Time-resolved FT-EPR study of photoreduction of duroquinone by triethylamine in methanol

Using time resolved Fourier transform EPR spectroscopy the photoreduction of duroquinone by triethylamine in methanol solution was investigated. It is found that the spin-polarized (CIDEP) duroquinone triplet deactivates by electron transfer from triethylamine generating duroquinone radical anion and amine radical cation, and by hydrogen transfer from the solvent generating durosemiquinone radical and hydroxymethyl radical, respectively. All radicals are observed at different conditions and are spin-polarized by triplet mechanism and partially by ST₀ radical pair mechanism. The time dependence of FT-EPR intensities of radical cation and radical anion on the amine concentration is investigated in the range of 1 to 100 mM triethylamine. The contribution of the triplet mechanism to the spin polarization of radicals changes with different triethylamine concentrations. The durosemiquinone radical is found to be transformed into duroquinone radical anion in the presence of triethylamine in the solution. CIDNP experiments indicate that the hydrogen back transfer between the durosemiquinone radical and hydroxymethyl radical pair has a significant influence on the time behaviour of duroquinone radical anion. The intensity of triethylamine radical cation is found to be decreased with the increase of triethylamine concentration, which is interpreted that the triethylamine radical cation is deprotonated by the amine. Based on the FT-EPR results, a new complete mechanism is proposed.     

Radical cyclization in heterocycle synthesis 15. Relationship between a Radical species and radical acceptors of three different types of double bond in radical addition-cyclization

Relationship between a radical species and radical acceptors of three different types of double bond in radical addition-cyclization was systematically investigated. Substrates carrying alpha,beta-unsaturated amide, isolated olefin, and oxime ether moieties underwent radical addition-cyclization to give differently substituted lactams depending upon the radicals used. The sulfanyl radical addition-cyclization of the substrate proceeded smoothly to give the 5-membered lactam having an alkoxyamino group as a result of preferable addition of an intermediary alpha-carbonyl radical to the oxime ether. On the other hand, the triethylborane-mediated radical addition-cyclization gave the lactam bearing an iodomethyl group as a result of addition to an intermediary alpha-carbonyl radical to isolated olefin. The different regioselectivity was explained by the stability of the intermediary radical and the interaction between SOMO and HOMO.     

Generation and absolute reactivity of an aryl enol radical cation in solution

Laser flash photolysis of 1-bromo-1-(4-methoxyphenyl)acetone in acetonitrile leads to the formation of the alpha-acyl 4-methoxybenzyl radical that under acidic conditions rapidly protonates to give detectable amounts of the radical cation of the enol of 4-methoxyphenylacetone. This enol radical cation is relatively long-lived in acidic acetonitrile (tau approximately equal to 200 micros), which is on the same order of magnitude as the radical cations of other 4-methoxystyrene derivatives. Rate constants for deprotonation of the radical cation and the acid dissociation constant for the enol radical cation were also determined using time-resolved absorption spectroscopy. Deprotonation is rapid, taking place with a rate constant of 3.9 x 10(6) s(-1), but the enol radical cation is found to be only moderately acidic in acetonitrile having a pK(a) = 3.2. The lifetime of the enol radical cation was also found to be sensitive to the presence of oxygen and chloride. The sensitivity toward oxygen is explained by oxygen trapping the vinyloxy radical component of the enol radical cation/vinyloxy equilibrium, while chloride acts as a nucleophile to trap the enol radical cation.     

分子间自由基正离子反应研究进展

自由基正离子含有一个正电荷和一对未成对电子,是很多有机化学反应的重要的活性中间体。文章综述了近几年自由基正离子反应研究进展,主要包括化学氧化剂诱导的自由基正离子反应、可见光诱导的自由基正离子反应、电诱导的自由基正离子反应等方面的研究。     

Ionizing radiation induced reactions of 2,6-dichloroaniline in dilute aqueous solution

In pulse radiolysis investigations the hydroxyl radical formed in water radiolysis reacts with 2,6-dichloroaniline in radical addition to the ring forming hydroxy-cyclohexadienyl radical and also in hydrogen atom abstraction from the amino group resulting in anilino radical. The hydroxy-cyclohexadienyl radical in the absence of dissolved O₂ partly transforms to anilino radical, when dissolved oxygen is present the radical transforms to peroxy radical. According to chemical oxygen demand measurements the reaction of one OH radical induces the incorporation of 0.6 O₂ into the products. It is a typical value for chlorine atom substituted aromatic molecules, and smaller than found for molecules without chlorine atom (1.0–2.0).     

Substituent effects on the rearrangements of cyclohexyl to cyclopentyl radicals involving avermectin-related radicals

The rearrangement of a substituted cyclohexyl radical to a cyclopentylmethyl radical on the skeleton of avermectin B1 has been investigated using density functional (UB3LYP/6-31G(d)) and G3MP2B3 computational methods. The rearrangement is preferred when highly radical stabilizing groups are present at the 2- and 3-positions of the cyclohexyl radical. A substituent on the 3-position of the cyclohexyl radical enables ring-cleavage of the cyclohexyl radical, while a radical stabilizing substituent on the 2-position of the cyclohexyl radical stabilizes the final cyclopentylmethyl radical, enabling the overall rearrangement and reversing the normal thermodynamic preference for the hexenyl radical ring closure.     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

LIGHT-INDUCED ELECTRON TRANSFER REACTIONS BETWEEN CHLOROPHYLL AND QUINONE IN LIPOSOMES: RADICAL FORMATION AND DECAY IN NEGATIVELY AND POSITIVELY CHARGED VESICLES*

Abstract— The incorporation of relatively small amounts (≤ 20 mol%) of a negatively charged surfactant into otherwise electrically neutral phosphatidylcholine vesicles containing chlorophyll in the presence of benzoquinone has been shown to produce large effects on radical formation and decay as measured by laser flash photolysis. When salt ions are present in the aqueous phase, increasing the level of negative surfactant leads to a small increase in radical yield, followed by a larger decrease in radical yield. When the salt concentration is low, increasing the negative surfactant concentration leads to a suppression of the fast radical recombination process, an increase in slow radical decay and, at the highest surfactant concentration, an approximately 35% increase in total radical yield. An analysis of these effects is given in terms of the influence of a negative electrostatic field on radical pair stabilization and recombination, radical pair separation and expulsion of the acceptor radical anion from the vesicle. The incorporation of relatively small amounts (≤ 20 mol%) of a positively charged surfactant into egg phosphatidylcholine (EPC) vesicles containing chlorophyll and benzoquinone also produces large effects on radical formation and decay. When the electrolyte concentration in the suspending aqueous medium is high, radical yields are decreased as the surfactant concentration is increased, without any appreciable effect on decay kinetics. When deionized water is used, the slow recombination component of the decay is specifically suppressed by the presence of the positive surfactant, whereas the fast decay component decreased and then increased in amount as the surfactant concentration is increased. In all cases, however, the total radical yield is less than in pure EPC vesicles. These results can be understood in terms of the influence of a positive electrostatic field on radical pair separation and acceptor radical anion mobility. When equimolar amounts of both positively and negatively charged surfactants are incorporated into EPC vesicles, the radical yields and decay kinetics are relatively unaffected, but a large effect is observed on the radical difference spectrum. This may be a consequence of clustering of oppositely charged molecules within the bilayer surface.     

Polymerization of coordinated monomers. XXII. Ab initio molecular orbital study on the binary radical complexes of 2-methoxycarbonyl propyl radical with boron trichloride and with boron trifluoride as a model of growing radical end

The geometrical and electronic structures of the binary radical complexes of 2-methoxycarbonyl propyl radical with boron trichloride and with boron trifluoride were determined by using an ab initio molecular orbital method. The 2-methoxycarbonyl propyl radical complex was a model of the growing radical end in the copolymerization of methyl methacrylate in the presence of boron halides. The most stable structure of the binary radical complex composed of 2-methoxycarbonyl propyl radical with boron trichloride was a twisted form in which the dihedral angle between the vinyl group and the ester group was 32°, while that of the binary radical complex composed of methyl methacrylate radical with boron trifluoride was a planar form as the free radical. The frontier orbital energy of 2-methoxycarbonyl propyl radical was lowered by 0.06 au by the coordination of boron trichloride, while that was lowered only by 0.02 au by the coordination of boron trifluoride. The polymerization mechanism was elucidated on the basis of these predictions. © 1993 John Wiley & Sons, Inc.     

Mapping photogenerated radicals in thin polymer films: fluorescence imaging using a prefluorescent radical probe

Prefluorescent radical probes, in which fluorescence is activated by radical trapping, and photoinitiators were used to detect radical generation in polymer films using fluorescence spectroscopy and microscopy. Prefluorescent radical probes are the foundation of a fluorescence imaging system for polymer films, that may serve both as a mechanistic tool in the study of photoinitiated radical processes in polymer films and in the preparation of functional fluorescent images.     

Expanding Radical SAM Chemistry by Using Radical Addition Reactions and SAM Analogues

Radical S‐adenosyl‐l ‐methionine (SAM) enzymes utilize a [4Fe‐4S] cluster to bind SAM and reductively cleave its carbon–sulfur bond to produce a highly reactive 5′‐deoxyadenosyl (dAdo) radical. In almost all cases, the dAdo radical abstracts a hydrogen atom from the substrates or from enzymes, thereby initiating a highly diverse array of reactions. Herein, we report a change of the dAdo radical‐based chemistry from hydrogen abstraction to radical addition in the reaction of the radical SAM enzyme NosL. This change was achieved by using a substrate analogue containing an olefin moiety. We also showed that two SAM analogues containing different nucleoside functionalities initiate the radical‐based reactions with high efficiencies. The radical adduct with the olefin produced in the reaction was found to undergo two divergent reactions, and the mechanistic insights into this process were investigated in detail. Our study demonstrates a promising strategy in expanding radical SAM chemistry, providing an effective way to access nucleoside‐containing compounds by using radical SAM‐dependent reactions.     

SPIN TRAPPING OF FREE RADICALS PRODUCED FROM NITROSOAMINE CARCINOGENS

Abstract— Using the spin trap 5,5-dimethylpyrroline-1-oxide we have demonstrated that the nitrosoamine carcinogen, 1-nitrosopiperidine, upon incubation with rat liver microsomes and nuclei produces two trapped free radical species. One trapped species is the hydroxyl free radical whereas the other is a free radical of unknown structure of the carcinogen itself. Three other nitrosoamine carcinogens tested, including diethylnitrosoamine, dimethylnitrosoamine and 1-nitrosopyrroline yielded similar results with the exception that the trapped carcinogen radical differs dependent upon the compound used. Oxygen was required to produce the hydroxyl free radical but its presence decreased the yield of the carcinogen radical. Both cyanide and α-tocopherol acetate caused a decrease in the yield of the carcinogen free radical. Some heat-labile inhibitor(s) of radical production was/were present in the cytosol. The amount of radical produced was not proportional to the P₄₅₀ content.     

Recent advances and perspectives on supramolecular radical cages

Supramolecular radical chemistry has been emerging as a cutting-edge interdisciplinary field of traditional supramolecular chemistry and radical chemistry in recent years. The purpose of such a fundamental research field is to combine traditional supramolecular chemistry and radical chemistry together, and take the benefit of both to eventually create new molecules and materials. Recently, supramolecular radical cages have been becoming one of the most frontier and challenging research focuses in the field of supramolecular chemistry. In this Perspective, we give a brief introduction to organic radical chemistry, supramolecular chemistry, and the emerging supramolecular radical chemistry along with their history and application. Subsequently, we turn to the main part of this topic: supramolecular radical cages. The design and synthesis of supramolecular cages consisting of redox-active building blocks and radical centres are summarized. The host–guest interactions between supramolecular (radical) cages and organic radicals are also surveyed. Some interesting properties and applications of supramolecular radical cages such as their unique spin–spin interactions and intriguing confinement effects in radical-mediated/catalyzed reactions are comprehensively discussed and highlighted in the main text. The purpose of this Perspective is to help students and researchers understand the development of supramolecular radical cages, and potentially to stimulate innovation and creativity and infuse new energy into the fields of traditional supramolecular chemistry and radical chemistry as well as supramolecular radical chemistry.

This Perspective summarizes the recent developments of supramolecular radical cages including the design and synthesis of radical cages, their interesting host–guest spin–spin interactions and applications in radical-mediated/catalyzed reactions.     

Radical heterolysis reactions. Dynamics of formation, collapse, and solvation of ion pairs determined by a competition kinetic method

The kinetics of radical heterolysis reactions, including rate constants for radical cation-anion contact ion pair formation, collapse of the contact pair back to the parent radical, and separation of the contact pair to a solvent-separated ion pair or free ions were obtained in several solvents for a beta-mesyloxy radical. Rate constants were determined from indirect kinetic studies using thiophenol as both a radical trapping agent via H-atom transfer and an alkene radical cation trapping agent via electron transfer. [reaction: see text].     

Reactions of hydrated electron with various radicals: spin factor in diffusion-controlled reactions

The reactions of hydrated electron (eaq-) with various radicals have been studied in pulse radiolysis experiments. These radicals are hydroxyl radical (*OH), sulfite radical anion (*SO3-), carbonate radical anion (CO3*-), carbon dioxide radical anion (*CO2-), azidyl radical (*N3), dibromine radical anion (Br2*-), diiodine radical anion (I2*-), 2-hydroxy-2-propyl radical (*C(CH3)2OH), 2-hydroxy-2-methyl-1-propyl radical ((*CH2)(CH3)2COH), hydroxycyclohexadienyl radical (*C6H6OH), phenoxyl radical (C6H5O*), p-methylphenoxyl radical (p-(H3C)C6H4O*), p-benzosemiquinone radical anion (p-OC6H4O*-), and phenylthiyl radical (C6H5S*). The kinetics of eaq- was followed in the presence of the counter radicals in transient optical absorption measurements. The rate constants of the eaq- reactions with radicals have been determined over a temperature range of 5-75 degrees C from the kinetic analysis of systems of multiple second-order reactions. The observed high rate constants for all the eaq- + radical reactions have been analyzed with the Smoluchowski equation. This analysis suggests that many of the eaq- + radical reactions are diffusion-controlled with a spin factor of 1/4, while other reactions with *OH, *N3, Br2*-, I2*-, and C6H5S* have spin factors significantly larger than 1/4. Spin dynamics for the eaq-/radical pairs is discussed to explain the different spin factors. The reactions with *OH, *N3, Br2*-, and I2*- have also been found to have apparent activation energies less than that for diffusion control, and it is suggested that the spin factors for these reactions decrease with increasing temperature. Such a decrease in spin factor may reflect a changing competition between spin relaxation/conversion and diffusive escape from the radical pairs.     

Use of a conformational radical clock for evaluating alkyllithium-mediated cyclization reactions

The reductive lithiation of nitrile 9 led to the cyclic product 11 as a single diastereomer in 42% ee. The intermediate radical is a conformational radical clock. The radical lifetime can be determined from the optical purity of the product 11. We show that the lifetime of the intermediate radical is too brief to allow a radical cyclization, and thus the cyclization proceeds through an alkyllithium intermediate.     

PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING II: REACTIONS INVOLVING PYRIMIDINE RADICAL ANION INTERMEDIATES

A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor Nα-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1′-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.     

Radical addition of alkyl halides to formaldehyde in the presence of cyanoborohydride as a radical mediator. A new protocol for hydroxymethylation reaction

Hydroxymethylation of alkyl halides was achieved using paraformaldehyde as a radical C1 synthon in the presence of tetrabutylammonium cyanoborohydride as a hydrogen source. The reaction proceeds via a radical chain mechanism involving an alkyl radical addition to formaldehyde to form an alkoxy radical, which abstracts hydrogen from a hydroborate anion.     

A DFT Study of the Nitric Oxide and Tyrosyl Radical Interaction: A Proposed Radical Mechanism

The present study employs a complete theoretical investigation, at the B3LYP/cc‐pVTZ level of theory, of the interactions between the tyrosyl radical and nitric oxide, exploring in detail the nitrotyrosine formation radical mechanism. Tyrosyl radicals play an essential role in catalytic reactions of numerous enzymes and biological systems have regulated appropriate mechanisms for their formation. Nitric oxide reacts with the tyrosyl radical and affords a weak intermediate complex which, through a sequence of non‐ionic water catalyzed and biologically feasible intermediate reactions, yields the iminoxyl radical. The iminoxyl radical further combines with hydroxyl radical, a species present in pathophysiological conditions, to yield nitrotyrosine.     

Heterolytic cleavage of a beta-phosphatoxyalkyl radical resulting in phosphate migration or radical cation formation as a function of solvent polarity

[formula: see text] The 2-(diethylphosphatoxy)-2-(p-methoxyphenyl)-1,1-dimethylethyl radical (1) reacted to give the benzylic radical product from phosphate migration or a radical cation (or a mixture of the two) as a function of solvent. Smooth acceleration in rates of reactions of 1 in solvents of increasing polarity and consistent entropies of activation indicate that radical 1 reacts by common mechanism irrespective of the final products formed, specifically by initial heterolysis to a radical cation-phosphate anion pair.