Direct Production of Enaminones from Terminal Alkynes via Rhodium-Catalyzed Reaction of Formamides with N-Sulfonyl-1,2,3-triazoles

Catalyst-free reactions of anilines with β-chloroenones: synthesis of α-chloroenaminones and 1,4-benzodiazepines

The Michael addition of anilines to β-chloroenones gives enaminones by the elimination of hydrochloric acid (HCl). These enaminones are transformed into α-chloroenaminones via in situ sp2 C-H functionalization. Anilines that are attached to an electron-donating group react more readily with β-chloroenone to give the corresponding products in excellent yields. A highly atom-economical method has been developed using dimethyl sulfoxide (DMSO) as a green oxidant and solvent. The desired α-functionalized enaminones are formed in good yields with excellent Z-selectivity. We have established the generality of this reaction with many substrates, and scaled-up reactions have been performed to showcase the practical applications. A catalyst-free double annulation of β-chloroenones with o-phenylenediamine has also been demonstrated for the synthesis of 1,4-benzodiazepine derivatives in moderate yields under mild reaction conditions.

Rh(II)-Catalyzed Denitrogenative Reaction of N-Sulfonyl-1,2,3-triazoles with Quinolones and Isoquinolones

Herein, we developed an efficient approach to access biologically relevant 2-aminoquinolines and 1-aminoisoquinolines from readily available N-sulfonyl-1,2,3-triazoles and 2-quinolones or 1-isoquinolones. This transformation involves the selective O-H insertion of these derivatives onto the in situ generated Rh-azavinyl carbenes (Rh-AVC) followed by rearrangement. The reaction proceeds smoothly under operationally simple conditions and the protocol was found to be scalable.

Dirhodium-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles to 2,3-Dehydropiperazines

The dirhodium(II)-catalyzed synthesis of a range of C2-substituted 2,3-dehydropiperazines using 1-mesyl-1,2,3-triazoles and β-haloalkylcarbamates is reported. The reaction is proposed to proceed through an α-imino rhodium carbene 1,3-insertion into N-H followed by a base-mediated cyclization. C-Substituted dehydropiperazines can also be conducted directly from terminal alkynes in a three-step, one-pot operation, forming the triazole in situ. This methodology has also been expanded to afford several 2,5-disubstituted 2,3-dehydropiperazines as well as a larger 4,5,6,7-tetrahydro-1H-1,4-diazepine derivative.

Continuous Flow Synthesis of N-Sulfonyl-1,2,3-triazoles for Tandem Relay Cu/Rh Dual Catalysis

The continuous flow synthesis of N-sulfonyl-1,2,3-triazoles, which are convenient reactive azavinyl carbene precursors, for tandem relay Cu/Rh dual catalysis has been developed. Most reactions readily proceeded at 75 °C in a short residence time of 13.09 min in the presence of 2.5 mol % of CuTC. The scope of the reactions was explored by synthesizing diversely functionalized N-sulfonyl and sulfamoyl triazoles in yields ranging from 92 to 98%. To demonstrate the scalability of the process, the reaction was conducted on a 5.4 mmol scale with residence and collection times of 13.09 and 60 min, respectively. Furthermore, a series of controlled experiments were performed to investigate the compatibility of Cu and Rh in a batch or a continuous flow system. Finally, the first integrated flow system using the azavinyl carbene intermediate under the tandem relay Cu/Rh dual catalysis was developed for the synthesis of various cis-diamino enones from alkynes and sulfonyl azides.

Sustainable catalyst-free N-formylation using CO2 as a carbon source

The development of new sustainable catalytic conversion methods of carbon dioxide (CO2) is of great interest in the synthesis of valuable chemicals. N-formylation of CO2 with amine nucleophiles as substrates has been studied in depth. The key to benign formylation is to select a suitable reducing agent to activate CO2. This paper showcases the activation modes of CO2 and the construction strategies of sustainable and catalyst-free N-formylation systems. The research progress of catalyst-free N-formylation of amines and CO2 is reviewed. There are two broad prominent categories, namely reductive amidation of CO2 facilitated by organic solvents and ionic liquids in the presence of hydrosilane. Attention is also paid to discussing the involved reaction mechanism with practical applications and identifying the remaining challenges in this field.

Insights into the Mechanisms and Chemoselectivities of Carbamates and Amides in Reactions Involving Rh(II)-Azavinylcarbene: A Computational Study

Reactions involving Rh(II)-azavinylcarbenes (Rh(II)-AVCs) to synthesize nitrogen-containing compounds have attracted significant research interest. Despite the importance of these reactions, controlling the chemoselectivities in the reactions involving Rh(II)-AVC remains a challenge. To understand the mechanisms and factors controlling the chemoselectivities between N-H and C═O groups of carbamates and amides in reactions involving Rh(II)-AVC, computational studies were employed. The results reveal that not only the greater nucleophilicity of the N-H group than that of the carbonyl group, but also the presence of H-bonding interactions, could favor the addition of the N-H group of primary carbamates to Rh(II)-AVC. However, for secondary carbamates and amides, they could undergo either chemoselective N-H or C═O addition. Secondary carbamates with less steric hindrance, such as oxazolidinone, prefer the N-H addition mode. However, a switch in chemoselectivity (preference for the C═O addition) was revealed for the sterically hindered secondary carbamates/amides. In addition, a possible O-H addition pathway via the keto-enol tautomerization for isatin and isatoic anhydride was disregarded due to the energetically demanding barrier. Instead, a pathway involving the chemoselective C═O addition, formal [3 + 2] cycloaddition, followed by ring opening was proposed. The origins of the chemoselectivity and the factors responsible were addressed.

Interception of Secondary Amide Ylide with Sulfonamides: Catalyst-Controlled Synthesis of N-Sulfonylamidine Derivatives

A novel, secondary amide activation strategy has been developed through the in situ generation of ylides from amides and diazoacetates. Under the developed reaction conditions, Mn-catalyzed ylide formation and interception reaction by sulfonamide delivered a variety of N-sulfonylamidines. Notably, when highly active Zn(OTf)₂ was used as the catalyst, further N-H insertion products were obtained. In contrast with traditional methods, our amide activation strategy is distinguished by accessible starting material, inexpensive catalyst, and broad substrate scope.

Silver-catalyzed ring-opening [3+2] annulation of cyclopropenones with amides

A silver(i)-catalyzed ring-opening [3+2] annulation between cyclopropenones and amides has been achieved to afford 5-amino-2-furanones.

Facile synthesis of pyrroloindoles via a rhodium(ii)-catalyzed annulation of 3-benzylidene-indolin-2-ones and α-imino carbenes

A novel carbenoid strategy to construct pyrrole-fused indole alkaloids.

Carbon-Carbon Bond Formation by Reaction of Rhodium Azavinylcarbenes with Secondary Amides: Access to Indigo Analogues from Isatins

A novel protocol for the synthesis of unsymmetrical indigo-like (E)-α-amino enaminones by rhodium-catalyzed transformations of isatins with 1-tosyl-1,2,3-triazoles has been developed. A plausible reaction mechanism is proposed to account for the formation of structurally important unsymmetrical indigo analogues, which represents a new strategy for C-C bond formation based on the transformations of secondary amides and rhodium azavinylcarbenes.

Rh(II) Catalyzed High Order Cycloadditions of 8-Azaheptafulvenes with N-Sulfonyl 1,2,3-Triazloes or α-Oxo Diazocompounds

A novel strategy was developed for the application of Rh carbenes generated from readily accessible N-sulfonyl 1,2,3,-triazoles or diazocompouds in the high order cycloadditions, which offered an efficient route to a variety of N-containing medium-sized rings. The process provided a wide range of cyclohepta[b]pyrazine and cyclohepta[b]pyrrolone derivatives with high yields.

Rhodium-catalyzed denitrogenative transannulation of 1,2,3-triazolyl-carbamates: efficient access to 4-aminooxazolidinones

The rhodium(ii)-catalyzed denitrogenative transannulation reaction of 1,2,3-triazolyl-carbamates was described providing an efficient access to biologically interesting 4-aminooxazolidinones in excellent yields and selectivities.

Synthesis of Multifunctionalized 2-Carbonylpyrrole by Rhodium-Catalyzed Transannulation of 1-Sulfonyl-1,2,3-triazole with β-Diketone

A facile rhodium-catalyzed transannulation of 1-sulfonyl-1,2,3-triazoles with β-diketones was realized, and a series of multisubstituted 2-carbonylpyrroles were synthesized efficiently (up to 94% yield). The protocol features several advantages, such as readily available materials, mild reaction conditions, a concise operating procedure, a broad reaction scope, and excellent regioselectivity when benzoylacetone derivatives were used.

Base-induced synthesis of N-dialkylaminomethyl-2H-1,2,3-triazoles from N-sulfonyl-1,2,3-triazoles

A facile synthetic method to access N-dialkylaminomethyl-2H-1,2,3-triazoles has been developed from N-sulfonyl-1,2,3-triazoles by a base induced reaction. Moreover, 4-phenyl-1H-1,2,3-triazoles can also give the same product even in the absence of a base at high temperature.

Electrochemical synthesis of α-enaminones from aryl ketones

A novel approach to realize the synthesis of α-enaminones via electrochemical oxidation was developed under mild conditions.

Rhodium(II)-Catalyzed Annulation of Azavinyl Carbenes Through Ring-Expansion of 1,3,5-Trioxane: Rapid Access to Nine-Membered 1,3,5,7-Trioxazonines

The rhodium(II)-catalyzed denitrogenative coupling of N-alkylsulfonyl 1,2,3-triazoles with 1,3,5-trioxane led to nine-membered-ringed trioxazonines in moderate-to-good yields. 1,3,5-Trioxane, acting as an oxygen nucleophile, reacted with the α-aza-vinylcarbene intermediate, giving rise to ylide formation, which was probably the key step in the reaction. Triazoles that contained aryl substituents with various electronic and steric features on the C4 carbon atom were well-tolerated. The synthesis of trioxazonine derivatives was achieved through a one-pot, two-step procedure from 1-mesylazide and a terminal alkyne by combining Cu(I)-catalyzed 1,3-dipolar cycloaddition and rhodium-catalyzed transformations.

Rhodium-catalyzed denitrogenative thioacetalization of N-sulfonyl-1,2,3-triazoles with disulfides: an entry to diverse transformation of terminal alkynes

An efficient and useful rhodium-catalyzed denitrogenative thioacetalization of N-sulfonyl-1,2,3-triazoles has been developed for the first time. The protocol uses readily available N-sulfonyl-1,2,3-triazoles and diaryl disulfides as the starting materials. The corresponding hydrolytic and reductive products with thioacetals were obtained in good to excellent yields, and the reactions were carried out easily under mild conditions with tolerance of some functional groups. Furthermore, the generated thioacetals could be transformed into some useful compounds. Therefore, the present method provides a novel and valuable strategy for the diverse transformation of alkynes.

Intramolecular annulation of aromatic rings with N-sulfonyl 1,2,3-triazoles: divergent synthesis of 3-methylene-2,3-dihydrobenzofurans and 3-methylene-2,3-dihydroindoles

The controllable synthesis of 3-methylene-2,3-dihydrobenzofurans 2 and 3-methylene-2,3-dihydroindoles 5 has been developed via cycloisomerization of N/O-tethered aryltriazoles.

A Rh-catalyzed 1,2-sulfur migration/aza-Diels–Alder cascade initiated by aza-vinyl carbenoids from sulfur-tethered N-sulfonyl-1,2,3-triazoles

A novel Rh(ii) catalyzed intramolecular 1,2-sulfur migration/intermolecular aza-Diels–Alder cascade of sulfur-tethered N-sulfonyl-1,2,3-triazoles has been developed, efficiently affording sulfur-containing tetrahydropyridine derivatives.

Divergent synthesis of indole-fused polycycles via Rh(ii)-catalyzed intramolecular [3 + 2] cycloaddition and C–H functionalization of indolyltriazoles

Intramolecular formal [3 + 2] cycloaddition and C–H functionalization of N-sulfonyl triazoles with indoles have been developed to synthesize potential indole-fused polycycles.

Cu-catalyzed transannulation reaction of pyridotriazoles: general access to fused polycyclic indolizines

An efficient intramolecular transannulation reaction of pyridotriazoles using internal alkynes en route to various fused polycyclic indolizines has been developed. For the first time it is shown that in addition to the well-established Rh- or Cu-catalyzed carbene mechanism, the transannulation reaction could also follow a Lewis acid-mediated electrophilic pathway.

An efficient approach to 1,2,3-trisubstituted indole via rhodium catalyzed carbene Csp3–H bond insertion

A method for convenient synthesis of N-alkyl-2-aryl-indole-3-carbaldehyde via carbene C–H bond insertion.

Electrosynthesis of enaminones directly from methyl ketones and amines with nitromethane as a carbon source

An efficient and mechanistically different method for the electrosynthesis of enaminone directly from methyl ketones, amines and nitromethane was developed.

Rhodium-catalyzed NH insertion of pyridyl carbenes derived from pyridotriazoles: a general and efficient approach to 2-picolylamines and imidazo[1,5-a]pyridines

A general and efficient NH insertion reaction of rhodium pyridyl carbenes derived from pyridotriazoles was developed. Various NH-containing compounds, including amides, anilines, enamines, and aliphatic amines, smoothly underwent the NH insertion reaction to afford 2-picolylamine derivatives. The developed transformation was further utilized in a facile one-pot synthesis of imidazo[1,5-a]pyridines.