Synthesis of 3-Fluoroindoles via Photoredox Catalysis

One-Pot Modified Madelung Synthesis of 3-Tosyl- and 3-Cyano-1,2-disubstituted Indoles

A One-pot, two-step procedure for the synthesis of 1,2-disubstituted-3-tosyl and 1,2-disubstituted-3-cyanoindoles from the corresponding N-(o-tolyl)benzamides is reported. The developed procedure is operationally simple, does not utilize any transition metals, and provides variably substituted indoles in good yields from readily available starting materials.

Synthesis of Medicinally Important Indole Derivatives: A Review

Indoles constitute a widely occurring functional group in nature and are present in an extensive number of bioactive natural products and medicinally important compounds. As a result, exponential increases in the development of novel methods for the formation of indole core along with site-specific indoles have been established. Conventional methods for the synthesis of indoles are getting replaced with green methods involving ionic liquids, water as a solvent, solid acid catalyst, microwave irradiation and the use of nanoparticles under solvent-free conditions. In addition, there are immense applications of the substituted indoles in diverse fields.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Alkene homologation via visible light promoted hydrophosphination using triphenylphosphonium triflate

A hydrophosphination reaction of alkenes with triphenylphosphonium triflate under photocatalytic conditions is described. The reaction is promoted by naphthalene-fused N-acylbenzimidazole and is believed to proceed through intermediate formation of a phosphinyl radical cation. The resulting phosphonium salts are directly involved in the Wittig reaction leading to homologated alkenes.

Ruthenium-Catalyzed C–H Activations for the Synthesis of Indole Derivatives

The synthesis of substituted indoles has received great attention in the field of organic synthesis methodology. C–H activation makes it possible to obtain a variety of designed indole derivatives in mild conditions. Ruthenium catalyst, as one of the most significant transition-metal catalysts, has been contributing in the synthesis of indole scaffolds through C–H activation and C–H activation on indoles. Herein, we attempt to present an overview about the construction strategies of indole scaffold and site-specific modifications for indole scaffold via ruthenium-catalyzed C–H activations in recent years.

A novel photoredox-active group for the generation of fluorinated radicals from difluorostyrenes

A 4-tetrafluoropyridinylthio group was suggested as a new photoredox-active moiety. The group can be directly installed on difluorostyrenes in a single step by the thiolene click reaction. It proceeds upon visible light catalysis with 9-phenylacridine providing various difluorinated sulfides as radical precursors. Single electron reduction of the C–S bond with the formation of fluoroalkyl radicals is enabled by the electron-poor azine ring. The intermediate difluorinated sulfides were involved in a series of photoredox reactions with silyl enol ethers, alkenes, nitrones and an alkenyl trifluoroborate.

A new photoredox-active group was applied for the generation of fluorinated radicals from difluorostyrenes under blue light irradiation.

Photoredox Reaction of 2-Mercaptothiazolinium Salts with Silyl Enol Ethers

A method for the generation of free radicals from thiazolinium salts upon photocatalytic reduction is described. The thiazolinium salts are generated by treatment with methyl triflate of 2-mercaptothiazolines, which can be readily obtained from alkyl bromides and tosylates via a nucleophilic substitution reaction or by hydrothiolation of alkenes. Silyl enol ethers were used to trap the radicals, furnishing ketones after successive single-electron oxidation and elimination of the silyl cation.

Regioselective Pd-catalyzed direct C1- and C2-arylations of lilolidine for the access to 5,6-dihydropyrrolo[3,2,1-ij]quinoline derivatives

The Pd-catalyzed C–H bond functionalization of lilolidine was investigated. The use of a palladium-diphosphine catalyst associated to acetate bases in DMA was found to promote the regioselective arylation at α-position of the nitrogen atom of lilolidine with a wide variety of aryl bromides. From these α-arylated lilolidines, a second arylation at the β-position gives the access to α,β-diarylated lilolidines containing two different aryl groups. The one pot access to α,β-diarylated lilolidines with two identical aryl groups is also possible by using a larger amount of aryl bromide. The synthesis of 5,6-dihydrodibenzo[a,c]pyrido[3,2,1-jk]carbazoles from lilolidine via three successive direct arylations is also described. Therefore, this methodology provides a straightforward access to several lilolidine derivatives from commercially available compounds via one, two or three C–H bond functionalization steps allowing to tune their biological properties.

Shielding Effect of Micelle for Highly Effective and Selective Monofluorination of Indoles in Water

Highly selective direct monofluorination of indoles and arenes was developed through an approach that allows site-specific solubility of substrate and fluorine source in the micelle. This approach was highly selective for a broad range of substrates with excellent functional group tolerance. Differences in binding constant and solubility of indoles and arenes in the micelle allowed the fine-tuning of selectivity. Control experiments suggested a radical pathway and provided insight into the role of micelles of the environmentally benign amphiphile PS-750-M. Dynamic light scattering experiments strongly indicated the site-specific solubility of the substrate and fluorine source. The methodology was successfully adapted to gram scale, and the E-factor established from a recycle study indicated that the process is environmentally responsible and sustainable.

Progress and prospects in the use of photocatalysis for the synthesis of organofluorine compounds

Data on the synthesis of fluorinated organic compounds by photocatalysis are systematically considered and analyzed. The attention is focused on the mechanisms of photocatalytic reactions and the selectivity problem.

The bibliography includes 173 references.

Visible light-mediated difluoroalkylation of electron-deficient alkenes

A method for the reductive difluoroalkylation of electron-deficient alkenes using 1,1-difluorinated iodides mediated by irradiation with blue light is described. The reaction involves radical addition of 1,1-difluorinated radicals at the double bond followed by hydrogen atom transfer from sodium cyanoborohydride.

Difluorocarbene as a Building Block for Consecutive Bond-Forming Reactions

Compounds containing a difluoromethylene unit have gained increasing attention due to their utility in drug design. Classic methods for the synthesis of these compounds rely on either harsh deoxofluorination reactions or laborious functional group manipulation sequences. In 2013, we proposed a method for assembling gem-difluorinated molecules from a difluorocarbene, a nucleophile, and an electrophile. In this process, a difluorocarbene can be considered an equivalent of a bipolar CF₂ unit. Performing consecutive bond-forming reactions by sequential attachment of a nucleophile and an electrophile to a difluorocarbene provides the opportunity for the synthesis of a wide variety of organofluorine compounds. Silicon reagents were the most effective sources of the difluoromethylene fragment, and among them (bromodifluoromethyl)trimethylsilane (Me₃SiCF₂Br) is the reagent of choice. Mildly basic activators such HMPA, DMPU, bromide and acetate ions can initiate the decomposition of the silane with concomitant generation of a difluorocarbene. Organozinc reagents can be employed as nucleophiles, and the CF₂ fragment can insert into the carbon-zinc bond. Primary and secondary benzyl and alkyl organozinc compounds work well. Generally, organozinc reagents tolerate a variety of functional groups. The resulting fluorinated organozinc species can be coupled with heteroatom- or carbon-centered electrophiles. Halogenation of the carbon-zinc bond leads to compounds with bromo- or iododifluoromethyl fragments, which are difficult to access by other means, whereas protonation of that bond generates a valuable difluoromethyl group. Despite the decrease in the reactivity of the carbon-zinc bond caused by the adjacent fluorines, organozinc compounds can effectively participate in copper-catalyzed cross-couplings with allylic and propargyl halides, 1-bromoalkynes, and S-acyl dithiocarbamates. Difluorocarbene can be inserted into the carbon-silicon bond of trimethylsilyl cyanide, and the resulting silane can react with aldehydes and imines to furnish difluorinated nitriles. Interactions of difluorocarbene with heteroatom nucleophiles, such as phosphines or halide ions, are reversible, but the adduct can be trapped by an electrophile. The use of halide ions allows the direct nucleophilic bromo- and iododifluoromethylation of aldehydes and iminium ions. The combination of triphenylphosphine with difluorocarbene generates a difluorinated phosphorus ylide, which can interact with a wide range of π-electrophiles (aldehydes, ketones, acyl chlorides, azomethines, and Michael acceptors) to provide gem-difluorinated phosphonium salts. In the latter species, the carbon-phosphorus bond can be readily cleaved under basic conditions, affording the difluoromethylation products. Primary products resulting from three-component couplings can subsequently be used for further transformations. Single-electron reduction of carbon-phosphorus or carbon-iodine bonds can be conducted under photocatalytic conditions to generate gem-difluorinated radicals. These radicals can be trapped by silyl enol ethers leading to β,β-difluorinated ketones as the primary products. Fluorinated radicals can also undergo intramolecular attacks adjacent to an aromatic ring or a double bond.

Photoredox generation of the trifluoromethyl radical from borate complexes via single electron reduction

A trifluoromethyl radical is generated by single electron reduction of CF₃-substituted borate complexes.