Higher acenes by on‐surfacedehydrogenation : from heptacene to undecacene

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements

The Principles of Gold-Catalyzed Molecular Gymnastics

L'or(I) dirigeix el contorsionisme molecular mitjançant reaccions intramoleculars i intermoleculars d'enins per mitjà d'intermedis ciclopropil o carbens altament distorsionats. La síntesi de productes naturals com (+)-orienalol F i (-)-englerin A mostra l'estat d'art de la catàlisi de l'or(I) per a construir complexitat molecular.Gold(I) orchestrates molecular gymnastics by intraand intermolecular reaction of enymes via highly distorted cyclopropyl gold carbenes as intermediates. The synthesis of natural products such as (+)-orientalol F and (−)-englerin A illustrates the state of the art of gold (I) catalysis for the buildup of molecular complexity

Shape‐shifting molecules: unveiling the valence tautomerism phenomena in bare barbaralones

Producción CientíficaWe report a state-of-the-art spectroscopic study of an archetypical barbaralone, conclusively revealing the valence tautomerism phenomena for this bistable molecular system. The two distinct 1- and 5-substituted valence tautomers have been isolated in a supersonic expansion for the first time and successfully characterized by high-resolution rotational spectroscopy. This work provides irrefutable experimental evidence of the [3,3]-rearrangement in barbaralones and highlights the use of rotational spectroscopy to analyze shape-shifting mixtures. Moreover, this observation opens the window toward the characterization of new fluxional systems in the isolation conditions of the gas phase and should serve as a reference point in the general understanding of valence tautomerism.Ministerio de Ciencia e Innovación y Agencia Estatal de Investigación (grants MCIN/A EI/10.13039/ 501100011 033, PID2019-111396GB-I00, PID2019-104815GB-I00, CEX2019-000925-S)Junta de Castilla y Leon (projects VA077U16 and VA244P20)Agencia de Gestión de Ayudas Universitarias y de Investigación y el Programa CERCA/Generalitat de Catalunya de apoyo financiero (2017 SGR 1257)Ministerio de Ciencia, Innovación y Universidades (grant FPU17/02987

Electronic decoupling of polyacenes from the underlying metal substrate by sp ³ carbon atoms

We report on the effect of sp3 hybridized carbon atoms in acene derivatives adsorbed on metal surfaces, namely decoupling the molecules from the supporting substrates. In particular, we have used a Ag(100) substrate and hydrogenated heptacene molecules, in which the longest conjugated segment determining its frontier molecular orbitals amounts to five consecutive rings. The non-planarity that the sp3 atoms impose on the carbon backbone results in electronically decoupled molecules, as demonstrated by the presence of charging resonances in dI/dV tunneling spectra and the associated double tunneling barriers, or in the Kondo peak that is due to a net spin S=1/2 of the molecule as its LUMO becomes singly charged. The spatially dependent appearance of the charging resonances as peaks or dips in the differential conductance spectra is further understood in terms of the tunneling barrier variation upon molecular charging, as well as of the different orbitals involved in the tunneling process

Coinage metal complexes bearing fluorinated N-Heterocyclic carbene ligands

The synthesis of novel fluorinated symmetrical and unsymmetrical imidazolium salts as well as their corresponding coinage metal complexes is described. The silver derivatives were prepared using Ag2O as the metal source and the subsequent (NHC)AgX complexes were successfully employed in the preparation of the corresponding Cu(I) and Au(I) complexes through transmetallation reactions. Halide extrusion from (NHC)AuCl complexes in the presence of labile ligands also allowed the isolation of Au(I) cationic complexes [(NHC)Au(L)]X. The molecular structures of relevant examples of the neutral and ionic complexes have been unambiguously determined by X-ray studies.The authors thank Agencia Estatal de Investigación (AEI)/FEDER (CTQ2017-82893-C2-1-R, CTQ2016- 75960-P), Red Intecat CTQ2016-81923-REDC, the AGAUR (2017 SGR 1257), and CERCA Program/Generalitat de Catalunya for financial support. M. D. R. thanks the Ministerio de Educación y Ciencia (MEC) for a FPU fellowship and C. G.-M. thanks Agencia Estatal de Investigación for a FPI predoctoral fellowship

Ferrocene derivatives of liquid chiral molecules allow assignment of absolute configuration by X-ray crystallography

The present study investigates a synthetically simple ferrocene derivatization of natural products and active pharmaceutical ingredients. Seven new crystal structures are analyzed together with 16 structures of ferrocene derivatives reported previously. In all cases, the unambiguous determination of the absolute structure was established from anomalous dispersion using the methods of Flack and Parsons. A comparison with other derivatization approaches shows the advantage of the described ferrocene derivatization for establishing the absolute configuration of novel compounds

Novel ortho-OPE metallofoldamers: binding-induced folding promoted by nucleating Ag(i)-alkyne interactions

We have developed a new family of ortho-oligophenylene ethynylene (o-OPE) metallofoldamers. The folding of these helicates is induced by nucleating carbon-metal interactions between Ag(i) cations and the alkynes of the inner core of the o-OPEs. These o-OPEs form metal-organic assemblies where at least three alkyne moieties are held in close proximity to form novel Ag(i)-complexes with the metal ion lodged into the helical cavity. NMR titration experiments and photokinetic studies have provided quantitative data about the thermodynamic and kinetic features of such binding/folding phenomena. X-ray diffraction and DFT studies have been performed to extract structural information on how the Ag(i) cation is accommodated into the cavity. The great simplicity and versatility of these new metallofoldamers open up the possibility to develop novel structures with applications in material science and/or in asymmetric catalysisThis research was funded by the Regional Government of Andalucía (project P09-FQM-4571) and the ICIQ Foundation. DM thanks Regional Government of Andalucía for her contract. AML thanks MICINN for her FPU fellowship. The authors thank the Centro de Servicios de Informática y Redes de Comunicaciones (CSIRC), Universidad de Granada, for providing the computing tim

On-surface synthesis of heptacene on Ag(001) from brominated and non-brominated tetrahydroheptacene precursors

Achieving the Ag(001)-supported synthesis of heptacene from two related reactants reveals the effect of the presence of Br atoms on the reaction process. The properties of reactants, intermediates and end-products are further characterized by scanning tunneling microscopy and spectroscopy.Fil: Colazzo, Luciano. Donostia International Physics Center; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Mohammed, Mohammed S. G.. Donostia International Physics Center; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Dorel, Ruth. Barcelona Institute of Science and Technology; EspañaFil: Nita, Pawel. Donostia International Physics Center; España. Consejo Superior de Investigaciones Científicas; EspañaFil: García Fernández, Carlos. Donostia International Physics Center; EspañaFil: Abufager, Paula Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Lorente Palacios, Nicolas. Donostia International Physics Center; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Echavarren, Antonio M.. Barcelona Institute of Science and Technology; España. Universitat Rovira I Virgili; EspañaFil: De Oteyza, Dimas G.. Donostia International Physics Center; España. Consejo Superior de Investigaciones Científicas; España. Ikerbasque; Españ

Cyclobutene vs 1,3-Diene Formation in the Gold-Catalyzed Reaction of Alkynes with Alkenes : The Complete Mechanistic Picture

The intermolecular gold(I)-catalyzed reaction between arylalkynes and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which takes place stepwise, first by formation of cyclopropyl gold(I) carbenes, followed by a ring expansion. However, 1,3-butadienes are also formed in the case of ortho -substituted arylalkynes by a metathesis-type process. The corresponding reaction of alkenes with aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively to cyclobutenes. A comprehensive mechanism for the gold(I)-catalyzed reaction of alkynes with alkenes is proposed on the basis of density functional theory calculations, which shows that the two pathways leading to cyclobutenes or dienes are very close in energy. The key intermediates are cyclopropyl gold(I) carbenes, which have been independently generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1 H -cyclopropa[ a ]naphthalenes

Gold(I)-Catalyzed Activation of Alkynes for the Construction of Molecular Complexity

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. Gold(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. In 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. figure Scheme 1. Anti-Nucleophilic Attack to η2-[AuL]+-Activated Alkynes Structurally, 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) Despite 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. Complexes [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. It 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) Often, 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. The 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. figure Figure 1. Increase in electrophilicity with decreased donating ligand ability in gold(I) complexes. Gold(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) The 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) 1.2Scope and Organization of the Review Homogeneous 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. Several 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. The 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