Rhodium-Catalyzed Synthesis of 4-Bromo-1,2-dihydroisoquinolines: Access to Bromonium Ylides by the Intramolecular Reaction of a Benzyl Bromide and an α-Imino Carbene

Copper-Catalyzed Synthesis of Furan-Tethered Benzocyclobutenes via Carbene-Mediated 1,4-Sulfinate Migration-Annulation

A copper-catalyzed intramolecular cascade reaction of conjugated enynones has been achieved via a pivotal 1,4-sulfinate migration step. This process leverages a cost-effective and ecofriendly copper salt as catalyst, enabling the efficient construction of five- and four-membered rings in a rapid, sequential manner, producing furan-tethered benzocyclobutenes in good to excellent yields under mild conditions. The reaction is characterized by 100% atom economy, outstanding efficiency, and excellent diastereoselectivity in the cases studied. The robustness of this method is evidenced by its compatibility with air exposure and the use of undistilled, commercially available solvents, further enhancing its practicality.

Catalyst-Free Synthesis of Polycyclic Spiroindolines by Cascade Reaction of 3-(2-Isocyanoethyl)indoles with 1-Sulfonyl-1,2,3-triazoles

A catalyst-free cascade reaction of 3-(2-isocyanoethyl)indoles and 1-sulfonyl-1,2,3-triazoles was realized. This dearomative spirocyclization provided an efficient protocol to synthesize a series of polycyclic indolines bearing spiro-α-carboline in moderate to high yields in one step under thermal reaction conditions.

Substituent-Controllable Cascade Regioselective Annulation of β-Enaminones with N-Sulfonyl Triazoles for Modular Access to Imidazoles and Pyrroles

A controllable synthesis of trisubstituted imidazoles and pyrroles has been developed through rhodium(II)-catalyzed regioselective annulation of N-sulfonyl-1,2,3-trizaoles with β-enaminones. The imidazole ring was formed through a 1,1-insertion of the N-H bond to α-imino rhodium carbene, followed by a subsequent intramolecular 1,4-conjugate addition. This occurred when the α-carbon atom of the amino group was bearing a methyl group. Additionally, the pyrrole ring was constructed by utilizing a phenyl substituent and undergoing intramolecular nucleophilic addition. The mild conditions, good tolerance towards functional groups, gram-scale synthesis capability, and ability to undergo valuable transformations of the products qualify this unique protocol as an efficient tool for the synthesis of N-heterocycles.

Rhodium-Catalyzed Transannulation of 4,5-Fused 1-Sulfonyl-1,2,3-triazoles with Nitriles. The Selective Formation of 1-Sulfonyl-4,5-fused Imidazoles versus Secondary C-H Bond Migration

The reactivity of readily available 4,5-fused-1-sulfonyl-1,2,3-triazoles was examined in the Rh(II)-catalyzed transannulation reaction with nitriles. We have come across the interesting observation that 1-sulfonyl cycloalkeno[d][1,2,3]triazoles that possess β-hydrogens resist intramolecular β-hydride migration and could serve as a new source of Rh-iminocarbenoids for intermolecular Rh(II)-catalyzed transannulation reactions. As a result, 1-sulfonyl cyclohexeno-, cyclohepteno-, dihydropyrano-, 5-phenyltetrahydrobenzo-, and 4,5-dihydronaphtho[d]imidazoles were synthesized from various nitriles in good yields. A one-pot methodology has also been executed for the synthesis of NH-imidazoles.

Synthesis of Azepane Derivatives via Formal 1,3-Migration of Hydroxy and Acyloxy Groups and Selective Annulation

Formal 1,3-migration of hydroxy and acyloxy groups initiated by α-imino rhodium carbene was achieved, and the following selective annulations of the corresponding zwitterions could efficiently afford azepane derivatives. Benefiting from a time-saving procedure as well as a good functional group tolerance, this unique migration-annulation protocol could provide an efficient tool for synthesizing seven-membered N-heterocycles. The plausible mechanism is discussed.

Synthesis of α-Amino Cyclobutanones via Formal 1,3-Hydroxy Migration Triggered by Formation of α-Imino Rhodium Carbene

Formal intramolecular 1,3-OH migration of α-imino carbene was achieved producing a unique zwitterion, and the subsequent selective annulation afforded α-amino cyclobutanone. Features such as readily available substrates, mild reaction conditions, a time-saving procedure, excellent functional group compatibility, and valuable transformations of the products qualified this unique protocol as an efficient tool for the synthesis of strained cyclic compounds. Density functional theory calculations were in good agreement with experimental observations, and a plausible mechanism is presented.

Access to tetrahydrocarbazoles and pyrrolo[3,4-b]carbazoles through sequential reactions of triazoles and indoles

Tetrahydrocarbazoles and pyrrolo[3,4-b]carbazoles could be synthesized conveniently through sequential reactions of ester-tethered 1-sulfonyl-1,2,3-triazoles and indoles. The reaction conditions were mild and the procedures were quite simple. Moreover, the key intermediate α,β-unsaturated imine acted as a [2C] synthon in the [4 + 2] cycloaddition reaction, and the imino group could be used as a nucleophile to construct the fourth ring.

1,2-Acyl migration with α-imino rhodium carbenoids leading to substituted 1-naphthols

A unique method for the synthesis of substituted 1-naphthols by a rhodium(II)-catalysed ring-expansion reaction of 2-triazolyl-1-indanone derivatives is reported. 1,2-Acyl migration occurs with an intermediate α-imino rhodium carbenoid generated from the triazolyl moiety.

Synthesis of dihydropyrroles from in situ-generated zwitterions via Rh2(adc)4/TBAI dual catalysis

A convenient rhodium/iodide dual catalysis strategy for the regioselective synthesis of dihydropyrroles was established.

Acid-catalyzed ring-expansion of 4-(1-hydroxycyclobutyl)-1,2,3-triazoles

A direct ring-opening/semipinacol rearrangement reaction catalyzed by the Brønsted acid of 4-(1-hydroxycyclobutyl)-1,2,3-triazole is reported, in which the usage of metal catalysts and the reservation of sulfonyl groups are avoided.

Access to (Z)-β-Substituted Enamides from N1-H-1,2,3-Triazoles

A direct ring-opening/nucleophilic substitution reaction of N1-H-1,2,3-triazoles has been described. Divergent (Z)-β-halogen- or sulfonyl-substituted enamides could be stereospecifically synthesized in a tunable manner. This strategy might not only enable a new ring-opening method of N1-H-1,2,3-triazoles under nonmetal catalysis and mild reaction conditions but also offer a good opportunity to reliably access versatile (Z)-β-substituted enamides that could be used as synthetic precursors for further synthetic transformations.

Synthesis of benzothiazonine by rhodium-catalyzed denitrogenative transannulation of 1-sulfonyl-1,2,3-triazole and thiochromone

A facile synthesis of multi-functionalized benzothiazonine was achieved by the rhodium-catalyzed denitrogenative annulation of 1-sulfonyl-1,2,3-triazole and thiochromone. In view of the excellent atom economy, broad substrate scope and easy availability of starting materials, the protocol provided an efficient strategy for the construction of medium N,S-heterocycles.

Two reaction modes of 1-sulfonyl-1,2,3-triazoles and pyridinium 1,4-zwitterionic thiolates: catalyst-free synthesis of pyrido[1,2-a]pyrazine derivatives and 1,4-thiazine derivatives

Efficient catalyst-free synthesis of pyrido[1,2-a]pyrazine and 1,4-thiazine derivatives from triazole and pyridinium 1,4-zwitterionic thiolate was realized.

The catalyst-free decarboxylative dearomatization of isoquinolines with β-keto acids and sulfonyl chlorides in water: access to dihydroisoquinoline derivatives

An efficient and concise catalyst-free one-pot synthetic protocol for obtaining dihydroisoquinoline derivatives has been developed via the three-component condensation of isoquinolines with β-keto acids and sulfonyl chlorides. This transformation involving decarboxylative dearomatization worked well under mild and water-mediated conditions. The protocol tolerates diverse functional groups, furnishing the dihydroisoquinoline products in good to excellent yields.

Rhodium (II)-Catalyzed Synthesis of Tetracyclic 3,4-Fused Indoles and Dihydroindoles

An efficient synthetic method of tetracyclic 3,4-fused indoles and dihydroindoles via rhodium-catalyzed (3+2) cycloaddition of N-tosyl-4-(2-phenoxyphenyl)-1,2,3-triazole was described. The aromatized xanthene derivatives can be achieved in a one-pot synthesis starting from 1-ethynyl-2-phenoxybenzene. The xanthene-based fused heterocycles were considered as the valuable fluorophore.

TMSCl-Catalyzed Tandem Reaction of Dihydroisobenzofuran Acetals with Indoles

A TMSCl-catalyzed tandem reaction of dihydroisobenzofuran acetals with indoles has been developped, which could provide an efficient and straightforward access to various tetrahydroisoquinolones in moderate to excellent yields. This process involved the first addition of the indoles to acetals, followed by skeletal rearrangement.

Efficient synthesis of cyclic amidine-based fluorophores via 6π-electrocyclic ring closure

Novel 10π-electron cyclic amidines with excellent fluorescence properties were synthesized by a general and efficient 6π-electrocyclic ring closure of ketenimine and imine starting from N-sulfonyl triazoles and arylamines. The photophysical properties of cyclic amidine fluorophores have been studied in detail and have shown good properties of a large Stokes shift, pH insensitivity, low cytotoxicity and higher photostability, which have great potential for biological imaging. Furthermore, this novel fluorophore was successfully applied to the localization of the NK-1 receptor in living systems.

Novel 10π-electron cyclic amidines with excellent fluorescence properties were synthesized by a general and efficient 6π-electrocyclic ring closure of ketenimine and imine starting from N-sulfonyl triazoles and arylamines.

Synthesis of α-cyano sulfone via thermal rearrangement of 1,4-disubstituted triazole mediated by carbene and radical species

α-Cyano sulfone is produced conveniently from 1,4-disubstituted triazole, which involves carbene, ketenimine and radical species as the key intermediates.

Synthesis of unsymmetrical urea derivatives via one-pot sequential three-component reactions of cyclic 2-diazo-1,3-diketones, carbodiimides, and 1,2-dihaloethanes

An effective and operationally simple one-pot strategy has been developed for the synthesis of unsymmetrical urea derivatives via sequential three-component reactions of cyclic 2-dizao-1,3-diketones with carbodiimides and 1,2-dihaloethane.

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.

Mechanistic studies for dirhodium-catalyzed chemoselective oxidative amination of alkynyl-tethered sulfamates

Theoretical studies revealed the detailed mechanisms for dirhodium-datalyzed chemoselective oxidative amination of alkynyl-tethered sulfamates and account for the origin of product selectivity.

Diastereoselective access to 2-aminoindanonesviathe rhodium(ii)-catalyzed tandem reaction involving O–H insertion and Michael addition

A rhodium(ii)-catalyzed tandem reaction involving O–H insertion ofN-sulfonyl 1,2,3-triazoles and Michael addition has been described. A series of 2-amino-1-indanones were afforded in acceptable to good yields with high diastereoselectivities.

Rhodium(ii)-catalyzed annulation of N-sulfonyl-1,2,3-triazoles with 1,3,5-triazinanes to produce octahydro-1H-purine derivatives: a combined experimental and computational study

The Rh(ii)-catalyzed annulation of N-sulfonyl-1,2,3-triazoles with 1,3,5-triazinanes leading to the octahydro-1H-purine derivatives with moderate to good yields is described in this communication.

Synthesis of α-aminoketones from N-sulfonyl-1,2,3-triazoles via N-sulfinyl imines generated by intramolecular oxygen transfer

A sulfinyl imine was generated by intramolecular oxygen transfer of an α-iminocarbene, which led to an α-aminoketone via nucleophilic addition and oxidation.

Rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones with 1-sulfonyl-1,2,3-triazole: synthesis of cyclopropa[cd]indole and benzofuran derivatives

A novel rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones and 1-sulfonyl-1,2,3-triazole is disclosed, affording cyclopropa[cd]indole and benzofuran derivatives in moderate to good yields.

Synthesis of [3a,7a]-Dihydroindoles by a Tandem Arene Cyclopropanation/3,5-Sigmatropic Rearrangement Reaction

Donor/acceptor carbenes provide a powerful platform for building molecular complexity, but the majority of their reactions have been limited to aryl and vinyl donor groups. We found that a N-containing donor/acceptor carbene precursor, 4-phthalimido- N-methanesulfonyl-1,2,3-triazole, reacts with unactivated arenes resulting in a mixture of [3+2]-cycloadducts, [3a,7a]-dihydroindoles, and formal C-H functionalization products in up to 82% yield upon heating. We also demonstrate that the formal C-H functionalization products arise from ring-opening of the [3+2]-cycloadducts. Computational studies suggest that the formal cycloaddition process takes places through a tandem arene cyclopropanation/6π electrocyclization/6π electrocyclic ring-opening/3,5-sigmatropic rearrangement reaction, which also accounts for the distinctive regioselectivity of the formal cycloaddition reaction.

The Expanding Utility of Rhodium-Iminocarbenes: Recent Advances in the Synthesis of Natural Products and Related Scaffolds

Rhodium-iminocarbenes that are derived from N-sulfonyl-1,2,3-triazoles have become an important class of reactive species and useful intermediates in organic synthesis. Over the last several years, many practical and versatile approaches involving rhodium-iminocarbene intermediates to synthetically challenging molecules (scaffolds) have been developed. This Minireview mainly summarizes the recent advance of rhodium-iminocarbene involved reactions in the synthesis of natural products and their related scaffolds by the end of 2017. Several applications in important pharmaceuticals are documented as well.

The Divergent Cascade Reactions of Arylalkynols with Homopropargylic Amines or Electron-Deficient Olefins: Access to the Spiro-Isobenzofuran- b-pyrroloquinolines or Bridged-Isobenzofuran Polycycles

Two divergent cascade reactions of arylalkynols with homopropargylic amines or electron-deficient olefins were developed to synthesize the spiro-isobenzofuran- b-pyrroloquinolines or bridged-isobenzofuran heterocycles in good yields, respectively. One reaction actually involved intramolecular 5- endo-dig hydroamination cyclization-protonation of homopropargylic amines to give cycloiminium ions and intramolecular 5- exo-dig hydroalkoxylation cyclization of arylalkynols to generate isobenzofuran with exocyclic double bond, followed by the nontypical Povarov-type reaction in the presence of PtCl₂/FeCl₃ cocatalysts. The other underwent intramolecular hydroalkoxylation cycloisomerization of alkynols with the subsequent normal [4 + 2] cycoaddition with dienophiles. Herein, the arylalkynols acted as both "masked" electron-rich olefins and "masked" electron-rich dienes.