Oxidations of amines with molecular oxygen using bifunctional gold–titania catalysts

Abstract

Over the past decades it has become clear that supported gold nanoparticles are surprisingly active and selective catalysts for several green oxidation reactions of oxygen-containing hydrocarbons using molecular oxygen as the stoichiometric oxidant. We here report that bifunctional gold-titania catalysts can be employed to facilitate the oxidation of amines into amides with high selectivity. Furthermore, we report that pure titania is in fact itself a catalyst for the oxidation of amines with molecular oxygen under very mild conditions. We demonstrate that these new methodologies open up for two new and environmentally benign routes to caprolactam and cyclohexanone oxime, both of which are precursors for nylon-6.

Aerobic Oxidation of Alcohols over Gold Catalysts: Role of Acid and Base

“…Following the discovery by Haruta et al, a variety of aerobic oxidation reactions have been reported [7,8]. For instance, gold nanoparticles were used in the oxidation of alcohols to aldehydes [9], carboxylic acids [10][11][12][13][14] or esters [15,16], in the oxidation of aldehydes to esters [17] or acids [18] in epoxidations of olefins [19,20], and in the oxidation of amines to amides [21]. Although Haruta et al [6] were able to achieve low temperature aerobic oxidation of carbon monoxide, most aerobic oxidations using gold catalysts require temperatures and oxygen pressures well above ambient conditions.…”

Highly Selective Acylation of Dimethylamine Mediated by Oxygen Atoms on Metallic Gold Surfaces

“…Indeed, there has been a resurgence of research on heterogeneous catalysis by gold recently due to its potential for developing environmentally benign processes, [1][2][3] since Harutas breakthrough observation of low-temperature CO oxidation on gold nanoparticles supported on reducible metal oxides. [4] Gold particles supported on oxide surfaces selectively promote a wide range of reactions under various conditions, including aerobic oxidation of alcohols [5][6][7] and amines, [8] as well as acylation of amines [9,10] as does unsupported gold powder. [11] Herein, we report for the first time the vapor-phase, surface mediated acylation of an amine to an amide on metallic gold, and we establish a molecular-level mechanism for this process based on a specific characteristic of the adsorbed amide intermediate that provides a general basis for predicting such reactions.…”

“…[12] Amine acylation reactions catalyzed by homogeneous transitionmetal complexes [13][14][15] in solution have been reported along with those on supported Au. [9,10] Ideally, direct synthesis of amides through heterogeneous catalytic processes with high selectivity under environmentally benign conditions would be possible.…”

Organic Synthesis Catalyzed by Supported Gold Nanoparticles

“…They demonstrated that it is a simple methodology to produce valuable caprolactam from the diamine, amine, and other precursors (Scheme 15). 153 A 70% yield of caprolactam could be achieved with the oxidation of 1,6-hexanediamine at 90 C and 2 bar O 2 for 18 h using Au/TiO 2 . A schematic illustration of the reaction mechanism was given by Friend et al (Scheme 16).…”