Copper-mediated intramolecular aza-Wacker-type cyclization of 2-alkenylanilines toward 3-aryl indoles

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Ir(III)-Catalysed electrooxidative intramolecular dehydrogenative C-H/N-H coupling for the synthesis of N-H indoles

Herein, an iridium(III)-catalysed electrooxidative intramolecular dehydrogenative C-H/N-H coupling of unprotected 2-alkenyl anilines is described. The developed method allows the synthesis of a variety of 3-substituted N-H indole scaffolds under undivided electrolytic conditions. Mechanistic studies suggest that the reaction proceeds through the electro-oxidation induced reductive elimination pathway.

Synthesis and Antimycobacterial Activity of 3-Phenyl-1H-indoles

Tuberculosis has been described as a global health crisis since the 1990s, with an estimated 1.4 million deaths in the last year. Herein, a series of 20 1H-indoles were synthesized and evaluated as in vitro inhibitors of Mycobacterium tuberculosis (Mtb) growth. Furthermore, the top hit compounds were active against multidrug-resistant strains, without cross-resistance with first-line drugs. Exposing HepG2 and Vero cells to the molecules for 72 h showed that one of the evaluated structures was devoid of apparent toxicity. In addition, this 3-phenyl-1H-indole showed no genotoxicity signals. Finally, time-kill and pharmacodynamic model analyses demonstrated that this compound has bactericidal activity at concentrations close to the Minimum Inhibitory Concentration, coupled with a strong time-dependent behavior. To the best of our knowledge, this study describes the activity of 3-phenyl-1H-indole against Mtb for the first time.

Electrocatalytic Dehydrogenative Cyclization of 2-Vinylanilides for the Synthesis of Indoles

Indole is prevalent in bioactive compounds and natural products. The development of efficient and sustainable methods to access this privileged structural scaffold has been a long-standing interest of synthetic chemists. Herein, we report an electrocatalytic method for the synthesis of indoles through dehydrogenative cyclization of 2-vinylanilides. The reactions employ an organic redox catalyst and do not require any external chemical oxidant, providing speedy and efficient access to 3-substituted and 2,3-disubstituted indoles.

Metal-free and regiospecific synthesis of 3-arylindoles

A convenient, metal-free, and organic acid-base promoted synthetic method to prepare 3-arylindoles from 3-aryloxirane-2-carbonitriles and arylhydrazine hydrochlorides has been developed. In the reaction, the organic acid catalyzes a tandem nucleophilic ring-opening reaction of aryloxiranecarbonitriles and arylhydrazine hydrochlorides and Fischer indolization. The organic base triethylamine plays a crucial role in the final elimination step in the Fischer indole synthesis, affording 3-arylindoles regiospecifically. The reaction features advantages of microwave acceleration, non-metal participation, short reaction time, organic acid-base co-catalysis, and broad substrate scope.

Copper-Catalyzed Aerobic 6-Endo-Trig Cyclization of β,γ-Unsaturated Hydrazones for the Divergent Synthesis of Dihydropyridazines and Pyridazines

A divergent synthetic strategy to 1,6-dihydropyridazines and pyridazines through Cu(II)-catalyzed controllable aerobic 6-endo-trig cyclization was developed. The selectivity can be rationally tuned via the judicious choice of reaction solvent. It was found that the 1,6-dihydropyridazines were obtained in moderate to high yields with CH₃CN as the reaction solvent, whereas employment of AcOH directly afforded pyridazines in up to 92% yields, probably arising from the oxidation of the in situ generated 1,6-dihydropyridazines.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Intramolecular Palladium-Catalyzed Oxidative Amination of Furans: Synthesis of Functionalized Indoles

Unconventional modification of palladium-catalyzed oxidative amination where a furan ring serves as a masked olefin is described. The designed chemical process provides 2-(2-acylvinyl)indole derivatives with up to a 93% yield and excellent E-selectivity. A highly reactive α,β-unsaturated carbonyl moiety of the obtained compounds allows for accessing various heteroaromatic scaffolds through simple synthetic procedures.

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.

Photoredox Cyanomethylation of Indoles: Catalyst Modification and Mechanism

The direct cyanomethylation of indoles at the 2- or 3-position was achieved via photoredox catalysis. The versatile nitrile synthon is introduced as a radical generated from bromoacetonitrile, a photocatalyst, and blue LED as a light source. The mechanism of the reaction is explored by determination of the Stern-Volmer quenching constants. By combining photophysical data and mass spectrometry to follow the catalyst decomposition, the catalyst ligands were tuned to enable synthetically useful yields of radical coupling products. A range of indole substrates with alkyl, aryl, halogen, ester, and ether functional groups participate in the reaction, affording products in 16-90% yields. The reaction allows the rapid construction of synthetically useful cyanomethylindoles, products that otherwise require several synthetic steps.

Synthesis of topologically constrained naphthalimide appended palladium(ii)–N-heterocyclic carbene complexes – insights into additive controlled product selectivity

Pd(ii)–NHC catalyzed regioselective heteroannulation of o-haloanilines and arylethynyl-trimethylsilane to yield indoles and additive controlled switchable product selectivity has been demonstrated.

Electronic effect of substituents on anilines favors 1,4-addition totrans-β-nitrostyrenes: access toN-substituted 3-arylindoles and 3-arylindoles

A simple and an efficient method for the regioselective synthesis ofN-alkyl/aryl/H 3-arylindole derivatives fromN-substituted anilines andtrans-β-nitrostyrenes has been described using 10 mol% of bismuth(iii) triflate as a catalyst in acetonitrile at 80 °C.

Stereoselective and Regioselective Synthesis of Heterocycles via Copper-Catalyzed Additions of Amine Derivatives and Alcohols to Alkenes

This Perspective describes the development of a family of copper(II)-catalyzed alkene difunctionalization reactions that enable stereoselective addition of amine derivatives and alcohols onto pendant unactivated alkenes to provide a range of valuable saturated nitrogen and oxygen heterocycles. 2-Vinylanilines and related substrates undergo alternative oxidative amination or allylic amination pathways, and these reactions will also be discussed. The involvement of both polar and radical steps in the reaction mechanisms have been implicated. Major product formation is a function of the lowest energy pathway, which in turn is a function of structural aspects of the various reaction components.