aerobic oxidative coupling of alcohols and amines from amidation to imination was realized for the first time by tuning the Au/Pd ratios in ion-exchange resin supported Au-Pd alloy catalysts (Au-Pd/resin). Amides were obtained with high yields on Au 6 Pd/resin while imines were obtained over AuPd 4 /resin. Various alcohols and amines underwent oxidative coupling smoothly in water to afford the desired products with good to excellent yields. Further investigation on the reaction mechanism suggested the synergistic effect between Au and Pd determined the adsorption strength of the aldehyde intermediate, which in turn dictated the reaction pathways. That is, on Au-rich alloys (e.g., Au 6 Pd) absorbed aldehyde species was formed, followed by further oxidation to yield amides, while on Pd-rich alloys (e.g., AuPd 4 ), free aldehyde was generated, which then underwent condensation with amines to produce imines.The discovery might provide avenues to develop new efficient catalysts for the green synthesis of special chemicals. Scheme 1 Pathways of oxidative coupling of alcohols and amines. † Electronic supplementary information (ESI) available: Experimental details, 1 H NMR and 13 C NMR spectra are provided. See

Recent Advances in Aerobic Oxidation of Alcohols and Amines to Imines

“…67 Interestingly, the reaction pathways can be switched by the ratio of Au/Pd: amides were produced on Au 6 Pd/resin and imines were obtained over AuPd 4 /resin. These results can be rationalized by the differences of adsorption strength of aldehyde intermediates on the catalyst surface: in the case of Au 6 Pd/resin, adsorbed aldehyde species tended to form because of the strong affinity of Au, and subsequently reacted with amines to form adsorbed hemiaminals, followed by further oxidation to give amides; as for AuPd 4 /resin, free aldehydes were preferentially formed owing to the isolation of Au atoms by Pd which weakened the adsorption of aldehydes on Au atoms.…”

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