Reactions Of Radical Cations Research Articles

Electrochemical Synthesis of Fused Indole Alkaloids By Controlling Radical Cation Reactivities

Pyrrolophenanthridone alkaloids are isolated from Amaryllidaceae and it has fused N-benzoyl indole as core structure. Many researchers targeted this simple alkaloids to demonstrate various synthetic method1), such as palladium- or hypervalent iodine-mediated intramolecular cross-coupling, tetrazine-promoted inverse electron-demand Diels-Alder reaction. All examples required DDQ oxidation to convert indoline to indole.We established metal- and oxidant-free electrochemical intramolecular C(sp2)-H cross-coupling and indole synthesis reaction. These reactions were initiated by same anodic oxidation for electron-rich indoline ring, but different electron-transfer pathway was occurred in both cyclization and indole forming reaction. Moreover, non-coordinative medium has crucial role to control reactivity of radical cation. Increasing electrophilicity of radical cation is key strategy to accomplish intramolecular cross-coupling. MeNO2-LiClO4 and HFIP (1,1,1,3,3,3,-hexafluoro-2-propanol), which known as suitable medium for radical cation-mediated reaction2), had not worked well for this reaction. However, MeNO2-HFIP (9:1) mixed solvent strongly enhanced the reactivity of radical cations. This phenomenon was occurred by synergetic stabilization effect of MeNO2 and HFIP3). The low donor number of MeNO2 and anion trapping ability of HFIP made non-coordinative environment and resulting electrophilic radical cation was maintained in this medium. Dehydrogenative indole forming reaction was accomplished in Lewis-basic condition (MeCN-Bu4NClO4), and collidine additive enhanced benzylic deprotonation from radical cation.Finally, we applied both reactions for the synthesis of seven naturally occurred pyrrolophenanthridone alkaloids4). References (1) (a) Black, DC.; Keller, PA.; Kumar, N. Tetrahedron Lett. 1989, 30, 5807-5808; (b) Ganton, MD.; Kerr, MA. Org. Lett. 2005, 21, 4777-4779; (c) Boger, DL.; Wolkenberg, SE. J. Org. Chem. 2000, 65, 9120-9124.(2) (a) Okada, Y.; Chiba, K. Chem. Rev. 2018, 118, 4592-4630; (b) Kirste, A.; Nieger, M.; Malkowsky, IM.; Stecker, F.; Fisher, A.; Waldvogel, SR. Chem. Eur. J. 2009, 15, 2273-2277.(3) (a) Shida, N.; Imada, Y.; Nagahara, S.; Okada, Y.; Chiba, K. Commun. Chem. 2019, 2, 24; (b) Shida, N.; Imada, Y.; Okada, Y.; Chiba, K. Eur. J. Org. Chem. 2020, 2020, 570-574.(4) Okamoto, K.; Chiba, K. Org. Lett. 2020, accepted (DOI: 10.1021/acs.orglett.0c01082). Figure 1

Redox-Neutral Radical-Cation Reactions: Multiple Carbon–Carbon Bond Formations Enabled by Single-Electron Transfer

Redox-Neutral Radical-Cation Reactions: Multiple Carbon–Carbon Bond Formations Enabled by Single-Electron Transfer

Mass spectrometry distinguishing C=C location and cis/trans isomers: A strategy initiated by water radical cations

We present an approach for the elucidation of C=C bond position and cis/trans isomers, which is achieved by the reaction of ambient water radical cations and double bonds, followed by the fragmentation of epoxide radical cations to generate diagnostic ions in tandem mass spectrometry. Hexenol double bond positional isomers and cis/trans isomers which exhibit different properties and biological functions are characterized as a proof of concept. The merits of the approach include the simplicity of experimental setup, rapid derivatization (within seconds), the obviation of organic solvents, as well as easy spectral interpretation.

Case studies of the radical cation reactivity in meso-aryl and octaethyl porphyrins

The reactivity of porphyrin radical cationic species derived from octaethyl porphyrin (OEP) or meso-aryl porphyrins with nucleophiles, envisioned as an access route to elaborate porphyrin dimers, has been studied and optimized in the case of OEP. Standardized conditions have been applied to various spacers to show that the success of the reaction is mostly nucleophile dependent and that the method has little chances to yield non-linear bis-porphyrins.

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.

Photoredox-catalyzed oxo-amination of aryl cyclopropanes

Cyclopropanes represent a class of versatile building blocks in modern organic synthesis. While the release of ring strain offers a thermodynamic driving force, the control of selectivity for C–C bond cleavage and the subsequent regiochemistry of the functionalization remains difficult, especially for unactivated cyclopropanes. Here we report a photoredox-coupled ring-opening oxo-amination of electronically unbiased cyclopropanes, which enables the expedient construction of a host of structurally diverse β-amino ketone derivatives. Through one electron oxidation, the relatively inert aryl cyclopropanes are readily converted into reactive radical cation intermediates, which in turn participate in the ensuing ring-opening functionalizations. Based on mechanistic studies, the present oxo-amination is proposed to proceed through an SN2-like nucleophilic attack/ring-opening manifold. This protocol features wide substrate scope, mild reaction conditions, and use of dioxygen as an oxidant both for catalyst regeneration and oxygen-incorporation. Moreover, a one-pot formal aminoacylation of olefins is described through a sequential cyclopropanation/oxo-amination.

Impact of the Replacement of a Triphenylamine by a Diphenylmethylamine Unit on the Electrochemical Behavior of Pentaerythritol‐Based Push‐Pull Tetramers

AbstractThe synthesis of a tetra‐functionalized pentaerythritol core decorated with N‐methyl‐N,N‐diphenylamine‐based push‐pull chromophores and its electropolymerization to 3D push‐pull networks are described. The electrochemical and absorption behaviors of the tetramer are compared with the one of two reference linear push‐pull compounds, carrying triphenylamine (TPA) or methyldiphenylamine (MeDPA) donor groups, a thienyl linker and a dicyanovinyl acceptor group (DCV). We found that substituting the outer phenyl with a methyl group causes important differences in the radical cation stability, such that MeDPA chromophore generates stable dimers and TPA is reversibly oxidized. Interestingly, DFT calculations suggest that steric hindrance and electrostatic interactions dominate the radical cation reactivity.

Interplay of arene radical cations with anions and fluorinated alcohols in hole catalysis

Chemical reactions via radical cation intermediates are of great interest in photoredox catalysis and electrosynthesis, while their reactivities are not clearly understood. For example, how the counter anions correlate with the reactivity of radical cations is still ambiguous. Here we report the effect of anions and fluorinated alcohols on the reactivity of organic radical cations in hole catalysis. The addition of salts in a radical cation Diels–Alder reaction under photoredox catalysis demonstrates that common anions significantly decease the efficiency of hole catalysis. The use of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) restores the reaction efficiency in the presence of salts, presumably due to solvation of the anions by HFIP to reduce their nucleophilicity. These findings enable hole catalysis under electrolytic conditions with greatly improved efficiency. The effect of anions and fluorinated alcohol described in this paper gives important insights on the fundamental understanding for the reactivity of arene radical cations.

Quantum Chemical Characterization of Factors Affecting the Neutral and Radical-Cation Newman-Kwart Reactions.

Details of the electronic and geometric structures of stationary points along the reaction coordinate of the Newman Kwart rearrangement, which transforms an O-arylthiocarbamate into an S-arylcarbamothioate, are examined using quantum-chemical methods for a large number of compounds considering both the thermal reactions of uncharged substrates as well as the corresponding reactions of radical-cation substrates generated under photoredox conditions. The uncharged mechanism, which has intrinsically high 298 K free energies of activation (in excess of 30 kcal mol-1), has the character of nucleophilic aromatic substitution and is thus accelerated by electron-withdrawing substituents on the aromatic ring. The radical-cationic mechanism, by contrast, has 298 K free energies of activation that are typically below 20 kcal mol-1 and is accelerated by electron donating substituents on the aromatic ring, which stabilize the hole character that is transferred to this fragment from the thiocarbamate fragment as the reaction proceeds. Opportunities to further accelerate the radical-cation reaction are revealed by computational assessment of alternative amino groups for the thiocarbamate functionality.

Biphotonic Ionization of DNA: From Model Studies to Cell.

Oxidation reactions triggered by low-intensity UV photons represent a minor contribution with respect to the overwhelming pyrimidine base dimerization in both isolated and cellular DNA. The situation is totally different when DNA is exposed to high-intensity UVC radiation under conditions where biphotonic ionization of the four main purine and pyrimidine bases becomes predominant at the expense of singlet excitation processes. The present review article provides a critical survey of the main chemical reactions of the base radical cations thus generated by one-electron oxidation of nucleic acids in model systems and cells. These include oxidation of the bases with the predominant formation of 8-oxo-7,8-dihydroguanine as the result of preferential hole transfer to guanine bases that act as sinks in isolated and cellular DNA. In addition to hydration, other nucleophilic addition reactions involving the guanine radical cation give rise to intra- and interstrand cross-links together with DNA-protein cross-links. Information is provided on the utilization of high-intensity UV laser pulses as molecular biology tools for studying DNA conformational features, nucleic acid-protein interactions and nucleic acid reactivity through DNA-protein cross-links and DNA footprinting experiments.

TiO2 Photocatalysis in Aromatic "Redox Tag"-Guided Intermolecular Formal [2 + 2] Cycloadditions.

Since the pioneering work by Macmillan, Yoon, and Stephenson, homogeneous photoredox catalysis has occupied a central place in new reaction development in the field of organic chemistry. While heterogeneous semiconductor photocatalysis has also been studied extensively, it has generally been recognized as a redox option in inorganic chemistry where such "photocatalysis" is most often used to catalyze carbon-carbon bond cleavage and not in organic chemistry where bond formation is usually the focal point. Herein, we demonstrate that titanium dioxide photocatalysis is a powerful redox option to construct carbon-carbon bonds by using intermolecular formal [2 + 2] cycloadditions as models. Synergy between excited electrons and holes generated upon irradiation is expected to promote the overall net redox neutral process. Key for the successful application is the use of a lithium perchlorate/nitromethane electrolyte solution, which exhibits remarkable Lewis acidity to facilitate the reactions of carbon-centered radical cations with carbon nucleophiles. The reaction mechanism is reasonably understood based on both intermolecular and intramolecular single electron transfer regulated by an aromatic "redox tag". Most of the reactions were completed in less than 30 min even in aqueous and/or aerobic conditions without the need for sacrificial reducing or oxidizing substrates generally required for homogeneous photoredox catalysis.

TiO2 Photocatalysis in the Aromatic “Redox Tag”-Guided Intermolecular Formal [2 + 2] Cycloadditions

Light has the potential to serve as green and sustainable reagent to catalyze chemical reactions. Stemming from the pioneering work of Honda and Fujishima in 1972, titanium dioxide (TiO2) has occupied a central place in the research field of semiconductor photocatalysis. The excited electron/hole pair generated upon irradiation in aqueous aerobic conditions produces reactive oxygen species (ROSs) at the surface, which are account for subsequent redox transformations. They rapidly convert organic compounds into CO2 and H2O via carbon-carbon bond cleavage, therefore, semiconductor photocatalysis has generally been recognized as redox options in inorganic chemistry rather than organic chemistry where carbon-carbon bond formation is a focal point for extensive research efforts. While semiconductor photocatalysis is a heterogeneous process, a homogeneous alternative can be provided by the use of transition metal complexes, which is now well- known as “photoredox catalysis”. The early works by MacMillan, Yoon, and Stephonson have triggered the explosive growth in the general area of new reaction developments in this class. Arguably, heterogeneous semiconductor photocatalysis has taken a backseat to homogeneous photoredox catalysis in the field of organic chemistry. For TiO2 photocatalysis to be effective in carbon-carbon bond formation, the reactions must be carried out in dry anaerobic conditions in order not to produce ROSs that can damage organic compounds. We have been developing carbon-carbon bond formations by electrocatalysis in lithium perchlorate (LiClO4)/nitromethane (CH3NO2) electrolyte solution.1 Our reaction design involves the aromatic “redox tag” concept,2 that is, an aromatic ring that provides both the oxidant and the reductant in the overall net redox neutral carbon-carbon bond formations. In most cases, the use of LiClO4/CH3NO2 electrolyte solution is critical, which exhibits remarkable property as a Lewis acid to facilitate the reactions of carbon-centered radical cations with carbon nucleophiles, even unactivated alkenes. Our reaction development is also characterized by the importance of an intramolecular electron transfer process, which is recently presented by Melchiorre as an elegant “electron relay” mechanism. We anticipated that the use of LiClO4/CH3NO2 electrolyte solution would pave the way for carbon-carbon bond formations by TiO2 photocatalysis. It should also be noted that overall net redox neutral processes would be benefited from the involvement of both reductive and oxidative electron transfers potentially enabled by excited electron and hole. In this talk, the aromatic redox tag-guided intermolecular formal [2 + 2] cycloadditions3 by TiO2 photocatalysis will be discussed in details. (1) Imada, Y.; Yamaguchi, Y.; Shida, N.; Okada, Y.; Chiba, K. Chem. Commun. 2017, 53, 3960–3963. (2) (a) Okada, Y.; Chiba, K. Chem. Rev. 2018, accepted. (b) Okada, Y.; Yamaguchi, Y.; Ozaki, A.; Chiba, K. Chem. Sci. 2016, 7, 6387–6393. (c) Okada, Y.; Nishimoto, A.; Akaba, R.; Chiba, K. J. Org. Chem. 2011, 76, 3470–3476. (3) Chiba, K.; Miura, T.; Kim, S.; Kitano, Y.; Tada, M. J. Am. Chem. Soc. 2001, 123, 11314–11315. Figure 1

Umpolung Diels-Alder Reactions By Electrocatalysis

Although the Diels–Alder reaction is a classic reaction, it is still at the forefront of the carbon–carbon bond formation toolbox in synthetic chemistry. In particular, the mechanism of the reaction has been well studied, and the scope has been expanded to a wide variety of diene/dienophile combinations. However, electronic matching needs to be considered; more specifically, that one reaction partners should be electron-rich and the other electron-deficient. To overcome this hurdle, "umpolung" strategy has been proven to be powerful approach. In this context, single electron transfer (SET) would be the most simple and straightforward process. For example, electron-deficient radical cation species can be produced from electron-rich reactant via SET oxidation. A typical example of the umpolung Diels-Alder reactions is the reaction between an electron-rich diene and a dienophile. These combinations are electronically mismatched, however, SET oxidation can produce a radical cation of one component, which is then able to be trapped by the other component. In addition to the use of chemical redox agents, photocatalysis and electrocatalysis are currently recognized as effective approaches to induce SET processes. Umpolung Diels-Alder reactions have also been extensively studied for almost half a century, however, scope of the reactions has been limited. In many cases, the reactions were triggered by SET oxidation of diene components, generating the radical cation species, and which were then trapped by dienophiles. In 2011, Yoon elegantly demonstrated the opposite version, namely, the reaction triggered by SET oxidation of dienophile components using photocatalysis. We have been developing carbon-carbon bond formations by electrocatalysis in lithium perchlorate (LiClO4)/nitromethane (CH3NO2) electrolyte solution.1 In most cases, the use of LiClO4/CH3NO2 electrolyte solution is critical, which exhibits remarkable property as a Lewis acid to facilitate the reactions of carbon-centered radical cations with carbon nucleophiles. We also have shown that the electrocatalysis in LiClO4/CH3NO2 electrolyte solution is effective to induce umpolung Diels-Alder reactions.2 In this talk, recent advancement of the umpolung Diels-Alder reactions by electrocatalysis will be discussed in details. (1) Imada, Y.; Yamaguchi, Y.; Shida, N.; Okada, Y.; Chiba, K. Chem. Commun. 2017, 53, 3960–3963.(2) (a) Ozaki, A.; Yamaguchi, Y.; Okada, Y.; Chiba, K. ChemElectroChem 2017, 4, 1852–1855. (b) Okada, Y.; Yamaguchi, Y.; Ozaki, A.; Chiba, K. Chem. Sci. 2016, 7, 6387–6393. Figure 1

Using the Complementarity of Electrochemistry and Photoelectron Transfer to Probe and Develop the Chemistry of Radical Cations

Through the years, we have utilized electrochemistry as a method to explore radical cation intermediates and the oxidative cyclization reactions they trigger. On the mechanistic side of things, we have uncovered a set of empirical rules that govern radical cation reactivity and allow for the design of new reactions. On the synthetic side of things, we have used the ensuing reactions to make heterocycles, fused-, spirocyclic-, and bridged molecules, and highly hindered tetrasubstituted carbons. Recently, we have found that these initial mechanistic models have to be adjusted to account for not only the initial cyclization reaction, but also the intermediates generated downstream of that step. In particular, the oxidation of a second electron from the molecule (k2 in Scheme 1) frequently plays a key role in the synthetic success of a transformation. While this observation was clear for cases for slower trapping reactions that might have a reversible first step, it was assumed that our very best trapping reactions that were known to be kinetically fast had no such issues. This assumption has also proven problematic. Photoelectron transfer reactions that are ideally suited for conducting one-electron oxidation based processes are ideal tools for probing cyclizations that require the loss of a second electron (Scheme 2). Note that all three substrates shown underwent a two electron oxidative cyclization nicely at an anode to give the cyclic products shown. The substrate with the alcohol trapping group also worked well providing a redox neutral complementary product to the electrolysis reaction. The substrate with the sulfonamide trapping group led to no reaction. This was not a surprise since the cyclization with the sulfonamide trapping group is slower than the alcohol trapping reaction. The reaction is dominated by a back electron transfer from the dithioketene acetal derived radical cation to the catalyst. The reaction with the enol ether trapping group led to polymer that showed evidence for consumption of both olefins in the starting material. This was a surprise since the cyclization is known to be very fast (much faster than the alcohol trapping of the radical cation). The reaction clearly required the loss of a second electron to form the desired cyclized product. This was a conclusion that could only be reached by comparing the photochemical and electrochemical reactions. In the talk to be presented, the historical context and future synthetic ramifications of this and other mechanistic studies that define the nature of the reactive intermediate needed for the initial cyclization will be discussed. Figure 1

Direct Microscopic Analysis of Individual C60 Dimerization Events: Kinetics and Mechanisms.

Modern transition state theory states that the statistical behavior of a chemical reaction is the sum of individual chemical events that occur randomly. Statistical analysis of each event for individual molecules in a three-dimensional space however is practically impossible. We report here that kinetics and mechanisms of chemical reactions can be investigated by using a one-dimensional system where reaction events can be observed in situ and counted one by one using variable-temperature (VT) atomic-resolution transmission electron microscopy (TEM). We thereby provide direct proof that the ensemble behavior of random events conforms to the Rice-Ramsperger-Kassel-Marcus theory, as illustrated for [2 + 2] cycloaddition of C60 molecules in carbon nanotubes (CNTs). This method gives kinetic and structural information for different types of reactions occurring simultaneously in the microscopic view field, suggesting that the VT-TEM opens a new dimension of chemical kinetics research on molecules and their assemblies in their excited and ionized states. The study carried out at 393-493 K showed that pristine CNT primarily acts as a singlet sensitizer of the cycloaddition reaction that takes place with an activation energy of 33.5 ± 6.8 kJ/mol. On the other hand, CNT suffers electron damage of the conjugated system at 103-203 K and promotes a reactive radical cation path that takes place with an activation energy of only 1.9 ± 0.7 kJ/mol. The pre-exponential factor of the Arrhenius plot gave us further mechanistic insights.

Reaction mechanism of the cation radicals of thymine and thymine derivatives in aqueous solution: A theoretical study

Mechanisms of decomposition reaction of thymine cation radicals (T[Formula: see text]), 1-methyl substituted T[Formula: see text] (1-MeT[Formula: see text]), and 1,3-dimethyl substituted T[Formula: see text] (1,3-Me2T[Formula: see text]) generated by ionizing radiation or one-electron oxidation to thymine, 1-methylthymine and 1,3-dimethylthymine in aqueous solution have been investigated by theoretical calculations. Seven reaction paths including H2O or OH[Formula: see text] addition, and deprotonation were considered. The most likely reaction for T[Formula: see text] decomposition is N1 deprotonation forming T(-N1H)[Formula: see text]. When N1 hydrogen is replaced by methyl, T[Formula: see text] would deprotonate from C5 methyl group generating 1-methyl substituted UCH[Formula: see text] in neutral solution, and reaction with OH[Formula: see text] producing 1-methyl substituted T6OH[Formula: see text] in basic solution. When N1 and N3 hydrogens are both replaced by methyl, T[Formula: see text] decomposition reaction is similar to 1-MeT[Formula: see text], indicating that N3 hydrogen barely influences the subsequent reaction of 1-MeT[Formula: see text]. These results can provide theoretical support for experimental observations.

Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall

In the context of generating reactive organic radical cations within a confined capsule and exploring photoinduced electron transfer from encapsulated organic molecules to organic and inorganic acceptors through an organic molecular wall, we have investigated electron transfer from the upper excited state (S2) of azulene (Az) and guaiazulene (GAz) enclosed within an octa acid (OA) capsule to water-soluble 4,4′-dimethyl viologen2+ (MV2+) and pyridinium+ (Py+) salts or colloidal TiO2. S2 fluorescence of OA encapsulated Az and GAz was quenched by electron acceptors such as MV2+ and Py+ salts. That electron transfer is responsible for S2 fluorescence quenching was established by recording the transient absorption spectrum of the MV●+ in the femtosecond time regime. Femtosecond time-resolved fluorescence experiments suggested that the time constant for the forward and reverse electron transfer from encapsulated Az and GAz to MV2+ is 4 and 3.6 ps, and 55.7 and 36.9 ps, respectively. The observed S2 fluorescence...

A Tetrasubstituted Hydroquinone Ether Catholyte for Non-Aqueous Redox Flow Batteries: Bicyclic Substitution Enabling High Stability

The practical application of non-aqueous redox flow batteries (NRFBs) sets stringent requirements on the electrochemical stability of the redox active materials. In the redox pair for NRFBs, the high-potential species is referred to as the catholyte, which is oxidized into the radical cation state in the charging process. Hydroquinone ethers are a key class of catholyte materials, but the intermolecular reaction between the radical cations is a dominating decomposition pathway of the catholyte during functioning. In order to increase the electrochemical stability of the catholyte materials, the introduction of bulky alkyl spacers at 2,5- positions of the arene ring is commonly used as a strategy in the molecular design to suppress this type of undesirable side reaction. However, the unsubstituted 3,6- positions that are favored for the electrochemical reversibility are responsible for the radical cation reaction in the long-time cycling. Tetrasubstitution of the arene core with either primary or secondary alkyl groups often leads to a compromised redox reversibility. This talk will describe a tetrasubstituted hydroquinone ether-based catholyte molecule that retains excellent redox reversibility and illustrates exceptional stability. Unique bicyclic alkyl groups are incorporated to the molecular scaffold, eliminating the bimolecular reaction between the radical cations. A hybrid NRFB using such catholyte has been operated for over 150 charge-discharge cycles with minimal loss of capacity.

Detection of Fleeting Amine Radical Cations and Elucidation of Chain Processes in Visible-Light-Mediated [3 + 2] Annulation by Online Mass Spectrometric Techniques.

Visible-light-mediated photoredox reactions have recently emerged as a powerful means for organic synthesis and thus have generated significant interest from the organic chemistry community. Although the mechanisms of these reactions have been probed by a number of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degrees of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) because of the lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in the intermolecular [3 + 2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture. In particular, the reactive CPA radical cation 1+•, the reduced photocatalyst Ru(I)(bpz)3+, and the [3 + 2] annulation product radical cation 3+• are all successfully detected and confirmed by high-resolution MS. More importantly, the post-irradiation reaction with an additional substrate, isotope-labeled CPA, following photolysis of 1, 2, and Ru catalyst provides strong evidence to support the chain mechanism in the [3 + 2] annulation reaction. Furthermore, the key step of the proposed chain reaction, the oxidation of CPA 1 to amine radical cation 1+• by product radical cation 3+• (generated using online electrochemical oxidation of 3), is successfully established. Additionally, the coupling of ESI-MS with online laser irradiation has been successfully applied to probe the photostability of photocatalysts.

Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy.

Delocalization of charges is one of the factors controlling charge transport in conjugated molecules. It is considered to play an important role in the performance of a wide range of molecular technologies, including organic solar cells and organic electronics. Dimerization reactions are well-suited as a model to investigate intermolecular spatial delocalization of charges. While dimerization reactions of radical cations are well investigated, studies on radical anions are still scarce. Upon dimerization of radical anions with neutral counterparts, an electron is considered to delocalize over the two molecules. Here, by using time-resolved infrared (TRIR) detection coupled with pulse radiolysis, we show that radical anions of 4-n-hexyl-4'-cyanobiphenyl (6CB) undergo such dimerization reactions, with an electron equally delocalized over the two molecules. We have recently demonstrated that nitrile ν(C≡N) vibrations respond to the degree of electron localization of nitrile-substituted anions: we can quantify the changes in the electronic charges from the neutral to the anion states in the nitriles by monitoring the ν(C≡N) IR shifts. In the first part of this article, we show that the sensitivity of the ν(C≡N) IR shifts does not depend on solvent polarity. In the second part, we describe how probing the shifts of the nitrile IR vibrational band unambiguously confirms the formation of dimer radical anions, with Kdim = 3 × 104 M-1. IR findings are corroborated by electronic absorption spectroscopy and electronic structure calculations. We find that the presence of a hexyl chain and the formation of π-π interactions are both crucial for dimerization of radical anions of 6CB with neutral 6CB. The present study provides clear evidence of spatial delocalization of electrons over two molecular fragments.