EPR analysis of the interacting properties and the degradation over time and irradiation of mastic resin used for painting protection

Mastic resin used as a covering film for painting protection was analyzed by electron paramagnetic resonance (EPR) spectroscopy, both as received and upon aging in sun-light. The effect of prolonged exposure to sun-light was mimicked by UV and, more so, by xenon lamp irradiation. Solid mastic presented EPR signals due to radicals trapped by PBN in solution. Data in the literature indicated the formation of acyl radicals (RCO·). These radicals preferentially dissolved in medium polarity solvents. The radical concentration in the solid mastic increased over time more than 50 times upon UV irradiation for 96 h and, even more, by xenon irradiation for 800 h. Also the PBN-trapped radicals in solution increased in concentration by irradiation. Small nitroxide radicals (TEMPO) interacted with a polar fraction of mastic dissolved in methanol, but mainly interacted with low polar mastic molecules in hydrophobic solvents. It was suggested, on the basis of both the PBN-spin trapping data and the TEMPO mobility variation in the solvents at different polarities, that terpenoid molecules partially polymerize by a radical mechanism to form low molecular weight products. A polyaromatic-radical (pyrene-TEMPO) and a biomolecule-radical (doxylcholestane) both interact weakly with mastic molecules in cyclohexane solutions. A positively charged surfactant radical (dimethylammonium-TEMPO bromide) was easily adsorbed onto the solid mastic surface suggesting that detergents are responsible for mastic degradation. In conclusion, this study provided information on the degradation mechanism of mastic resin and on its interacting ability towards external molecules and pollutants.     

A radical mediated approach to the core structure of huperzine A

The synthesis of the core structure of huperzine A by cyclisation of 2-pyridylmethyl radicals is described. (2-Methylpyridin-3-yl)cyclohexenols are directly selenated at the benzylic position by deprotonation/selenation and the products undergo either 5-exo-trig or 6-exo-trig radical cyclisations giving access to hexahydroindenopyridines and the bicyclo[3.3.1]nonane core of huperzine A, respectively.     

Photochemical Reactions of Linear Alkane Radical Cations in Low-Temperature Matrices

The radical cations of linear alkanes (n-pentane, n-heptane) trapped in various matrices (Freon-11, Freon-113, Freon-113a, mixture of Freon-11 and Freon-114B2, and sulfur hexafluoride) were found to undergo the following types of photochemical reactions: (1) charge transfer to the matrix followed by neutralization, (2) isomerization and unimolecular decomposition, and (3) deprotonation. The absorption spectra of the radical cations were characterized, and the quantum yields of reactions occurring in different matrices at 77 K were determined. It was shown that the reaction pathway and efficiency of the photochemical processes observed for a given radical cation in different matrices with similar physical and chemical characteristics could considerably differ.     

Radical products of γ-radiolysis of 12-crown-4 at 77 K

Paramagnetic products of γ-radiolysis of 12-crown-4 and its solutions in CFCl₃ and CFCl₂CF₂Cl at 77 K were studied by ESR spectroscopy. It was found that the ESR spectra of 12-crown-4 irradiated with γ-rays at 77 K contained superimposing signals of at least two species, the radicals resulting from macrocycle opening -?H-C(H) = O and macrocyclic radicals -O-?H-CH₂-, which are formed with nearly equal yields. It was shown that -O-?H-CH₂-radicals rapidly decayed at temperatures above 140 K. However, the -?H-C(H)=O radicals are stable almost up to the matrix softening temperature. The radical cations of 12-crown-4 are not stabilized in the matrices of Freon 11 and Freon 113, since they undergo transformation to macrocyclic radicals -O-?H-CH₂-via the deprotonation reaction.     

Quinone sensitized electron transfer photooxidation of nucleic acids: chemistry of thymine and thymidine radical cations in aqueous solution

The 2-methyl-1,4-naphthoquinone (MQ) sensitized photooxidation of nucleic acid derivatives has been studied by laser flash photolysis and steady state methods. Thymine and thymidine, as well as other DNA model compounds, quench triplet MQ by electron transfer to give MQ radical anions and pyrimidine or purine radical cations. Although the pyrimidine radical cations cannot be directly observed by flash photolysis, the addition of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) results in the formation of the TMPD radical cation via scavenging of the pyrimidine radical cation. The photooxidation products for thymine and thymidine are shown to result from subsequent chemical reactions of the radical cations in oxygenated aqueous solution. The quantum yield for substrate loss at limiting substrate concentrations is 0.38 for thymine and 0.66 for thymidine. The chemistry of the radical cations involves hydration by water leading to C(6)-OH adduct radicals of the pyrimidine and deprotonation from the N(1) position in thymine and the C(5) methyl group for thymidine. Superoxide ions produced via quenching of the quinone radical anion with oxygen appear to be involved in the formation of thymine and thymidine hydroperoxides and in the reaction with N(1)-thyminyl radicals to regenerate thymine. The effects of pH were examined in the range pH 5-8 in both the presence and absence of superoxide dismutase. Initial C(6)-OH thymine adducts are suggested to dehydrate to give N(1)-thyminyl radicals.     

ESR study of radical processes in irradiated DNA: The effect of the complexation with cis-dichloro diamino Pt(II)

The room temperature esr spectra of irradiated neat duplex hydrated, dehydrated and thermally denaturated DNA are interpreted in terms of the radicals formed by protonation of the thymine and cytosine anion radicals, by deprotonation of the guanine cation radicals, by addition of OH radicals at C8 of guanine and by loss of a methyl hydrogen from thymine. The complexation of DNA with cis-dichlorodiamino Pt(II) causes a drastic alteration of the radical distribution interpreted in terms of electron scavenging by the Pt(II) complex, a partial inhibition of the radical-cation chemistry of guanine, and enhancement of the reactivity of thymine toward hydrogen abstractions.     

Primary radicals in the photo-oxidation of aromatics — reactions of xylenols with OH, N3 and H radicals and formation and characterization of dimethylphenoxyl, dihydroxydimethylcyclohexadienyl and hydroxydimethylcyclohexadienyl radicals by pulse radiolysis

Photo-oxidations of environmental organics in illuminated TiO₂ dispersions have implicated surface-bound ^•OH radicals and/or valence band holes. To explore the implications of the former oxidizing entity, six isomeric xylenols (dimethylphenols) were examined by pulsed (nanoseconds to milliseconds) radiolysis methods. The spectral and kinetic characteristics of formation and decay of the transients formed by the reaction of N₃^•, ^•OH and H^• radicals with these xylenols were assessed in buffered (pH 4, 10⁻³ M phosphate) aqueous media, where the xylenols exist in their protonated form (pK ≈ 10.19–10.65). The products from the reaction of N₃^• with 2,6- and 3,4-xylenol were exclusively the corresponding dimethylphenoxyl radicals, formed via electron transfer followed by deprotonation. In contrast, except with 3,4-xylenol, the principal radical intermediates formed initially upon reaction with ^•OH were the corresponding ^•OH adducts, the dihydroxydimethylcyclohexadienyl radicals. 3,4-Xylenol was examined in the pH range 4–10. At pH 8 the initial ^•OH adduct (dihydroxy-3,4-dimethylcyclohexadienyl radical) was subsequently transformed (about 20%–40%) via water elimination into the dimethylphenoxyl radical. In contrast, at pH 9 and 10 the ^•OH adduct and the dimethylphenoxyl radical were formed concurrently (about 60% ^•OH adduct and about 40% dimethylphenoxyl species), the latter through an inner-sphere electron transfer pathway. The switch in behaviour from pH 8 to pH 9 suggests that the pK_a of the dihydroxy-3,4-dimethylcyclohexadienyl radical is about 8–9, about 2 pK units below the pK_a of the parent substrate (10.4). A mechanism for the conversion of the ^•OH adduct to the dimethylphenoxyl radical is proposed. Reaction of 2,6-xylenol with H^• radicals gave exclusively the H^• adduct (hydroxycyclohexadienyl radical), whose spectral characteristics are similar to those of the related ^•OH adduct.     

Mechanism and kinetics studies on the antioxidant activity of sinapinic acid

The OOH radical scavenging activity of sinapinic acid (HSA) has been studied in aqueous and lipid solutions, using the Density Functional Theory. HSA is predicted to react about 32.6 times faster in aqueous solution than in lipid media. The overall rate coefficients are predicted to be 5.39 × 10(5) and 1.66 × 10(4) M(-1) s(-1), respectively. Branching ratios for the different channels of reaction are also reported for the first time, as well as the UV-Vis spectra of the main products of reaction. It was found that the reactivity of sinapinic acid towards OOH radicals takes place almost exclusively by H atom transfer from its phenolic moiety. However it was found to react via SET, at diffusion-limit controlled rate constants, with ˙OH, ˙OCCl(3) and ˙OOCCl(3) radicals. It was found that the polarity of the environment and the deprotonation of HSA in aqueous solution, both increase the reactivity of this compound towards peroxyl radicals.     

tBu4NCl/K2S2O8 System Induced C-2 Functionalization of 8-Aminoquinoline Amides with Primary Alcohols

A metal and light-free Minisci-type acylation approach of 8-aminoquinoline amides with general primary alcohols was demonstrated under ^tBu₄NCl/K₂S₂O₈ system. Simultaneously, the C2−H alkylation can be realized by accident for special benzyl alcohols bearing strong electron-donating groups. Most N-heterocycles were also compatible to afford corresponding acylated products. Furthermore, the mechanism investigation reveals that chlorine radical induced the original ketyl radical generation through abstracting the α-C−H from the alcohols, then underwent nucleophilic carbon-centered radicals addition, deprotonation, and oxidation to provide the target acylation products.     

Photocatalytic Generation of N‐Centered Hydrazonyl Radicals: A Strategy for Hydroamination of β,γ‐Unsaturated Hydrazones

A visible‐light photocatalytic generation of N‐centered hydrazonyl radicals has been accomplished for the first time. This approach allows efficient intramolecular addition of hydrazonyl radical to terminal alkenes, thus providing hydroamination and oxyamination products in good yields. Importantly, the protocol involves deprotonation of an N? H bond and photocatalytic oxidation to an N‐centered radical, thus obviating the need to prepare photolabile amine precursors or the stoichiometric use of oxidizing reagents.     

Controlling regioselectivity in cyclization of unsaturated amidyl radicals: 5-exo versus 6-endo

Intramolecular cyclization of an amidyl radical onto an olefin provides an appealing method for the synthesis of lactams and other nitrogen-containing heterocycles. Here we conducted the first, systematic theoretical study on the regioselectivity in the cyclization of various types of pent-4-enamidyl radicals that carried synthetically relevant substituents. It was found that the cyclization of most of the substituted pent-4-enamidyl radicals produced the 5-exo products (gamma-lactams) almost exclusively. Marcus theory analysis showed the involvement of both the thermodynamic (stabilization of the starting double bond or the resulting radical center) and intrinsic (mainly steric effects) contributions in determining the 5-exo selectivity. Nonetheless, in two types of systems we found that the delta-lactams became the favored products through the 6-endo cyclization. In one of the systems an aromatic substituent was placed at the C4-position, whereas in the other system an electron-rich aromatic ring was incorporated into the pent-4-enamidyl radical backbone at the C2- and C3-positions. This unprecedented 6-endo mode of amidyl radical cyclization provided an interesting route for the preparation of mono- and bicyclic delta-lactams (pyridinones).     

Acid-, base-, and lewis-acid-catalyzed heterolysis of methoxide from an alpha-hydroxy-beta-methoxy radical: models for reactions catalyzed by coenzyme B12-dependent diol dehydratase

[Reaction: see text].A model for glycol radicals was employed in laser flash photolysis kinetic studies of catalysis of the fragmentation of a methoxy group adjacent to an alpha-hydroxy radical center. Photolysis of a phenylselenylmethylcyclopropane precursor gave a cyclopropylcarbinyl radical that rapidly ring opened to the target alpha-hydroxy-beta-methoxy radical (3). Heterolysis of the methoxy group in 3 gave an enolyl radical (4a) or an enol ether radical cation (4b), depending upon pH. Radicals 4 contain a 2,2-diphenylcyclopropane reporter group, and they rapidly opened to give UV-observable diphenylalkyl radicals as the final products. No heterolysis was observed for radical 3 under neutral conditions. In basic aqueous acetonitrile solutions, specific base catalysis of the heterolysis was observed; the pK(a) of radical 3 was determined to be 12.5 from kinetic titration plots, and the ketyl radical formed by deprotonation of 3 eliminated methoxide with a rate constant of 5 x 10(7) s(-1). In the presence of carboxylic acids in acetonitrile solutions, radical 3 eliminated methanol in a general acid-catalyzed reaction, and rate constants for protonation of the methoxy group in 3 by several acids were measured. Radical 3 also reacted by fragmentation of methoxide in Lewis-acid-catalyzed heterolysis reactions; ZnBr2, Sc(OTf)3, and BF3 were found to be efficient catalysts. Catalytic rate constants for the heterolysis reactions were in the range of 3 x 10(4) to 2 x 10(6) s(-1). The Lewis-acid-catalyzed heterolysis reactions are fast enough for kinetic competence in coenzyme B12 dependent enzyme-catalyzed reactions of glycols, and Lewis-acid-catalyzed cleavages of beta-ethers in radicals might be applied in synthetic reactions.     

Stereochemistry of hexenyl radical cyclizations with tert-butyl and related large groups: substituent and temperature effects

The long held notion that hexenyl radicals bearing large substituents on the radical carbon cyclize to give 1,2-trans-substituted cyclopentanes is experimentally disproved by study of the radical cyclization of an assortment of simple and complex substrates coupled with careful product analysis and rigorous assignment of configurations. X-ray studies and syntheses of authentic samples establish that the published assignments for cis- and trans-1-tert-butyl-2-methylcyclopentane must be reversed. The original assignment based on catalytic hydrogenation of 1-tert-butyl-2-methylenecyclopentane was compromised by migration of the double bond prior to hydrogenation. The cyclization of 1-tert-butylhexenyl radical is moderately cis selective, and the selectivity is increased by geminal substitution on carbon 3. This selectivity trend is general and extends to relatively complex substrates. It has allowed Ihara to reduce the complexity of an important class of round trip radical cyclizations to make linear triquinanes to the point where two tricyclic products-cis-syn-cis and cis-anti-cis-account for about 80% of the products. However, the further increase in selectivity that was proposed by lowering the temperature is shown to be an artifact of the analysis methods and is not correct. This work solidifies "1,2-cis selectivity" in cyclizations of 1-subsituted hexenyl radicals as one of the most general stereochemical trends in radical cyclizations.     

On the mechanism of the gif system for the oxidation of saturated hydrocarbons

The high selectivity of the Gif system for hydrocarbon oxidation is shown to depend upon the capture of $\text{tert.}$ radicals by pyridine; the mechanism for the secondary oxidation products has only a minor radical component as judged by competitive trapping.     

Phenyl trifluorovinyl sulfide: a radical acceptor for preparation of gem-difluoromethylene compounds

Phenyl trifluorovinyl sulfide was prepared from the reaction of trifluorovinyllithium and S-phenyl benzenethiosulfonate. The fluorinated olefin showed reactivity with alkyl radicals generated from halogen-abstraction from alkyl halides. Reactions with alkyl halides required tris(trimethylsilyl)silane as a chain transfer reagent to improve selectivity of the products. Initiation of radical reaction was effected by thermal decomposition of AIBN. Oxidation of the thioether products gave the corresponding sulfoxides, which were successively converted into α,α-difluoroalkanecarboxylic acid thiol esters by Pummerer reaction.     

Radical formation in pulse radiolysis of organic solutions of 9,10-dihydrophenanthrene

Formation and decay of neutral radicals from 9,10-dihydrophenanthrene (DHPT) in irradiated organic solutions was studied by pulse radiolysis method. Major species of the neutral radicals are H-adducts (H-DHP ) in ethanol solution of DHPT, whereas they are 9-hydrophenanthryl radicals (9-HP ) in 1,2-dichloroethane(DCE) solution of DHPT. In ethanol solution, H-DHP is generated through proton transfer to the anion radical of DHPT and through reaction between atomic hydrogen and DHPT. In the DCE solution, 9-HP is generated mainly through deprotonation from cation radicals of DHPT. Reaction between solvent radicals and DHPT plays a minor role for the formation of 9-HP .     

One product, two pathways: initially divergent radical reactions reconverge to form a single product in high yield

The paper describes examples of net diastereotopic-group-selective radical processes having the unusual feature that a single product is formed even though the key reaction of the two diastereotopic radical precursors is nonselective. For example, reaction of (R)-N-(cyclohex-2-en-1-yl)-N-(2,6-diiodo-4-methylphenyl)acetamide with tributyltin hydride produces 1-((4aR,9aR)-6-methyl-2,3,4,4a-tetrahydro-1H-carbazol-9(9aH)-yl)ethanone with high product selectivity and in high yield. Analysis of the concentration profiles of the closed-shell intermediates at the halfway point of the reaction shows that nonselective abstraction of diastereotopic iodides by tin radicals occurs, leading to diastereomeric aryl radicals. These isomeric intermediates evolve via two nonintersecting reaction pathways, cyclization and bimolecular trapping or vice versa, into the same final product. Origins of the selectivity are suggested on the basis of conformational analysis of the products using both X-ray crystallography and density functional theory calculations.     

Acyclic diastereoselection in prochiral radical addition to prochiral olefins

The stereochemical preference (syn or anti) when prochiral radicals add to prochiral acceptors is of fundamental interest. The primary focus of this research was to determine which factors influence the relative stereochemistry between the beta and gamma chiral centers when these are formed concurrently. While moderate diastereoselectivity was found for addition of alkyl (6a-d) and alpha-alkoxy radicals (16a-c) (< or =6:1 syn) to acceptors 4, 7, 8, 10, and 14, consistently high selectivity was observed with less reactive halogenated radicals (6f,g) (>15:1 anti). Steric influence in alkyl radical additions was difficult to evaluate due to decreased reactivity when using bulky reaction partners; however, more reactive alpha-alkoxy radicals, it was found that increasing steric bulk leads to moderate increases in selectivity. In addition, higher selectivity was observed when employing lanthanide Lewis acids whose environment (reactivity) was modified using achiral additives, suggesting a potentially simple means for selectivity enhancements in radical reactions. Overall these results indicate that significant stereoelectronic effects are necessary to achieve high levels of selectivity in prochiral radical additions to prochiral acceptors.     

Antioxidative properties of hydroxycinnamic acid derivatives and a phenylpropanoid glycoside. A pulse radiolysis study

Spectral and redox properties of the phenoxyl radicals from hydroxycinnamic acid derivatives and one selected component of phenylpropanoid glycosides, verbascoside, were studied using pulse radiolysis techniques. On the basis of the pH dependence of phenoxyl radical absorptions, the pK_a values for deprotonation of sinapic acid radical and ferulic acid radical are 4.9 and 5.2. The rate constants of one electron oxidation of those antioxidants by azide radical and bromide radical ion were determined at pH 7. The redox potentials of those antioxidants were determined as 0.59–0.71 V vs NHE at pH 7 with reference standard 4-methoxyphenol and resorcinol.