Electrifying Phthalimide-N-Oxyl (PINO) Radical Chemistry: Anodically Induced Dioxygenation of Vinyl Arenes with N-Hydroxyphthalimide

Data-Driven Workflow for the Development and Discovery of N-Oxyl Hydrogen Atom Transfer Catalysts

N-oxyl species are promising hydrogen atom transfer (HAT) catalysts to advance C-H bond activation reactions. However, because of the complex structure-activity relationship within the N-oxyl structure, catalyst optimization is a key challenge, particularly for simultaneous improvement across multiple parameters. This paper describes a data-driven approach to optimize N-oxyl hydrogen atom transfer catalysts. A focused library of 50 N-hydroxy compounds was synthesized and characterized by three parameters-oxidation peak potential, HAT reactivity, and stability-to generate a database. Statistical modeling of these activities described by their intrinsic physical organic parameters was used to build predictive models for catalyst discovery and to understand their structure-activity relationships. Virtual screening of 102 synthesizable candidates allowed for rapid identification of several ideal catalyst candidates. These statistical models clearly suggest that N-oxyl substructures bearing an adjacent heteroatom are more optimal HAT catalysts compared to the historical focus, phthalimide-N-oxyl, by striking the best balance among all three target experimental properties.

In Situ Electroreorganization of Self-Assembled Perylene Diimide Enables Ultrahigh-Rate and Ultralong-Cycle-Life Sodium Organic Batteries

Morphology optimization of organic electrodes with more exposed redox-active sites and large contact area between active materials and conductive carbon is crucial for higher actual capacity, and improved rate performance, however, is also challenging. In this work, we report a self-assembled perylene diimide disodium salts (PDI-ONa)-based cathode for sodium organic batteries (SOBs) and unveil an in situ electro-induced reorganization process that optimizes the nanostructured morphology and significantly improves the ionic and electronic transport. Experimental and theoretical data suggest that such reorganization is driven by the continuous dissociation/restacking of PDI-ONa during cycling. The resulting reorganized PDI-ONa electrode exhibits excellent rate (136 and 126 mA h g-1 at 0.2 and 100 A g-1) and cycling performance (141 mA h g-1 after 24,000 cycles at 3 A g-1). Even at -30 °C, it can also perform well (140 mA h g-1 after 500 cycles at 0.1 A g-1). Our findings provide a fresh perspective on optimizing organic nanostructured electrodes for high-performance SOBs.

Electrochemically Induced Synthesis of N-Allyloxyphthalimides via Cross-Dehydrogenative C-O Coupling of N-Hydroxyphthalimide with Alkenes Bearing the Allylic Hydrogen Atom

The electrochemically induced reaction between alkenes, bearing an allylic hydrogen atom, and N-hydroxyphthalimide was investigated. Cross-dehydrogenative C-O coupling with phthalimide-N-oxyl radical, derived from N-hydroxyphthalimide, occurs instead of oxidation of the allylic site, with the formation of a carbonyl group or functionalization of the double C=C bond. The discovered transformation proceeds in an undivided electrochemical cell equipped with a carbon felt anode and a platinum cathode. Coupling products were obtained with yields up to 79%. The developed process is based on the abstraction of hydrogen atom from the allylic position for functionalization while the C=C bond remains unreacted. The method exploits the ability of the phthalimide-N-oxyl radical to abstract hydrogen atoms with the following interception of the intermediate C-centered radical.

Synthesis of ω-functionalized ketones from strained cyclic alcohols by ring-opening and cross-recombination between alkyl and N-oxyl radicals

Radical ring-opening oxyimidation of cyclobutanols and cyclopropanols with the formation of ω-functionalized ketones was discovered. The oxidative C-O coupling proceeds via the interception of a primary alkyl radical generated from a cyclic alcohol with a reactive radical generated in situ, which is an electron-deficient N-oxyl radical. The developed conditions allow for the balanced generation rates of carbon- and N-oxyl radicals, which are necessary for their selective cross-recombination. Thus, typical competitive dimerization processes of carbon-centered radicals, their intermolecular cyclization, and N-oxyl radical self-decay are suppressed. The method is applicable to a wide range of cyclobutanols and results in oxyimidated ketones in yields of up to 82%.

Hydrogen-Bond-Assisted Electrocatalytic Semi-Oxidation of 5-Hydroxymethylfurfural into 2,5-Diformylfuran by Operando Dissociated N-Oxyl Mediator

The conversion of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) is a promising approach for enhancing biomass utilization. Nevertheless, traditional methods using noble metal catalysts face challenges due to high costs and poor selectivity towards DFF. Herein, we developed a novel catalytic electrode integrating N-hydroxyphthalimide (NHPI) into a metal-organic framework on a hydrophilic carbon cloth. This design significantly enhances the selective adsorption of HMF due to stronger hydrogen-bond interaction between the electrode's hydrophilic surface and the C(sp3)-OH group in HMF compared to the C(sp2)=O in DFF. Additionally, the electro-driven dissociation of the NHPI-linker generates stabilized N-Oxyl radicals that promote selective semi-oxidation of HMF under neutral conditions. As a result, this approach achieves a high yield rate of 138.2 mol molcat -1 h-1 with a selectivity of 96.7 % for the HMF-to-DFF conversion. This work introduces a novel strategy for designing catalytic electrodes with stabilized N-Oxyl radicals, and offers a promising method for electrocatalytic DFF synthesis, leveraging hydrogen-bond interaction between electrode surface and HMF.

The diacetyliminoxyl radical in oxidative functionalization of alkenes

The intermolecular oxime radical addition to CC bonds was observed and studied for the first time. The diacetyliminoxyl radical was proposed as a model radical reagent for the study of oxime radical reactivity towards unsaturated substrates, which is important in the light of the active development of synthetic applications of oxime radicals. In the present work it was found that the diacetyliminoxyl radical reacts with vinylarenes and conjugated dienes to give radical addition products, whereas unconjugated alkenes can undergo radical addition or allylic hydrogen substitution by diacetyliminoxyl depending on the substrate structure. Remarkably, substituted alkenes give high yields of C-O coupling products despite the significant steric hindrance, whereas unsubstituted alkenes give lower yields of the C-O coupling products. The observed atypical C-O coupling yield dependence on the alkene structure was explained by the discovered ability of the diacetyliminoxyl radical to attack alkenes with the formation of a C-N bond instead of a C-O bond giving side products. This side process is not expected for sterically hindered alkenes due to lower steric availability of the N-atom in diacetyliminoxyl than that of the O-atom.

Free Radicals in the Queue: Selective Successive Addition of Azide and N-Oxyl Radicals to Alkenes

The selective successive addition of azide (•N3) and N-oxyl radicals to alkenes is demonstrated, despite each of the two radicals being known to attack C═C bonds and the mixture of radical adducts possibly being expected. The proposed radical mechanism was supported by density functional theory calculations, electron paramagnetic resonance, and radical trapping experiments. The reaction proceeds at room temperature with the available reagents: NaN3, N-hydroxy compounds, and PhI(OAc)2 as the oxidant. The method can be applied for N-hydroxyimides, N-hydroxyamides, N-hydroxybenzotriazole, and oximes as N-oxyl radical precursors. Vinylarenes, aliphatic alkenes, and even electron-deficient methyl methacrylate were successfully functionalized.

5,8-Di-tert-butyl-2-hydroxy-1H-benzo[de]isoquinoline-1,3(2H)-dione—A New Lipophilic N-oxyl Radical Precursor

N-hydroxyimides are widely known as organocatalysts for aerobic oxidation and oxidative coupling reactions, in which corresponding imide-N-oxyl radicals play the role of catalytically active hydrogen atom abstracting species. The drawbacks of many N-hydroxyimides are poor solubility in low polarity solvents and limited activity in the cleavage of unactivated C–H bonds. To overcome these shortcomings, we have synthesized a new lipophilic N-hydroxyimide, 5,8-di-tert-butyl-2-hydroxy-1H-benzo[de]isoquinoline-1,3(2H)-dione, with high solubility in low-polarity solvents such as DCM. According to the EPR study, the stability of the corresponding imide-N-oxyl radical is comparable to that of the non-tert-butylated analogue, naphthalimide-N-oxyl radical. DFT calculations showed that the NO–H bond dissociation enthalpy (BDE) in the synthesized tert-butylated-N-hydroxynaphthalimide is one of the highest in N-hydroxyimide series, which corresponds to high hydrogen atom abstracting reactivity and may be useful in catalysis of strong C–H bond oxidative cleavage. The synthesized compound can be considered as catalyst for liquid-phase free-radical oxidation and oxidative coupling reactions in non-polar media where solubility was previously the limiting factor.

2-(2,5-Dimethoxyphenoxy)isoindoline-1,3-dione

In this work, the direct C-H functionalization reaction of 1,4-dimethoxybenzene with N-hydroxyphthalimide has been disclosed. A previously unknown product of the C-O coupling of 1,4-dimethoxybenzene and N-hydroxyphthalimide was obtained. The reaction proceeded under mild conditions, in which a commercially available manganese-based oxidizing agent was used for generation of a phthalimide-N-oxyl radical. The obtained compound is a promising valuable precursor of O-aryl hydroxylamine.

Photoinduced Silylation of N-Heteroarenes and Unsaturated Benzamides with Naphthalimide-Based Organic Photocatalysts

Described herein is the development of a general strategy for the silylation of N-heteroaromatics and unsaturated benzamides via the rational designing of an efficient organic photocatalyst. The process features operational simplicity, mild reaction conditions, and the use of readily prepared naphthalimide (NI)-based organic photocatalysts. Notably, both inert trialkylhydrosilanes and arylhydrosilanes are well tolerated with this protocol.