Homogeneous Gold-Catalyzed Cyclization Reactions of Alkynes with N- and S-Nucleophiles
TL;DR: In this article, the formation of N-and S-containing heterocycles, initiated by gold-catalyzed nucleophilic attack of N or S-nucleophiles onto alkynes, is discussed.
Abstract: This review covers the formation of N- and S-containing heterocycles, initiated by gold-catalyzed nucleophilic attack of N- or S-nucleophiles onto alkynes. These types of nucleophiles have been somewhat overlooked as compared to their C- or O-counterparts in other reviews. In this particular work, their intramolecular gold-mediated attack onto alkynes is reviewed in depth. It is structured in such a fashion that the reader will get a clear view of which substrates react in which cyclization mode.
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TL;DR: This review provides an overview of the chemistry developed with all classes of specially activated carbon π-systems by discussing their general and specific reactivities, presenting and commenting on their gold-catalyzed transformations as well as their applications.
Abstract: This review describes the gold-catalyzed reactions of specially activated alkynes, allenes, and alkenes. Such species are characterized by the presence of either electron-donating or electron-withdrawing groups as substituents of the carbon π-system. They are intrinsically polarized, and when compared to their nonspecially activated counterparts can therefore be involved in gold-catalyzed transformations featuring increased regio-, stereo-, and chemoselectivities. The chemistry of specially activated carbon π-systems under homogeneous gold catalysis is extremely rich and varied. The reactivity observed with nonspecially activated unsaturated systems can often be transposed to specially activated ones without loss of efficiency. However, specially activated carbon π-systems also exhibit specific reactivities that cannot be attained with regular substrates. In this family of carbon π-systems, ynamides and their analogs, along with alkynyl carbonyl derivatives, are the classes of substrates that have retained the most attention. This review provides an overview of the chemistry developed with all classes of specially activated carbon π-systems by discussing their general and specific reactivities, presenting and commenting on their gold-catalyzed transformations as well as their applications.
145 citations
TL;DR: Accounts on Au-mediated cyclization giving notable 7-membered and medium-sized (8-11- Membered ring) structures are presented.
Abstract: Compounds having cyclic molecular frameworks are highly regarded for their abundance and diverse utilities. In particular, medium-sized carbocycles and heterocycles exist in a broad spectrum of natural products, bioactive therapeutics, and medicinally significant synthetic molecules. Metal-mediated methods have been developed for the preparation of compounds containing a medium-sized ring (MSR) through cyclization of different classes of substrates and acyclic precursors. This review focuses on the methodologies for construction of MSRs via gold catalysis. Given the challenges in enabling the assembly of different ring sizes, we present here accounts on Au-mediated cyclization giving notable 7-membered and medium-sized (8-11-membered ring) structures. Emphasis on the pathway and mode of cyclization and the selection of precursors ranging from structurally biased compounds were outlined. Reactivity patterns and the choice of efficient Au catalysts for controlling reaction performance and selectivity in addition to mechanistic attributes are examined.
95 citations
TL;DR: The synthesis and characterization of hitherto hypothetical AuIII π‐alkyne complexes are reported and Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry.
Abstract: The synthesis and characterization of hitherto hypothetical AuIII π-alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of PtII and their drastically more reactive AuIII congeners. Lack of back-bonding facilitates alkyne slippage, which is energetically less costly for gold than for platinum and explains the propensity of gold to facilitate C−C bond formation. Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry.
44 citations
TL;DR: Depending on the size of the heterocyclic ring and substitution of the substrates, two unprecedented forms of nucleophilic attack on the sulfonyl group were exploited, that is, a N-desulfonylation in the presence of an external protic O’nucleophile and a unique N-to-O 1,5-sulfonyl migration.
Abstract: Valuable 1-azabicycloalkane derivatives have been synthesized through a novel gold(I)-catalyzed desulfonylative cyclization strategy. An ammoniumation reaction of ynones substituted at the 1-position with an N-sulfonyl azacycle took place in the presence of a gold cation by intramolecular cyclization of the disubstituted sulfonamide moiety onto the triple bond. Depending on the size of the heterocyclic ring and substitution of the substrates, two unprecedented forms of nucleophilic attack on the sulfonyl group were exploited, that is, a N-desulfonylation in the presence of an external protic O nucleophile (37–87 %, 10 examples) and a unique N-to-O 1,5-sulfonyl migration (60–98 %, 9 examples).
42 citations
TL;DR: In this article, commercial gold nanoparticles supported on titanium dioxide (TiO2) were found to be a highly efficient catalyst for alkyne hydroamination and Indoles were efficiently synthesized using microwave heating through intramolecular hydroamination.
Abstract: Commercial gold nanoparticles supported on titanium dioxide (TiO2) were found to be a highly efficient catalyst for alkyne hydroamination. Terminal alkynes could easily undergo intermolecular hydroamination with low catalyst loadings (0.2 mol% Au) under solvent-free conditions. Indoles were efficiently synthesized using microwave heating through intramolecular hydroamination.
35 citations
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TL;DR: The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization and the application of platinum- and gold-catalyzed transformations in natural product synthesis is discussed.
Abstract: 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.
1,938 citations
TL;DR: The ways in which selectivity can be controlled in homogeneous Au catalysis are enumerated, in the 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 homogeneity AuCatalysis.
Abstract: 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.
1,783 citations
TL;DR: 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 cycloprostyl palladiumCarbenes undergo [4 + 2] cycloaddition with the double bond of the conjugate enyne.
Abstract: 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
1,728 citations
TL;DR: Thanks to gold-based catalysts, various organic transformations have been accessible under facile conditions with both high yields and chemoselectivity.
Abstract: 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.
1,698 citations
TL;DR: This Review draws on experimental and computational data to present the current understanding of homogeneous gold catalysis, focusing on previously unexplored reactivity and its application to the development of new methodology.
Abstract: Transition-metal catalysts containing gold present new opportunities for chemical synthesis, and it is therefore not surprising that these complexes are beginning to capture the attention of the chemical community. Cationic phosphine-gold(i) complexes are especially versatile and selective catalysts for a growing number of synthetic transformations. The reactivity of these species can be understood in the context of theoretical studies on gold; relativistic effects are especially helpful in rationalizing the reaction manifolds available to gold catalysts. This Review draws on experimental and computational data to present our current understanding of homogeneous gold catalysis, focusing on previously unexplored reactivity and its application to the development of new methodology.
1,534 citations