Bromo- or Methoxy-Group-Promoted Umpolung Electron Transfer Enabled, Visible-Light-Mediated Synthesis of 2-Substituted Indole-3-glyoxylates

Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Enantioselective Copper-Catalyzed Fukuyama Indole Synthesis from 2-Vinylphenyl Isocyanides

Enantioenriched chiral indoles are of high interest for the pharmaceutical and agrochemical industries. Herein, we present an asymmetric Fukuyama indole synthesis through a mild and efficient radical cascade reaction to access 2-fluoroalkylated 3-(α-cyanobenzylated) indoles by stereochemical control with a chiral copper-bisoxazoline complex using 2-vinylphenyl arylisocyanides as radical acceptors and fluoroalkyl iodides as C-radical precursors. Radical addition to the isonitrile moiety, 5-exo-trig cyclization, and Cu-catalyzed stereoselective cyanation provide the targeted indoles with excellent enantioselectivity and good yields. Due to the similar electronic and steric properties of the two aryl substituents to be differentiated, the enantioselective construction of the cyano diaryl methane stereocenter is highly challenging. Mechanistic studies reveal a negative nonlinear effect which allows proposing a model to explain the stereochemical outcome. Scalability and potential utility of the enantioenriched 3-(α-cyanobenzylated) indoles as hubs for chiral tryptamines, indole-3-acetic acid derivatives, and triarylmethanes are demonstrated, and a formal synthesis of a natural product analogue is disclosed.

Metal-catalyzed reactions of organic nitriles and boronic acids to access diverse functionality

The nitrile or cyano (-CN) group is one of the most appreciated and effective functional groups in organic synthesis, having a polar unsaturated C-N triple bond. Despite sufficient stability and being intrinsically inert, the nitrile group can be easily transformed into many other functional groups, such as amines, carboxylic acids, ketones, etc. which makes it a vital group in organic synthesis. On the other hand, despite several boronic acids having a low level of genotoxicity, they have found wide applicability in the field of organic synthesis, especially in transition metal-catalyzed cross-coupling reactions. Recently, transition-metal-catalyzed cascade additions or addition/cyclization processes of boronic acids to the nitrile group open up exciting and useful strategies to prepare a variety of functional molecules through the formation of C-C, C-N and CO bonds. Boronic acids can be added to the cyano functionality through catalytic carbometallation or through a radical cascade process to provide newer pathways for the rapid construction of various important acyclic ketones or amides, carbamidines, carbocycles and N,O-heterocycles. The present review focuses on various transition-metal-catalyzed additions of boronic acids via carbometallation or radical cascade processes using the cyano group as an acceptor.

DMF-Assisted Radical Cyclization of o-Isocyanodiaryl Ethers via 1,5-Aryl Migration: Construction of 2-Arylbenzoxazoles

A novel DMF-assisted radical cyclization of o-isocyanodiaryl ethers via 1,5-aryl migration has been developed for the synthesis of a series of 2-arylbenzoxazoles by the FeCl₃/TBHP/Et₃N catalytic system in DMF. However, N,N-dimethylbenzo[d]thiazole-2-carboxamide and N,N-dimethylbenzo[d]selenazole-2-carboxamide were obtained from the corresponding substrate 2-isocyanophenyl p-methoxyphenyl thioether and 2-isocyanodiphenyl selenoether under the same conditions. A possible mechanism may involve aryl 1,5-migration and DMF-assisted radical cyclization of o-isocyanodiaryl ethers.

2,3-Difunctionalized Indoles via Radical Acylation or Trifluoromethylation of ortho-Alkynylphenyl Isonitriles

A radical cascade to 2,3-disubstituted indoles proceeding via acylation or trifluoromethylation of ortho-alkynylphenyl isonitriles is presented. In these cascades, two C-C bonds and one C-O bond are formed using an inexpensive oxidant and a catalytic copper or iron salt. The starting isonitriles are easily accessible, and commercially available aldehydes and fluoromethylation reagents serve as reaction partners. Functional group tolerance is high, as documented by the successful preparation of a series of 2,3-disubstituted indoles.

Mn(III)-Mediated Radical Cyclization of o-Alkenyl Aromatic Isocyanides with Boronic Acids: Access to N-Unprotected 2-Aryl-3-cyanoindoles

The synthesis of N-unprotected 2-aryl-3-cyanoindoles was realized via the Mn(III)-mediated radical cascade cyclization of o-alkenyl aromatic isocyanides with boronic acids. A possible mechanism involving a sequential intermolecular radical addition, intramolecular cyclization, and cleavage of the C-C bond under mild reaction conditions is proposed. Mechanism studies show that H₂O or O₂ might provide the oxygen source for the elimination of benzaldehyde.

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.

Photo-Mediated Intermolecular Coupling of Alkenes with Ketones via Acyloxy Nitroso Compounds

An atom-economic intermolecular radical addition reaction of acyloxy nitroso compounds to electron-deficient alkenes mediated by visible light is reported. The starting nitroso derivatives are readily prepared by oxidation of the corresponding oximes prepared from ketones and the overall transformation represents an oxidative coupling of a ketone with a Michael acceptor. The cascade proceeds smoothly under mild conditions, providing a series of valuable functionalized oximes in moderate to good yields. Mechanistic studies suggest that these cascades proceed via addition/coupling processes that are controlled by the persistent radical effect (PRE) with NO acting as the persistent species.

Self-cyclization vs. dimerization of o-alkenyl arylisocyanides: chemodivergent synthesis of quinolines and pyrrolo-fused diindoles

Solvent-dependent chemoselective MBH-type self-cyclization and dimerization of o-alkenyl arylisocyanides were developed for the efficient and chemodivergent synthesis of various 3-acylquinoline and pyrrolo-fused diindole frameworks.

Mn(iii)-mediated cascade cyclization of 1-(azidomethyl)-2-isocyanoarenes with organoboronic acids: construction of quinazoline derivatives

A novel and efficient Mn(iii)-mediated oxidative radical cascade reaction of 1-(azidomethyl)-2-isocyanoarenes with organoboronic acids is reported.

Shining Visible Light on Vinyl Halides: Expanding the Horizons of Photocatalysis

ConspectusOver the past decade, photoredox catalysis has blossomed as a powerful methodology because of its wide applicability in sustainable free-radical-mediated processes, in which light is used as a cleaner energy source to alter the redox properties of organic molecules and to drive unique chemical transformations. Numerous examples of highly selective C-C and C-heteroatom bond formation processes have been achieved this way in an efficient and waste-reducing way. Therein, the activation of widely available organic halides via single-electron reduction has been broadly applied for organic synthesis. However, in comparison with alkyl and aryl halides, the analogous utilization of vinyl halides is less developed, most likely as a consequence of the highly unstable vinyl radicals generated as intermediates along with their strong tendency to abstract hydrogen atoms from a suitable source (e.g., the solvent), resulting in a synthetically less useful reduction.Nevertheless, during the last years, a number of photocatalytic processes involving vinyl halides have been developed, featuring the generation of vinyl radicals, diradicals, or radical cations as the key transient species. Moreover, photoredox processes in which a radical reacts with a vinyl halide or with an in situ-generated vinylmetal halide have been developed. Thus, identifying suitable conditions to generate and manipulate these reactive species has resulted in novel synthetic processes in a controllable manner. Moreover, in view of the great versatility of vinyl halides in palladium-catalyzed cross-coupling reactions, their activation by visible light might provide an attractive alternative to such processes, especially when non-noble metals could be used as photoinitiators in the future.In this Account, we discuss the various strategies of photoredox processes involving vinyl halides, classifying the material into four categories: (a) formation of a vinyl radical upon receipt of an electron from the photocatalyst, (b) formation of a radical cation after donation of an electron to the photocatalyst, (c) energy transfer corresponding to diradical formation upon triplet-triplet sensitization, and (d) dual transition metal and photocatalysis employing vinyl halides as precursors. While in the first three approaches the activation of vinyl halides is part of the photochemical step, the fourth one involves the interaction of a photochemically generated radical with a vinylnickel(II) halide obtained in turn by the oxidative addition of nickel(0) to the vinyl halide. Therefore, we highlight these important developments for conceptual comparison to the direct activation of vinyl halides by light, but they are not covered in depth in this Account.

Synthesis of pyrazolo[1,5-c]quinazoline derivatives through the copper-catalyzed domino reaction of o-alkenyl aromatic isocyanides with diazo compounds

Various o-alkenyl aromatic isocyanides were prepared from readily available reactants for their double annulation with diazo compounds for a one-pot synthesis of pyrazolo[1,5-c]quinazolines under mild reaction conditions.

Rhodium/copper-cocatalyzed coupling-cyclization of o-alkenyl arylisocyanides with vinyl azides: one-pot synthesis of α-carbolines

A novel rhodium/copper-cocatalyzed coupling–cyclization reaction of o-alkenyl arylisocyanides with vinyl azides has been developed. The reaction provides a new route to α-carbolines by the formation of two C–C bonds, one C–N bond and two aromatic rings in a single step.

Total Synthesis and Antibacterial Activity Evaluation of Griseofamine A and 16- epi-Griseofamine A

The first total synthesis of griseofamine A and its diastereomer, 16- epi-griseofamine A, is described over seven steps with yields of 23% and 7%, respectively. Their antibacterial activities are also disclosed for the first time. Griseofamine A exhibited in vitro activities against a panel of drug-resistant Gram-positive bacteria with minimum inhibitory concentration (MIC) values of 8-16 μg/mL. Notably, 16- epi-griseofamine A was 2-3 times more potent than griseofamine A with MIC values of 2-8 μg/mL.

Mn(III)-Mediated Cascade Cyclization of 3-Isocyano-[1,1'-biphenyl]-2-carbonitrile with Arylboronic Acid: Construction of Pyrrolopyridine Derivatives

A Mn(III) mediated cascade cyclization of new designed multifunctionalized 3-isocyano-[1,1'-biphenyl]-2-carbonitrile with arylboronic acid to construct pyrrolopyridine derivatives is developed. A series of pyrroloporidine compounds have been constructed through the formation of two new C-C bonds and one C-N bond via a radical pathway.

Visible light-mediated chemistry of indoles and related heterocycles

The impact of visible light-promoted chemistry on the functionalization of indoles and related heterocycles is reviewed.