Distonic Radical Cations Research Articles

Photocatalytic (3 + 3) Annnulation of Vinyldiazo Compounds and Aminocyclopropanes.

A (3 + 3) annulation of aminocyclopropanes and vinyldiazo compounds enabled by organo-photocatalysis is described. The reaction allows the regioselective synthesis of cyclohexenes bearing adjacent amino and carbonyl groups with broad functional group tolerance. In a departure from previous reports, our work demonstrated that a distonic radical cation can be preferentially intercepted by weakly nucleophilic vinyldiazo compounds, followed by an exclusive 6-endo radical cyclization for ring closure. Based on the interaction between adjacent amino and ester groups, the products can be further converted to cyclohexene-fused 1,3-oxazinane and azetidine.

Sequential radical and cationic reactivity at separated sites within one molecule in solution.

Distonic radical cations (DRCs) with spatially separated charge and radical sites are expected to show both radical and cationic reactivity at different sites within one molecule. However, such "dual" reactivity has rarely been observed in the condensed phase. Herein we report the isolation of crystalline 1λ2,3λ2-1-phosphonia-3-phosphinyl-cyclohex-4-enes 2a,b˙+, which can be considered delocalized DRCs and were completely characterized by crystallographic, spectroscopic, and computational methods. These DRCs contain a radical and cationic site with seven and six valence electrons, respectively, which are both stabilized via conjugation, yet remain spatially separated. They exhibit reactivity that differs from that of conventional radical cations (CRCs); specifically they show sequential radical and cationic reactivity at separated sites within one molecule in solution.

Syn-Selective Difunctionalization of Bicyclobutanes Enabled by Photoredox-Mediated C-S σ-Bond Scission.

Given the importance of cyclic frameworks in molecular scaffolds and drug discovery, it is intriguing to precisely forge and manipulate ring systems in synthetic chemistry. In this field, the intermolecular synthesis of densely substituted cyclobutanes with precise diastereocontrol under simple reaction conditions remains a challenge. Herein, a photoredox strategy for the difunctionalization of bicyclo[1.1.0]butanes (BCBs) under high regio- and syn-selectivity is disclosed. C-S σ-bond cleavage of partially unsaturated sulfur-containing bifunctional reagents in an overall strain-release-driven process enables the thio-alkynylation, -alkenylation, and -allylation of BCBs under mild conditions and demonstrates the generality of this protocol. Mechanistic studies suggest that the intermediacy of cyclic distonic radical cations might be key for the efficient scission of C-S σ-bonds and the origin of diastereoselectivity.

Olefination of carbonyls with alkenes enabled by electrophotocatalytic generation of distonic radical cations.

The conversion of carbonyls to olefins is a transformation of great importance for complex molecule synthesis. Standard methods use stoichiometric reagents that have poor atom economy and require strongly basic conditions, which limit their functional group compatibility. An ideal solution would be to catalytically olefinate carbonyls under nonbasic conditions using simple and widely available alkenes, yet no such broadly applicable reaction is known. Here, we demonstrate a tandem electrochemical/electrophotocatalytic reaction to olefinate aldehydes and ketones with a broad range of unactivated alkenes. This method involves the oxidation-induced denitrogenation of cyclic diazenes to form 1,3-distonic radical cations that rearrange to yield the olefin products. This olefination reaction is enabled by an electrophotocatalyst that inhibits back-electron transfer to the radical cation intermediate, thus allowing for the selective formation of olefin products. The method is compatible with a wide range of aldehydes, ketones, and alkene partners.

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation.

6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product's electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation-cycloaddition-reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

(Digital Presentation) TiO2 Photoelectrochemical Radical-Cation Vinylcyclopropane Rearrangements

In the field of synthetic organic chemistry, photochemical and electrochemical methods are often considered to be competing techniques that induce single electron transfer (SET). Recently, metal complexes and organic dyes have been extensively used as molecular sensitizers that can induce homogeneous bidirectional SET. On the other hand, heterogeneous monodirectional SET can be induced by electrodes, which differs from the photochemical methods. In this context, semiconductors are unique alternatives, since they can induce heterogeneous but bidirectional SET, potentially enjoying advantages of both photochemical and electrochemical methods. Such semiconductor "photoelectrochemical" methods may trigger the reactions that are otherwise difficult to achieve.We have been developing electrochemical SET-triggered cycloadditions in lithium perchlorate (LiClO4)/nitromethane (CH3NO2) solution, which facilitates the generation of radical cations from electron-rich alkenes and styrenes. Recently, we also have been focusing on TiO2 photoelectrochemical SET-triggered cycloadditions, expanding a scope of the electrochemical versions. During the course of our study to develop new TiO2 photoelectrochemical SET-triggered cycloadditions, we unexpectedly found that vinylcyclopropane rearrangements was also possible in LiClO4/CH3NO2 solution. The reactions are initiated by oxidative SET by hole, which is followed by immediate ringopening of the cyclopropanes to generate distonic radical cations. It is expected that reductive SET by excited electron is also involved to realize effective net redox-neutral transformations. Experimental details, including preliminary mechanistic studies, will be presented in this talk.

Reactivity Trends in the Gas-Phase Addition of Acetylene to the N-Protonated Aryl Radical Cations of Pyridine, Aniline, and Benzonitrile.

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta, and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10-12 cm3 molecule-1 s-1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10-12 cm3 molecule-1 s-1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10-11 cm3 molecule-1 s-1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height.

A Room-Temperature Stable Distonic Radical Cation.

Distonic radical cations (DRCs) with spatially separated charge and radical sites have, so far, largely been observed by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Herein, we disclose the isolation of a crystalline dicarbondiphosphide-based β-distonic radical cation salt 3.+ (BARF) (BARF=[B(3,5-(CF3 )2 C6 H3 )4 )]- ) stable at room temperature and formed by a one-electron-oxidation-induced intramolecular skeletal rearrangement reaction. Such a species has been validated by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray diffraction, UV/Vis spectroscopy and density functional theory (DFT) calculations. Compound 3.+ (BARF) exhibits a large majority of spin density at a two-coordinate phosphorus atom (0.74 a.u.) and a cationic charge located predominantly at the four-coordinate phosphorus atom (1.53 a.u.), which are separated by one carbon atom. This species represents an isolable entity of a phosphorus radical cation that is the closest to a genuine phosphorus DRC to date.

Electron Transfer-Catalyzed Vinylcyclopropane Rearrangements

Recently, electron has been recognized as the simplest catalyst in the field of synthetic organic chemistry. Addition or removal of electron can activate small molecules for further chemical transformations, which is referred to as redox catalysis. Reductive and/or oxidative single electron transfer (SET) can be induced by means of electro- and photochemistry, where an electron can play a role of catalysts. Radical ions are primarily generated via SET, offering unique reactive intermediates.Distonic radical ions are transient species with formally separated radical and charge cites. They potentially exhibit radical and ion reactivities independently, which may differ from usual radical ions. However, distonic radical ions are not commonly used as reactive intermediates in the field of synthetic organic chemistry, probably because of lack of their simple generation methods.We have been developing oxidative SET-triggered cycloadditions in a lithium perchlorate/nitromethane solution. Radical cations are involved in the reactions as distinctive reactive intermediates, facilitating intermolecular carbon-carbon bond formations. We questioned whether the distonic radical cation can also be generated by oxidative SET, leading to novel chemical transformations. In this presentation, oxidative SET-catalyzed vinylcyclopopane rearrangements will be described. Figure 1

Radical-Cation Vinylcyclopropane Rearrangements by TiO2 Photocatalysis

Radical cation vinylcyclopropane rearrangements by TiO2 photocatalysis in lithium perchlorate/nitromethane solution are described. The reactions are triggered by oxidative single electron transfer, which is followed by immediate ring-opening of the cyclopropanes to generate distonic radical cations as unique reactive intermediates. This approach can also be applied to vinylcyclobutane, leading to the construction of six-membered rings. A stepwise mechanism via distonic radical cations is proposed based on preliminary mechanistic studies, which is supported by density functional theory calculations.

An N-Heterocyclic-Carbene-Derived Distonic Radical Cation.

We present the discovery of a novel radical cation formed through one-electron oxidation of an N-heterocyclic carbene-carbodiimide (NHC-CDI) zwitterionic adduct. This compound possesses a distonic electronic structure (spatially separate spin and charge regions) and displays persistence under ambient conditions. We demonstrate its application in a redox-flow battery exhibiting minimal voltage hysteresis, a flat voltage plateau, high Coulombic efficiency, and no performance decay for at least 100 cycles. The chemical tunability of NHCs and CDIs suggests that this approach could provide a general entry to redox-active NHC-CDI adducts and their persistent radical ions for various applications.

An N‐Heterocyclic‐Carbene‐Derived Distonic Radical Cation

AbstractWe present the discovery of a novel radical cation formed through one‐electron oxidation of an N‐heterocyclic carbene–carbodiimide (NHC–CDI) zwitterionic adduct. This compound possesses a distonic electronic structure (spatially separate spin and charge regions) and displays persistence under ambient conditions. We demonstrate its application in a redox‐flow battery exhibiting minimal voltage hysteresis, a flat voltage plateau, high Coulombic efficiency, and no performance decay for at least 100 cycles. The chemical tunability of NHCs and CDIs suggests that this approach could provide a general entry to redox‐active NHC–CDI adducts and their persistent radical ions for various applications.

Structure fragmentation studies of ring-substituted N-trifluoroacetyl-N-benzylphenethylamines related to the NBOMe drugs.

The halogenated derivatives of N-(2-methoxy)benzyl-2,5-dimethoxyphenethylamine (25-NBOMe) such as the 4-bromo analogue (25B-NBOMe) represent a new class of hallucinogenic or psychedelic drugs. The purpose of this study was to determine the role of the electron-donating groups (halogen and dimethoxy) in the pathway of decomposition for the distonic molecular radical cation in the electron ionization mass spectrometry (EI-MS) process of the trifluoroacetamide (TFA) derivatives. The systematic removal of substituents from the 4-halogenated 2,5-dimethoxyphenethylamine portion of the N-dimethoxybenzyl NBOMe analogues allowed an evaluation of structural effects on the formation of major fragment ions in the EI-MS of the TFA derivatives. All six regioisomeric dimethoxybenzyl-substituted analogues (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dimethoxy) for the four series of phenethyl aromatic ring substitution patterns were prepared, derivatized and analyzed via gas chromatography coupled with EI-MS. The analogues yield two unique radical cation fragments from the decomposition of the common distonic molecular radical cation. The substituted phenylethene radical cation (m/z 164) is the base peak or second most abundant ion in all six TFA-2,5-dimethoxyphenethylamine isomers. The dimethoxybenzyltrifloroacetamide radical cation (m/z 263) is the base peak or second most abundant ion in the 2- and 3-monomethoxyphenethylamine isomers. However, the 2- and 3-methoxyphenylethene radical cation (m/z 134) is among the five most abundant ions for each of these twelve isomers. Only one isomer in the phenethylamine series yields the corresponding unsubstituted phenylethene radical cation at m/z 104. The decomposition of the hydrogen-rearranged distonic molecular radical cation favors formation of the dimethoxybenzyltrifloroacetamide (m/z 263) species for the less electron-rich phenethyl aromatic rings. The addition of electron-donating groups to the aromatic ring of the phenethyl group as in the NBOMe-type molecules shifts the decomposition of the common distonic molecular radical cation to favor the formation of the electron-rich substituted phenylethene radical cation.

Stereoelectronic and Resonance Effects on the Rate of Ring Opening of N-Cyclopropyl-Based Single Electron Transfer Probes.

N-Cyclopropyl-N-methylaniline (5) is a poor probe for single electron transfer (SET) because the corresponding radical cation undergoes cyclopropane ring opening with a rate constant of only 4.1 × 104 s-1, too slow to compete with other processes such as radical cation deprotonation. The sluggish rate of ring opening can be attributed to either (i) a resonance effect in which the spin and charge of the radical cation in the ring-closed form is delocalized into the phenyl ring, and/or (ii) the lowest energy conformation of the SET product (5•+) does not meet the stereoelectronic requirements for cyclopropane ring opening. To resolve this issue, a new series of N-cyclopropylanilines were designed to lock the cyclopropyl group into the required bisected conformation for ring opening. The results reveal that the rate constant for ring opening of radical cations derived from 1'-methyl-3',4'-dihydro-1'H-spiro[cyclopropane-1,2'-quinoline] (6) and 6'-chloro-1'-methyl-3',4'-dihydro-1'H-spiro[cyclopropane-1,2'-quinoline] (7) are 3.5 × 102 s-1 and 4.1 × 102 s-1, effectively ruling out the stereoelectronic argument. In contrast, the radical cation derived from 4-chloro-N-methyl-N-(2-phenylcyclopropyl)aniline (8) undergoes cyclopropane ring opening with a rate constant of 1.7 × 108 s-1, demonstrating that loss of the resonance energy associated with the ring-closed form of these N-cyclopropylanilines can be amply compensated by incorporation of a radical-stabilizing phenyl substituent on the cyclopropyl group. Product studies were performed, including a unique application of EC-ESI/MS (Electrochemistry/ElectroSpray Ionization Mass Spectrometry) in the presence of 18O2 and H218O to elucidate the mechanism of ring opening of 7•+ and trapping of the resulting distonic radical cation.

Reaction of Distonic Aryl and Alkyl Radical Cations with Amines: The Role of Charge and Spin Revealed by Mass Spectrometry, Kinetic Studies, and DFT Calculations

AbstractGas‐phase reaction of the aromatic distonic radical cations 4‐Pyr+. and 3‐Pyr+. with amines led to formation of the corresponding amino pyridinium ions 4‐Pyr+NR2 and 3‐Pyr+NR2 through amine addition at the site of the radical, followed by homolytic β‐fragmentation. The reaction efficiencies range from 66–100 % for 4‐Pyr+. and 57–86 % for 3‐Pyr+., respectively, indicating practically collision‐controlled processes in some cases. Computational studies revealed that the combination of positive charge and spin makes nucleophilic attack by the amine at the site of the radical barrierless and strongly exothermic by about 175±15 kJ mol−1, thereby rendering ‘conventional’ radical pathways through hydrogen abstraction or addition onto π systems less important. Exemplary studies with 4‐Pyr+. showed that this reaction can be reproduced in solution. A similar addition/radical fragmentation sequence occurs also in the gas‐phase reaction of amines with the aliphatic distonic radical cation Oxo+C., showing that the observed charge‐spin synergism is not limited to aromatic systems.

A DFT study on radical-cationic methanol clusters: Structure, bonding and H-transfer

Radical-cationic methanol clusters (CH3OH) +n, n = 2–5, are studied in the gas phase and solvent methanol using hybrid B3LYP, CAM-B3LYP and BHandHLYP exchange-correlation functionals. The CAM-B3LYP predictions are the most consistent. Ionization of parental neutral rings and chains yields distonic radical cations with spin residing on the CH3O link. Higher energy OO hemibonded structures are also located. The intra-cluster H-transfer from the methyl group of the CH3OH-terminus of the protonated moiety to the O-atom of the CH3O terminus in chain radical cations leads to the lowest-energy distonic isomers with the CH2OH terminus. The CAM-B3LYP activation enthalpies for this reaction in the gas phase are 9.5, 11.1, 13.7, and 17.1 kcal/mol for n = 5, 4, 3, 2, respectively. The inter-cluster H-transfer from the CH3O terminus to a neutral methanol cluster occurs spontaneously, with formation of a protonated cluster weakly bound to the radical cluster. The protonated cluster may serve as a proton carrier.

Molecular Weight Growth in the Gas-Phase Reactions of Dehydroanilinium Radical Cations with Propene.

Protonated aniline-one of the simplest nitrogen-bearing aromatic molecules-is speculated to be present within Titan's atmosphere, where it could play a role in molecular weight growth chemistry. To investigate this possibility, this paper examines the reactions of propene with distonic radical cations derived from protonated aniline. The reaction kinetics, products, and branching ratios of these distonic radical cations (i.e., 2-, 3-, and 4-dehydroanilinium radical cations) are measured in the gas phase using ion-trap mass spectrometry, and calculations (M06-2X/6-31G(2df,p) and G3X-K) are used to rationalize the results. The reaction efficiencies for the 2-, 3-, and 4-dehydroanilinium radical cations are reported as 30.7 (±3.1)%, 2.4 (±0.5)%, and 2.1 (±0.4)%, respectively (with ±50% absolute accuracy). The 4- and 3-dehydroanilinium radical cations undergo addition of propene followed by either (i) methyl radical loss forming ammonio styrene cations (m/z 120) or (ii) ethylene loss pathways forming the ammonio benzyl radical cation (m/z 107). The 4-dehydroanilinium reaction favors methyl loss over ethylene loss, whereas the reverse is true for 3-dehydroanilinium. In contrast, the 2-dehydroanilinium radical cation reacts with propene to purportedly form amino tropylium cation (m/z 106) with ethyl radical loss and methyl amino tropylium cation (m/z 120) with methyl radical loss. The energies of all minima and transition states leading to predicted dissociation products are below the energy of the initial reactants providing plausible pathways at the low temperatures of Titan's atmosphere.

Isotactic Regulation in the Radical Polymerization of Calcium Methacrylate: Is Multiple Chelation the Key to Stereocontrol?

ABSTRACTAccurate quantum chemistry is used to explain the origins of isospecificity in radical polymerization of calcium methacrylate hydrate (CaMA). Distonic radical–cation interactions are shown to be crucial in determining the reactivity of different coordination structures. Cation coordination to the terminal and incoming monomer side chains reduces radical‐cation separation, enhancing the reactivity of these modes over the stereocontrolling terminal‐penultimate binding modes. This explains why Lewis acid‐mediated radical polymerization often fails to produce highly isotactic polymer for common monomers such as methyl methacrylate. However, theoretical calculations suggest that the poly(CaMA) terminus forms a chelated bridging scaffold in N,N‐dimethylformamide (DMF), which involves the terminal, penultimate and incoming monomer carboxylate groups. This scaffold simultaneously activates the incoming monomer toward propagation and regulates the relative orientation of the terminal and penultimate side chains. The bridging scaffold is disrupted in more polar solvents and/or if alternative nonchelating counter‐cations are employed, leading to loss of isotactic control. These results suggest that higher levels of isotactic control may be achievable if reaction conditions are optimized to favor bridging scaffold formation. The broader importance of these findings to stereocontrol in radical polymerization is also discussed. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 52–61

Detection and follow‐up reactions of distonic β,β‐dimesityl enol radical cations containing nitrogen heterocyclic bases

Abstractβ,β‐Dimesityl enols E2‐E4 containing various nitrogen heterocyclic bases in the α‐position were investigated by laser flash photolytic oxidation and density functional theory in order to identify distonic enol radical cations in solution and characterize their immediate follow‐up reactions. Enol E1 with a tert‐butyl group in the α‐position served as a reference. Experimental and computational efforts reveal that one‐electron oxidation of enol E1 furnishes the enol radical cation, whereas oxidation of E2‐E4 provides distonic radical cations in solution. While the distonic radical cations of E2‐E3 undergo intermolecular acid‐base reactions, the distonic radical cation of E4 experiences an intramolecular hydrogen abstraction followed by subsequent ring closure and ring‐opening steps. As a final product on the laser flash photolysis time scale, the rearranged radical cation E4d‐R is generated that is characterized by a dimesitylmethyl radical subunit.

Difunctionalization of Cyclopropylanilines and Cyclobutylanilines

We have previously developed photoredox conditions for ring opening of cyclopropylanilines and cyclobutylanilines. Distonic radical cations are presumably the intermediates. Although commonly producing in gas phase such as in the ionization chamber of a mass spectrometer, in solution phase, the methods for generating them are quite limited. Bearing a spatially separated radical and cation within the same molecule, these species possess bimodal reactivities that are distinct from those of radicals, ions, or classical radical ions (the radical and the ion reside at the same atom). In the [3+2] annulation of cyclopropylanilines and the [4+2] annulation of cyclobutylanilines developed by us, only one pi bond of alkenes, alkynes, dienes, enynes, or diynes reacts with the radical and the cation moiety of the distonic radical cations sequentially to form a five-membered and six-membered carbocycle respectively. Therefore, the two reactions don’t maximize the distonic radical cations’ bimodal reactivity. We have recently focused on identifying a suitable nucleophile and a radical acceptor to orthogonally intercept the cation and the radical moiety, which allows us to capitalize the distonic radical cations’ bimodal reactivity fully. Herein, we report our efforts in harvesting the distonic radical cations' bimodal reactivity, which results in 1,3-difunctionalization of cyclopropylanilines and 1,4-difunctionalization of cyclobutylanilines.