Metal-free regioselective nitration of quinoxalin-2(1H)-ones with tert-butyl nitrite

NXS/TBHP as a Reagent System for Expedient Generation of Halogen Radicals: Site-Selective Halogenation of Quinoxalin-2(1H)-ones and Conversion of Terminal Alkynes to gem-Dihaloketones

We report an efficient and practical protocol for the generation of halogen (Br and Cl) radicals using inexpensive and readily available N-bromo/N-chlorosuccinimide (NXS) and t-butylhydroperoxide (TBHP) at rt. The NXS/TBHP reagent systems are demonstrated for site-selective bromination and chlorination of the benzene-core of quinoxalin-2(1H)-ones. It is shown that the reagent system permits regioselective halogenation at the C7 position of the benzene core over the readily-functionalized C3 position of quinoxalin-2(1H)-ones under mild reaction conditions. We further show that terminal alkynes can be converted to gem-dihaloketones in a facile manner by NXS/TBHP. Mechanistically, single electron transfer between TBHP and NXS leads to instantaneous formation of electrophilic halogen radicals, which attack arenes and alkynes.

Photoredox-Catalyzed Radical Coupling of C7-Chloromethyl-Substituted Thiazolino Ring-Fused 2-Pyridones with Quinoxalinones

We have developed an Ir(PPy)3 photoredox-catalyzed cross-coupling reaction that allows installation of quinoxalinones at the C7 position of thiazolino ring-fused 2-pyridones (TRPs) under mild conditions. The methodology tolerates various substituted quinoxalinones and biologically relevant substituents on the C8 position of the TRP. The TRP scaffold has large potential in the development of lead compounds, and while the coupled products are interesting from a drug-development perspective, the methodology will be useful for developing more potent and drug-like TRP-based candidates.

Redox Relay-Induced C-S Radical Cross-Coupling Strategy: Application in Nontraditional Site-Selective Thiocyanation of Quinoxalinones

Oxidative cross-coupling is a powerful strategy to form C-heteroatom bonds. However, oxidative cross-coupling for constructing C-S bond is still a challenge due to sulfur overoxidation and poisoning transition-metal catalysts. Now, electrochemical redox relay using sulfur radicals formed in situ from inorganic sulfur source offers a solution to this problem. Herein, electrochemical redox relay-induced C-S radical cross-coupling of quinoxalinones and ammonium thiocyanate with bromine anion as mediator is presented. The electrochemical redox relay comprised initially the formation of sulfur radical via indirect electrochemical oxidation, simultaneous electrochemical reduction of the imine bond, electro-oxidation-triggered radical coupling involving dearomatization-rearomatization, and the reformation of the imine bond through anodic oxidation. Applying this strategy, various quinoxalinones bearing multifarious electron-deficient/-rich substituents at different positions were well compatible with moderate to excellent yields and good steric hindrance compatibility under constant current conditions in an undivided cell without transition-metal catalysts and additional redox reagents. Synthetic applications of this methodology were demonstrated through gram-scale preparation and follow-up transformation. Notably, such a unique strategy may offer new opportunities for the development of new quinoxalinone-core leads.

Site-selective direct nitration of 2H-indazoles: easy access to 7-nitroindazoles

A new site-selective methodology for C-H nitration of 2H-indazoles has been accomplished at the C7 position using iron(III) nitrate. This strategy enables practical access to an array of 7-nitroindazoles with broad functional group tolerance in good yields. The synthesized products have been proven as valuable synthetic intermediates by demonstrating the synthetic utility. Mechanistic investigations indicate that the reaction goes through a radical pathway.

Oxidative functionalization of alkylidenecyclopropanes and alkylidenecyclobutanes: a versatile platform to access nitrated cyclopropanes and cyclobutanes

A divergent radical nitration of alkylidenecyclopropanes (ACPs) and alkylidenecyclobutanes (ACBs) with Fe(NO₃)₃·9H₂O or AgNO₂ has been achieved, affording three categories of products including β-nitro alcohol, α-nitro ketone and nitro nitratosation products with yields up to 90%. Particularly, the cyclopropyl and cyclobutyl rings were conserved in the products. The applicability of this method was demonstrated by the scale-up experiment and reduction of the nitro into an amino group. Preliminary mechanistic studies suggested that the nitro radical was involved in the reaction process.