Regioselective Electrooxidative [3+2] Annulation between Indole and Aniline Derivatives to Construct Functionalized Indolo[2,3-b]indoles

A protocol for the electrooxidative [3+2] annulation to generate indolo[2,3-b]indoles in an undivided cell is reported. It exhibits good yields with excellent regioselectivities and tolerates various functional groups without external chemical oxidants. Cyclic voltammetry and density functional theory calculations indicate that the [3+2] annulation is initiated by the simultaneous anodic oxidation of indole and aniline derivatives, and the step to determine the rate relies on the combination of radical cations.

Substrate-directed divergent synthesis of fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones

Herein, we report a substrate-directed diverse synthetic strategy toward two kinds of structurally intriguing fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones with enaminones. The subtle structural changes in enaminones lead to different reactivities. Cyclic enaminones produce indolo[2,3-b]indoles, whereas acyclic ones afford oxazolo[3,2-a]indoles.

Recent Advances towards the Synthesis and Material Applications of Indoloindoles

An important class of N -heteroacenes is indoloindoles which are air-stable, electron-rich and possesses many tuneable properties. Initially, indoloindoles were explored for potential biological applications but current interest is based on their performance as photovoltaic materials. With growing applications of indoloindoles across multiple facets as organic functional materials, the need for efficient methods to synthesize and functionalize indoloindoles has taken a centre stage. Over the years, synthetic routes leading to indoloindoles have evolved from multistep protocols to one-pot multicomponent synthesis. Present literature boasts of a variety of reports that employ metals such as Cu, Ru, Rh, Pd, or Au to mediate the reaction towards indoloindoles. As alternatives to such metal-mediated methods, researchers have also developed metal-free and catalyst-free conditions. Indoloindoles, which are fundamentally fused-indoles, are often synthesized by transforming indole derivatives but methods that employ anilines or arynes as the starting substrates are equally abundant. The present review highlights the rich diversity and versatility of recent literature for the synthesis of indolo[3,2- b ]indoles, indolo[2,3- b ]indoles, indolo[7,6- g ]indoles, and indolo[5,4- e ]indoles. This review discusses protocols that were explicitly designed to obtain the above-mentioned indoloindoles and also explores several other methods that can be adapted to access said heteroacenes. Available mechanistic details pertaining to novel transformations have been detailed for the readers. Various applications where indoloindoles function as organic light-emitting diodes, organic field-effect transistors, solar cells, etc. have also been delved into before concluding with an outlook on future research.

Synthesis of polysubstituted pyrroles via isocyanide-based multicomponent reactions as an efficient synthesis tool

The present review covers all synthetic methods based on isocyanide-based multicomponent reactions for the preparation of polysubstituted pyrroles as the parent cores of many essential drugs, biologically active compounds, and compounds with wide application in materials science.

Straightforward access to novel indolo[2,3-b]indoles via aerobic copper-catalyzed [3+2] annulation of diarylamines and indoles

Herein, we report an unprecedented aerobic copper-catalyzed [3+2] annulation reaction of diarylamines with indoles, which allows direct access to novel 2-diarylaminoindolo[2,3-b]indoles, a class of potential photoelectric device molecules. The developed transformation proceeds with broad substrate scope, good functional group tolerance, high chemo-selectivity, and no need for pre-preparation of specific agents, which offers a practical route for diverse and atom-economic synthesis of the desired products that are difficult to prepare with the conventional approaches.

New efficient synthesis of benzofuro[2,3-b]pyrroles utilizing a reactive nitrilium trapping approach by an acid-promoted cascade addition/cyclization sequence

The first p-TSA·H₂O-catalyzed highly efficient one-pot cascade reaction for the synthesis of novel fused tricyclic benzofuro[2,3-b]pyrroles has been discovered.

Divergent synthesis of N-heterocycles via controllable cyclization of azido-diynes catalyzed by copper and gold

Gold-catalyzed intermolecular alkyne oxidation by an N–O bond oxidant has proven to be a powerful method in organic synthesis during the past decade, because this approach would enable readily available alkynes as precursors in generating α-oxo gold carbenes. Among those, gold-catalyzed oxidative cyclization of dialkynes has received particular attention as this chemistry offers great potential to build structurally complex cyclic molecules. However, these alkyne oxidations have been mostly limited to noble metal catalysts, and, to our knowledge, non-noble metal-catalyzed reactions such as diyne oxidations have not been reported. Herein, we disclose a copper-catalyzed oxidative diyne cyclization, allowing the facile synthesis of a wide range of valuable pyrrolo[3,4-c]quinolin-1-ones. Interestingly, by employing the same starting materials, the gold-catalyzed cascade cyclization leads to the divergent formation of synthetically useful pyrrolo[2,3-b]indoles. Furthermore, the proposed mechanistic rationale for these cascade reactions is strongly supported by both control experiments and theoretical calculations.

Fused N-heterocycles are structural motifs observed in natural products and bioactive compounds. Here, the authors developed divergent copper- and gold-catalyzed oxidative cyclizations of diynes to two types of tricyclic N-heterocycles and rationalized the product selectivity by theoretical calculations.

Exploiting Pincer Ligands to Perturb the Geometry at Boron

Boranes are ubiquitous in synthesis and materials but advancements in their development have been primarily restricted to the geometric energetic minima, trigonal planar complexes. This report discloses a class of boranes with expanded bond angles achieved by taking advantage of the structural rigidity of tridentate pincer ligands. The bonding of these novel boranes is investigated by X-ray crystallography and computationally.

Metal-free C–H sulfonamidation of pyrroles by visible light photoredox catalysis

Blue light and catalytic amounts of an acridinium dye are sufficient for the C–H sulfamidation of pyrroles.

Iodine mediated intramolecular C2-amidative cyclization of indoles: a facile access to indole fused tetracycles

A novel and metal-free I₂-mediated intramolecular C2 amidation of indoles under mild reaction conditions is developed for synthesis of indole fused tetracycles.

A Pd(ii)-catalyzed C–H activation approach to densely functionalized N-heteroaromatics related to neocryptolepine and their evaluation as potential inducers of apoptosis

Bioactive indoloquinolines are synthesized via Pd(ii)-catalyzed intramolecular oxidative C–H alkenylation as the key step.

Regioselective Csp2–H dual functionalization of indoles using hypervalent iodine(iii): bromo-amination via 1,3-migration of imides on indolyl(phenyl)iodonium imides

A regioselective C_sp2–H dual functionalization of indoles, which underlies bromo-amination via the 1,3-migration of imide groups on indolyl(phenyl)iodonium imides, has been developed to provide 2-bis(sulfonyl)amino-3-bromo-indoles under the metal-free conditions.

Creation of molecular complexities via a new Cu-catalyzed cascade reaction: a direct access to novel 2,2′-spirobiindole derivatives

A Cu-mediated cascade reaction of cyclopenta[b]indoles furnished an array of 2,2′-spirobi[indolin]-3-one based novel and complex molecules.

Pd-catalyzed isocyanide insertion/nucleophilic attack by indole C-3/desulfonylation in the same pot: a direct access to indoloquinolines of pharmacological interest

A Pd-catalyzed new cascade reaction furnished indoloquinolin based small molecules as potential cytotoxic agents.

KO(t)Bu-mediated aerobic transition-metal-free regioselective β-arylation of indoles: synthesis of β-(2-/4-nitroaryl)-indoles

A KO(t)Bu-mediated intermolecular oxidative C-C coupling of nitroarenes with indoles is presented in DMSO at room temperature in an open flask. By using this mild and economical methodology, syntheses of β-(2/4-nitroaryl)-indoles with sensitive functionalities such as bromo, iodo, cyano, and nitro were achieved chemo- and regioselectively. Synthesized β-(2/4-nitroaryl) indoles were transformed into densely functionalized biindoles, indoloindoles, and (4-aminoaryl)-indoles which demonstrate post-transformation utility of the developed methodology.