Wolff Rearrangement of Oxidatively Generated α-Oxo Gold Carbenes: An Effective Approach to Silylketenes

Reactivities of α-Oxo BMIDA Gold Carbenes Generated by Gold-Catalyzed Oxidation of BMIDA-Terminated Alkynes

An oxidative strategy is reported to access α-oxo BMIDA gold carbenes directly from BMIDA-terminated alkynes. Besides offering expedient access to seldom studied boryl metal carbenes, these BMIDA gold carbene species undergo facile insertions into methyl, methylene, methine, and benzylic C-H bonds in the absence of the Thorpe-Ingold effect. They also undergo efficient OH insertion, cyclopropanation, and F-C alkylations. This chemistry provides rapid access to structurally diverse α-BMIDA ketones, which are scarcely documented. In combination with DFT studies, the role of BMIDA is established to be an electron-donating group that attenuates the high electrophilicity of the gold carbene center.

Highly Site-Selective Oxidative Cyclization of Ene-ynamides via Non-Noble-Metal Catalysis: Access to Functionalized Lactams

Herein, an unprecedented non-noble-metal-catalyzed oxidation/cyclization of ene-ynamides is developed, allowing the synthesis of diversely functionalized lactams in moderate to good yields with excellent diastereoselectivities without the observation of typical cyclopropanation products. In combination with Ellman's tert-butylsulfinimine chemistry, chiral γ-lactams containing three contiguous stereocenters are obtained with high diastereo- and enantioselectivity. Moreover, density functional theory (DFT) calculations indicate that this protocol probably undergoes a carbon cation or proton transfer process.

Reactions of thioalkynes with diarylketenes via [3+2]-annulation versus benzannulation using Au and P(C6F5)3 catalysts

Two catalytic annulations of non-symmetric diarylketenes with thioalkynes are described using gold and phosphine catalysts respectively. We employed α-aryldiazo ketones to generate gold-π-ketenes, ultimately yielding azulen-1-one derivatives. With the same reactants, we utilized P(C₆F₅)₃ to increase the yields of 1-naphthols, notably with a complete regioselectivity.

Gold-Catalyzed Regioselective Synthesis of Crowded Cyclopentadienes by Migratory Cycloisomerization of Vinylallenes

We report the regioselective synthesis of silyl-substituted cyclopentadienyl esters through gold-catalyzed migratory cycloisomerization of silyl-substituted vinylallenes. This transformation is proposed to proceed through a perfectly orchestrated sequence of events including Nazarov-like cyclization and several silyl and hydrogen rearrangements. Furthermore, exploiting the multifaceted nature of the gold catalyst, we have also identified suitable conditions for the synthesis of these cyclopentadienes in a more straightforward manner through gold-catalyzed reaction of propargyl esters and alkynylsilanes.

Gold self-relay catalysis for accessing functionalized cyclopentenones bearing an all-carbon quaternary stereocenter

A new gold(i) self-relay catalysis consisting of a 3,3-rearrangement, Nazarov cyclization and Michael addition cascade of 1,3-enyne acetates with aurones and their derived azadienes is reported, producing functionalized cyclopentenones.

Efficient synthesis of tetracyclic γ-lactams via gold-catalyzed oxidative cyclization of alkenyl diynes

An efficient gold-catalyzed cascade cyclization of alkenyl diynes involving alkyne oxidation, carbene-alkyne metathesis and cyclopropanation has been developed, furnishing a series of tetracyclic γ-lactams bearing one quaternary carbon center and...

Silver-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds to construct polysubstituted furans

An efficient synthesis of polysubstituted furans by Ag(i)-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds is reported.

Theoretical insights into the mechanism and origin of chemoselectivity in the catalyst- and directing group-dependent oxidative cyclization of diynes with pyridine N-oxides

The mechanisms and origin of the chemoselectivity of Cu(i)- and Au(i)-catalyzed oxidation of diynes for the divergent syntheses of two different N-heterocycles, substituted pyrroles and dihydroindeno[1,2-c]pyrrol-3(2H)-ones, were elucidated using density functional theory.

Cascade Wolff Rearrangement/Acylation: A Metal-Free and Eco-Friendly Approach for 4-Hydroxy-pyrazolo[3,4-b]pyridin-6-ones and N-Pyrazole Amides Synthesis from 5-Aminopyrazoles and α-Diazoketones

A highly chemoselective cascade Wolff rearrangement/acylation reaction between 5-aminopyrazoles and diazo compounds has been developed. The protocol can facilitate the switchable synthesis of 4-hydroxy-pyrazolo[3,4-b]pyridin-6-ones and N-pyrazole amides with the merits of a broad substrate scope, high functional group compatibility, and green and sustainable performance manner. All reactions proceeded efficiently without any catalyst and additives (acid and base) and resulted in the release of benign N₂, wherein diethyl carbonate served as a green benign solvent.

Phosphine-Catalyzed Annulations Based on [3+3] and [3+2] Trapping of Ketene Intermediates with Thioamides

With the aim of developing novel annulations via ketene intermediates, allenyl imide and alkynoates bearing good leaving groups are used for their function in a tandem conjugate addition-elimination reaction (SN2' type) promoted by nucleophilic phosphine catalysts. By utilizing thioamides as 1S,3N-bis-nucleophiles, [3+3] and [3+2] annulations have been established to allow rapid access to 1,3-thiazin-4-ones and 5-alkenyl thiazolones in high yields, respectively. Furthermore, the possible reaction mechanisms are proposed on the basis of deuterium labeling experiments and density functional theory calculations.

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.

Biomimetic Carbene Cascades Enabled Imine Derivative Migration from Carbene-Bearing Thiocarbamates

Inspired by the body circulation of Omeprazole (irreversible proton pump inhibitor), we disclose the carbene-triggered cascades for the synthesis of 2-aminobenzofuran derivatives from N-sulfonyl-1,2,3-triazoles or benzothioazole-bearing thiocarbamates, which represents an unprecedented imine derivative migration process. Furthermore, the desulfurizing reagent-free Barton-Kellogg-type reactions starting from N-sulfonyl-1,2,3-triazoles have also been achieved for the first time, and elemental sulfur is confirmed as a byproduct during this transformation. Both experimental data and DFT calculations further thoroughly explained the unique reactivity.

Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes

We have prepared and characterized a series of unprecedented group 6-group 11, N2-bridged, heterobimetallic [ML4(η1-N2)(μ-η1:η1-N2)Au(NHC)]+ complexes (M = Mo, W, L2 = diphosphine) by treatment of trans-[ML4(N2)2] with a cationic gold(I) complex [Au(NHC)]+. The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M-N-N-Au motif. To verify how far the analogy goes, we computed the electronic structures of [W(depe)2(η1-N2)(μ-η1:η1-N2)AuNHC]+ (10W+) and [W(depe)2(η1-N2)(μ-η1:η1-N2)B(C6F5)3] (11W). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N2 with a d orbital of the group 6 metal overlaps in 10W+ with an empty sd hybrid orbital at gold, whereas in 11W with an sp3 hybrid orbital at boron. The bent N-N-LA arrangement maximizes these interactions, providing a similar level of N2 "push-pull" activation in the two compounds. In the gold case, the HOMO-2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the μ-N2-[Au(NHC)]+ bond.

Gold(I)-Catalyzed Oxidative 1,4-Additions of 3-En-1-ynamide with Nitrones via Carbon- versus Nitrogen-Addition Chemoselectivity

This work reports gold-catalyzed 1,4-oxofunctionalizations of 3-en-1-ynamides with nitrones, yielding two distinct E-configured products. We obtained 1,4-oxoarylation products from 3-en-1-ynamides bearing C(4)-electron-donating substituents and 1,4-oxoamination products from those analogues bearing C(4)-aryl substituents. We propose that if vinylgold carbenes are stable, imines undergo a para-arylation on these gold carbenes. If vinylgold carbenes are highly electron-deficient, this N-attack is irreversible to enable 1,4-oxoaminations.

Synthesis of 4-amino-5-allenylisoxazoles via gold(I)-catalysed propargyl aza-Claisen rearrangement

Propargyl aza-Claisen rearrangement of 4-propargylaminoisoxazoles 1 proceeded in the presence of cationic gold(i) catalysts to give 4-amino-5-allenylisoxazoles 2 in good to high yields. The silyl group at the terminal alkyne and a cationic gold(i) catalyst bearing a sterically bulky ligand are essential for the generation of isolable allene intermediates. The N-protection of the generated 4-amino-5-allenylisoxazoles 2 allowed the isolation of 5-allenylisoxazoles 4 that have never been synthesized. N-Propargyl aniline 5 was successfully converted to the corresponding ortho-allenyl aniline 6 under the current reaction conditions.

Gold-Catalyzed Oxidative Aminocyclizations of Propargyl Alcohols and Propargyl Amines to Form Two Distinct Azacyclic Products: Carbene Formation versus a 3,3-Sigmatropic Shift of an Initial Intermediate

Gold-catalyzed oxidations of propargyl alcohols with nitrones by using a P(tBu)₂ (o-biphenyl)Au⁺ catalyst, afforded bicyclic annulation products from the Mannich reactions of gold enolates. The same reactions of propargyl amines with nitrones by using the same gold catalyst gave distinct oxoarylation products. Our DFT calculations indicate that oxidation of propargyl alcohols with nitrones by using electron-rich gold catalysts lead only to gold carbenes, which can generate gold enolates or oxoarylation intermediates with enolate species having a barrier smaller than that of oxoarylation species.

Cu(I)-Catalyzed Oxidative Cyclization of Enynamides: Regioselective Access to Cyclopentadiene Frameworks and 2-Aminofurans

An efficient Cu(I)-catalyzed oxidative cyclization of alkynyl-tethered enynamides for the construction of fused bicyclic cyclopentadiene derivatives is disclosed. The cascade proceeds through alkyne oxidation, carbene/alkyne metathesis, and formal (3 + 2) cycloaddition. Employing aryl-tethered enynamides as starting materials, substituted 2-aminofurans can be exclusively formed.

Non-Diazo C-H Insertion Approach to Cyclobutanones through Oxidative Gold Catalysis

Cyclobutanones are synthetically versatile compounds that often require extensive effort to access. Herein, we report a facile synthesis of cyclobutanones based on the C(sp³ )-H insertion chemistry of oxidatively generated gold carbenes. Various cyclobutanones were obtained in synthetically useful yields from substrates with minimal structural prefunctionalization. This discovery reveals new synthetic utilities of gold-catalyzed oxidative transformations of alkynones.

Nitrene Transfer and Carbene Transfer in Gold Catalysis

Catalytic transformations involving metal carbenes are considered one of the most important aspects of homogeneous transition metal catalysis. Recently, gold-catalyzed generation of gold carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. This Review summarizes the advances in the gold-catalyzed nitrene-transfer reactions of alkynes with nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils, and gold-catalyzed carbene-transfer reactions, involving oxygen atom-transfer reactions of alkynes with nitro compounds, nitrones, sulfoxides, and pyridine N-oxides, through the presumable α-imino gold carbene and α-oxo gold carbene intermediates, respectively. Gold-catalyzed processes are reviewed by highlighting their product diversity, selectivity, and applicability, and the mechanistic rationale is presented where possible.

Recent Progress in N-Heterocyclic Carbene Gold-Catalyzed Reactions of Alkynes Involving Oxidation/Amination/Cycloaddition

Recent rapid development in homogeneous gold catalysis affords an alternative and particularly thriving strategy for the generation of gold carbenes through gold-catalyzed oxidation/amination/cycloaddition of alkynes, while it avoids the employment of hazardous and potentially explosive diazo compounds as starting materials for carbene generation. In addition to facile and secure operation, gold carbenes generated in this strategy display good chemoselectivity distinct from other metal carbenes produced from the related diazo approach. N-heterocyclic carbene (NHC) gold is a special metal complex that can be used as ancillary ligands, which provides enhanced stability and can also act as an efficient chiral directing group. In this review, we will present an overview of these recent advances in alkyne oxidation/amination/cycloaddition by highlighting their specificity and applicability, aiming to facilitate progress in this very exciting area of research.

Gold-Catalyzed Oxidation of Thioalkynes To Form Phenylthio Ketene Derivatives via a Noncarbene Route

Gold-catalyzed oxidations of thioalkynes with 8-methylquinoline oxides afford 2-phenylthioketenes that can be trapped efficiently with alcohols. The synthetic utility is manifested by terminal and internal thioalkynes over a wide scope, bearing esters, ketones, alkyl, and oxime substituents. Our density functional theory calculations suggest that gold-catalyzed oxidations of terminal and internal thioalkynes with 8-methylquinoline oxides generate gold-bound ketene intermediates without the intermediacy of α-oxo gold carbene.