Intramolecular Single Electron Transfer Research Articles

Probing Electron Transfer Events in Radical Cation Cycloadditions: Intramolecular vs. Intermolecular Single Electron Transfer

AbstractAlthough radical ion chain pathways have long been recognized in the field of synthetic organic chemistry and are expected to be operative in various reactions, it is somewhat complicated to verify their involvement, since counting the number of electrons is not straightforward. Herein, a series of bis‐styrenes is designed and synthesized to investigate electron transfer events by using radical cation [4+2] cycloadditions as probes. It is demonstrated that the intramolecular single electron transfer seems to be ineffective, while the intermolecular variants are highly effective. Therefore, a truly catalytic amount of electricity is enough for full conversions, confirming that chain pathways were definitely involved.

Photochemical palladium-catalyzed methylation and alkylation reactions in cascade reactions of isonitriles

Photochemistry of palladium catalysis has been proved as an efficient strategy for cross coupling and multifunctionalization reaction of olefins via alkyl radicals. Herein, we demonstrate that isonitriles can serve as radical acceptors, which engage in a further radical cascade cyclization to furnish phenanthridine derivatives. We have also extended this photocatalytic process to cascade methylation reaction in one-pot with TMSCH2I as the radical precursor. Further experimental and computational studies indicate an intramolecular single electron transfer process to obtain alkyl radicals is involved in this transformation.

The Lewis Acid Induced Formation of a Stable Diradical with an Intramolecular Ion Pairing State.

A stable cross-conjugated diradical was prepared by the reaction of a donor-acceptor-donor (D-A-D) molecule with B(C6F5)3. Its geometry and electronic structure were characterized by single crystal X-ray diffraction, EPR spectroscopy, SQUID measurement, UV/vis spectroscopy, and DFT calculation. It has an open-shell singlet ground state with a thermally excited triplet state. It can be viewed as an intramolecular radical ion pair, and the formation mechanism is proposed as an intramolecular single electron transfer that occurs from the bis(triarylamine) donor fragment to the central dioxophenyl acceptor moiety, induced by the acidic boron atom. This work provides a Lewis acid induced approach to the formation of neutral and cross-conjugated diradicals.

Radical Cation [4+2] Cycloaddition of Non‐Conjugated Tetrasubstituted Alkenes by an FeCl3/AgSbF6 Co‐Initiator

AbstractRadical cation [4+2] cycloaddition is an alternative strategy for constructing various six‐membered rings that cannot be easily accessed by thermal [4+2] cycloaddition. Here, we developed an FeCl3/AgSbF6 co‐initiator to promote radical cation [4+2] cycloaddition of non‐conjugated tetrasubstituted alkenes with 2,3‐dimethyl‐1,3‐butadiene. In the presence of 10 mol% of FeCl3 and 30 mol% of AgSbF6, the reaction proceeded smoothly in MeCN to provide the cycloadducts from tetrasubstituted alkenes having an electron‐rich aromatic group, which improved the yield. We demonstrate the efficiency of an FeCl3/AgSbF6 co‐initiator by comparing our finding to the results with previously reported iron(III) initiators in an investigation of the substrate scope. In addition, a kinetic study was conducted to elucidate the detailed reaction mechanism, in which the rate‐determining step can be facilitated by intramolecular single electron transfer.

Intramolecular Reductive Cyclization of o-Nitroarenes via Biradical Recombination.

A visible-light-induced/thiourea-mediated intramolecular cyclization of o-nitroarenes under mild conditions is realized for the first time, which provides an efficient and environmentally friendly way to access pharmaceutical relevant quinazolinone derivatives. The reaction can be easily extended to gram level by using a continuous-flow setup with high efficiency. Mechanistic investigation including control experiments, transient fluorescence, UV-vis spectra, and DFT calculations suggests that the formation of active biradical intermediates via intramolecular single electron transfer (SET) is key stage in the catalytic cycle.

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.

Intramolecular Photoreactions of 9-Cyanophenanthrene-Linked Arylcyclopropanes.

With the aim of developing efficient and useful processes for the preparation of polycyclic organic compounds, intramolecular [3 + 2] photoreactions of 9-cyanophenanthrene-linked arylcyclopropanes were investigated. Photoreactions of 6a,b, which contain respective p-methoxyphenylcyclopropane and phenylcyclopropane moieties, form the intramolecular [3 + 2] photocycloadducts, endo- and exo-7a,b, along with the dihydroisochroman derivatives, cis- and trans-8a,b. The efficiency of the photoreaction of 6a is higher when benzene rather than acetonitrile is used as a solvent. Interestingly, this solvent effect is reversed in the photoreaction of 6b, where the efficiency is higher in acetonitrile than that in benzene. On the basis of the observed effects of substituents and solvents, fluorescence emission from intramolecular exciplexes, and ΔGs for intramolecular single electron transfer (SET), we propose that the photoreactions proceed through pathways involving the initial formation of singlet intramolecular exciplexes and/or SET between the excited 9-cyanophenanthrene and the ground-state arylcyclopropane moieties.

Nitrene Radical Intermediates in Catalytic Synthesis.

Nitrene radical complexes are reactive intermediates with discrete spin density at the nitrogen‐atom of the nitrene moiety. These species have become important intermediates for organic synthesis, being invoked in a broad range of C−H functionalization and aziridination reactions. Nitrene radical complexes have intriguing electronic structures, and are best described as one‐electron reduced Fischer type nitrenes. They can be generated by intramolecular single electron transfer to the “redox non‐innocent” nitrene moiety at the metal. Nitrene radicals generated at open‐shell cobalt(II) have thus far received most attention in terms of spectroscopic characterization, reactivity screening, catalytic nitrene‐transfer reactions and (computational and experimental) mechanistic studies, but some interesting iron and precious metal catalysts have also been employed in related reactions involving nitrene radicals. In some cases, redox‐active ligands are used to facilitate intramolecular single electron transfer from the complex to the nitrene moiety. Organic azides are among the most attractive nitrene precursors in this field, typically requiring pre‐activated organic azides (e.g. RSO2N3, (RO)2P(=O)N3, ROC(=O)N3 and alike) to achieve efficient and selective catalysis. Challenging, non‐activated aliphatic organic azides were recently added to the palette of reagents useful in synthetically relevant reactions proceeding via nitrene radical intermediates. This concept article describes the electronic structure of nitrene radical complexes, emphasizes on their usefulness in the catalytic synthesis of various organic products, and highlights the important developments in the field.

(Anthracen-9-yl)methyl sulfides as a mechanistic probe for chemical as well as photochemical electron-transfer reactions

We have examined the intermediacy of sulfur-centred radical cations in the one-electron oxidation of (anthracen-9-yl)methyl sulfides. Reaction of (anthracen-9-yl)methyl sulfides with ceric ammonium nitrate (CAN) gave predominantly nitration and oxidation products along with products arising through sulfur-centred radical cations in minor amounts. Upon irradiation, (anthracen-9-yl)methyl sulfides underwent efficient intramolecular single electron transfer leading to a variety of products arising through the intermediacy of sulfur-centred radical cations.

Stopped flow kinetic studies on reductive half-reaction of histamine dehydrogenase from Nocardioides simplex with histamine

Histamine dehydrogenase from Nocardioides simplex (HmDH) which catalyzes the oxidative deamination of histamine is an iron-sulphur-containing flavoprotein. For our further understanding on the intramolecular electron transfer process, the reductive half reaction of HmDH with histamine has been studied by stopped flow spectrophotometry at pH 7.5 and 10. The reaction at pH 7.5 is found to be analysed on a kinetic model composed of three sequential first-order reactions. The first fast phase, of which the rate constant shows a hyperbolic dependence on the histamine concentration, is assigned to a direct two-electron reduction of the oxidized flavin (CFMN(O)) by histamine with no involvement of the semiquinone form of the flavin (CFMN(S)). The second moderate process is the substrate-independent intramolecular single-electron transfer from the reduced flavin to the oxidized iron-sulphur cluster. The third slow process is considered to reflect the second binding of histamine to CFMN(S), which is responsible for the substrate inhibition. At pH 10, the reaction is analysed with one pseudo-first-order reaction phase which is substrate-dependent two-electron reduction of CFMN(O) coupled with the subsequent fast intersubunit single-electron transfer. The UV-vis spectroscopy of HmDH suggests the deprotonation of Tyr residues, which seems to cause the switching of the electron transfer property.

Lariat-crown ether based fluorescence sensors for heavy metal ions

Exploratory investigations were conducted to probe several aspects of a new strategy for the design of metal ion fluorescence sensors. The results of the investigation show that lariat-crown ethers that contain amine and thioether side chains, and a naphthalene chromophore can be efficiently prepared by using sequences that rely on key single electron transfer promoted photocyclization reactions. Members of this novel family of lariat-crown ethers serve as selective fluorescence sensors for the divalent metal cation of Mg, Hg, and Pb. The response of the sensors to the divalent metal ion is modulated by the nature of the heteroatom(s) incorporated into the side chains. Specifically, lariat-crown ethers that contain tertiary amine groups in their side chains display an off–on type response to Mg(II), Hg(II), and Pb(II). In contrast, thioether side chain containing lariat-crown ethers behave differently in that their fluorescence intensities decrease in the presence of increasing concentrations of these divalent metal cations. These responses can be understood on the basis of selective divalent metal ion induced disruption of intramolecular single electron transfer (SET)-quenching (for side chain amine containing lariat-crown ethers) and the enhancement of intersystem crossing (for side chain thioether containing lariat-crown ethers) of the singlet excited state of the naphthalene fluorophore.

Intrachain Electron Transfer in Conducting Oligomers and Polymers: The Mixed Valence Approach

Organic mixed valence compounds consisting of bisdiarylamino charge-bearing units with an oligothiophene bridge and oligothiophene radical cations have been compared using molecular modeling. The study has been performed with oligomers of 1 to 22 thiophene units. These two series of molecules have several properties in common, and intramolecular Single Electron Transfer (SET) in both series can be described within the same theoretical framework. Conducting oligomer radical cations and slightly doped conducting polymers appear as special cases of the vast ensemble of organic mixed valence compounds. Short oligomers are class III, whereas longer oligomers and conducting polymers are class II. Therefore, doped conducting polymers cannot be correctly modeled using oligomers with a short conjugation length. Experimental evidence extracted from the literature confirms these findings. Single electron transfer theories can thus be used when studying interchain and intrachain electron transfer in slightly doped conducting polymers and in materials consisting of short oligomers. This makes it possible to extract from the UV-vis-near-IR spectra the electron-transfer constant rate along or between the pi-conjugated chain. The main differences among inorganic, organic, and conducting oligomer or polymer mixed valence compounds lies in the H(ab) and lambda values associated with these different series. Inorganic mixed valence compounds have small H(ab) and lambda values; organic mixed valence compounds have large H(ab) and lambda values, whereas conducting oligomers and polymers have large H(ab) but small lambda values. This induces charge delocalization to occur for systems larger than those of inorganic and nitrogen-centered organic mixed valence compounds.

ChemInform Abstract: Highly Polar Metal-Metal Bonds in “Early-Late” Heterodimetallic Complexes

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes.

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Photofragmentation in linked donor-acceptor molecules. Intramolecular single electron transfer induced cleavage of a 1,2-diamine

Two intramolecular donor-acceptor molecules which fragment by a single electron transfer initiated cation radical carbon-carbon bond cleavage have been synthesized and their photoreactivity studied. The intramolecular [open quotes]diads[close quotes] consist of a 1,2-diamine linked via an ester bond to either an anthraquinone or a 9,10-dicyanoanthracene electron-acceptor chromophore. As the covalent linkage between the donor and acceptor chromophores prevents solvent separation of the photogenerated radical ion pair, these systems provide a [open quotes]clock[close quotes] to examine directly competition between fragmentation and back electron transfer. The linked anthraquinone molecule fragments efficiently, with quantum yields approaching 80%, despite the inability of the photoproduced radical ions to separate. These high yields may be attributed to a slow, spin-forbidden back electron transfer and a rapid fragmentation. In contrast, the quantum yields for the dicyanoanthracene diad (reactive singlet) are markedly lower, less than 0.001 in benzene. The reactivity of comparable intermolecular donor-acceptor combinations is also reported. 54 refs., 3 figs., 2 tabs.

Spin-charge exchange in allodial radical ions, a novel intramolecular single electron transfer equilibrium

The intramolecular single-electron transfer in radical anions ane radical cations, constituted by two identical triphenylmethyl moieties, has been studied. The preparation of the radical ions has been effected by mixing the corresponding stable diions (tetra-n-butylammonium or hexachloroantimonate salts) and chemically inert diradicals: by partial oxidation of the dianion salts with iodine (radical anions) and by partial oxidation of the diradicals with SbCl 5 (radical cations). The ESR 13 C hyperfine coupling constants and linewidths and UV-vis absorptivity spectra afford compelling evidence for a rapid spin-charge exchange equilibrium: (C 6 Cl 5 ) 2 C-C 6 Cl 4 -Sp-C 6 Cl 4 -C(C 6 Cl 5 ) 2 ⇔(C 6 Cl 5 ) 2 C-C 6 Cl 4 -Sp-C 6 Cl 4 -C(C 6 Cl 5 ) 2 (Sp=none, CH 2 CH 2 , C≡C. *=−,+). Relevant structural aspects, such as steric inhibition of resonance, electron paths, and counterion involvement, are discussed. The synthesis and isolation of related disalts from the corresponding diradicals are also described