Synthesis of sterically congested unsymmetrical 1,2-dicarbonyl radicals through a stepwise approach

A simplified and stepwise synthetic method for producing sterically congested unsymmetrical 1,2-dicarbonyl radicals was successfully demonstrated including detailed characterization of each radical cation. Using this approach, an aryl- and N-heterocyclic carbene-substituted 1,2-dicarbonyl radical in its neutral form is generated, revealing the stabilizing role of N-heterocyclic carbenes.

An air-stable radical with a redox-chameleonic amide

An air-stable (amino)(amido)radical was synthesized by reacting a cyclic (alkyl)(amino)carbene with carbazoyl chloride, followed by one-electron reduction. We show that an adjacent radical center weakens the amide bond. It enables the amino group to act as a strong acceptor under steric contraint, thus enhancing the stabilizing capto-dative effect.

The curious case of a sterically crowded Stenhouse salt

We report a peculiar Stenhouse salt. It does not evolve into cyclopentenones upon basification, due to the steric hindrance of its bulky stable carbene patterns. This allowed for the observation and characterization of the transient open-chain neutral derivative, which was isolated as its cyclized form. The latter features an unusually long reactive C–O bond (150 pm) and a rich electrochemistry, including oxidation into an air-persistent radical cation.

Bulky substituents of a Stenhouse salt prevent the usual formation of a cyclopentenone upon deprotonation. For the first time, a transient open-chain neutral derivative was observed; the cyclized form and an air-persistent radical were isolated.

Diamidocarbene-Based Thiele and Tschitschibabin Hydrocarbons: Carbonyl Functionalized Kekulé Diradicaloids

Herein, we report diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, diradicaloids that contain four carbonyl/amido functional groups. The impact of two different π-conjugated spacers, p-phenylene vs p,p'-biphenylene, has been realized. The quantum chemical calculations suggest diamidocarbene (DAC)-based Thiele hydrocarbon (p-phenylene bridged) closed-shell singlet is the ground state, whereas for the diamidocarbene (DAC)-based Tschitschibabin hydrocarbon (p,p'-biphenylene bridged), open-shell singlet is the ground state. The influence of two different π-conjugated spacers also has been reflected in their UV-vis spectra. To gain more information on the diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, we have also carried out cyclic voltammetry investigations along with UV-vis-NIR-spectroelectrochemical studies of their corresponding 2-e oxidized product.

Critical Assessment of the Reducing Ability of Breslow-type Derivatives and Implications for Carbene-Catalyzed Radical Reactions*

We report the synthesis of acyl azolium salts stemming from thiazolylidenes C_NS, triazolylidenes C_TN, mesoionic carbenes C_MIC and the generation of their corresponding radicals and enolates, covering about 60 Breslow‐type derivatives. This study highlights the role of additives in the redox behavior of these compounds and unveils several critical misconceptions about radical transformations of aldehyde derivatives under N‐heterocyclic carbene catalysis. In particular, the reducing ability of enolates has been dramatically underestimated in the case of biomimetic C_NS. In contrast with previous electrochemical studies, we show that these catalytic intermediates can transfer electrons to iodobenzene within minutes at room temperature. Enols derived from C_MIC are not the previously claimed super electron donors, although enolate derivatives of C_NS and C_MIC are powerful reducing agents.

Air-/Heat-Stable Crystalline Carbon-Centered Radicals Derived from an Annelated N-Heterocyclic Carbene

Organic radicals are open-shell species and have been extensively applied to functional materials due to their unique physicochemical properties with unpaired electrons; however, most of them are highly reactive and short-lived. Herein, a series of stable radicals were readily accessed in two steps from a bis(imino)acenaphthene-supported N-heterocyclic carbene (IPr(BIAN)) through enhancing the delocalization of spin density. The IPr(BIAN)-based radicals 3a-c, obtained by reduction of the corresponding iminium salts 2a-c with KC₈, have been spectroscopically and crystallographically (3a,c) characterized. DFT calculations indicate that increasing the electron-withdrawing properties of the para substituent on the carbene carbon atom results in the spin density evolving from the acenaphthene ring to the phenyl ring. The IPr(BIAN)-based radicals 3a-c show excellent stability: they have half-lives of 1 week in well-aerated solutions and feature a high thermal decomposition temperature up to 200 °C.

Sydnone Methides-A Forgotten Class of Mesoionic Compounds for the Generation of Anionic N-Heterocyclic Carbenes

Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π-electron rich anionic N-heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C-C coupling reactions with 1-fluoro-4-iodobenzene under Pd(PPh3 )4 and CuBr catalysis. 77 Se NMR resonance frequencies and 1 JC4-Se as well as 1 JC4-H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N-heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in 13 C NMR spectroscopy at -40 °C which is extremely shifted upfield in comparison to classical N-heterocyclic carbenes.

Synthesis, crystal structure determination, and spectroscopic analyses of 1-chloro-2-(2,6-diisopropylphenyl)-4,4-dimethyl-2-azaspiro[5.5]undecane-3,5-dione: an unyielding precursor to a cyclic (alkyl)(amido)carbene

The synthesis, single-crystal X-ray structure, and ¹H and ¹³C NMR spectrocopic analyses of an unyielding precursor molecule to a cyclic (alkyl)(amido)carbene, 1-chloro-2-(2,6-diisopropylphenyl)-4,4-dimethyl-2-azaspiro[5.5]undecane-3,5-dione, C₂₄H₃₄ClNO₂ (1), is reported. Despite the use of several bases, 1 could not be deprotonated to afford the corresponding carbene. The crystal structure of 1 was compared to the crystal structures of two structurally similar HCl adducts of stable carbenes (compounds 4 and 5), which revealed no significant differences in the geometries about the `carbene' C atoms. To better understand the reactivity differences observed for 1 when compared to 4 and 5, modified percent buried volume (%V_bur) calculations were performed. These calculations revealed that the H atom bound to the carbene C atom is the most sterically hindered in compound 1 when compared to 4 and 5 (%V_bur = 84.9, 81.3, and 79.3% for 1, 4, and 5, respectively). Finally, close inspection of the quadrant-specific %V_bur values indicated that the approach of a deprotonating base to the H atom bound to the carbene C atom is significantly blocked in 1 (69.9%) when compared to 4 and 5 (50.4 and 56.5%, respectively).

Highly Stable 1,2-Dicarbonyl Radical Cations Derived from N-Heterocyclic Carbenes

Stable organic radicals have been of great academic interest not only in the context of fundamental understanding of reactive intermediates but also because of their numerous applications as functional materials. Apart from the early examples of triphenylmethyl and TEMPO derivatives, reports on air- and water-stable organic radicals are scarce, and their development remains a challenge. Herein, we present the design and synthesis of a novel organic radical based on a 1,2-dicarbonyl scaffold supported by N-heterocyclic carbenes (NHCs). The presented radical cations exhibit remarkable stability toward various harsh conditions, such as the presence of reactive chemicals (reductants, oxidants, strong acids, and bases) or high temperatures, by far exceeding the stability of triphenylmethyl and TEMPO radicals. In addition, physiological conditions including aqueous buffer and blood serum are tolerated. The steric and electronic stabilization provided by the two NHC moieties enabled the successful design of the highly stable radical.

A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent

Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO₂ to methoxyborane, all under ambient temperature and pressure.

Tunable Redox-Active Triazenyl-Carbene Platforms: A New Class of Anolytes for Non-Aqueous Organic Redox Flow Batteries

Non-aqueous all organic redox flow batteries (NORFBs) are one of the promising options for large-scale renewable energy storage systems owing to their scalability with energy and power along with the affordability. The discovery of new redox-active organic molecules (ROMs) for the anolyte/catholyte would bring them one step closer to the practical application, thus it is highly demanded. Here, we report a new class of ROMs based on cationic triazenyl systems supported by N-heterocyclic carbenes (NHCs) and demonstrate, for the first time, that the triazenyl can serve as a new redox motif for ROMs and could be significantly stabilized for the use in NORFBs by the coupling with NHCs even at radical states. A series of NHC-triazenyl ROM families were successfully synthesized via the reaction of a synthon, N-heterocyclic carbene azido cation, with various Lewis bases including NHCs. Remarkably, it is revealed that NHCs substituted on the triazenyl fragments can serve as a versatile platform for tailoring the electrochemical activity and stability of triazenyl-based compounds, introducing various ROMs exploiting triazenyl redox motif, as demonstrated in the full cell of NORFBs for an anolyte.

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.

Stable dicationic dioxoliums and fate of their dioxolyl radicals

A glimpse into uncharted territory: the synthesis and study of dicationic dioxolium salts allow for assessing the fate of the corresponding elusive dioxolyl radicals.

Cyclic (aryl)(amido)carbenes: pushing the π-acidity of amidocarbenes through benzannulation

Cyclic(aryl)(amido)carbenes were synthesized, and studied via a combination of experimental and computational approaches.

What Are the Radical Intermediates in Oxidative N-Heterocyclic Carbene Organocatalysis?

The oxidation of the Breslow intermediate resulting from the addition of an N-heterocyclic carbene (NHC) to benzaldehyde triggers a fast deprotonation, followed by a second electron transfer, directly affording the corresponding acylium at E > -0.8 V (versus Fc/Fc⁺). Similarly, the oxidation of the cinnamaldehyde analogue occurs at an even higher potential and is not a reversible electrochemical process. As a whole, and contrary to previous beliefs, it is demonstrated that Breslow intermediates, which are the key intermediates in NHC-catalyzed transformations of aldehydes, cannot undergo a single electron transfer (SET) with mild oxidants ( E < -1.0 V). Moreover, the corresponding enol radical cations are ruled out as relevant intermediates. It is proposed that oxidative NHC-catalyzed radical transformations of enals proceed either through SET from the corresponding electron-rich enolate or through coupled electron-proton transfer from the enol, in any case generating neutral capto-dative radicals. Relevant electrochemical surrogates of these paramagnetic species have been isolated.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

An air-persistent oxyallyl radical cation with simple di(methyl)amino substituents

We report an experimental and theoretical study of the 1,1,3,3-tetrakis-di(methylamino)oxyallyl radical cation. Despite simple substituents with minimal steric hindrance, this radical was found to be stable under an inert atmosphere and persistent for several hours in well-aerated solutions.

An air-stable N-heterocyclic carbene iminoxyl borate radical zwitterion

The presented N-heterocyclic carbene iminoxyl borate radical zwitterion shows remarkable stability toward air, moisture, and silica gel.

The serendipitous discovery of a readily available redox-bistable molecule derived from cyclic(alkyl)(amino)carbenes

A simple redox bistable system is available in one step from a stable carbene and a bis(acyl chloride).

The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals

A cyclic and an acyclic di(amino)carbene as well as a cyclic and an acyclic (alkyl)(amino)carbene cleanly react with benzoyl chloride to give the corresponding adducts 1⁺_cyc , 1⁺_acy , 2⁺_cyc , and 2⁺_acy , respectively. The reduction of 1⁺_cyc and 2⁺_cyc derived from cyclic carbenes affords the corresponding radicals 1_cyc and 2_cyc that are stable at room temperature. In contrast, radicals 1_acy and 2_acy , derived from acyclic carbenes, cannot be isolated. It is shown that 1_acy is as thermodynamically stabilized as its cyclic counterpart 1_cyc , but its instability is the result of β-hydrogens of the nitrogen substituent, along with the enhanced flexibility around C-N bonds, which allow for a H^. -migration-elimination process. Radical 2_acy is thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2⁺_acy and enolate 2^-_acy . This is due to the excessive steric hindrance, which prevents electron-delocalization on the NCCO fragment, and thus, the capto-dative stabilization. This work suggests general guidelines for the design of highly persistent (amino)(carboxy)radicals, especially by emphasizing the key advantage of cyclic patterns.