Gold-catalyzed carbene transfer to alkynes: access to 2,4-disubstituted furans

Molecular basis of hyper-thermostability in the thermophilic archaeal aldolase MfnB

Methanogenic archaea are chemolithotrophic prokaryotes that can reduce carbon dioxide with hydrogen gas to form methane. These microorganisms make a significant contribution to the global carbon cycle, with methanogenic archaea from anoxic environments estimated to contribute > 500 million tons of global methane annually. Archaeal methanogenesis is dependent on the methanofurans; aminomethylfuran containing coenzymes that act as the primary C1 acceptor molecule during carbon dioxide fixation. Although the biosynthetic pathway to the methanofurans has been elucidated, structural adaptations which confer thermotolerance to Mfn enzymes from extremophilic archaea are yet to be investigated. Here we focus on the methanofuran biosynthetic enzyme MfnB, which catalyses the condensation of two molecules of glyceralde-3-phosphate to form 4‑(hydroxymethyl)-2-furancarboxaldehyde-phosphate. In this study, MfnB enzymes from the hyperthermophile Methanocaldococcus jannaschii and the mesophile Methanococcus maripaludis have been recombinantly overexpressed and purified to homogeneity. Thermal unfolding studies, together with steady-state kinetic assays, demonstrate thermoadaptation in the M. jannaschii enzyme. Molecular dynamics simulations have been used to provide a structural explanation for the observed properties. These reveal a greater number of side chain interactions in the M. jannaschii enzyme, which may confer protection from heating effects by enforcing spatial residue constraints.

Enantioselective Synthesis of α-Quaternary Isochromanes by Oxidative Aminocatalysis and Gold Catalysis

A novel strategy that combines oxidative aminocatalysis and gold catalysis allows the preparation of chiral α-quaternary isochromanes, a motif that is prevalent in natural products and synthetic bioactive compounds. In the first step, α-branched aldehydes and propargylic alcohols are transformed into α-quaternary ethers with excellent optical purities (>90 % ee) via oxidative umpolung with DDQ and an amino acid-derived primary amine catalyst. Subsequent gold(I)-catalyzed intramolecular hydroarylation affords the isochromane products with retention of the quaternary stereocenter. A second approach explores the use of allylic alcohols as reaction partners for the oxidative coupling to furnish α-quaternary ethers with generally lower enantiopurities. Stereoretentive cyclization to isochromane products is achieved via intramolecular Friedel-Crafts type alkylation with allylic acetates as a reactive handle. A number of synthetic elaborations and a biological study on these α-quaternary isochromanes highlight the potential applicability of the presented method.

Continuous isomerisation of 2,5-dimethylfuran to 2,4-dimethylfuran over Ga-silicate

2,4-dimethylfuran has a rare disubstitution pattern in the five-membered heterocyclic furan ring that is highly interesting chemically but challenging to access synthetically. We present a heterogeneously catalysed route to synthesise 2,4-dimethylfuran from commonly available 2,5-dimethylfuran using a zeolite packed-bed flow reactor. As supported by DFT calculations, the reaction occurs inside the zeolite channels, where the acid sites catalyse proton transfer followed by migration of a methyl group. The zeotype Ga-silicate (MFI type) appears superior to an aluminium-containing ZSM-5 by demonstrating higher selectivities and slower catalyst deactivation. This work provides new opportunities for the continuous valorisation of bio-feedstock molecules in the perspective of the emerging biorefinery era.

Catalyst-free diazo cross-coupling to access useful 3(2H)-furanone derivatives

Catalyst-free synthesis of 3(2H)-furanone derivatives has been achieved from metal-free cross-coupling of α-diazo ester and α-aryldiazo ketones. This reaction sequence comprises a prior Wolff rearrangement of α-aryldiazo ketones to 2-arylketenes, which are trapped in situ with α-diazo esters to form cyclopropanones before undergoing oxa-Nazarov cyclization.

Lewis Acid-Catalyzed Synthesis of Polysubstituted Furans from Conjugated Ene-yne-ketones and 1,3,5-Triazinanes

An efficient and metal-free approach to the construction of diverse functionalized furan derivatives has been developed from ene-yne-ketones and 1,3,5-triazinanes. As an atom-economical and environment-friendly protocol, the new conditions are suitable for selective C-N and C-O bond formation of a wide range of polysubstituted furans in one pot.

Silver oxide(I) promoted Conia-ene/radical cyclization for a straightforward access to furan derivatives

A novel access to fused furan cores using silver oxide(I) has been developed. Mechanistic investigations indicate the involvement of a Conia-ene reaction/radical cyclization for an expedient path to complex furan derivatives. The reaction is broad in scope with interesting atom economy and can also be conducted in a one-pot fashion from readily accessible α,β-unsaturated ketones.

Recent progress towards the transition-metal-catalyzed Nazarov cyclization of alkynes via metal carbenes

In recent years, transition-metal-catalyzed tandem cyclization reactions of alkynes, especially those involving a metal carbene intermediate, have received worthwhile interest, as this type of reaction does not require the use of risky and potentially explosive diazo compounds as starting materials for carbene generation. A significant and general strategy for the stereospecific synthesis of 5-membered cycles is Nazarov cyclization based on the 4π-conrotatory electrocyclization of a conjugated pentadienyl cation to afford a cyclopentenyl cation. In this review, we introduce an overview of recent advances in the transition-metal-catalyzed Nazarov cyclization of alkynes via a metal carbene intermediate, and categorize these reactions according to the structure of the metal carbene. Our aim is to accelerate advancements in this enchanting area of research.

(2 + 1 + 1 + 1)-Annulation Reactions of Aryldiazonium Tetrafluoroborates with Sulfur Ylides to Polysubstituted Pyrazoles

Novel divergent domino annulation reactions of sulfur ylides with aryldiazonium tetrafluoroborates have been developed, affording various tetra- and trisubstituted pyrazole derivatives in moderate to good yields. Three molecules of sulfur ylides were applied as C1 synthon to construct the complex products with five new chemical bonds formed in these one-pot reactions.

Challenges in the Highly Selective [3 + 1]-Cycloaddition of an Enoldiazoacetamide to Form a Donor-Acceptor Cis-Cyclobutenecarboxamide

A substituted donor-acceptor cyclobutenecarboxamide is synthesized with modest enantiocontrol through a chiral copper(I) complex catalyzed [3 + 1]-cycloaddition reaction of α-acyl diphenylsulfur ylides with 3-siloxy-2-diazo-3-butenamides. With a methyl substituent on the 4-position of the 3-butenamide, the cis-vicinal-3,4-disubstituted cyclobutenecarboxamide is formed with >20:1 diastereocontrol. Donor-acceptor 3-methyl-2-siloxycyclopropenecarboxamide is rapidly formed from the reactant enoldiazoamide and undergoes catalytic ring opening to give only the Z-γ-substituted metallo-enolcarbene. Elimination from 3-siloxy-2-diazo-3-pentenamide to form the conjugated 3-siloxy-2,4-pentadienamide is competitive but minimized at low temperature.

Tetra-substituted furans by a gold-catalysed tandem C(sp3)–H alkynylation/oxy-alkynylation reaction

A gold-catalysed cascade C(sp3)–H alkynylation/oxy-alkynylation of acceptor-substituted carbonyl compounds with hypervalent iodine(iii) reagents for the synthesis of tetra-substituted furans, offering distinct advantages over previous methods.

Gold-Catalyzed Cycloisomerization of Sulfur Ylides to Dihydrobenzothiepines

The metal-promoted nucleophilic addition of sulfur ylides to π-systems is a well-established reactivity. However, the driving force of such transformations, elimination of a sulfide moiety, entails stoichiometric byproducts making them unfavorable in terms of atom economy. In this work, a new take on sulfur ylide chemistry is reported, an atom-economical gold(I)-catalyzed synthesis of dihydrobenzo[b]thiepines. The reaction proceeds under mild conditions at room temperature.

α-Imino Gold Carbene Intermediates from Readily Accessible Sulfilimines: Intermolecular Access to Structural Diversity

Catalytic approaches to pharmaceutically important bioactive skeletons through gold carbene intermediates have experienced a dramatic development in the last decade. Although various carbene precursors continue to play an important role in heterocyclic syntheses, these reagents are associated with some drawbacks in terms of functional group tolerance, synthetic methods and safety limitations. A new generation of nitrene transfer reagents was established in 2019: the sulfilimines. These are safe, inexpensive and readily available. They can conveniently be stored and handled, and thus represent ideal reagents for the fast and modular modification of scaffolds and the preparation of libraries by intermolecular reactions of two components. Both the practical methods for synthesizing sulfilimines and the versatility of these ylidic species in gold-catalyzed preparation of structural diversity, for both heterocycles and carbocycles, will be outlined in this Concept article.

Acyl Migration versus Epoxidation in Gold Catalysis: Facile, Switchable, and Atom-Economic Synthesis of Acylindoles and Quinoline Derivatives

We report a switchable synthesis of acylindoles and quinoline derivatives via gold-catalyzed annulations of anthranils and ynamides. α-Imino gold carbenes, generated in situ from anthranils and an N,O-coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π-bond, followed by unexpected and highly selective 1,4- or 1,3-acyl migrations to form 6-acylindoles or 5-acylindoles. With the (2-biphenyl)di-tert-butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3-hydroxylquinolines, quinolin-3(4H)-ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma-complexes support the observed substitution pattern in the final product.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

Efficient access to fluorescent benzofuro[3,2-b]carbazoles via TFA-promoted cascade annulations of sulfur ylides, 2-hydroxy-β-nitrostyrenes and indoles

An efficient, TFA-promoted cascade annulation of sulfur ylides, 2-hydroxy-β-nitrostyrenes and indoles was established.

Ruthenium(ii)-catalyzed chemoselective deacylative annulation of 1,3-diones with sulfoxonium ylides via C-C bond activation

The first successful example of deacylative annulation of 1,3-diones with sulfoxonium ylides was achieved through Ru(ii)-catalyzed C-C bond activation. The excellent chemoselectivity and broad substrate scope render this method a practical and versatile approach for the preparation of (hetero)aryl and alkenyl substituted furans, which are valuable units in many biologically active compounds and functional materials. A preliminary mechanistic study reveals that this process involves a deacylative α-ruthenation to generate key alkyl Ru(ii) intermediates with the release of a benzoic acid fragment.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Gold(iii)-catalyzed chemoselective annulations of anthranils with N-allylynamides for the synthesis of 3-azabicyclo[3.1.0]hexan-2-imines

We herein report the gold(iii)-catalyzed selective annulation of anthranils with N-allylynamides under mild conditions. By trapping the in situ-generated α-imino gold carbenes, 3-azabicyclo[3.1.0]hexan-2-imines were obtained in high synthetic efficiency. The reaction, which can be conducted in the gram scale, tolerates electron-rich and electron-deficient anthranils as well as a diverse set of functionalized ynamides (aryl- and alkyl-substituted terminal).

Ylide-Functionalization via Metalated Ylides: Synthesis and Structural Properties

The α-metallated ylides [Ph3P-C-Z]-M+ (with Z=SO2Tol or CN and M=Na or K) were used as versatile nucleophiles for the facile access to ylide-substituted compounds. Halogenations, alkylations, carbonylations and functionalization reactions with main group element halides were easily accomplished by simple trapping reactions with the appropriate electrophiles. X-ray crystallographic studies of all compounds - including the first structures of α-fluorinated P-ylides - showed remarkable differences in the ylide backbone depending on the substituents. In the fluorinated compounds, a change from a fully planar to a pyramidalized ylidic carbon centre was observed despite the strongly anion-stabilizing ability of the yldiide substituent. π-Donation from the ylide substituent also resulted in geometric restrictions depending on the steric and electronic properties of the introduced substituents.

Ruthenium(ii)-catalyzed selective C–H bond activation of imidamides and coupling with sulfoxonium ylides: an efficient approach for the synthesis of highly functional 3-ketoindoles

Ruthenium-catalyzed selective C–H bond activation of imidamides and annulation of sulfoxonium ylides were achieved, which afforded a series of 3-ketoindole derivatives in good yields, with good functional group compatibility.

Regiospecific synthesis of polysubstituted furans with mono- to tricarboxylates from various sulfonium acylmethylides and acetylenic esters

Polysubstituted furans with mono- to tricarboxylates were prepared in moderate to good yields from various dimethylsulfonium acylmethylides and acetylenic esters.

Gold-catalyzed heterocyclic syntheses through α-imino gold carbene complexes as intermediates

This review reports a comprehensive compilation of gold-catalyzed heterocyclic synthesis by using α-imino gold carbene complexes as the proposed intermediates.

Substrate-switched dual functionalization of alkenes: catalyst-free synthetic route for β-hydroxy and β-keto thioethers

A substrate-controlled dual functionalization of alkenes under catalyst-free and solvent-free conditions to provide β-hydroxy thioethers and β-keto thioethers is described.

A Catalytic Cross-Olefination of Diazo Compounds with Sulfoxonium Ylides

A ruthenium‐catalysed cross‐olefination of diazo compounds and sulfoxonium ylides is presented. Our reaction design exploits the intrinsic difference in reactivity of diazo compounds and sulfoxonium ylides as both carbene precursors and nucleophiles, which results in a highly selective reaction.

Iron-Catalyzed Carbenoid-Transfer Reactions of Vinyl Sulfoxonium Ylides: An Experimental and Computational Study

A method for the generation of unprecedented vinyl carbenoids from sulfoxonium ylides has been developed and applied in the synthesis of a diverse array of heterocycles such as indolizines, pyrroles, 3-pyrrolin-2-ones, and furans. The reactions proceed by FeBr₂ catalysis under mild reaction conditions with a broad substrate scope. A reaction pathway involving iron carbenoids is proposed based on a series of control experiments and DFT calculations.

Sulfur-Based Ylides in Transition-Metal-Catalysed Processes

Traditionally employed in the synthesis of small ring systems and rearrangement chemistry, sulfur-based ylides occupy a unique position in the toolbox of the synthetic organic chemist. In recent years a number of pioneering researchers have looked to expand the application of these unorthodox reagents through the use of transition metal catalysis. The strength and flexibility of such a combination have been shown to be of key importance in developing powerful novel methodologies. This chapter summarises recent developments in transition metal-catalysed sulfonium/sulfoxonium ylide reactions, as well as providing a historical perspective. In overviewing the successes in this area, the authors hope to encourage others into this growing field.

Silver & gold-catalyzed routes to furans and benzofurans

Silver and gold have become incredibly versatile and mild catalysts for numerous transformations, especially in heterocycle synthesis. For the most prominent of them, i.e. furans, silver and gold, with their unique reactivity and mildness, allow numerous possible routes to highly substituted and/or functionalized furans from a large variety of starting materials. Silver and gold catalysis provide thus the most flexible way to this important family of compounds. The present review describes these silver and gold-catalyzed routes, with some emphasis on mechanistic aspects, and proposes a comparison of both the silver and the gold-catalyzed syntheses of furans.

Rhodium-Catalyzed Synthesis of Sulfur Ylides via in Situ Generated Iodonium Ylides

A convenient strategy for the synthesis of sulfur ylides via rhodium-catalyzed coupling of in situ generated iodonium ylides with sulfides or sulfoxides has been developed. A wide range of sulfur ylides were obtained in moderate to good yields from inexpensive sulfur compounds and active methylene compounds with a short reaction time (MW, 5-10 min) or 12-16 h at rt. Furthermore, these sulfoxonium ylides were used as novel acceptor/acceptor carbenes for N-H insertion reactions.