(428c) 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.