Copper-Catalyzed Carboarylation of Alkynes via Vinyl Cations

Aluminum-Catalyzed Intramolecular Vinylation of Arenes by Vinyl Cations

This study addresses the challenges associated with vinyl cation generation, a process that traditionally requires quite specific counterions. Described herein is a novel intramolecular vinylation of arenes catalyzed by aluminum(III) chloride, utilizing practical conditions and readily available vinyl triflates derived from 2-aceto-3-arylpropionates. Comprehensive experimental data support diverse carbocycle synthesis, exemplified by indenes and higher analogues. Control experiments verify the applicability of the vinylation protocol, and synthetic applications showcase a potent tubulin polymerization inhibitor with anticancer properties. Density functional theory computations reveal a Lewis-acid-driven mechanism involving triflate moiety abstraction to generate a reactive vinyl cation.

Gold-Catalyzed 1,2-Dicarbofunctionalization of Alkynes with Organohalides

Herein, we report the first gold-catalyzed 1,2-dicarbofunctionalization of alkynes using organohalides as non-prefunctionalized coupling partners. The mechanism of the reaction involves an oxidative addition/π-activation mechanism in contrast to the migratory insertion/cis-trans isomerization pathway that is predominantly observed with other transition metals yielding products with anti-selectivity. Mechanistic insights include several control experiments, NMR studies, HR-MSMS analyses, and DFT calculations that strongly support the proposed mechanism.

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Regiodivergent and Stereoselective Synthesis of Highly Substituted 1,3-Dienes via Arylative Acyloxy Migration of Propargyl Esters

We report the first catalyst-controlled regiodivergent method that enables the synthesis of structurally diverse 1,2,3,4-tetrasubstituted conjugated dienes with excellent regio- and stereochemical outcomes from the same set of readily available propargyl esters and diaryliodonium salts. In this diene chemistry, the in situ generated, highly electrophilic aryl-CuIII complex serves not only as a π-Lewis acid catalyst for alkyne activation/acyloxy migration but also as an aryl electrophile equivalent. The competitive arylative 1,2- and 1,3-acyloxy migration patterns are exquisitely dictated by Cu and Au/Cu relay catalyses, respectively, providing a modular and attractive approach to traditionally inaccessible tetrasubstituted 1,3-dienes in a regiodivergent manner. Finally, the synthetic utility of this method is demonstrated by further synthetic derivatization of 1,3-dienes into an array of useful compounds.

α-Vinylation of Ester Equivalents via Main Group Catalysis for the Construction of Quaternary Centers

A methodology for the construction of sterically congested quaternary centers via the trapping of vinyl carbocations with silyl ketene acetals is disclosed. This main group-catalyzed α-vinylation reaction is advantageous as methods to access these congested motifs are limited. Moreover, β,γ-unsaturated carbonyl moieties and tetrasubstituted alkenes are present in various bioactive natural products and pharmaceuticals, and this catalytic platform offers a means of accessing them using simple and inexpensive materials.

I2-Mediated [6 + 1] Annulation of Alkynes with MsONH3OTf: Direct Synthesis of Benzo[b]azepines

The synthesis of benzo[b]azepines using protonated aminating reagent (MsONH₃OTf) and alkynes through I₂-mediated [6 + 1] annulation reaction has been developed. This protocol features excellent functional group tolerance and mild reaction conditions and affords the benzo[b]azepines in moderate to good yields under metal-free reaction conditions.

Design, development and applications of copper-catalyzed regioselective (4 + 2) annulations between diaryliodonium salts and alkynes

Diaryliodonium salts have been extensively applied in organic synthesis as aryl cation equivalents. However, in the electrophilic reactions with alkenes or alkynes, only the electrophilic carbon of the diaryliodonium salts was involved while the other part of the aryl ring was not utilized. Herein, a reaction pattern of diaryliodonium was reported as oxa-1,4-dipoles to undergo (4 + 2) cycloaddition reactions with alkynes. Broad spectrum of the two reaction partners could be utilized in this protocol, enabling an operationally simple, high yielding, and regioselective synthetic approach to isocoumarins. Particularly, good to excellent regioselectivities were achieved for the sterically unbiased unsymmetrical diaryl acetylenes, which was challenging for other transition metal-catalyzed processes. The reaction could be scaled up with the ideal 1:1 stoichiometry and the isocoumarin type natural products Oospolactone and Thunberginol A could be obtained in one or three steps through this methodology.

Vinyl cation-mediated intramolecular hydroarylation of alkynes using pyridinium reagents

Once considered to be exotic species of limited synthetic utility, vinyl cations have recently been shown to be highly versatile intermediates in a variety of processes. Here, we report a method for the synthesis of aryl-substituted benzocycloheptenes and -hexenes using the hydrotriflate salt of an electron-poor pyridine as a uniquely efficient proton source for a vinyl cation mediated Friedel-Crafts cyclization. The mild conditions made possible by this reagent allowed a range of simple and functionalized alkynes bearing pendant aryl groups to serve as suitable substrates for this scalable and convenient protocol.

Copper-Catalyzed Chemo-, Regio-, and Stereoselective Multicomponent 1,2,3-Trifunctionalization of Internal Alkynes

Herein, we report the first diaryliodonium salts promoted multicomponent 1,2,3-trifunctionalization of alkynes, where both the acetylenic bond and the adjacent nonactivated propargylic C(sp³)-H bond were functionalized synergistically to generate α-arylated enones with high chemo-, regio-, and stereoselectivity. A broad spectrum of diaryliodonium salts and internal alkynes could be utilized in this protocol, and a diverse collection of highly substituted and stereochemically defined linear and cyclic complex structures could be elaborated from the enone products.

Refining boron-iodane exchange to access versatile arylation reagents

Aryl(Mes)iodonium salts, which are multifaceted aryl transfer reagents, are synthesized via boron-iodane exchange. Modification to both the nucleophilic (aryl boron) and electrophilic (mesityl-λ³-iodane) reaction components results in improved yield and faster reaction time compared to previous conditions. Mechanistic studies reveal a pathway that is more like transmetallation than S_EAr.

Diaryliodonium(III) Salts in One-Pot Double Functionalization of C–IIII and ortho C–H Bonds

Since the 1950s, diaryliodonium(III) salts have been demonstrated to participate in various arylation reactions, forming aryl–heteroatom and aryl–carbon bonds. Incorporating the arylation step into sequential transformations would provide access to...

Recent advances in Cu-catalyzed transformations of internal alkynes to alkenes and heterocycles

Numerous metal-catalyzed reactions involving internal alkynes and aimed towards synthetically and pharmacologically important alkenes and heterocycles have appeared in the literature. Among these, Cu-catalyzed reactions have a special place, which has prompted the investigation and development of carbon-carbon and carbon-heteroatom bond-forming reactions. These reactions possess wide scope, and during the paths of these reactions, either stable or in situ intermediates are formed via the addition of Cu as a core catalyst or synergistic catalyst. In this review, we aim to report different contributions relating to Cu-catalyzed reactions of internal alkynes for the synthesis of different valuable alkenes and heterocycles which have appeared in the literature in the last decade. We anticipate that this appraisal will deliver basic insights for the further advancement of Cu-catalyzed reactions in organic chemistry.

Binary-Acid Catalysis with Sc(OTf)3/TfOH in the Alkenylation of Arenes with Alkynes

Herein we report binary acid Sc(OTf)₃/TfOH-catalyzed alkenylation of arenes with alkynes. In this system, the high-energy vinyl carbocations with activated and weakly coordinating trifluoromethanesulfonate anions by Lewis acid Sc(III) can undergo facile Friedel-Crafts reactions with arenes to give the desired adducts in up to 90% yield and with high Z-selectivity.

Iron-catalyzed carboarylation of alkynes via activation of π-activated alcohols: rapid synthesis of substituted benzofused six-membered heterocycles

An Fe(OTf)3-catalysed carboarylation of alkynes is reported for the straightforward synthesis of densely substituted 1,2-dihydroquinolines from N-propargyl anilides and π-activated alcohols. The reaction provides a new method for the synthesis of highly substituted benzofused six-membered heterocycles by the formation of two carbon-carbon bonds and one ring in a single step. The power of the methodology was further extended to the synthesis of substituted chromene and thiochromene derivatives in high yields. In addition, substituted quinoline derivatives were also achieved in a single step in the presence of FeCl3 through detosylation/aromatisation. A number of control experiments have been performed and a plausible mechanism has also been proposed to explain the formation of the products.

Vinyl and Alkynyl Triazenes: Synthesis, Reactivity, and Applications

Aromatic compounds containing triazenyl groups (N₃ RR') have a profound impact on synthetic organic and medicinal chemistry. In contrast, the chemistry of vinyl and alkynyl triazenes was a largely uncharted territory until recently. The situation has changed over the last five years, and it has become apparent that vinyl and alkynyl triazenes are highly interesting compounds with a unique reactivity. The electron-donating properties of the triazenyl group provide alkynyl triazenes with an ynamide-like reactivity, which can be exploited in reactions of the triple bond. Vinyl triazenes, on the other hand, can be used for electrophilic vinylation reactions. The foundation for this new triazene chemistry are synthetic pathways which allow preparing vinyl and alkynyl triazenes in few steps from readily available starting materials. In this Minireview, we summarize recent developments in this area.

Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents

The discovery of safe, general, and practical procedures to prepare vinyl fluorides from readily available precursors remains a synthetic challenge. The metal-free hydrofluorination of alkynes constitutes an attractive though elusive strategy for their preparation. Introduced here is an inexpensive and easily handled reagent that enables the development of simple and scalable protocols for the regioselective hydrofluorination of alkynes to access both the E and Z isomers of vinyl fluorides. These reaction conditions were suitable for a diverse collection of alkynes, including several highly functionalized pharmaceutical derivatives. Computational and experimental mechanistic studies support C-F bond formation through vinyl cation intermediates, with the E- and Z-hydrofluorination products forming under kinetic and thermodynamic control, respectively.

Bimolecular vinylation of arenes by vinyl cations

Styrene derivatives can be easily synthesized from vinyl triflates and arenes under mild reaction conditions, using [Li][Al(OC(CF₃)₃)₄] as a catalyst and LiHMDS as a base.

Metallomimetic Chemistry of Boron

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H₂, CO, alkynes, alkenes and even with N₂.

Copper-catalyzed regioselective cyclization of vinyl azides by gem-difluoromethylene for trisubstituted pyridines

A novel copper-catalyzed cyclization of readily available vinyl azides with gem-difluoromethylene is described.

Intramolecular Vinylation of Aryl Rings by Vinyl Cations

A Lewis acid mediated intramolecular electrophilic vinylation of aryl rings by vinyl cations is reported. This reaction takes advantage of β-hydroxy-α-diazo ketones as vinyl cation precursors and provides good yields of tricyclic 1-indenones that contain a seven-membered ring. Extending the alkane chain that tethers the vinyl cation to the aromatic ring leads to 2-napthol and 2-indenone products.

Teaching an old carbocation new tricks: Intermolecular C-H insertion reactions of vinyl cations

Vinyl carbocations have been the subject of extensive experimental and theoretical studies over the past five decades. Despite this long history in chemistry, the utility of vinyl cations in chemical synthesis has been limited, with most reactivity studies focusing on solvolysis reactions or intramolecular processes. Here we report synthetic and mechanistic studies of vinyl cations generated through silylium-weakly coordinating anion catalysis. We find that these reactive intermediates undergo mild intermolecular carbon-carbon bond-forming reactions, including carbon-hydrogen (C-H) insertion into unactivated sp³ C-H bonds and reductive Friedel-Crafts reactions with arenes. Moreover, we conducted computational studies of these alkane C-H functionalization reactions and discovered that they proceed through nonclassical, ambimodal transition structures. This reaction manifold provides a framework for the catalytic functionalization of hydrocarbons using simple ketone derivatives.

DFT calculations on the mechanism of copper-catalysed tandem arylation-cyclisation reactions of alkynes and diaryliodonium salts

We present a computational mechanistic study on the copper(III)-catalysed carboarylation-ring closure reactions leading to the formation of functionalised heterocycles. We have performed DFT calculations along selected routes and compared their free energy profiles. The calculations considered two viable options for the underlying mechanism which differ in the order of the oxazoline ring formation and the aryl transfer steps. In our model transformation, it was found that the reaction generally features the aryl transfer-ring closing sequence and this sequence shows very limited sensitivity to the variation of the substituent of the reactants. On the basis of the mechanism the origin of the stereoselectivity is ascribed to the interaction of the Cu ion with the oxazoline oxygen driving the ring-closure step selectively.

Development of concise two-step catalytic approach towards lasofoxifene precursor nafoxidine

We have elaborated a two-step catalytic approach to nafoxidine, a key precursor to lasofoxifene. Firstly, an efficient α-arylation of 6-methoxy-3,4-dihydronaphthalen-1(2H)-one with chlorobenzene was developed, which operates at low 0.1 mol% Pd-132 catalyst loading in the presence of 1.9 equivalents of sodium tert-butoxide at 60 °C in 1,4-dioxane and provides 6-methoxy-2-phenyl-3,4-dihydronaphthalen-1(2H)-one in 90% yield. Secondly, we have demonstrated that 6-methoxy-2-phenyl-3,4-dihydronaphthalen-1(2H)-one can be converted to nafoxidine in 61% yield via CeCl₃ promoted reaction with (4-(2-(pyrrolidin-1-yl)ethoxy)phenyl)lithium, which is formed in-situ from the corresponding arylbromide precursor and n-butyllithium. Altogether, the shortest two-step approach to nafoxidine from simple tetralone commodity starting material has been developed with overall 55% yield. The developed synthetic approach to nafoxidine has several beneficial aspects over the one used in the synthetic route primarily developed for the preparation of lasofoxifene.

In(III)-TMSBr-Catalyzed Cascade Reaction of Diarylalkynes with Acrylates for the Synthesis of Aryldihydronaphthalene Derivatives

A combined Lewis acid system comprising of two or more Lewis acids occasionally exhibits augmented catalytic activity in organic transformations which are otherwise unrealizable by either of the components exclusively. On the other hand, the efficient construction of multiple new C-C bonds and polycyclic structures in minimal steps remains a subject of great interest in both academia and industry. Herein we report an efficient method to assemble aryldihydronaphthalene derivatives via a cascade reaction of diarylalkynes with acrylates under the catalysis of a combined Lewis acid derived from In(III) salt and TMSBr.

Palladium-catalyzed carbene/alkyne metathesis with enynones as carbene precursors: synthesis of fused polyheterocycles

An unprecedented palladium-catalyzed novel carbene/alkyne metathesis cascade reaction of alkyne-tethered enynones is described. This reaction affords fused polyheterocycles in moderate to good yields. The transformation begins with Pd-catalyzed 5-exo-dig cyclization of the enynone to form the donor/donor metal carbene, which then undergoes metathesis with the alkyne followed by electrophilic aromatic substitution.

Cationic polycyclization of ynamides: building up molecular complexity

Polycyclization reactions are among the most efficient synthetic tools for the synthesis of complex, polycyclic molecules in a single operation from simple starting materials. We report in this manuscript a full account on the discovery and development of a novel cationic polycyclization from readily available ynamides. Simple activation of these building blocks under acidic conditions enables the generation of highly reactive activated keteniminium ions, which triggers an unprecedented cationic polycyclization yielding highly substituted polycyclic nitrogen heterocycles possessing up to seven fused cycles and three contiguous stereocenters.