Role of Surface-Bound Intermediates in the Oxygen-Assisted Synthesis of Amides by Metallic Silver and Gold

Abstract

A general mechanism for the oxygen-assisted synthesis of amides over metallic gold and silver surfaces has been derived from the study of acetaldehyde and dimethylamine in combination with previous work, allowing detailed comparison of the two surfaces' reactivities. Facile acetylation of dimethylamine by acetaldehyde occurs with high selectivity on oxygen-covered silver and gold (111) crystals via a common overall mechanism with different rate-limiting steps on the two metals. Adsorbed atomic oxygen activates the N-H bond of the amine leading to the formation of an adsorbed amide, which attacks the carbonyl carbon of the aldehyde, forming an adsorbed hemiaminal. Because aldehydes are known to form readily from partial oxidation of alcohols, our mechanism also provides insight into the related catalytic coupling of alcohols and amines. The hemiaminal β-H eliminates to form the coupled amide product. On silver, β-H elimination from the hemiaminal is rate-limiting, whereas on gold desorption of the amide is the slow step. Silver exhibits high selectivity for the coupling reaction for adsorbed oxygen concentrations between 0.01 and 0.1 monolayer, whereas gold exhibits selectivity more strongly dependent on oxygen coverage, approaching 100% at 0.03 monolayer. The selectivity trends and difference in rate-limiting steps are likely due to the influence of the relative stability of the adsorbed hydroxyl groups on the two surfaces. Low surface coverages of oxygen lead to the highest selectivity. This study provides a general framework for the oxygen-assisted coupling of alcohols and aldehydes with amines over gold- and silver-based catalysts in either the vapor or the liquid phase.

XPS and STM studies of the oxidation of hydrogen chloride at Cu(100) surfaces

“…surfaces in particular [1][2][3][4][5][6]. The ability of oxygen to act as a base to ab-41 stract hydrogen was recognized early on [7][8][9][10], and a huge number of 42 reactions have been studied subsequently, ranging from amines and 43 sulfides to alcohols, alkenes, and acids [9,10,5,11].…”

Size of Gold Nanoparticles Driving Selective Amide Synthesis through Aerobic Condensation of Aldehydes and Amines

“…[21] As imilar cascade process without dissociation of ah emiaminal in the presence of molecular oxygen and an externally added base could proceed on Au-NPs of medium size (see SI, Figures S11 Aa nd B). Friend et al proposed ac ontinuous reaction of hemiaminal formation and subsequent oxidation on an oxygen-covered metallic Au surface.…”

Landgang

“…No coupling agent (e.g., strong base) or organic solvent was added (methanol acted as both solvent and reactant) and “green” reaction conditions were used. As indicated by the model studies on single crystals, the presence of oxygen on the gold surface was crucial for the activity and selectivity 18. According to the mechanism of the initial phase of the coupling reaction (Scheme ), the reactant should only be activated in the presence of this active surface oxygen 17.…”

“…Despite the complexity of the subject of oxygen supply (mass transport inside the porous structure and transfer from the gas phase into solution), both findings indicated that the concentration of oxygen in solution determined the reaction rate. Notably, in model reactions the selectivity of the reaction was found to be dependent on the concentration of reactive surface oxygen,18–19 implying that under experimental conditions even with high oxygen pressures (e.g., 8 bar) the steady‐state concentration of reactive oxygen on npAu is low (<0.1 monolayers).…”

“…We chose silver and ruthenium as reasonable dopants for npAu. Silver was demonstrated to efficiently dissociate molecular oxygen, generating atomic oxygen on the catalyst surface as a Brønsted base and strong nucleophile 18. 22 Ruthenium, on the other hand, has been the most commonly used material for this reaction in the homogeneous phase owing to the generation of highly active ruthenium‐bound methoxy species, which were even more active in coupling with amines than with the free aldehyde 23.…”