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Summary tert-(Hetero)cyclobutanols are important synthetic building blocks. However, previous synthesis of these compounds has mainly focused on the nucleophilic addition of organometallic reagents to (hetero)cyclobutanones. Herein, we disclose a single-electron process for preparing allyl tert-(hetero)cyclobutanols with readily available organosilicon reagents. The success of the reaction is attributed to the favored radical 1,2-silyl transfer over β-carbon elimination of the alkoxyl radicals. The mechanism of the generation of carbon radicals from alkoxyl radicals is distinct from previous photoredox-mediated oxidation of hypervalent silicates. Our reaction shows broad substrate scope and wide functional-group tolerance. With the highly functionalized allyl tert-(hetero)cyclobutanols, we have achieved ring-opening reactions to synthesize thiolated 1,4-diketones and ring-expansion reactions to prepare seven-membered ring compounds.

Alkoxyl radicals play important roles in various fields of chemistry. Understanding their reactivity is essential to applying their chemistry for industrial and biological purposes. Hydrogen-atom t...

Peroxyl radicals participate in multiple processes involved in critical changes to cells, tissues, pharmacueticals and foods. Some of these reactions explain their association with degenerative pathologies, including cardiovascular and neurological diseases, as well as cancer development. Azocompounds, and particularly AAPH (2,2'-Azobis(2-methylpropionamidine) dihydrochloride), a cationic water-soluble derivative, have been employed extensively as sources of model peroxyl radicals. A considerable number of studies have reported mechanistic data on the oxidation of biologically-relevant targets, the scavenging activity of foods and natural products, and the reactions with, and responses of, cultured cells. However, despite the (supposed) experimental simplicity of using azocompounds, the chemistry of peroxyl radical production and subsequent reactions is complicated, and not always considered in sufficient depth when analyzing experimental data. The present work discusses the chemical aspects of azocompounds as generators of peroxyl (and other) radicals, together with their contribution to our understanding of biochemistry, pharmaceutical and food chemistry research. The evidence supporting a role for the formation of alkoxyl (RO•) and other radicals during thermal and photochemical decomposition of azocompounds is assessed, together with the potential influence of such species on the reactions under study.

This article demonstrates that ionizing radiation induces simultaneous crosslinking and scission in poly(trimethylene carbonate-co-d-lactide) diblock and random copolymers. Copolymer films were electron-beam (EB) irradiated up to 300 kGy under anaerobic conditions and subsequently examined by evaluation of their structure (FT-IR, NMR), molecular weight, intrinsic viscosities, and thermal properties. Radiation chemistry of the copolymers is strongly influenced by the content of ester linkages of the lactide component. At low lactide content, crosslinking reaction is the dominant one, however, as the lactide ratio increases, the ester linkages scission becomes more competent and exceeds the crosslinking. Electron paramagnetic resonance (EPR) measurements indicate that higher content of amorphous carbonate units in copolymers leads to a reduction in free radical yield and faster radical decay as compared to lactide-rich compositions. The domination of scission of ester bonds was confirmed by identifying the radiolytically produced alkoxyl and acetyl radicals, the latter being more stable due to its conjugated structure.

A direct approach to β-iodophosphonates and β-iodophosphine oxides from 2,3-dideoxy-3-phosphoryl carbohydrate derivatives has been achieved by using the anomeric alkoxyl radical 1,2-fragmentation protocol. The reaction has been conducted on carbohydrate derivatives under mild conditions with (diacetoxyiodo)benzene and molecular iodine. Subsequent dehydroiodination afforded the corresponding vinylphosphonates and vinylphosphine oxides.

Four diastereomers of 2-[(trimethylammonium)-methyl]-5-methyltetrahydrofuran-3-ol, trivially named isomuscarine, allo-isomuscarine, epi-isomuscarine, and epiallo-isomuscarine, were prepared as bromide salts from 2,4-like and 2,4-unlike diastereomers of 3-(4-hydroxyhex-5-en-2-oxy)-4-methylthiazole-2(3H)-thione. The strategy for constructing the tetrahydrofuran nucleus of the isomuscarines uses alkenoxyl radical 5-exo-bromocyclization, occurring 2,3-cis-selectively for the 2,4-like-4-hydroxyhex-5-en-2-oxyl radical, and 2,3-trans-selectively for the 2,4-unlike diastereomer. A fraction of 4-hydroxyhex-5-en-2-oxyl radicals cyclizes 6-endo-selectively providing 5-bromo-2-methyltetrahydropyran-4-ols in yields between 3 and 15%. Substituting trimethylamine for bromide in 5-exo-bromocyclized products furnishes isomuscarine bromides, which were structurally characterized by X-ray diffraction and NMR-spectroscopy.

The carbonyl products of the reactions of hydroxyl radicals with three vicinal diols (ethane-1,2-diol, propane-1,2-diol and butane-2,3-diol) have been identified and quantified. Hydroxyl radicals were produced by gamma-radiolysis of N2O-saturated aqueous solutions. The reactions result in the formation of alkoxyl radicals (similar to 15%) followed by beta-fragmentation, and alpha-hydroxyl alkyl radicals that undergo H2O elimination. The latter process is part of a radical chain reaction at higher diol concentrations.

The oxidation of cumene in the presence of a mixture of two initiators, azobisisobutyronitrile and benzoyl peroxide, revealed the antagonistic effect of their action. This phenomenon was explained using mathematical simulation of the process by means of analyzing kinetic schemes. It was shown that the antagonistic effect is a result of formation of alkoxyl radicals at the stage of combination of cumene peroxyl radicals and their further participation in cross-termination reactions with initiator radicals. The temperature dependence of the initiators’ effect has been analyzed.

The investigated set of 6-exo-trig cyclizations occurs irreversibly and justifies the use of tetrahydropyran-derived transition structures for rationalizing 2,6-cis, 2,5-trans, and 2,4-cis stereoselectivity in ring-closure reactions of 1-, 2-, and 3-phenyl-6-methyl-5-hepten-1-oxyl radicals. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Primary, secondary, and tertiary alkoxyl radicals add exo -selectively to the olefinic π-bond in bicyclo[2.2.1]heptene to afford exo -2-alkoxybicyclo[2.2.1]hept-3-yl radicals, which are trapped with BrCCl 3 preferentially from the endo face to furnish β-bromohydrine ethers in 23–67% yield.

Polar bromocyclizations of bishomoallylic alcohols (nucleophilic oxygen), which have methyl and/or phenyl substituents located at positions 1, 4, and/or 5, and cyclizations of the derived alkenoxyl radicals (electrophilicoxygen) followed by bromine atom trapping from BrCCl 3 , proceed with complementary regio-selectivities.

Addition of various organic O-centered radicals XO• {i.e. (alkoxycarbonyl)oxyl radicals, ROC(O)O•, [(alkoxycarbonyl)acyl]oxyl radicals, ROC(O)C(O)O•, (carbamoyl)oxyl radicals, R2NC(O)O•, alkoxyl radicals, RO•, and nitroxyl radicals, R2NO•} to alkynes initiates an oxidative radical cyclization cascade, which is terminated by release of a C- or N-centered radical X•. This finding clearly highlights the generality of the recently discovered concept of self-terminating radical oxygenations, through which C≡C triple bonds may be converted into carbonyl groups under very mild reaction conditions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The oxidation of proteins by free radicals is thought to play a major role in many oxidative processes within cells and is implicated in a number of human diseases as well as ageing. This review summarises information on the formation of radicals on peptides and proteins and how radical damage may be propagated and transferred within protein structures. The emphasis of this article is primarily on the deleterious actions of radicals generated on proteins, and their mechanisms of action, rather than on enzymatic systems where radicals are deliberately formed as transient intermediates. The final section of this review examines the control of protein oxidation and how such damage might be limited by antioxidants.

The paper describes a new source of surface organic radicals whose special feature is a low rate of the initiation of alkyl radicals in reactions of H atoms with the surface of alkanes. A special reactor design was used to accumulate radicals at room temperature and observe alkoxyl radicals RO by the EPR method. For this purpose, alkanes were deposited on aerosil and placed into an EPR cavity. Thanks to the large area of aerosil loads (∼103 cm2), we were able to obtain a stable signal corresponding to a ∼1017 total amount of alkoxyl radicals (the degree of surface coverage ∼0.1%). When O2 was introduced into the reactor in concentrations of 1015–1016 cm−3, a sharp decrease in the signal from surface organic radicals was observed. The process was described as the first-order (RO)s + O2 → HO2 + ketone reaction with a 1.7 × 10−17 cm3 s−1 rate constant.

Nitrate esters, prepared by treatment of β-hydroxy-α-amino acid derivatives with nitric acid, react with tributyltin hydride to give the corresponding alkoxyl radicals. These radicals readily undergo β-scission, providing a convenient route for the regiocontrolled production of α-carbon-centred amino acid radicals. By examining the partitioning of the alkoxyl radicals between the β-scission process and the competing hydrogen transfer reaction, it has been possible to evaluate the influence of electronic and steric effects on the β-scission reaction and the formation of the carbon-centred radicals.

A novel fluorogenic probe for tert-butoxy radicals based on the hydrazino-naphthalimide system is reported. Interestingly, different regioisomers exhibited significantly different optical properties toward ROS, which suggested 4-hydrazinyl naphthalimide as a potential new platform for the in vitro and in cellulo detection of alkoxyl radicals.

Substituted 5-hexen-1-oxyl radicals have been generated from N -(5-hexen-1-oxy)-5-( p -methoxyphenyl)-4-methylthiazole-2(3 H )-thiones under tin-free conditions and have been successfully applied as reactive intermediates in a mechanistic study on the formation of bromomethyl-substituted tetrahydropyrans via 6- exo - trig selective cyclizations.

A time-resolved kinetic study on the effect of metal ions (M(n+)) on hydrogen abstraction reactions from C-H donor substrates by the cumyloxyl radical (CumO(•)) was carried out in acetonitrile. Metal salt addition was observed to increase the CumO(•) β-scission rate constant in the order Li(+)Mg(2+)Na(+). These effects were explained in terms of the stabilization of the β-scission transition state determined by Lewis acid-base interactions between M(n+) and the radical. When hydrogen abstraction from 1,4-cyclohexadiene was studied in the presence of LiClO(4) and Mg(ClO(4))(2), a slight increase in rate constant (k(H)) was observed indicating that interaction between M(n+) and CumO(•) can also influence, although to a limited extent, the hydrogen abstraction reactivity of alkoxyl radicals. With Lewis basic C-H donors such as THF and tertiary amines, a decrease in k(H) with increasing Lewis acidity of M(n+) was observed (k(H)(MeCN)k(H)(Li(+))k(H)(Mg(2+))). This behavior was explained in terms of the stronger Lewis acid-base interaction of M(n+) with the substrate as compared to the radical. This interaction reduces the degree of overlap between the α-C-H σ* orbital and a heteroatom lone-pair, increasing the C-H BDE and destabilizing the carbon centered radical formed after abstraction. With tertiary amines, a2-order of magnitude decrease in k(H) was measured after Mg(ClO(4))(2) addition up to a 1.5:1 amine/Mg(ClO(4))(2) ratio. At higher amine concentrations, very similar k(H) values were measured with and without Mg(ClO(4))(2). These results clearly show that with strong Lewis basic substrates variations in the nature and concentration of M(n+) can dramatically influence k(H), allowing for a fine control of the substrate hydrogen atom donor ability, thus providing a convenient method for C-H deactivation. The implications and generality of these findings are discussed.

Reaction of polyunsaturated fatty acid hydroperoxides with metal complexes generates lipid alkoxyl radicals and metal-oxo complexes. Lipid alkoxyl radicals are presumed to be the species responsible for metal-amplified lipid peroxidation because of the chemical analogy of simple organic alkoxyl radicals to the hydroxyl radical. However, polyunsaturated fatty acid alkoxyl radicals exhibit a rich and diverse chemistry that is dominated by intramolecular cyclization to epoxyallylic radicals. Studies described herein demonstrate that the equilibrium between cyclization and ring-opening of epoxyallylic radicals lies overwhelmingly toward cyclization. Thus lipid alkoxyl radicals have a steady-state concentration that is so low that their contribution to metal-amplified lipid peroxidation is insignificant. In fact, the species responsible for metal amplification of lipid peroxidation appears to be the epoxyperoxyl radical formed by coupling the epoxyallylic radical to molecular oxygen.

Biologically/medicinally important compounds, when metabolized, can generate free radicals from which electronically excited products—often in the triplet state—are generated. Peroxidases are particularly apt to catalyze such processes, which usually entail oxidations by electron transfer. In the latter case, the chemiluminescence may derive from peroxyl and alkoxyl radicals or excited states derived from dioxetanes. Besides peroxidases, prostaglandin-H synthase and lipoxygenase may catalyze the formation of excited carbonyls. The pronounced similarity in the chemical behavior and reactivity of radicals and excited species derives in part from the biradical nature of the latter. Usually in analyzing the biological effects of xenobiotics, only radicals and/or reactive ground state products have been considered. However, in such processes the generation of excited species is possible, which should be tested for by direct and/or sensitized emission or by photochemical transformation.

A strategy for comparing the 1,5- and 1,6-cyclisation reactions of 3-phenylpropan-1-oxyl radicals is described. Iron(II)-catalysed reduction of 3-(p-methylphenyl)prop-1-yl hydroperoxide and its para-chloro and para-methoxy-substituted analogues, carried out in the presence of copper(II), has been found to give in each case the appropriate para-substituted 3-phenylpropan-1-ol, 3-phenylpropanal and a low yield of a mixture of isomeric 6- and 7-substituted chromans. The alcohols are proposed to form via reduction of either the hydroperoxide or the resulting alkoxyl radical or its cyclised intermediates, and the aldehydes as a result of rearrangement of the alkoxyl radical to an α-hydroxy alkyl radical which subsequently undergoes oxidation. The 7-substituted chromans, which arise directly from 1,6-cyclisation of the alkoxyl radical, were found to dominate the 6-substituted isomers which result from rearrangement of 1,5-cyclised intermediates. This effect is attributed to inefficient interception of the 1,5-cyclised radical intermediate which permits equilibration to the thermodynamically more stable 1,6-cyclised radical isomer to occur. The effect of pH on the reactions has been investigated and although no products typical of the intermediary of aryl radical rations were detected (even under highly acidic conditions), the formation of such intermediates cannot be excluded. Semiempirical MO calculations have been carried out (at the PM3 level of approximation) on a series of model compounds, yielding results which have clarified our understanding of the effect of substituents on the stabilities of the various intermediates arising from the cyclisation reactions of 3-phenylpropan-1-oxyl radicals. Furthermore, these calculations have supported our assumptions regarding the probability and specificity of rearrangements of the spirodienyl intermediates.

This article reviews the progress in the area of radicals in carbohydrate chemistry with special attention to new developments published during the past decade. No attempt has been made to present a comprehensive review of the literature but, rather, some specific examples are given to highlight the most synthetically valuable processes. The major focus is on inter- and intramolecular carbon–carbon bond-forming radical reactions, with emphasis on the preparation of C-glycosides, C-ketosides, C-disaccharides, and branched-chain sugars, as well as on the synthesis of polyfunctionalized carbocycles by radical cyclization of acyclic carbohydrate derivatives. We also describe methods for the formation of carbon–hydrogen bonds, and synthesis in which a heteroatom-centered radical plays a clear role in the carbon–heteroatom (C–N, C–P, C–O, C–S/Se, and C–Br) bond formation. Finally, a few selected examples of alkoxyl radical β-fragmentation reactions, carried out under oxidative or reductive conditions, are included to illustrate their potential in the preparation of chiral synthons. Keywords: carbohydrates; radicals; C-glycosides; C-disaccharides; carbasugars; glycos-1-yl radicals; carbon-centered radicals; heteroatom-centered radicals

EPR spectroscopic observations and product studies showed that the trimethylsilyl group undergoes 1,2-migration from carbon to nitrogen in aminyl radicals, Me 3 SiCh 2 O . → . CH 2 NHSiMe 3 , and from carbon to oxygen in alkoxyl radicals, Me 3 SiCh 2 O . → . CH 2 OSiMe 3 .

A time-resolved kinetic study on the reactions of the tert-butoxyl (t-BuO*), cumyloxyl (CumO*), and benzyloxyl (BnO*) radicals with alkylferrocenes has been carried out in MeCN solution. With all radicals, clear evidence for an electron transfer (ET) process has been obtained, and with the same ferrocene donor, the reactivity has been observed to increase in the order t-BuO*CumO*BnO*, with the difference in reactivity approaching 3 orders of magnitude on going from t-BuO* to BnO*. With BnO*, an excellent fit to the Marcus equation has been obtained, from which a value of the reduction potential of BnO* (E degrees(BnO*/BnO(-)) = 0.54 V/SCE) has been derived. The latter value appears, however, to be significantly higher than the previously determined reduction potential values for alkoxyl radicals and in contrast with the differences in the computed solution-phase electron affinities determined for t-BuO*, CumO*, and BnO*, indicating that the reaction of BnO* with ferrocene donors may not be described in terms of a straightforward outer sphere ET mechanism. From these data, and taking into account the available value of the reduction potential for CumO*, a value of E degrees (BnO*/BnO(-)) = -0.10 V/SCE has been estimated. On the basis of computational evidence for the formation of a pi-stacked prereaction complex in the reaction between BnO* and DcMFc, an alternative ET mechanism is proposed for the reactions of both CumO* and BnO*. In these cases, the delocalized nature of the unpaired electron allows for the aromatic ring to act as an electron relay by mediating the ET from the ferrocene donor to the formal oxygen radical center. This hypothesis is also in line with the observation that both BnO* and CumO* react with the ferrocene donors with rate constants that are in all cases at least 2 orders of magnitude higher than those measured for t-BuO*, wherein the radical is well-localized.

Reaction of polyunsaturated fatty acid hydroperoxides with metal complexes generates lipid alkoxyl radicals and metal-oxo complexes. Lipid alkoxyl radicals are presumed to be the species responsible for metal-amplified lipid peroxidation because of the chemical analogy of simple organic alkoxyl radicals to the hydroxyl radical. However, polyunsaturated fatty acid alkoxyl radicals exhibit a rich and diverse chemistry that is dominated by intramolecular cyclization to epoxyallylic radicals. Studies described herein demonstrate that the equilibrium between cyclization and ring-opening of epoxyallylic radicals lies overwhelmingly toward cyclization. Thus lipid alkoxyl radicals have a steady-state concentration that is so low that their contribution to metal-amplified lipid peroxidation is insignificant. In fact, the species responsible for metal amplification of lipid peroxidation appears to be the epoxyperoxyl radical formed by coupling the epoxyallylic radical to molecular oxygen.

Nitrate esters, prepared by treatment of β-hydroxy-α-amino acid derivatives with nitric acid, react with tributyltin hydride to give the corresponding alkoxyl radicals. These radicals readily undergo β-scission, providing a convenient route for the regiocontrolled production of α-carbon-centred amino acid radicals. By examining the partitioning of the alkoxyl radicals between the β-scission process and the competing hydrogen transfer reaction, it has been possible to evaluate the influence of electronic and steric effects on the β-scission reaction and the formation of the carbon-centred radicals.

The rates and selectivities of the hydrogen-atom abstraction reactions of electrically-neutral free radicals are known to depend on polar effects which operate in the transition state. Thus, an electrophilic species such as an alkoxyl radical abstracts hydrogen much more readily from an electron-rich C–H bond than from an electron-deficient one of similar strength. The basis of polarity-reversal catalysis (PRC) is to replace a single-step abstraction, that is slow because of unfavourable polar effects, with a two-step process in which the radicals and substrates are polarity-matched. This review explores the concept of PRC and describes its application in a variety of situations relevant to mechanistic and synthetic organic chemistry.

EPR spectroscopic observations and product studies showed that the trimethylsilyl group undergoes 1,2-migration from carbon to nitrogen in aminyl radicals, Me 3 SiCh 2 O . → . CH 2 NHSiMe 3 , and from carbon to oxygen in alkoxyl radicals, Me 3 SiCh 2 O . → . CH 2 OSiMe 3 .

A computer-aided design of alkoxyl radical precursors is performed. The new precursors should combine the advantages of N-alkoxypyridine-2(1H)thiones (less reactive radicals) and N-alkoxythiazole-2(3H)thiones (stable with respect to daylight). Additionally, the radical liberation process should be initiated by light with a wavelength of around 350 nm. To find promising compounds, 18 test candidates were obtained by a systematic variation of the parent compound N-alkoxythiazole-2(3H)thione. The properties of the test molecules were computed by a protocol that was already successfully used to rationalize the photochemical behavior of N-alkoxypyridine-2(1H)thiones and N-alkoxythiazole-2(3H)thiones. The computations identify two promising new compounds. For N-methoxy-(1,3)dihydro-[1,3]azaphosphole-2-thione (6a), they predict that the fragmentation process will be initiated by an absorption at 348 nm. An analysis of its fragmentation process indicates that the free excess energy of the resulting radicals should more resemble the situation found for N-alkoxypyridine-2(1H)thiones. For N-methoxy-(1,3)dihydro-pyrrole-2-thione (3a), the excitation energy is somewhat higher (330 nm), but the computed fragmentation paths again indicate that the remaining excess energy of the released radicals is quite favorable. The test molecules also contained the experimentally well-known N-methoxypyridine-2(1H)one (1b). For this molecule, our computed data rationalizes nicely the experimental findings.

The reaction of derivatives of 3-acetyl-d-glucal, 3-acetyl-l-rhamnal, 3-acetyl-d-galactal, and 3-acetyl-d-lactal with sodium benzenesulfinate in acid medium catalyzed by HgSO4 afforded diastereoisomeric mixtures of the corresponding 2,3-dideoxy-3-(phenylsulfonyl)-hexopyranoses through a Ferrier rearrangement. The anomeric alkoxyl radical fragmentation of these γ-hydroxy sulfones using the system (diacetoxyiodo)benzene and iodine gave vinyl sulfones with structures of 1,2-dideoxy-4-O-formyl-2-(phenylsulfonyl)-pent-1-enitol and configurations d-erythro, l-erythro, and d-threo at the two stereogenic centers.

[reaction: see text] The absolute rate constants for beta-scission of a series of benzocycloalken-1-oxyl radicals and of the 2-(4-methylphenyl)-2-butoxyl radical have been measured directly by laser flash photolysis. The benzocycloalken-1-oxyl radicals undergo ring opening with rates which parallel the ring strain of the corresponding cycloalkanes. In the 1-X-indan-1-oxyl radical series, ring opening is observed when X = H, Me, whereas exclusive C-X bond cleavage occurs when X = Et. The factors governing the fragmentation regioselectivity are discussed.

[reaction: see text] A product and time-resolved kinetic study of the effect of ring substitution on the reactivity of 1,1-diarylalkoxyl radicals has been carried out. The radicals undergo an O-neophyl shift to give the isomeric 1-aryl-1-aryloxyalkyl radicals from which the corresponding aromatic ketones are formed. The rearrangement rate constants are influenced by ring substitution, increasing in the presence of electron-withdrawing substituents and decreasing in the presence of electron-donating ones. From the results of product and kinetic studies, the following migratory aptitudes have been obtained: 4-trifluoromethylphenylphenyl approximately = 4-methylphenyl4-methoxyphenyl. Excellent Hammett-type correlations between the sigma+ substituent constants and both the visible absorption band maxima and the rearrangement rate constants have been obtained. The experimental results indicate that the rearrangement is governed by electronic effects in the starting 1,1-diarylalkoxyl radicals, whereas the stability of the rearranged carbon-centered radical plays a minor role, in line with a reactant-like transition state, strongly supporting the hypothesis that the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals proceeds through a concerted mechanism.

5 pages, 1 table, 6 schemes.-- Printed version published Feb 9, 2004., The fragmentation of primary alkoxyl radicals, often described as low yielding and plagued by side reactions, proceeded in good to excellent yields when aminol derivatives were used as substrates. Remarkably, no side reactions such as hydrogen abstraction or oxidation were observed. The fragmentation can be coupled with an alkylation reaction to give 2-substituted pyrrolidine and piperidine rings such as alkaloid analogues and functionalized, chiral nitrogen heterocycles., This work was supported by the Investigation Programme PPQ2000-0728 of the Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica, Dirección General de Investigación, Ministerio de Ciencia y Tecnología, Spain. We also acknowledge financial support from FEDER funds. A.M. thanks the C.S.I.C. (M.C.yT.) for an I3P fellowship.

The C-C beta-scission reactions of 1-alkylcycloalkoxyl radicals, generated photochemically by visible light irradiation of CH2Cl2 solutions containing the parent 1-alkylcycloalkanols, (diacetoxy)iodobenzene (DIB), and I2, have been investigated through the analysis of the reaction products. The 1-alkylcycloalkoxyl radicals undergo competition between ring opening and C-alkyl bond cleavage as a function of ring size and of the nature of the alkyl substituent. With the 1-propylcycloheptoxyl, 1-propylcyclooctoxyl,and 1-phenylcyclooctoxyl radicals, formation of products deriving from an intramolecular 1,5-hydrogen atom abstraction reaction from the cycloalkane ring has also been observed. The results are discussed in terms of release of ring strain associated to ring opening, stability of the alkyl radical formed by C-alkyl cleavage, and with cycloheptoxyl and cyclooctoxyl radicals, also in terms of the possibility of achieving a favorable geometry for intramolecular hydrogen atom abstraction.

Ab initio calculations have been used to design radical-resistant amino acid residues. Optimized structures of free and protected amino acids and their corresponding alpha-carbon-centered radicals were determined with B3-LYP/6-31G(d). Single-point RMP2/6-31G(d) calculations on these structures were then used to obtain radical stabilization energies, to examine the effect of steric repulsion between the side chains and amide carbonyl groups on the stability of alpha-carbon-centered peptide radicals. Relative to glycine, the destabilization for alanine and valine residues was found to be approximately 9 and 18 kJ mol(-1), respectively, which correlates with the reactivity of analogous amino acid residues in peptides toward hydrogen atom abstraction in conventional free radical reactions. To design amino acid residues that would resist radical reactions, strategies by which the steric effects could be magnified were considered. This resulted in the identification of tert-leucine and 3,3,3-trifluoroalanine as suitable molecules. With these amino acid residues, the destabilization of the alpha-carbon-centered radicals relative to that of glycine is increased substantially to approximately 36 and 41 kJ mol(-1), respectively. The theoretical predictions have been supported by experimental observations: a tert-leucine derivative was shown to be very slow to react with N-bromosuccinimide, while the corresponding trifluoroalanine derivative was found to be inert.

Utilising gaseous and aqueous thermodynamic quantities, estimates (in water versus NHE) have been made of the one-electron reduction potentials of alkyl peroxyl radicals including CCl3OO˙, percarboxyl and carboxyl radicals, and alkoxyl radicals, including CCl3O˙ and two-electron reduction potentials of alkyl hydroperoxides including CCl3OOH, alkyl peroxyl radicals including CCl3OO˙ and of percarboxyl radicals.

Product distributions resulting from the OH radical induced oxidation of the following hydrocarbons have been determined: 2-methyl-propane, 2,3-dimethyl-butane, 2-methyl-butane, n-pentane, cyclohexane, methylsubstituted 1-butenes, isoprene, toluene. Whenever possible, branching ratios for the self-reactions of alkylperoxy radicals and decomposition rate coefficients for alkoxyl radicals were derived.

The oxidation of cyclooctane using tert-alkyl hydroperoxides under GoAggV conditions has been examined using selected alkyl hydroperoxides as mechanistic probes. The results indicate that the products are formed by alkoxyl radical-induced reactions, there being no need to invoke any chemistry involving high valent iron-oxo species.

It was reported that the electron paramagnetic resonance (EPR) spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/lipid alkoxyl radical exhibited a quartet with 1:2:2:1 relative intensity that is identical to that of DMPO/hydroxyl radical (K. M. Schaich and D. C. Borg, 1990, Free Radicals Res. Commun. 9, 267-278). We repeated these EPR experiments using HPLC separation of radical adducts and isotope substitution. We found that the HPLC/EPR chromatogram of the radical adduct with a 1:2:2:1 quartet obtained by the reduction of methyl linoleate hydroperoxide (MLOOH) with Fe2+ exhibited identical retention time to that of the DMPO/OH radical adduct obtained from the Fenton reaction in two different solvent systems. Upon performing the same reaction in 17O-enriched water, the 17O-hyperfine coupling constants due to DMPO/17OH were identified. Ultimately, approximately 80-90% of the total DMPO/OH is derived from water by an iron-dependent nucleophilic addition reaction. Initially, a water-independent mechanism also significantly contributes to DMPO/OH formation. Regardless of its mechanism of formation, the 1:2:2:1 quartet radical adduct of DMPO formed during the reduction of MLOOH by Fe2+ is in fact DMPO/OH.

The inhibition of the autoxidation of hydrocarbons and polypropylene by aliphatic, aromatic, sterically hindered and cyclic phosphites has been studied by means of volumetric and 31P-NMR techniques. The antioxidant activity of phosphites depends on the rate of their reactions with peroxyl radicals and on the way they react with alkoxyl radicals. Only those phosphites which react by substitution to give free aryloxyl radicals are effective as chain-breaking antioxidants. The reaction modes of various phosphites with various peroxyl and alkoxyl radicals have been studied in some model reactions and the relationship between structure, reaction mechanism and antioxidant activity has been elucidated.

The low-pressure gas-phase thermolysis of several dialkyl peroxides has been investigated in the 250–350°C range. The free radicals were trapped at 77 K and identified by ESR. No primary alkoxyl radicals have been detected. There is evidence for the decomposition of methoxyl and ethoxyl radicals with formation of H and CH 3 radicals, respectively, which readily combine with traces of O 2 to yield HO 2 and CH 3 O 2 . For higher homologues only RCH 2 O 2 radicals have been detected and identified from the hyperfine structure.

1. Alkoxy derivatives of 2,2,6,6-tetramethylpiperidine interact with peroxide and alkoxyl radicals, forming the corresponding nitroxyl radicals. The kinetic criterion of effectiveness of inhibition on account of regeneration of nitroxyl radicals was formulated. 2. Reacting with alkyl radicals, 2,2,6,6-tetramethylpiperidino-l-oxyl inhibits the process of oxidation of isopropylbenzene and cyclohexyl methyl ether. 3. 4,4'-Dimethoxydiphenylnitroxyl interacts both with peroxide and with alkyl radicals of cyclohexyl methyl ether.

Methanethiyl radicals (CH3S.) are shown to react with cyclic ethers containing double bonds to give addition and hydrogen abstraction from the allylic positions. The esr spectra of the corresponding radicals have been determined. In some cases radicals arising from a rearrangement (probably 1,4 H-shift) were observed. t-Butoxyl radicals also give abstraction and addition with the same substrates. In two cases the barriers to ring inversion for six-membered ring radicals have been determined.

The e.s.r. spectra derived from 7-bromo-5,6-epoxyheptene derivatives such as epoxygeranyl bromide (7) showed that the main free radical intermediates were tetrahydrofuranylmethyl radicals (11), formed by ring opening of the epoxide and subsequent exocyclisation of the unsaturated alkoxyl radicals. The 7-oxabicyclo[2.2.1]heptanyl methyl radicals (12) were also formed via a double cyclisation. Products derived from both these intermediates were isolated from the reaction of (7) with tri-n-butyltin hydride.