Although homogeneous gold catalysis was known previously, an exponential growth was only induced 12 years ago. The key findings which induce that rise of the field are discussed. This includes early reactions of allenes and furanynes and intermediates of these conversions as well as hydroarylation reactions. Other substrate types addressed are alkynyl epoxides and N-propargyl carboxamides. Important vinylgold intermediates, the transmetalation from gold to other transition metals, the development of new ligands for gold catalysis, and significant contributions from computational chemistry are other crucial points for the field highlighted here.

Gold-Catalyzed Organic Reactions

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by π acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric π acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization—from the design of new transformations to the improvement to known reactions—are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

1.1. Context and Meta-Review
Despite the ubiquity of metallic gold (Au) in popular culture, its deployment in homogeneous catalysis has only recently undergone widespread investigation. In the past decade, and especially since 2004, great progress has been made in developing efficient and selective Au-catalyzed transformations, as evidenced by the prodigious number of reviews available on various aspects of this growing field. Hashmi has written a series of comprehensive reviews outlining the progression of Au-catalyzed reaction development,1 and a number of more focused reviews provide further insight into particular aspects of Au catalysis. A brief meta-review of the available range of perspectives published on Au catalysis helps to put this Chemical Reviews article in context.

The vast majority of reactions developed with homogeneous Au catalysts have exploited the propensity of Au to activate carbon-carbon π-bonds as electrophiles. Gold has come to be regarded as an exceedingly mild, relatively carbophilic Lewis acid, and the broad array of newly developed reactions proceeding by activation of unsaturated carbon-carbon bonds has been expertly reviewed.2

Further reviews and highlights on Au catalysis focus on particular classes of synthetic reactions. An excellent comprehensive review of Au-catalyzed enyne cycloisomerizations is available.3 Even more focused highlights on hydroarylation of alkynes,4 hydroamination of C-C multiple bonds,5 and reactions of oxo-alkynes6 and propargylic esters7 provide valuable perspectives on progress and future directions in the development of homogeneous Au catalysis.

Most of the reviews on Au catalysis emphasize broad or specific advances in synthetic utility. Recently, we have invoked relativistic effects to provide a framework for understanding the observed reactivity of Au catalysts, in order to complement empirical advancements.8 In this Chemical Reviews article, we attempt to enumerate the ways in which selectivity can be controlled in homogeneous Au catalysis. It is our hope that lessons to guide catalyst selection and the design of new catalysts may be distilled from a thorough evaluation of ligand, counterion, and oxidation state effects as they influence chemo-, regio-, and stereoselectivity in homogeneous Au catalysis.

http://www.cchem.berkeley.edu/toste/publications/cr068430g.pdf

Ligand effects in homogeneous Au catalysis.

Gold salts and complexes have emerged in the past few years as the most powerful catalysts for electrophilic activation of alkynes toward a variety of nucleophiles under homogeneous conditions. In a simplified form, nucleophilic attack on the [AuL]-activated alkyne proceeds via π complexes 1 to give trans-alkenyl gold complexes of type 2 as intermediates (Scheme 1). This type of coordination is also a common theme in gold-catalyzed cycloisomerizations of enynes, in which the alkene function acts as the nucleophile. In the reaction of enynes with complexes of other transition metals, an Alder-ene cycloisomerization can take place by simultaneous coordination of the alkyne and the alkene to the metal followed by an oxidative cyclometalation. In contrast, this process does not occur for gold(I) since oxidative addition processes are not facile for this metal. 6 In addition, the [AuL] fragment, which is isolobal to H and HgL, adopts a linear coordination and binds to either the alkene or the alkyne. Thus, cycloisomerizations of enynes catalyzed by gold proceed by an initial coordination of the metal to the alkyne, and as illustrated in Scheme 2, the resulting complex 3 reacts with the alkene by either the 5-exo-dig or 6-endo-dig pathway to form the exoor endocyclopropyl gold carbene 4 or 5, respectively, as has been established with other electrophilic transition-metal complexes or halides MXn as catalysts. The proposed involvement of cyclopropyl metal carbenes of type 4 in the electrophilic activation of enynes by transition metals was first substantiated in reactions catalyzed by Pd(II), in which the initially formed cyclopropyl palladium carbenes undergo [4 + 2] cycloaddition with the double bond of the conjugate enyne. Strong evidence for the existence of cyclopropyl metal carbenes as intermediates was also obtained in the reaction of enynes bearing additional double bonds at the alkenyl chain with Ru(II) and Pt(II) catalysts. In these reactions, the cyclopropyl metal carbenes are trapped intramolecularly by the terminal alkene to give tetracycles containing two cyclopropanes. Gold(I) complexes usually surpass the reactivity shown by Pt(II) and other electrophilic metal salts and complexes for the activation of enynes. They are highly reactive yet uniquely selective Lewis acids that have a high affinity for π bonds. This high π-acidity is linked to relativistic effects, which reach a maximum in the periodic table with gold. However, on occasion, the stronger Lewis acidity of gold complexes can be detrimental in terms of selectivity and because of their low tolerance to certain functional groups. In these instances, the less-strongly Lewis acidic Pt(II) complexes could be the catalysts of choice. * To whom correspondence should be addressed. E-mail: aechavarren@ iciq.es. † Additional affiliation: Departamento de Quı́mica Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. Scheme 1 Chem. Rev. 2008, 108, 3326–3350 3326

Gold-catalyzed cycloisomerizations of enynes: a mechanistic perspective.

Thanks to its unusual stability, metallic gold has been used for thousands of years in jewelry, currency, chinaware, and so forth. However, gold had not become the chemists’ “precious metal” until very recently. In the past few years, reports on gold-catalyzed organic transformations have increased substantially. Thanks to gold-based catalysts, various organic transformations have been accessible under facile conditions with both high yields and chemoselectivity.

Gold-catalyzed organic transformations.

An auraoxetane , obtained from the reaction of norbornene with a gold( III ) oxo complex, has been isolated and fully characterized (see structure). Action of the olefin leads to elimination of the epoxide from the auraoxetane.

Reactions of gold(III) oxo complexes with cyclic alkenes.

Activation of a C−H Bond in a Pyridine Ring. Reaction of 6-Substituted 2,2‘-Bipyridines with Methyl and Phenyl Platinum(II) Derivatives: N‘,C(3)-“Rollover” Cyclometalation

Cyclometalation of 2,2′-Bipyridine. Mono- and Dinuclear C,N Platinum(II) Derivatives

Novel T-Shaped 14-Electron Platinum(II) Complexes Stabilized by One Agostic Interaction†

Synthesis of the First C-2 Cyclopalladated Derivatives of 1,3-Bis(2-pyridyl)benzene. Crystal Structures of [Hg(N-C-N)Cl], [Pd(N-C-N)Cl], and [Pd2(N-C-N)2(μ-OAc)]2[Hg2Cl6]. Catalytic Activity in the Heck Reaction

Antonio Zucca

Papers

Reactions of gold(III) oxo complexes with cyclic alkenes.

Activation of a C−H Bond in a Pyridine Ring. Reaction of 6-Substituted 2,2‘-Bipyridines with Methyl and Phenyl Platinum(II) Derivatives: N‘,C(3)-“Rollover” Cyclometalation

Cyclometalation of 2,2′-Bipyridine. Mono- and Dinuclear C,N Platinum(II) Derivatives

Novel T-Shaped 14-Electron Platinum(II) Complexes Stabilized by One Agostic Interaction†

Synthesis of the First C-2 Cyclopalladated Derivatives of 1,3-Bis(2-pyridyl)benzene. Crystal Structures of [Hg(N-C-N)Cl], [Pd(N-C-N)Cl], and [Pd2(N-C-N)2(μ-OAc)]2[Hg2Cl6]. Catalytic Activity in the Heck Reaction