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DOI: 10.1055/sos-SD-103-00038

Hopkinson, M. N.; Gouverneur, V.Science of Synthesis Knowledge Updates, (20112101.

General Introduction

Homo- and cross-coupling reactions catalyzed by transition metals are an increasingly important class of carbon–carbon and carbon–heteroatom bond-forming processes.[‌1‌] These transformations allow for the efficient coupling of diversely functionalized organic molecules with high selectivity and functional-group tolerance. As such, coupling reactions have become powerful tools for synthetic chemists with palladium-catalyzed processes, such as the Suzuki–Miyaura, Stille, and Sonogashira reactions, being the subject of the 2010 Nobel Prize in Chemistry.[‌2‌,‌3‌] Most coupling reactions proceed via a redox cycle where the metal catalyst Mn undergoes a two-electron oxidation to afford an Mn + 2 species amenable to reductive elimination. Whilst palladium is ideally suited to such cycles, other transition metals such as nickel and copper have emerged as alternative catalysts for homo- and cross-coupling reactions, expanding the range of transformations available to synthetic chemists.

The use of gold complexes as catalysts for coupling reactions is challenging. Unlike palladium and other late transition metals, gold rarely changes oxidation state during the course of a reaction and most commonly acts as a redox-neutral, carbophilic π-acid, activating multiple bonds toward nucleophilic attack.[‌4‌‌9‌] This reactivity can be attributed to the large redox potential of the gold(I)/gold(III) couple (Eθ = +1.41 V), which is significantly higher than that of palladium(0)/palladium(II) (Eθ = +0.92 V).[‌10‌] As a result, strongly oxidizing conditions are required to effect the oxidation of gold(I) to gold(III) essential for coupling catalysis. Despite the challenges, several homo- and cross-coupling reactions mediated by gold complexes have been reported in the literature.[‌11‌‌14‌] In this chapter the development of gold-catalyzed coupling reactions proceeding via proposed gold(I)/gold(III) redox cycles under homogeneous conditions will be reviewed. The material is divided into four subcategories according to the type of coupling reaction (homo- or cross-) and the method of oxidation of gold(I) to gold(III). In Sections and, transformations where gold(III) acts as a stoichiometric oxidant and selected examples of some conventional catalytic coupling reactions involving oxidative addition to gold(I) are presented. This is followed in Sections and by a discussion of how the use of sacrificial external oxidants has enabled gold to catalyze a wide-range of synthetically useful oxidative homo- and cross-coupling reactions.

SAFETY: Gold complexes are generally mild irritants and care should be taken to avoid contact with eyes or skin, or inhalation of dust particles. Gold(I) chloride and tetrachloroauric acid (HAuCl4) cause burns and may cause sensitization by skin contact.