Synthesis of Carbazoles from 2-Iodobiphenyls by Palladium-Catalyzed C−H Activation and Amination with Diaziridinone

Synthesis of Functionalized 4-Hydroxy Carbazoles and Carbazole Alkaloids via Ring Expansion of Indole Cyclopentanone

The exploration of a ring expansion reaction from indole cyclopentanone to generate a range of diversely functionalized 4-hydroxyl carbazole frameworks, representing the core structure of numerous carbazole alkaloids, has been conducted under mild reaction conditions. This approach exhibits broad functional group tolerance and moderate to good yields. The practical applicability of this strategy has been demonstrated through the concise syntheses of carbazomycins A, D, and G.

Pd-catalyzed three-component [2 + 2 + 1] cycloamination toward carbazoles

Conventional approaches using hydroxylamine derivatives as single nitrogen sources for the preparation of N-heterocyclic molecules rely on two chemical processes involving sequential nucleophilic and electrophilic C-N bond formations. Herein, we report a novel Suzuki reaction/C-H activation/amination sequence for building a myriad of carbazoles in a single transformation using bifunctional secondary hydroxylamines. It is noteworthy that the synthetic utility of this methodology is highlighted by the total synthesis of clausine V and glycoborine by incorporating the title [2 + 2 + 1] cycloamination as the key step. Control experiments were performed to gain a better understanding of the reaction mechanism.

Synthesis of Fluoren-9-ones via Pd-Catalyzed Annulation of 2-Iodobiphenyls with Vinylene Carbonate

A palladium-catalyzed reaction for intermolecular selective C-H cyclocarbonylation of 2-iodobiphenyls is described. Intriguingly, the vinylene carbonate acts as a carbon monoxide transfer agent to enable the annulation reaction. Moreover, as a versatile synthon, fluoren-9-one can be transformed into a variety of functionalized organic molecules, such as [1,1'-biphenyl]-2-carboxylic acid, 1'H,3'H-spiro[fluorene-9,2'-perimidine] and N-tosylhydrazones.

Pd-Catalyzed Aryl C-H Amination with Diaziridinone

This work describes an efficient Pd-catalyzed ortho-C-H amination of N-(quinolin-8-yl)benzamides with di-t-butyldiaziridinone, providing a variety of anthranilic amides in good yields. The reaction likely involves the formation of a pallada(II)heterocycle via aryl C-H activation and subsequent amination with di-t-butyldiaziridinone.

Intermolecular Difunctionalization of C,C-Palladacycles Obtained by Pd(0)-Catalyzed C-H Activation

C,C-Palladacycles are an important class of organometallic compounds in which palladium is σ-bonded to two carbon atoms. They have three notable features that make them attractive in organic synthesis and organometallic chemistry: (1) C,C-Palladacycles are reactive intermediates that can be accessed via Pd(0)-catalyzed C-H activation of organic halides. Compared to Pd(II)-catalyzed heteroatom-directed C-H activation, C-H activation catalyzed by Pd(0) has some distinct advantages. In this type of catalytic reaction, the halo groups of readily available organic halides act as traceless directing groups. Furthermore, this strategy avoids the use of stoichiometric external oxidants. (2) C,C-Palladacycles have differentiated reactivities from common open-chain Pd(II) species. In particular, C,C-palladacycles have high reactivity toward electrophiles including alkyl halides. This unique reactivity can be utilized to develop novel reactions. (3) C,C-Palladacycles have two C-Pd bonds, providing a unique platform for developing novel reactions.Although a number of reactions of C,C-palladacycles had been developed prior to our work, the scope was largely limited to intramolecular cyclization reactions. Although Catellani reactions are intermolecular reactions of C,C-palladacycles, only one of the C-Pd bonds is functionalized. Our laboratory has sought to develop intermolecular difunctionalization reactions of C,C-palladacycles that exploit their unique reactivity and open new possibilities in organic synthesis. Aiming to develop synthetically useful reactions, we primarily focus on ring-forming reactions. In this Account, we summarize our laboratory's efforts to exploit intermolecular difunctionalization reactions of C,C-palladacycles that are obtained through Pd(0)-catalyzed C-H activation. We have developed a wide array of new reactions that represent facile and efficient methods for the synthesis of cyclic organic compounds, including functional materials and drug molecules. A range of C,C-palladacycles have been studied, including C(aryl),C(aryl)-palladacycles from 2-halobiaryls, C(aryl),C(alkyl)-palladacycles from ortho-iodo-tert-butylbenzenes or ortho-iodoanisole derivatives, and those obtained by cascade reactions. C,C-Palladacycles have been found to react with a variety of oxidants to furnish Pd(IV) intermediates, such as alkyl halides, aryl halides, diazo compounds, and N,N-di-tert-butyldiaziridinone, ultimately affording various cyclic structures, including 5-10-membered rings, carbo- and azacycles, spirocycles, and fused rings. Furthermore, novel reactivity of C,C-palladacycles has been discovered. For example, we found that C,C-palladacycles have unusually high reactivity toward disilanes, which can be leveraged to disilylate a variety of C,C-palladacycles with very high efficiency. These results should provide inspiration to develop other C-Si bond-forming reactions in the future. We hope that this Account will stimulate further research into the rich chemistry of C,C-palladacycles, in particular reactions that find practical applications in the synthesis of bioactive and functional molecules and those that advance the state of the art in C-H functionalization.

Lewis Acid-Catalyzed 2,3-Dihydrofuran Acetal Ring-Opening Benzannulations toward Functionalized 1-Hydroxycarbazoles

The development of a Lewis acid-catalyzed, intramolecular ring-opening benzannulation of 5-(indolyl)2,3-dihydrofuran acetals is described. The resulting 1-hydroxycarbazole-2-carboxylates are formed in up to 90% yield in 1 h. The dihydrofuran acetals are readily accessed from the reactions of enol ethers and α-diazo-β-indolyl-β-ketoesters. To highlight the method's synthetic utility, a formal total synthesis of murrayafoline A, a bioactive carbazole-containing natural product, was undertaken.

Construction of carbazole-based unnatural amino acid scaffolds via Pd(II)-catalyzed C(sp3)-H functionalization

We report the synthesis of carbazole-based unnatural α-amino acid and non-α-amino acid derivatives via a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided β-C(sp³)-H activation/functionalization method. Various N-phthaloyl, DL-, L- and D-carboxamides derived from their corresponding α-amino acids, non-α-amino acids and aliphatic carboxamides were subjected to the β-C(sp³)-H functionalization with 3-iodocarbazoles in the presence of a Pd(II) catalyst to afford the corresponding carbazole moiety installed unnatural amino acid derivatives and aliphatic carboxamides. Carbazole motif-containing racemic (DL) and enantiopure (L and D) amino acid derivatives including phenylalanine, norvaline, leucine, norleucine and 2-aminooctanoic acid with anti-stereochemistry and various non-α-amino acid derivatives including GABA have been synthesized. Removal of the 8-aminoquinoline directing group, deprotection of the phthalimide moiety and the preparation of carbazole amino acid derivatives containing free amino- and carboxylate groups are shown. The carbazole motif is prevalent in alkaloids and biologically active molecules and functional materials. Thus, this work on the synthesis of carbazole-based unnatural amino acid derivatives would enrich the libraries of unnatural amino acid derivatives and carbazoles.

Palladium-catalyzed triple coupling of 2-iodoanisoles with aryl iodides to access 6H-dibenzopyrans

A palladium-catalyzed triple coupling of 2-iodoanisoles with aryl iodides has been developed. 3-Methyl-2-pyridone was used as a ligand to accelerate the cross-coupling and suppress the homo-coupling of 2-iodoanisoles. A variety...

Decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy to access dibenzo[a,c]carbazoles

A novel decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy for the assembly of dibenzo[a,c]carbazoles has been reported.

Palladium-catalyzed coupling reaction of 2-iodobiphenyls with alkenyl bromides for the construction of 9-(diorganomethylidene)fluorenes

An atom economical protocol for the construction of 9-(diorganomethylidene)fluorenes through palladium-catalyzed coupling reactions of 2-iodobiphenyls with alkenyl bromides has been reported. The reaction proceeds through the C-H activation/oxidative addition/reduction elimination/intramolecular Heck coupling reaction to afford a series of 9-(diorganomethylidene)fluorenes with good yields. Control experiments demonstrate that a five-membered palladacycle acts as a key intermediate and β-H elimination serves as the rate-limiting step.

Bay-Region Annulative π-Extension of o-Iodobiphenyls with Aliphatic Anhydrides Catalyzed by Pd(OAc)2

Bay-region annulative π-extension of o-iodobiphenyls with aliphatic anhydrides was developed. Many o-iodobiphenyls and aliphatic anhydrides can react well under the optimized conditions. A lot of phenanthrol derivatives can be efficiently prepared by this approach. The control experiments support that dibenzopalladacyclopentadienes may be the reaction intermediates.

Pd-Catalyzed Indole Synthesis via C-H Activation and Bisamination Sequence with Diaziridinone

This work describes an efficient Pd-catalyzed indole synthesis. A wide variety of indoles can be obtained in good yields from readily available vinyl bromides. The reaction likely proceeds through a sequential aryl C-H activation and bisamination of a resulting pallada(II)cycle with diaziridinone.

Synthesis of Tribenzo[b,d,f]azepines via Palladium-Catalyzed Annulation Reaction of 2-Iodobiphenyls with 2-Halogenoanilines

A palladium-catalyzed annulation reaction of 2-iodobiphenyls with 2-halogenoanilines has been developed. A variety of 2-iodobiphenyls and 2-halogenoanilines can undergo this transformation. Diversified tribenzo[b,d,f]azepine derivatives can be synthesized in moderate to excellent yields according to this method.

Palladium-Catalyzed Sequential C-H Activation/Amination with Diaziridinone: An Approach to Indoles

Indoles are an important class of molecules. This paper describes an efficient palladium-catalyzed synthesis of indoles from 2-iodostyrenes and di-t-butyldiaziridinone with a simultaneous installation of two C-N bonds. The reaction process likely proceeds through the oxidative insertion of Pd to aryl iodide and subsequent vinyl C-H activation to from a pallada(II)cycle intermediate, which is bisaminated by di-t-butyldiaziridinone to give the indole product.

Domino C-N Bond Formation via a Palladacycle with Diaziridinone. An Approach to Indolo[3,2-b]indoles

Palladium-catalyzed C-N bond formation is one of the widely used transformations for the synthesis of structurally diverse N-heterocycles. This work describes an efficient palladium-catalyzed multiple-C-N bond formation reaction for the synthesis of highly π-conjugated N-heterocycles, indolo[3,2-b]indoles with di-tert-butyldiaziridinone. The reaction likely proceeds through the initial formation of an indole-fused palladacycle by nucleophilic aminopalladation and subsequent bisamination to give indolo[3,2-b]indoles.

A tris-spiro metalla-aromatic system featuring Craig-Möbius aromaticity

As aromaticity is one of the most fundamental concepts in chemistry, the construction of aromatic systems has long been an important subject. Herein, we report the synthesis and characterization of a tris-spiroaromatic complex, hexalithio spiro vanadacycle 2. The delocalization of the four electrons within the two V 3d orbitals and the π* orbitals of the three biphenyl ligands leads to a 40π Craig-Möbius aromatic system with three metalla-aromatic rings, as revealed by both experimental measurements and theoretical analyses. For comparison, if Cr is used instead of V, a similar Craig-Möbius aromatic system can not be generated. In this case, pentalithio spiro chromacycle 3 is obtained, and the Cr center uses its two 3d orbitals to form two independent metalla-aromatic rings. This work presents a type of aromatic systems that will contribute to both aromaticity theory and organometallic chemistry.

Spiroaromatic compounds are advantageous platforms for designing expanded aromatic systems. Herein, the authors present a tris‐spiro metalla‐aromatic Vanadium compound which forms a 40π Craig‐Möbius aromatic system.

Palladium catalyzed annulation of 2-iodobiphenyl with a non-terminal alkene enabled by neighboring group assistance

In this work, an efficient palladium catalyzed annulation of 2-iodobiphenyl with a non-terminal alkene was developed. The key factor in this transformation was the formation of a highly reactive oxo-palladacycle intermediate, which was enabled by a neighboring hydroxyl group, and remarkably restrained the β-H elimination process. Mechanistically, control experiments demonstrated that the hydroxyl group may act as an anionic ligand, which was irreplaceable in this reaction. This transformation presented good reactivity and selectivity, and no simple Heck coupling products were detected for all of the explored substrates.

C–H hetero-functionalization of arenes through palladacyclopentane-type intermediates

In this review article, we summarized recent advances in C–H hetero-functionalization of arenes through palladacyclopentane-type intermediates.

Palladium-Catalyzed [2 + 2 + 1] Annulation of Alkyne-Tethered Aryl Iodides with Diaziridinone: Synthesis of 3,4-Fused Tricyclic Indoles

A novel palladium-catalyzed [2 + 2 + 1] annulation of alkyne-tethered aryl iodides with diaziridinone was developed, leading to the formation of 3,4-fused tricyclic indoles. From a mechanistic standpoint, the formation of fused tricyclic indole scaffolds involved C,C-palladacycles, which were synthesized through the intramolecular reaction of aryl halides and alkynes. The cascade reaction described herein could be carried out with a broad range of substrates and provided various 3,4-fused tricyclic indoles with yields up to 98%.

Synthesis of 3,4-Fused Tricyclic Indoles through Cascade Carbopalladation and C-H Amination: Development and Total Synthesis of Rucaparib

3,4-Fused tricyclic indole scaffolds are ubiquitous in bioactive natural products and pharmaceuticals. A new protocol for the synthesis of 3,4-fused tricyclic indoles has been developed through cascade carbopalladation and C-H amination with N,N-di-tert-butyldiaziridinone. The protocol allows access to a range of 3,4-fused tricyclic indoles, including those containing various linkers and fused with medium-sized rings. Rucaparib can be synthesized via this reaction, providing an advantageous synthetic method for the FDA-approved cancer medicine.

Hydroxylamines As Bifunctional Single-Nitrogen Sources for the Rapid Assembly of Diverse Tricyclic Indole Scaffolds

Conventional approaches on using hydroxylamine derivatives as single nitrogen sources, for the construction of n-membered (n > 3) N-heterocycles, rely upon two chemical operations by involving sequential nucleophilic and electrophilic C-N bond formations. Here, we report a highly efficient cascade of alkyne insertion/C-H activation/amination for the rapid preparation of a myriad of tricyclic indoles, in a single-step transformation, by using bifunctional secondary hydroxylamines. It is noteworthy that judicious selection of applicable amino agents, for enabling the prior oxidative addition of aryl iodide to initial Pd(0) species and subsequent two C-N bonds formation, was the key to the success of this reaction. Control experiments indicated that a five-membered palladacyclic intermediate played a crucial role in promoting the final aminative ring closure.

Palladium-catalyzed C–H bond activation for the assembly of N-aryl carbazoles with aromatic amines as nitrogen sources

A convenient and efficient palladium-catalyzed C–H bond activation for the assembly of N-aryl carbazole is reported, in which two C–N bonds were formed under one set of conditions.

Mixed er-NHC/phosphine Pd(ii) complexes and their catalytic activity in the Buchwald–Hartwig reaction under solvent-free conditions

A single catalyst for solvent-free Buchwald–Hartwig amination with both primary and secondary amines.

Ruthenium-catalyzed ortho-selective CAr–H amination of heteroaryl arenes with di-tert-butyldiaziridinone

Application of an oxidative amination reagent (di-tert-butyldiaziridinone) to the Ru₃(CO)₁₂-catalyzed ortho-selective C_Ar–H amination reaction is described.

Synthesis of Indoles through Palladium-Catalyzed Three-Component Reaction of Aryl Iodides, Alkynes, and Diaziridinone

The three-component reaction of aryl iodides, alkynes, and diaziridinone is described. The reaction provides an innovative synthetic approach for indoles. The approach features high efficiency, broad substrate scope, and excellent regioselectivity. C, C-Palladacycles should act as the intermediates. The C, C-palladacycles are obtained from simple aryl halides and alkynes and then reacted with diaziridinone to afford indoles.

Recent Progress in the Transition Metal Catalyzed Synthesis of Indoles

Indole is the most frequently found heterocyclic core structures in pharmaceuticals, natural products, agrochemicals, dyes and fragrances. For about 150 years, chemists were absorbed in finding new and easier synthetic strategies to build this nucleus. Many books and reviews have been written, but the number of new syntheses that appear in the literature, make necessary continuous updates. This reviews aims to give a comprehensive overview on indole synthesis catalyzed by transition metals appeared in the literature in the years 2016 and 2017.