A dual photoredox-nickel strategy for remote functionalization via iminyl radicals: radical ring-opening-arylation, -vinylation and -alkylation cascades

A divergent strategy for the remote arylation, vinylation and alkylation of nitriles is described. These processes proceed through the photoredox generation of a cyclic iminyl radical and its following ring-opening reaction. The distal nitrile radical is then engaged in nickel-based catalytic cycles to form C-C bonds with aryl bromides, alkynes and alkyl bromides.

Oxygen atom transfer: a mild and efficient method for generating iminyl radicals

Treating iminoxyl species with oxygen acceptors such as PPh₃ resulted in oxygen atom transfer and afforded the corresponding iminyl radicals. DFT and other calculations revealed that association between the oxygen atom acceptors and the iminoxyl species results in the formation of key intermediates during the reaction. Subsequent dissociation is accompanied with homolytic cleavage of the N-O bond and generates iminyl radicals with spin densities that are localized on exocyclic nitrogen atoms.

Visible-light-induced radical trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides

An efficient and general visible-light-mediated trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides has been successfully accomplished using N-trifluoromethylthiosaccharin as an effective source of SCF3 radicals. The reaction was proposed to proceed via a domino radical trifluoromethylthiolation/cyano insertion/cyclization to afford the corresponding SCF3-containing ring-fused phenanthridine derivatives in moderate to good yields.

Copper-catalyzed oxidative benzylic C-H cyclization via iminyl radical from intermolecular anion-radical redox relay

Base-promoted C-H cleavage without transition metals opens a practical alternative for the one based on noble metals or radical initiators. The resulting carbanion can pass through radical addition to unsaturated bonds like C-N or C-C triple bonds, in which stoichiometric oxidants are needed. When in situ C-H cleavage meets catalytic carbanion-radical relay, it turns to be challenging but has not been accomplished yet. Here we report the combination of base-promoted benzylic C-H cleavage and copper-catalyzed carbanion-radical redox relay. Catalytic amount of naturally abundant and inexpensive copper salt, such as copper(II) sulfate, is used for anion-radical redox relay without any external oxidant. By avoiding using N-O/N-N homolysis or radical initiators to generate iminyl radicals, this strategy realizes modular synthesis of N-H indoles and analogs from abundant feedstocks, such as toluene and nitrile derivatives, and also enables rapid synthesis of large scale pharmaceuticals.

The synthesis of high value heterocycles from cheap starting materials remains challenging. Here, the authors develop a new strategy to realize practical and modular synthesis of N-H indoles and analogs from toluene and nitrile derivatives using naturally abundant copper salt as catalyst.

Recent advances in iminyl radical-mediated catalytic cyclizations and ring-opening reactions

Iminyl radicals have emerged as versatile synthons for N-heterocycle constructions and ring-opening reactions.

Preparation of 2-arylquinolines from β-arylpropionitriles with aryllithiums and NIS through iminyl radical-mediated cyclization

Treatment of β-arylpropionitriles with aryllithiums, followed by the reaction with water and then with NIS under irradiation with a tungsten lamp gave 2-arylquinolines in good to moderate yields.

Radical-induced ring-opening and reconstruction of cyclobutanone oxime esters

The first example of intramolecular ring-opening and reconstruction of cyclobutanone oxime esters via selective C–C bond cleavage leading to the synthesis of 3,4-dihydronaphthalene-2-carbonitriles in the presence of a cheap copper catalyst has been reported.

Visible-light-promoted oxidation/condensation of benzyl alcohols with dialkylacetamides to cinnamides

An oxidative cross-coupling reaction of benzyl alcohols with dialkylacetamides was developed to construct cinnamides under visible-light-enabled photocatalytic conditions.

Photo- and dioxygen-enabled radical C(sp3)–N(sp2) cross-coupling between guanidines and perfluoroalkyl iodides

The utilization of visible light and dioxygen (air) as green and sustainable mediators in photocatalyzed synthetic chemistry is demonstrated.

Visible photocatalysis of novel oxime phosphonates: synthesis of β-aminophosphonates

A novel oxime phosphonate was synthesized and used in the difunctionalization of alkenes to access β-functionalized phosphonates via visible-light-driven N-centered iminyl radical-mediated multiple bond cleavage and a recombination process.

Leaving Group Assisted Strategy for Photoinduced Fluoroalkylations Using N-Hydroxybenzimidoyl Chloride Esters

Redox-active esters (RAEs) as alkyl radical precursors have been extensively developed for C-C bond formations. However, the analogous transformations of fluoroalkyl radicals from the corresponding acid or ester precursors remain challenging because of the high oxidation potential of the fluoroalkyl carboxylate anions. The newly developed N-hydroxybenzimidoylchloride (NHBC) ester provides a general leaving group assisted strategy to generate a portfolio of fluoroalkyl radicals, and can be successfully applied in photoinduced decarboxylative hydrofluoroalkylation and heteroarylation of unactivated olefins. In addition, DFT calculations revealed that the NHBC ester proceeds by the fluorocarbon radical pathway, whereas other well-known RAEs proceed by the nitrogen radical pathway.

Visible-Light-Promoted Synthesis of Fluoroalkylated Oximes

A method has been developed for the synthesis of fluoroalkylated oximes, potential fluoroalkyl building-blocks for the synthesis of various organofluorine compounds, from easily available amino substrates and fluoroalkylated alkenes. tBuONO was utilized both as a diazotizing agent and as a NO radical source for the oxime synthesis in the process, and the use of a photocatalyst under visible-light irradiation increased the efficiency of the reaction. Various fluoroalkylated oximes were prepared by a tandem process of aryl radical addition to fluoroalkylated alkene and consecutive oxime generation process, albeit in moderate yields. This differentiated approach, transferring an aromatic system into an electron-deficient fluoroalkylated alkene, expands the scope of substrates where electron-poor aromatic systems could be utilized.

Photoredox-promoted alkyl radical addition/semipinacol rearrangement sequences of alkenylcyclobutanols: rapid access to cyclic ketones

Two photo-catalytic tandem alkyl radical addition/semipinacol rearrangement reactions of cycloalkanol-substituted styrenes with N-acyloxyphthalimides and O-acyl oximes have been documented. These protocols provide efficient access to functionalized cyclic ketones, and feature mild conditions (i.e., visible light irradiation, redox neutral and room temperature), broad substrate scope and excellent functional group tolerance.