Throughout the ages, gold has been highly valued because of its seeming chemical inertness, its luster and beauty resulting from its resistance to bulk tarnishing reactions. However, the surface of gold is not completely inert, particularly in the presence of adsorbed oxygen. 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.
Amides are widely used in chemical synthesis, in pharmaceutical production, and in the synthesis of polymers, including nylon. [11] Conventional methods for synthesizing amides use either activated acid derivatives, such as acid chlorides or anhydrides, or rearrangement reactions induced by an acid or base, which often produce toxic chemical waste. [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.
The performance of catalytic processes can be improved through understanding the reaction mechanism at a molecular level so that the kinetics and selectivity of the overall process can be accurately predicted. A first step is to deconvolute the roles of gold and the oxide support. Our approach is to investigate O-covered Au(111), since without oxygen, Au is inert towards many reactions, including those of alcohols, aldehydes, and amines. [8,[16][17][18][19][20] The general concept for amine acylation on O/Au(111) originates in the chemical nature of adsorbed oxygen and other nucleophilic adsorbates formed by selective deprotonation of their conjugate acids. For example, adsorbed O on Au surfaces activates alcohols, [17] ammonia, [19,20] and amines [8] through Brønsted acid-base reactions