Institut Català d'Investigació Química

ADVERTISEMENT RETURN TO ISSUEPREVReviewNEXTN-Heterocyclic Carbenes in Late Transition Metal CatalysisSilvia Díez-González†, Nicolas Marion‡†, and Steven P. Nolan*†§View Author Information Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain, and School of Chemistry, University of St Andrews, North Haugh, St Andrews KY16 9ST, U.K.*[email protected]†Institute of Chemical Research of Catalonia.‡Present address: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.§University of St Andrews.Cite this: Chem. Rev. 2009, 109, 8, 3612–3676Publication Date (Web):July 9, 2009Publication History Received24 February 2009Published online9 July 2009Published inissue 12 August 2009https://pubs.acs.org/doi/10.1021/cr900074mhttps://doi.org/10.1021/cr900074mreview-articleACS PublicationsCopyright © 2009 American Chemical SocietyRequest reuse permissionsArticle Views36042Altmetric-Citations2763LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Addition reactions,Catalysts,Chemical reactions,Hydrocarbons,Ligands Get e-Alerts

ADVERTISEMENT RETURN TO ISSUEPREVReviewNEXTGold-Catalyzed Cycloisomerizations of Enynes: A Mechanistic PerspectiveEloísa Jiménez-Núñez and Antonio M. Echavarren*†View Author Information Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain* To whom correspondence should be addressed. E-mail: [email protected]†Additional affiliation: Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain.Cite this: Chem. Rev. 2008, 108, 8, 3326–3350Publication Date (Web):July 18, 2008Publication History Received17 December 2007Published online18 July 2008Published inissue 1 August 2008https://pubs.acs.org/doi/10.1021/cr0684319https://doi.org/10.1021/cr0684319review-articleACS PublicationsCopyright © 2008 American Chemical SocietyRequest reuse permissionsArticle Views17523Altmetric-Citations1919LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Catalysts,Cyclization,Gold,Hydrocarbons,Rearrangement Get e-Alerts

The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).

ADVERTISEMENT RETURN TO ISSUEPREVReviewNEXTAromatic Trifluoromethylation with Metal ComplexesOlesya A. Tomashenko and Vladimir V. Grushin*View Author Information The Institute of Chemical Research of Catalonia (ICIQ), Tarragona 43007, SpainE-mail: [email protected]Cite this: Chem. Rev. 2011, 111, 8, 4475–4521Publication Date (Web):April 1, 2011Publication History Received15 December 2010Published online1 April 2011Published inissue 10 August 2011https://pubs.acs.org/doi/10.1021/cr1004293https://doi.org/10.1021/cr1004293review-articleACS PublicationsCopyright © 2011 American Chemical SocietyRequest reuse permissionsArticle Views25169Altmetric-Citations1552LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Alkyls,Aromatic compounds,Metals,Reagents,Trifluoromethylation Get e-Alerts

For centuries, gold had been considered a precious, purely decorative inert metal. It was not until 1986 that Ito and Hayashi described the first application of gold(I) in homogeneous catalysis.(1) More than one decade later, the first examples of gold(I) activation of alkynes were reported by Teles(2) and Tanaka,(3) revealing the potential of gold(I) in organic synthesis. Now, gold(I) complexes are the most effective catalysts for the electrophilic activation of alkynes under homogeneous conditions, and a broad range of versatile synthetic tools have been developed for the construction of carbon–carbon or carbon–heteroatom bonds.\nGold(I) complexes selectively activate π-bonds of alkynes in complex molecular settings,(4-10) which has been attributed to relativistic effects.(11-13) In general, no other electrophilic late transition metal shows the breadth of synthetic applications of homogeneous gold(I) catalysts, although in occasions less Lewis acidic Pt(II) or Ag(I) complexes can be used as an alternative,(9, 10, 14, 15) particularly in the context of the activation of alkenes.(16, 17) Highly electrophilic Ga(III)(18-22) and In(III)(23, 24) salts can also be used as catalysts, although often higher catalyst loadings are required.\nIn general, the nucleophilic Markovnikov attack to η2-[AuL]+-activated alkynes 1 forms trans-alkenyl-gold complexes 2 as intermediates (Scheme 1).(4, 5a, 9, 10, 12, 25-29) This activation mode also occurs in gold-catalyzed cycloisomerizations of 1,n-enynes and in hydroarylation reactions, in which the alkene or the arene act as the nucleophile.\nfigure\nScheme 1. Anti-Nucleophilic Attack to η2-[AuL]+-Activated Alkynes\nStructurally, Au(I) predominantly forms linear two-coordinate complexes, although higher coordination numbers are also possible.(30) A significant number of alkyne-gold complexes have been characterized(31, 32) and studied either in solution(32, 33) or theoretically.(34) This selective activation of the alkyne moiety can explain a vast majority of the results experimentally observed for gold(I)-catalyzed cyclization of 1,n-enynes. Nevertheless, complexes of gold(I) with the alkene moiety of the enynes are also formed in equilibrium with the alkyne-gold complexes.(35) Indeed, well-characterized complexes of gold(I) with alkenes have been reported,(36) as well as with allenes(37) and 1,3-dienes.(38)\nDespite the fact that simple gold salts such as NaAuCl4 or AuCl are active enough to catalyze several transformations, gold(I) complexes bearing phosphines or N-heterocyclic carbenes as ligands have found more wide-ranging applications.(39) The active species are often generated in situ by chloride abstraction from [LAuCl] upon treatment with a silver salt bearing a weakly coordinating anion.\nComplexes [LAuY] only exist as neutral species when Y– is a coordinating anion (halides, carboxylates, sulfonates, and triflimide). The corresponding complexes with less coordinating anions, such as SbF6–, PF6–, or BF4–, are in most of the cases not stable. Although, species [AuL]+ (also known as “naked gold complexes”) are often suggested in mechanistic proposals, structural proof for their existence as stable, isolable species is still lacking. Here, for the sake of simplicity in mechanistic schemes throughout this review, LAu+ is used as a surrogate of [LAuL′]+ complexes, where L′ states for a relatively weakly bound ligand such as the substrate (alkyne or alkene), product, or solvent molecule.\nIt is important to remark that when the catalytically active species are generated in situ by chloride abstraction from complexes [LAuCl] in the absence of the alkyne or other unsaturated substrate, much less reactive chloride-bridged dinuclear species [LAuClAuL]Y are readily formed.(40) Formation of these dinuclear complexes could explain, at least partly, the erratic results that have been ascribed as the “silver effects” in reactions in which Ag(I) salts are used in situ to activate neutral gold(I) complexes [LAuY].(41)\nOften, the most convenient catalysts for the activation of alkynes are complexes [LAuL′]X or [LAuX] bearing weakly coordinating neutral (L′)(42) or anionic ligand (X–).(43) These complexes can enter catalytic cycles by ligand exchange with the unsaturated substrate, which proceed by associative mechanisms as observed for Au(I) and other diagonal d10 metal centers.(44) Thus, large negative activation entropies characteristic of associative mechanisms have been determined for the rate determining ligand exchange reactions of substituted alkyne(45, 46) and alkenes(36o) on commonly used Au(I) catalysts. Although nitriles are frequently used as weakly coordinating neutral ligands, 1,2,3-triazole(46, 47) or other related ligands(48) have also been employed.\nThe properties of gold(I) complexes can be easily tuned sterically or electronically depending on the ligand, consequently modulating their reactivity in the activation of alkynes, alkenes, and allenes.(27, 29f, 49) Thus, complexes containing more donating N-heterocyclic carbenes (3) are less electrophilic than those with phosphine ligands (4, 5) (Figure 1).(28) Complexes with less donating phosphite ligands (6) and related species are the most electrophilic catalysts.\nfigure\nFigure 1. Increase in electrophilicity with decreased donating ligand ability in gold(I) complexes.\n\nGold(I) complexes bearing weak-coordinated ligands such as Me2S, thiodiglycol, or tetrahydrothiophene (tht) have been widely used for the preparation of soluble gold(I) complexes, commonly starting from a gold(III) source.(50) Complex [Au(tmbn)2]SbF6 (tmbn = 2,4,6-trimethoxybenzonitrile), in which gold(I) is supported by two nitrile ligands, can be used for the in situ preparation of a variety of chiral and achiral cationic complexes [LAu(tmbn)]SbF6, including complexes immobilized on a polymeric support.(42a) Other immobilized gold(I) complexes have also been prepared.(51) The use of gold complexes bearing chiral ligands has led to the development of efficient asymmetric gold-catalyzed transformations.(52) Less common precatalysts used in gold(I)-catalyzed transformations are gold hydroxo complex IPrAuOH, which is activated in the presence of Brønsted acids,(53) open carbenes,(39c, 54) and other related complexes,(55) which give rise to selective catalysts of moderate electrophilicity. Cyclopropenylylidene-stabilized phospenium cations, which behave similarly to classical triaryl- and trialkylphosphines, have also been used as ligands in gold-catalyzed reactions.(56)\nThe effect of the counteranion has been studied in detail for several gold(I)-catalyzed transformations.(57, 58) Thus, for the intermolecular reaction of phenylacetylene with 2-methylstyrene catalyzed by [t-BuXPhosAu(NCMe)]Y, it was found that yields increase depending on the counteranion in the order Y = OTf– < NTf2– < BF4– < SbF6– < BARF (BARF = 3,5,bis(trifluoromethyl)phenylborate). By using the bulky and noncoordinating anion BARF, yields are increased by 10–30% compared to those obtained when Y = SbF6–, probably due to a decrease in the formation of the unproductive σ,π-(alkyne)digold(I) complexes from the initial alkyne.(57)\n1.2Scope and Organization of the Review\nHomogeneous gold(I)-catalysis has experienced an outbreak in the past decade leading to the discovery of a remarkable amount of new synthetically useful transformations. Thus, in recent years many groups have used gold catalysis in key steps of total synthesis taking advantage of the unique catalytic ability of gold to build molecular complexity under mild reaction conditions.\nSeveral reviews have been published on gold(I)-catalyzed reactions of alkynes, enynes, and related substrates,(5, 7, 25-28, 59) as well as on gold(I)-catalyzed reactions of allenes(60) and cascade gold-catalyzed reactions.(61) Moreover, specific reviews focused on gold-catalyzed carbon-heteroatom bond formation(62) and on the use of gold catalysis in total synthesis(63) have also been published. In this review, we will cover reactions of alkynes activated by gold(I) complexes, including recent applications of these transformations in the synthesis of natural products. According to the aim of this thematic issue, the main focus is on the application of gold(I)-catalyzed reactions of alkynes in organic synthesis, although reactions are organized mechanistically. Reactions of gold(I)-activated alkenes and allenes, as well as gold(III)-activated alkynes, will not be covered.\nThe discussion has been primarily organized based on the different reactions catalyzed by gold(I) complexes that alkynes can undergo. When possible, inter- and intramolecular processes, as well as the applications in total synthesis, are treated in specific subsections.

valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.

The association of an electron-rich substrate with an electron-accepting molecule can generate a new molecular aggregate in the ground state, called an electron donor-acceptor (EDA) complex. Even when the two precursors do not absorb visible light, the resulting EDA complex often does. In 1952, Mulliken proposed a quantum-mechanical theory to rationalize the formation of such colored EDA complexes. However, and besides a few pioneering studies in the 20th century, it is only in the past few years that the EDA complex photochemistry has been recognized as a powerful strategy for expanding the potential of visible-light-driven radical synthetic chemistry. Here, we explain why this photochemical synthetic approach was overlooked for so long. We critically discuss the historical context, scientific reasons, serendipitous observations, and landmark discoveries that were essential for progress in the field. We also outline future directions and identify the key advances that are needed to fully exploit the potential of the EDA complex photochemistry.

Metal-catalyzed cross-coupling reactions, notably those permitting C-C bond formation, have witnessed a meteoritic development and are now routinely employed as a powerful synthetic tool both in academia and in industry. In this context, palladium is arguably the most studied transition metal, and tertiary phosphines occupy a preponderant place as ancillary ligands. Seriously challenging this situation, the use of N-heterocyclic carbenes (NHCs) as alternative ligands in palladium-catalyzed cross-coupling reactions is rapidly gaining in popularity. These two-electron donor ligands combine strong sigma-donating properties with a shielding steric pattern that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the number of well-defined NHC-containing palladium(II) complexes is growing, and their use in coupling reactions is witnessing increasing interest. In this Account, we highlight the advantages of this family of palladium complexes and review their synthesis and applications in cross-coupling chemistry. They generally exhibit high stability, allowing for indefinite storage and easy handling. The use of well-defined complexes permits a strict control of the Pd/ligand ratio (optimally 1/1), avoiding the use of excess costly ligand that usually requires end-game removal. Furthermore, it partly removes the "black box" character often associated with cross-coupling chemistry and catalyst formation. In the present Account, four main classes of NHC-containing palladium(II) complexes will be presented: palladium dimers with bridging halogens, palladacycles, palladium acetates and acetylacetonates, and finally pi-allyl complexes. These additional ligands are best described as a protecting shell that will be discarded going from the palladium(II) precatalyst to the palladium(0) true catalyst. The synthesis of all these precatalysts generally requires simple and short synthetic procedures. Their catalytic activity in different cross-coupling reactions is discussed and put into context. Remarkably, some NHC-containing catalytic systems can achieve extremely challenging coupling reactions such as the formation of tetra-ortho-biphenyl compounds and perform reactions at very low loadings of palladium (ppm levels). The chemistry described here, combining fundamental organometallic, catalysis, and pure organic methodology, remains rich in opportunities considering that only a handful of palladium(II) architectures have been studied. Hence, en route to an "ideal catalyst", [(NHC)Pd(II)] compounds exhibit remarkable stability and allow for fine-tuning of the NHC and of surrounding ligands in order to control the activation and the catalytic activity. Finally, unlike [Pd(PPh(3))(4)], [(NHC)Pd(II)] compounds have so far been examined only in palladium-mediated reactions (most often cross-coupling such as the Suzuki-Miyaura and Heck reactions), leaving a treasure trove of exciting discoveries to come.

<p> The catalytic formation of cyclic organic carbonates<br /> (COCs) using carbon dioxide (CO2) as a renewable carbon feed<br /> stock is a highly vibrant area of research with an increasing amount of<br /> researchers focusing on this thematic investigation. These organic<br /> carbonates are highly useful building blocks and nontoxic reagents<br /> and are most commonly derived from CO2 coupling reactions with<br /> oxirane and dialcohol precursors using homogeneous catalysis methodologies. The activation of suitable reaction partners using<br /> catalysis as a key technology is a requisite for efficient CO2 conversion as its high kinetic stability poses a barrier to access<br /> functional organic molecules with added value in both academic and industrial laboratories. Although this area of science has<br /> been flourishing for at least a decade, in the past 2−3 years, significant advancements have been made to address the general<br /> reactivity and selectivity issues that are associated with the formation of COCs. Here, we present a concise overview of these<br /> activities with a primary focus to highlight the most important progress made and the opportunities that catalysis can bring about<br /> when the synthesis of these intermediates is optimized to a higher level of sophistication. The attention will be limited to those<br /> cases in which homogeneous metal-containing systems have been employed because they possess the highest potential for<br /> directed organic synthesis using CO2 as molecular building block. This review discusses examples of exceptional reactivity and<br /> selectivity, taking into account the challenging nature of the substrates that were involved, and mechanistic understanding guiding<br /> the optimization of these protocols is also highlighted.</p>

We report the preparation and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.

Eighteen years ago in Angewandte Chemie John K. Stille reviewed a novel methodology, which eventually became known by his name, for the coupling of organostannanes with organic electrophiles. Since then that seed has blossomed into a multifaceted methodology full of hidden possibilities to explore, discover, and enjoy. Very recent modifications are making synthetic wishes come true that were only dreamed of a few years ago. Moreover, as important advances are being made in the understanding of the mechanistic details of the process, it is becoming increasingly possible to apply this essential reaction and its new variants in a less empirical way. The purpose of this Review is to give a critical account of this progress.

The fast-moving fields of photoredox and photocatalysis have recently provided fresh opportunities to expand the potential of synthetic organic chemistry. Advances in light-mediated processes have mainly been guided so far by empirical findings and the quest for reaction invention. The general perception, however, is that photocatalysis is entering a more mature phase where the combination of experimental and mechanistic studies will play a dominant role in sustaining further innovation. This Review outlines the key mechanistic studies to consider when developing a photochemical process, and the best techniques available for acquiring relevant information. The discussion will use selected case studies to highlight how mechanistic investigations can be instrumental in guiding the invention and development of synthetically useful photocatalytic transformations.

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.

The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidise water and generate carbohydrates (solar fuel) for over a billion years. Inspired, but not constrained, by nature, artificial systems can be designed to capture light and oxidise water and reduce protons or other organic compounds to generate useful chemical fuels. This tutorial review covers the primary topics that need to be understood and mastered in order to come up with practical solutions for the generation of solar fuels. These topics are: the fundamentals of light capturing and conversion, water oxidation catalysis, proton and CO2 reduction catalysis and the combination of all of these for the construction of complete cells for the generation of solar fuels.

In this work, we study the characteristics of dye-sensitized solar cells using an ionic liquid as the electrolyte and compare them with the response of a solvent-containing electrolyte cell. Impedance spectroscopy is used to derive the key circuit elements determining the photovoltaic performance of the cell. On the basis of this data, photocurrent voltage curves are calculated and compared with experimental results.

The monumental “Elegy to the Horizon” by the Basque artist Eduardo Chillida oversees the Atlantic coast at the town of Gijón (Asturias, Spain). A similar structural shape is involved in the enantiodiscrimination of alkenes through a chiral iodine(III) catalyst. In their Communication on page 413 ff., K. Muñiz et al. discuss selective hydrogen bonding toward chirally induced supramolecular scaffolds in iodine(III) catalysts and their performance in an intermolecular enantioselective diacetoxylation reaction. The monumental “Elegy to the Horizon” by the Basque artist Eduardo Chillida oversees the Atlantic coast at the town of Gijón (Asturias, Spain). A similar structural shape is involved in the enantiodiscrimination of alkenes through a chiral iodine(III) catalyst. In their Communication on page 413 ff., K. Muñiz et al. discuss selective hydrogen bonding toward chirally induced supramolecular scaffolds in iodine(III) catalysts and their performance in an intermolecular enantioselective diacetoxylation reaction. NMR Spectroscopy Photoswitchess Environmental Chemistry

Metal complexes of salen ligands are an important class of compounds, and they have been widely studied in the past. Among their successful catalytic applications, the synthesis of cyclic carbonates by the coupling reaction of epoxides with CO(2) has received increased attention; this is mostly due to the importance of using a greenhouse gas as a feedstock for the synthesis of useful molecules. Herein the most relevant past and present research surrounding this topic is presented.

In this feature article we cover most recent efforts in gold-catalysed transformations, highlighting the wide molecular diversity that can be achieved, in particular with regard to the formation of C-C bonds. Mechanistic interpretations of some cyclisations are based on our own work on the skeletal rearrangement of 1,6-enynes.

Palladium-mediated cross-coupling reactions are attractive organometallic transformations for the generation of C--C, C--N, C--O, and C--S bonds. Despite being widely employed in small-scale syntheses, cross-coupling reactions have not found important industrial applications because until recently, only reactive aryl bromides and iodides could be used as substrates. These substrates are generally more expensive and less widely available than their chloride counterparts. Over the past few years, new catalytic systems with the ability to activate unreactive and sterically hindered aryl chlorides have been developed. The new catalysts are based on palladium complexes that contain electron-rich and bulky phosphine or carbene ligands. The enhanced reactivity observed with these new systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].