Gold-catalyzed ketene dual functionalization and mechanistic insights: divergent synthesis of indenes and benzo[d]oxepines

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

Catalytic Asymmetric Difluoroalkylation Using In Situ Generated Difluoroenol Species as the Privileged Synthon

A robust and practical difluoroalkylation synthon, α,α-difluoroenol species, which generated in situ from trifluoromethyl diazo compounds and water in the presence of dirhodium complex, is disclosed. As compared to the presynthesized difluoroenoxysilane and in situ formed difluoroenolate under basic conditions, this difluoroenol intermediate displayed versatile reactivity, resulting in dramatically improved enantioselectivity under mild conditions. As demonstrated in catalytic asymmetric aldol reaction and Mannich reactions with ketones or imines in the presence of chiral organocatalysts, quinine-derived urea, and chiral phosphoric acid (CPA), respectively, this relay catalysis strategy provides an effective platform for applying asymmetric fluorination chemistry. Moreover, this method features a novel 1,2-difunctionalization process via installation of a carbonyl motif and an alkyl group on two vicinal carbons, which is a complementary protocol to the metal carbene gem-difunctionalization reaction.

Gold-catalyzed carbocyclization and imidization of alkyne-tethered diazo compounds with nitrosoarenes for the synthesis of nitrones and naphthalene derivatives

A gold-catalyzed carbocyclization/imidization cascade reaction has been developed,leading a facile access to the synthesis of functionalized nitrones in moderate to good yields under mild conditions. The reaction initiated by a catalytic 6-endo-dig diazo-yne carbocyclization to form the key endocyclic vinyl carbene from alkyne-tethered diazo compounds, followed by addition with nitrosoarenes that features an imidization process. Notably, these resulting nitrone products could be smoothly converted into different substituted naphthalenol analogues, such as 4-aminonaphthalen-1-ol, naphthalene-1,4-dione, and naphthalene-1,4-diol derivatives, in high yields. Moreover, the generated products exhibited potential tumor suppression activity in tested cancer celllines; compound 3c (HCT116 cells, IC50 = 7.41 μM; MCF-7 cells, IC50 = 14.28 μM) exhibits higher anticancer potency than other tested compounds.

Gold(I)-Catalyzed Substitution-Controlled Syntheses of Spiro[indoline-3,3'-pyrrolidine] and Spiro[indoline-3,3'-piperidine] Derivatives

Spiro[indoline-3,3'-pyrrolidine] and spiro[indoline-3,3'-piperidine] derivatives were synthesized in a substitution-controlled manner under the catalysis of cationic gold(I) species in the presence of Hantzsch ester (HEH). The optimal reaction condition was determined by screening, and the functional group tolerances of these two pathways were examined by readily synthetic substrates. The endo and exo selectivities of these cyclizations were elucidated by density functional theory calculations, and a plausible mechanism for these transformations was proposed.

Direct Access to Trifluoromethylated Benzo[d]oxepines from o-Alkynylaryl Aldehydes and Trifluorodiazoethane

Reported in this Letter is a silver-catalyzed reaction between o-alkynylaryl aldehydes and trifluorodiazoethane that enables an expedient synthesis of trifluoromethylated benzo[d]oxepines. The reaction works through a silver-promoted 6-endo-dig cyclization of o-alkynylbenzaldehydes for the generation of an isochromenylium intermediate, which upon a ring-expansive addition of trifluorodiazoethane delivers a novel class of trifluoromethylated benzoxepine frameworks. This strategy was applied to the synthesis of phosphonylated benzo[d]oxepines using the Seyferth-Gilbert reagent.

Computational insight into gold(I)-catalyzed intramolecular regioselectivity of tryptamine-ynamide cycloisomerizations

The regioselectivity for gold(I)-catalyzed intramolecular cycloisomerizations of tryptamine-ynamides has long been elusive despite various synthetic examples of similar substrates being available. Computational studies were carried out to provide insight into the mechanisms and the origin of the substrate-dependent regioselectivity of these transformations. Based on the analyses of non-covalent interactions, distortion/interaction, and energy decomposition on the interactions between the terminal substituent of alkynes and the gold(I) catalytic ligand, the electrostatic effect was determined to be the key factor for α-position selectivity while the dispersion effect was determined to be the key factor for β-position selectivity. Our computational results were consistent with the experimental observations. This study provides useful guidance for understanding other similar gold(I)-catalyzed asymmetric alkyne cyclization reactions.

Divergent Construction of N-Doped Polycyclic Aromatic Hydrocarbons with Indole as the Nitrogen Source Building Block

An Ag/Au-catalyzed divergent cascade reaction of alkyne embedded diazoketones with indoles has been described. Preliminary mechanistic studies indicate that the reaction goes through a [4+2]-cycloaddition of an in situ formed isobenzopyrylium intermediate with indole, followed by a sequential retro-Michael addition/carbene N-H insertion process to give the benzo[i]phenanthridines products with gold catalysis; whereas a dearomatization/rearomatization sequence occurs favourably when the reaction is catalyzed by a silver catalyst, delivering benzo[b]carbazoles in generally high to excellent yields. Notably, this is a rare example of using indole as the dienophile for cycloaddition with the isobenzopyrylium species, providing a concise and practical approach for the selective construction of N-doped polycyclic aromatic hydrocarbons (PAHs) with structural diversity and broad functional-group compatibility.

Gold-Catalyzed Alkyne Multifunctionalization through an Oxidation-Oxyalkylation-Aryloxylation Sequence

A gold-catalyzed oxidative three-component reaction of terminal alkynes with alcohols and quinone monoimines has been disclosed, affording α-ketoacetals in good to excellent yields. By using quinone monoimines as electrophiles for the interception of the in situ generated gold enolate intermediate, this one-pot process provides an unprecedented method for the polyfunctionalization of terminal alkynes through an oxidation-oxyalkylation-aryloxylation sequence, installing three oxygen atoms on the C-C triple bond.

Recent Advances in Gold(I)-Catalyzed Approaches to Three-Type Small-Molecule Scaffolds via Arylalkyne Activation

Gold catalysts possess the advantages of water and oxygen resistance, with the possibility of catalyzing many novel chemical transformations, especially in the syntheses of small-molecule skeletons, in addition to achieving the rapid construction of multiple chemical bonds and ring systems in one step. In this feature paper, we summarize recent advances in the construction of small-molecule scaffolds, such as benzene, cyclopentene, furan, and pyran, based on gold-catalyzed cyclization of arylalkyne derivatives within the last decade. We hope that this review will serve as a useful reference for chemists to apply gold-catalyzed strategies to the syntheses of related natural products and active molecules, hopefully providing useful guidance for the exploration of additional novel gold-catalyzed approaches.

Cu-Catalyzed Three-Component Cascade Synthesis of 1,3-Benzothiazines from ortho-Aminohydrazones and Bromodifluoroacetamides

An efficient and convenient synthesis of benzo[d][1,3]thiazine has been developed by employing a copper-catalyzed transformation of readily available ketone-derived hydrazones with elemental sulfur and bromodifluoroalkylative reagents. The strategy involves an S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides, which acts as a C1 synthon at the 2-position of benzo[d][1,3]thiazine. A mechanism proceeding through a Cu-carbene intermediate is proposed for the C-S bond formation.

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.

An asymmetric oxidative cyclization/Mannich-type addition cascade reaction for direct access to chiral pyrrolidin-3-ones

An efficient gold and chiral phosphoric acid cooperatively catalyzed enantioselective oxidative cyclization/Mannich-type addition reaction of homopropargyl amides with nitrones has been developed, which provides chiral pyrrolidin-3-ones in high yields with excellent enantioselectivities under mild conditions. This reaction employed stable and readily available alkynes as non-diazo carbene precursors, which provides a 100% atom economy method with high bond formation efficiency.

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.

Stereoselective copper-catalyzed heteroarene C-H functionalization/Michael-type annulation cascade with α-diazocarbonyls

A stereoselective, copper-catalyzed, arene C(sp²)-H functionalization/Michael-type annulation reaction involving α-diazocarbonyl compounds has been developed. The method features low catalyst loadings, high yields, and excellent regio and stereoselectivity, in the synthesis of various heteroaromatic frameworks by employing indoles as the arene partner.