Rh(I)-Catalyzed Coupling of Conjugated Enynones with Arylboronic Acids: Synthesis of Furyl-Containing Triarylmethanes

Transition metal-catalyzed reactivity of carbenes with boronic acid derivatives for arylation (alkylation) and beyond

This comprehensive review article discussed the reactivity of carbenes with boronic acid derivatives for the one-pot synthesis of diarylmethanes, difluoromethylated arenes, aryl and alkyl boron compounds, arylacetic acid derivatives, furan derivatives, and many other compounds. We have summarized the arylation, vinylation, and alkylation of carbenes utilizing various transition metals, viz. palladium, rhodium, copper, and platinum, for the construction of carbon-carbon bonds, carbon-boron bonds, and beyond through the cross-coupling strategy. The reason for the increasing popularity of these novel methodologies is their application in the synthesis and late-stage functionalization of biologically active compounds and natural products. Notably, organoboron compounds are exemplified as versatile synthetic intermediates for constructing various bonds.

Ready Access to Densely Substituted Furans Using Tsuji-Wacker-Type Cyclization

A competent method for the construction of highly substituted furans catalyzed by Pd(II) and Cu(II) chloride has been developed. The method provides easy access to di-, tri-, and tetrasubstituted furans from corresponding diols with relatively mild conditions in a unified strategy. The developed method has been successfully tested with more than 25 substrates, which resulted in furans of multiple substitution patterns with up to 84% isolated yields.

AgOTf/I2-Mediated Cyclization/Cross-Coupling/Isomerization of Enynones with Phosphorus Ylides: An Expedient Route to Stereoselective Synthesis of (E)-2-Alkenylfurans

Ag(I)-catalyzed cascade reactions involving enynone cyclization and cross-coupling with phosphorus ylides have been achieved for the first time. Subsequent treatment of the reaction mixture with I₂ afforded the corresponding (E)-α-alkenylfurans in 73-95% yields with excellent stereoselectively. A reasonable mechanism has been proposed.

Oxymetalation or oxidative cyclization? mechanism of Pd-catalyzed annulation of enynones

Enynones are powerful synthons for constructing furan derivatives in the presence of transition metal catalysts. Unlike the conventional intramolecular nucleophilic attack with the activation of coinage metals, we propose that enynones undergo an oxidative cyclization process with a Pd(0) species. The full catalytic cycle involves oxidative cyclization, isocyanide insertion, and reductive elimination, which was supported by DFT calculations. Geometric and electronic analyses confirmed the oxidative cyclization process, which proceeds via a Pd(ii) intermediate.

Silver(i)-catalyzed tandem reaction of enynones and 4-alkynyl isoxazoles: regioselective synthesis of highly functionalized 4H-furan[3,4-c]pyrroles

This work reports a silver(i)-catalyzed tandem reaction of enynones with 4-alkynyl isoxazoles.

Gold(iii) promoted formation of dihydroquinazolinones: double X-H activation by gold

An efficient 2-furyl gold-carbene promoted synthetic method was developed for the formation of dihydroquinazolinones from enynones by dual insertion of anthranilamides. In this organic transformation a new C-O and two C-N bond formations occurred and dihydroquinazolinones were obtained with a quaternary centre in moderate to very good yields in one-pot synthesis.

Palladium-catalyzed cascade reactions of enynones and isocyanides: access towards functionalized ketenimine and its application

Palladium-catalyzed cascade reactions of enynones and isocyanides have been disclosed. This strategy allows for the synthesis of structurally unusual furan-incorporated ketenimines, which are stable and can undergo a formal [4+1] annulation with another isocyanide to yield indenone derivatives.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Tritylium assisted iodine catalysis for the synthesis of unsymmetrical triarylmethanes

The combined Lewis acid catalytic system, generated from molecular iodine and tritylium tetrafluoroborate effectively catalyzed the Friedel-Crafts (FC) arylation of diarylmethyl sulfides providing an efficient access to various unsymmetrical triarylmethanes. The addition of tritylium and iodine created a more active catalytic system to promote the cleavage of sulfidic C-S bonds.

Access to Polysubstituted (Furyl)methylthioethers via a Base-Promoted S-H Insertion Reaction of Conjugated Enynones

A convenient and applicable approach to the construction of diverse functionalized (2-furyl)methylthioether derivatives via base-promoted S-H insertion of conjugated enynones with thiophenols or thiols has been developed. This reaction features readily available starting materials, high atom economy, broad substrate scope, and versatile operation. Moreover, the synthetic utility of this method has been demonstrated by the efficient synthesis of the CNKSPR1 inhibitor precursor and late-stage functionalization of glutathione.

Gold-carbene assisted formation of tetraarylmethane derivatives: double X-H activation by gold

An efficient gold-carbene promoted generation of tetraarylmethane derivatives from enynones and indoles was accomplished by the formation of a new C-O bond and two C-C bonds. It is significant that (2-furyl) gold-carbene assisted addition of two nucleophiles resulted in the formation of tetraarylmethane derivatives with a quaternary centre in moderate to good yields in one pot.

Ag(I)-Catalyzed Tandem Reaction of Conjugated Ene-yne-ketones in the Presence of PhI(OAc)2 and Triethylamine: Synthesis of 2-Alkenylfurans

Silver-catalyzed tandem cyclization-elimination reactions of conjugated ene-yne-ketones in PhI(OAc)₂/triethylamine system lead to the formation of 2-alkenylfurans. 2-Furylsilver carbene and phenyliodonium ylide are proposed as the key intermediates in these transformations.

A facile iron-catalyzed dual C–C bond cleavage: an approach towards triarylmethanes

An efficient iron-catalyzed dual C–C bond cleaving reaction has been developed for the synthesis of triarylmethanes.

A one-pot metal-free protocol for the synthesis of chalcogenated furans from 1,4-enediones and thiols

Transition-metal-free synthesis of chalcogenated furans through the sequential thiol-Michael/Paal–Knorr reaction of 1,4-enediones in the presence of a catalytic amount of p-toluene sulfonic acid has been developed.

Palladium-catalyzed oxidative borylation of conjugated enynones through carbene migratory insertion: synthesis of furyl-substituted alkenylboronates

A new method for the synthesis of furyl-substituted alkenylboronates has been developed by palladium-catalyzed oxidative borylation reaction of conjugated enynones.

Pd-catalyzed Oxidative Cross-coupling of Alkyl Chromium(0) Fischer Carbene Complexes with Organoboronic Acids

Alkyl chromium(0) carbene complexes have been explored as the cross-coupling partners in the palladium-catalyzed reaction with aryl or alkenyl boronic acids. This coupling reaction displays the versatile reactivities of alkyl chromium(0) carbenes under palladium catalysis. Mechanistically, this transformation is proposed to involve deprotonation of the alkyl chromium carbene substrate to generate a vinyl chromium anion intermediate that undergoes transmetalation to organopalladium species and reductive elimination.

Copper-Catalyzed Intramolecular Annulation of Conjugated Enynones to Substituted 1 H-Indenes and Mechanistic Studies

Herein, a copper-catalyzed intramolecular cascade reaction of conjugated enynones to deliver substituted 1 H-indenes is reported. The inexpensive and less toxic copper salt served as the only catalyst in the transformation, affording the 3-(2-furyl)-substituted 1 H-indenes in good to excellent yields under mild reaction conditions with broad functional group tolerance and making it highly appealing for synthetic organic chemistry. Notably, detailed DFT calculations have been carried out to elucidate that the reaction undergoes a copper-mediated 5- exo-dig cyclization of enynones to afford copper-(2-furyl)-carbene intermediate, followed by diene-carbene cyclization (one step but involving 6π cyclization of Cu-carbene and reductive elimination) and 1,5-hydrogen shift to provide the 1 H-indenes.

Synthesis of Multisubstituted Furans via a Catalyst- and Additive-Free Tandem Reaction of Enynones with Sulfinic Acids in Water

A facile and efficient tandem reaction of enynones with arylsulfinic acids was developed. A variety of multisubstituted furans were obtained in satisfactory yields via an O₂-oxidative single-electron-transfer process without any catalyst and additive in water.

Acid-Promoted One-Pot Synthesis of Substituted Furan and 6-Methylpyrazin-2(1 H)-one Derivatives via Allene Intermediate Formed in Situ

Under the acidic conditions, substituted furans were constructed from γ-alkynyl ketones through corresponding allene intermediates in one-pot. The methodology was also tailored to a series of the Ugi reaction products for the synthesis of 6-methylpyrazin-2(1 H)-one derivatives. The current method offered significant advantages for the combinatorial applications of these chemical scaffolds.

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.

Directing group assisted ZnI2-catalyzed cyclopropanation of indoles via 2-furylcarbenoids

An efficient method for the synthesis of cyclopropane-fused indolines via a Zn₂I catalyzed cyclopropanation of indoles and enynones was reported.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C-C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C-C bond cleavage, and C-H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C-C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven to be general: various transition metals including Pd, Cu, Rh, Ni, Co, and Ir are effective catalysts; the scope of the reaction has also been extended to substrates other than diazo compounds; and various cascade processes have also been devised based on the carbene migratory insertion. This review will summarize the achievements made in this field since 2001.

A highly efficient one-pot trifluoromethylation/cyclization reaction of electron-deficient 1,3-conjugated enynes: modular access to trifluoromethylated furans and 2,3-dihydrofurans

A highly efficient one-pot trifluoromethylation/cyclization reaction of conjugated enyne aldehydes and ketones was developed, which provides modular access to highly substituted trifluoromethylated furans and 2,3-dihydrofurans.