268294 Mechanistic Aspects of Coupling Reactions On Metallic Silver and Gold
Noble metal catalysts have long been employed to facilitate the selective conversion of abundant natural resources into commodity and specialty chemicals. Among these metals platinum, palladium and silver are used for a variety of reactions, relying on abilities of each metal for activating specific bonds within feedstocks. Platinum, for example, activates C-H bonds in alkanes, facilitating catalytic reforming in a reducing atmosphere or combustion in the presence of oxygen, whereas silver, being inert toward C-H bond activation, is known for its ability for selective oxidation. A relative newcomer to the arsenal of catalytic materials is metallic gold, long appreciated for its chemical inertness and therefore valued as currency and an object of art. Surprisingly, in an oxidizing environment the surface of metallic gold mediates complex catalytic reactions, partially because of the difficulty in activating C-H, C-O and C-N bonds on gold. These reactions include selective oxidation of alcohols to aldehydes, esterification, cross-coupling of mixed alcohols, and coupling of amines and alcohols to form amides. Furthermore, there are strong parallels between reactions in metallic gold and silver that evolve from the same basic set of principles of reactivity. We have elucidated the fundamental principles underlying this paradox. The core principles are as follows: (1) the substrate (reactant) can be viewed as a gas phase acid; (2) adsorbed atomic oxygen acts as a Broensted base, activating the most acidic hydrogen within the substrate to form an adsorbed conjugate base and water; (3) this adsorbed species acts as a nucleophile toward coadsorbed electrophiles, leading to coupling. These principles have been developed from studies of model surfaces under highly controlled conditions, and they are directly applicable to catalytic application under normal catalytic conditions, even in the liquid phase. They also can be used to predict new reactions.