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Silane3 is an effective mediator in organic synthesis via radicals. Reactions with this silane give smaller amounts of unwanted reduction products than reactions with toxic stannanes.

The absolute rate constants for the reaction of Et3Si· radicals with a large number of unsaturated compounds have been measured by laser flash photolysis techniques. The reactivities of C=C double bonds have a wide range, e.g., the rate constants at ca. 300 K are 1.1 × 109, 2.2 × 108, 1.0 × 107, 3.7 × 106, and 9.4 × 105 M-1 s-1 for acrylonitrile, styrene, tetrachloroethylene, 1-hexene, and cyclohexene, respectively. The range of reactivities for addition to aromatic and heteroaromatic compounds is rather small, the rate constants at ca. 300 K varying from 4.6 × 105 M-1 s-1 for benzene to 5.0 × 106 M-1 s-1 for thiophene and α-methylnaphthalene. Acetylenes are slightly less reactive than structurally analogous 1-olefins. For the other types of multiple bonds examined, reactivities decreased in the order isocyanide > nitrone > nitro > isocyanate > nitrile. Arrhenius parameters were determined for a few olefins including ethylene, for which an EPR spectroscopic competition procedure was required. For styrene, log (A/(M-1 s-1)) = 9.35 ± 0.23 and Ea = 1.37 ± 0.29 kcal/mol; for ethylene, log (A/(M-1 s-1)) = 8.40 ± 0.60 and Ea = 1.40 ± 0.80 kcal/mol. © 1983 American Chemical Society.

tert-Butoxyl abstracts hydrogen from M-H bonds (M = Si, Ge, Sn) with rate constants (300 K) of (0.6-2) × 10 7 (M = Si), ∼9 × 10 7 (M = Ge), and (2-4) × 10 8 M -1 s -1 (M = Sn), respectively. The radicals (alkyl) 3M· show a strong absorption band at λ < 300 nm and a weaker band/shoulder at longer λ that shifts from 320 (M = C) to 390 nm (M = Sn). The spectra of Ph 3M show little sensitivity to the heteroatom. Arylsilyl radicals add readily to their precursors; for example, Ph 3Si· adds to Ph 3SiH with k 4 = (2.1 × 0.4) × 10 5 M -1 s -1 at 300 K.

On mesure par des techniques de photolyse eclair laser les constantes de vitesse absolue des reactions de radicaux t-butoxyles avec des aldehydes organiques. Parametres d'Arrhenius. Les effets polaires sont tres importants pour determiner la reactivite des aldehydes vis-a-vis des radicaux t-butoxyles

The absolute rate constants for the reactions of Et3Si• radicals with a large number of carbonyl-containing compounds have been measured in solution by using laser flash photolysis for compounds having rate constants > 105 M-1 s-1 and kinetic EPR spectroscopy for compounds having rate constants

Rate constants for the overall rearrangement of the 2,2-dimethyl-3-buten-1-yl radical to the 1,1 -dimethyl-3-buten-1-yl radical, kC=C, have been measured from −145 °C to −101 °C by kinetic epr spectroscopy and at 40 °C by spin trapping with 1-methyl-4-nitroso-3,5-diphenylpyrazole. The temperature dependence can be represented by[Formula: see text]where θ = 2.3RT kcal mol−1. This rearrangement, which must proceed via a 2,2-dimethylcyclopropylcarbinyl radical as an intermediate, is one of the fastest rearrangements involving a primary alkyl radical. At 25 °C, kC=C = 4.3 × 107 s−1, which makes this rearrangement about 10 000 times faster than the rearrangement of the simplest homoallyl radical, 3-buten-1-yl. This rate enhancement is attributed to steric acceleration of ring closure by the gem dimethyl groups (Thorpe–Ingold effect).

The epr spectral parameters for some [Formula: see text] radicals have been measured over a range of temperatures. These radicals are probably non-planar at nitrogen. Their OR groups are in the eclipsed position with respect to the N 2pz orbital, a conformational preference which is attributed to a combination of steric factors and the anomeric effect. For [Formula: see text] the H hyperfine splittings (hfs) are of unusually small magnitude. It is pointed out that all other known [Formula: see text] also have anomalously low H hfs. It is suggested that this is due to the anomeric effect which not only promotes the eclipsed conformation but also causes the OR group to bend towards the semioccupied orbital, thereby moving H and R2 towards the nodal plane of this orbital.

The formation of radicals by a "spontaneous" reaction between nitroso compounds and certain organic materials has often been reported in the literature and has generally been ascribed to a process of molecule assisted homolysis (MAH). Some selected "spontaneous" radical-forming processes involving trifluoronitrosomethane, nitrosobenzene, and 2,4,6-trichloronitrosobenzene have been reexamined. In many cases it has been found that the radicals are formed only when the sample is exposed to ordinary laboratory light. It is suggested that the role of light on other "spontaneous" radical-forming reactions should be investigated. © 1981 American Chemical Society.

Addition des radicaux triethylsilyles, triphenylsilyles et triphenylgermyles avec les di-«t»-butyl-2,6-, dimethoxy-2,6 et dimethyl-2,6 benzoquinones-1,4, le dimethyl-3,6 thieno [3,2-b] thiophene, l'anthraquinonemethide et la benzylidene-5 dimethyl-3,6 thieno [3,2-b] thiophenone-2

NADPH-oxidase is an enzyme that represents, when activated, the major source of non-mitochondrial reactive oxygen species. In phagocytes, this production is an indispensable event for the destruction of engulfed pathogens. The functional NADPH-oxidase complex consists of a catalytic membrane flavocytochrome (Cytb(558)) and four cytosolic proteins p47(Phox), p67(Phox), Rac and p40(Phox). The NADPH-oxidase activity is finely regulated spatially and temporally by cellular signaling events that trigger the translocation of the cytosolic subunits to its membrane partner involving post-translational modifications and activation by second messengers such as arachidonic acid (AA). Arachidonic acid in its natural cis-poly unsaturated form (C20:4) has been described to be an efficient activator of the enzyme in vivo and in vitro. In this work, we examined in a cell-free system whether a change of the natural cis geometry to the trans configuration, which could occur either by diet or be produced by the action of free radicals, may have consequences on the functioning of NADPH-oxidase. We showed the inability of mono-trans AA isomers to activate the NADPH-oxidase complex and demonstrated the inhibitory effect on the cis-AA-induced NADPH oxidase activation. The inhibition is mediated by a direct effect of the mono-trans AA which targets both the membrane fraction containing the cytb(558) and the cytosolic p67(Phox). Our results suggest that the loss of the natural geometric feature (cis-AA) induces substantial structural modifications of p67(Phox) that prevent its translocation to the complex. (C) 2012 Elsevier B.V. All rights reserved.

Apolipoprotein E (apoE) is a major lipid carrier of the lipoprotein transport system that plays critical roles in various pathologies. Human apoE has three common isoforms, the apoE4 being associated with Alzheimer's disease. This is the first study in the literature investigating the effects of apoE (apoE3 and apoE4 isoforms) on membrane fatty acid profile in neuroblastoma SK-N-SH cells. Fatty acid analyses were carried out by gas chromatography of the corresponding methyl esters (FAME). We observed the occurrence of membrane fatty acid remodeling in the presence of each of the two apoE isoforms. ApoE3 increased the membrane level of stearic acid and dihomo-gamma-linolenic acid (DGLA), whereas apoE4 had opposite effects. Both apoE3 and apoE4 increased saturated and monounsaturated fatty acids (SFA and MUFA), omega-6/omega-3 ratio and decreased total polyunsaturated fatty acid (PUFA) amount, but with various intensities. Moreover, both apoE isoforms decreased membrane homeostasis indexes such as PUFA balance, unsaturation index and peroxidation index. Our results highlight membrane property changes connected to the apoE isoforms suggesting membrane lipidomics to be inserted in further model studies of apolipoproteins in health and disease.

The EPR parameters for (CF3)2N· have been determined to be g = 2.00374, aN = 13.84 G, and aF = 13.84 G in CF2Cl2 from 150 to 220 K. The (CF3)2NO· radical has been generated from (CF3)2Ṅ by using isotopically enriched 17O2. It has a17O equal to 23.6 G which is higher than a17O values found for dialkyl nitroxides (19-20 G). This serves to confirm that (CF3)2NO· has a higher spin density on oxygen than other nitroxides. © 1980 American Chemical Society.

The hyperfine splitting by 17O in (CF3)2N170• is 23.6 G which is appreciably larger than the 19.4 G reported for [(CH3)3C] 2N170• and indicates that the former radical has a relatively high spin density on oxygen. This is attributed to the electron withdrawing effect of the CF3 groups which destabilize the canonical structure, \({\left( {C{F_3}} \right)_2}{\dot N^ + } - \overline {\ddot O} \) relative to, \({\left( {C{F_3}} \right)_2}\ddot N - \dot O\). The greater localization of the unpaired electron gives (CF3) 2NOH a higher O-H bond strength than dialkyl nitroxides, viz., 82.6 vs. ca. 70 kcal/mol. The configuration and conformation of a number of alkoxyalkyl- and hydroxyalkyl trifluoromethyl nitroxides have been examined. It is concluded that these radicals are probably non-planar at nitrogen and that the anomeric effect plays an important role in determining their conformation.

The infrared and Raman spectra of N,N-bis(trifluoromethyl)hydroxylamine ((CF3)2NOH) and of bis(trifluoromethyl) nitroxide ((CF3)2NO·) have been recorded and vibrational assignments proposed. In the vapor phase (CF3)2NOH exists as a mixture of two conformers which have an enthalpy difference of ca. 12 kJ mol-1. The most stable conformer has Cs symmetry. The spectra of (CF3)2NO· are indicative of a planar, C2v, structure though a quasi-planar structure with a low barrier to inversion cannot be ruled out. A frequency of 133 cm-1 has been tentatively assigned to the CNO bending mode which is out of the CNC plane. Since (CF3)2NO· is less likely to be planar than an acyclic dialkyl nitroxide, it is concluded that the latter also should be considered to be planar species. The CF3 rotational barriers of (CF3)2NOH and (CF3)2NO· are discussed. © 1981 American Chemical Society.

The absolute rate constants for the reactions of Et3Si• radicals with a number of organic halides have been measured in solution by using laser flash photolysis techniques. The reactivities cover a wide range; e.g., the rate constants at ca 300 K are 4.3 × 109, 1.1 × 108, 2.5 × 106, and

The epr spectral parameters for some radicals have been measured over a range of temperatures. These radicals are probably non-planar at nitrogen. Their OR groups are in the eclipsed position with respect to the N 2pz orbital, a conformational preference which is attributed to a combination of steric factors and the anomeric effect. For the H hyperfine splittings (hfs) are of unusually small magnitude. It is pointed out that all other known also have anomalously low H hfs. It is suggested that this is due to the anomeric effect which not only promotes the eclipsed conformation but also causes the OR group to bend towards the semioccupied orbital, thereby moving H and R2 towards the nodal plane of this orbital.

Absolute rate constants for the reactions of tert-butoxyl radicals and some ketone triplets with a variety of silanes have been measured by a laser flash technique. © 1982 American Chemical Society.

The radicals formed during photolysis of di-t-butyl peroxide and a number of alkenyldimethylsilanes have been examined by e.s.r. spectroscopy. Only carbon-centred radicals were observed. These were either secondary alkyl radicals formed by the addition of the initially formed silyl radical to a double bond and/or allyl radicals formed by hydrogen-atom abstraction from the alkenyl group. In most cases addition to the double bond was an intermolecular process. However, pent-4-enylsilyl radicals undergo intramolecular addition with endo-cyclization to form the six-membered ring being strongly favoured over exo-cyclization to form the five-membered ring. This preference was confirmed by product studies on 3,3-dimethylpent-4-enyldimethylsilane. The factors that control homolytic exo : endo cyclization rates for species in which the radical centre is located on a second row element are adumbrated.

Examples of free radical reactions are described for which convincing evidence for quantum-mechanical tunneling has been reported. They include: H-atom abstraction reactions by thermalized methyl radicals and H-atoms in solids (crystalline and glass phases) at low temperatures; radical-pair conversions at 4.2 K; isomerization of hindered aryl radicals and intermolecular H-transfer reactions in fluid solution.

Rate constants for the reaction of primary alkyl radicals with a variety of germanium hydrides have been measured by using the one-carbon ring expansion of cyclopentanones as a timing device. The r...

The involvement of free radicals in life sciences has constantly increased with time and has been connected to several physiological and pathological processes. This subject embraces diverse scientific areas, spanning from physical, biological and bioorganic chemistry to biology and medicine, with applications to the amelioration of quality of life, health and aging. Multidisciplinary skills are required for the full investigation of the many facets of radical processes in the biological environment and chemical knowledge plays a crucial role in unveiling basic processes and mechanisms. We developed a chemical biology approach able to connect free radical chemical reactivity with biological processes, providing information on the mechanistic pathways and products. The core of this approach is the design of biomimetic models to study biomolecule behavior (lipids, nucleic acids and proteins) in aqueous systems, obtaining insights of the reaction pathways as well as building up molecular libraries of the free radical reaction products. This context can be successfully used for biomarker discovery and examples are provided with two classes of compounds: mono-trans isomers of cholesteryl esters, which are synthesized and used as references for detection in human plasma, and purine 5',8-cyclo-2'-deoxyribonucleosides, prepared and used as reference in the protocol for detection of such lesions in DNA samples, after ionizing radiations or obtained from different health conditions.

A 1,5-radical translocation from a conveniently situated alkoxyl or vinyl radical next to the C-1′ position is the key step for the successful radical-based cascade reactions that allow the construction of two classes of anomeric spironucleosides 1 and 2 in the ribo and 2′-deoxyribo series.

The unsaturated fatty acyl moieties of TAG present in natural oils of borage, olive, and rice were converted to their corresponding geometrical trans isomers by thiyl radical-catalyzed isomerization. Thiyl radicals were generated from 2-mercaptoethanol under photolytic or thermal conditions. A relevant feature of this method is the absence of double-bond shifts, so that no positional trans isomers or conjugated polyenes are formed. Oils obtained after the isomerization were winterized to further increase their trans fatty acid content. Methanolysis and hydrolysis of the trans oil mixtures using an enzymatic method (lipase B from Candida antarctica) gave good conversions to the corresponding trans FAME and fatty acids, respectively. These results are relevant for the studies of lipid isomerism and trans fatty acid recognition, which is a growing concern in biochemistry and nutrition, and open new perspectives for the synthesis of glycerides and studies of their structure-activity relationships.

The versatility of the palladium(II) chloride/triethylsilane system has been tested in the transformation of alcohols. The conversion to the corresponding halides and alkanes has been achieved in good yields and in the absence of solvent for a variety of substrates.

Tris(trimethylsily1)silane is an effective reducing agent for organic halides, selenides, xanthatee, and isocyanide, as well as an effective hydrosilylating agent for dialkyl ketones and alkenes. The silane functions as a mediator in the formation of intermolecular carbon-carbon bonds via radicals and allows a variety of organic substrates to be used as alkyl radical percursors. Absolute rate constants for the reaction of (MeSSi),@' radicals with a variety of organic compounds have been measured in solution by laser flash photolysis. At 294 K rate constants are >5 X 10' M-' s-l for C=C double bonds that are activated by neighboring ?r-electron systems or by electron-withdrawing groups. For other substrates, reactivities decreased in the order xanthate > selenide > isocyanide > nitro > sulfide.

Formation de radicaux CH 3 CO 2 • , C 6 H 5 SO 2 • , C 6 H 5 CH 2 SO 2 • par elimination d'halogene par les radicaux (C 2 H 5 ) 3 Si • a partir d'halogenures de sulfonyle appropries

The low-temperature 13C n.m.r. spectra (25.16 MHz) of thiophene-2- and -3-carbaldehyde [(1) and (2)] display line-broadening effects due to the exchange between their S,O-syn- and S,O-anti-conformers. In both cases the free energy of activation for rotation of the –CHO moiety was measured, the values being 10.15 and 8.5 kcal mol–1 for (1) and (2), respectively. In the case of thiophene-3-carbaldehyde (2) the signals of both conformers were detected, the ratios being 97:3 at –140 and 93:7 at –100 °C in favour of the S,O-anti-conformer. In thiophene-2-carbaldehyde (1) the minor conformer (S,O-anti) was present in too small an amount to be unambiguously detected by a conventional spectrometer, but spectra taken with a superconducting instrument (75.48 MHz) revealed ca. 1.5% of this conformer at –100 °C.

The e.s.r. spectra are described of a variety of arenesulphonyl radicals (ArSO2·) which have been generated photolytically over a wide range of temperature. It is shown that the spin distribution in these σ-radicals resembles that in the benzoyl radical; radicals without ortho-substituents, which undergo relatively rapid rotation about the C–S bond at all accessible temperatures, have a(m-H) > a(o-H), a(p-H). For some di-ortho-substituted radicals, rotation around C–S is slow at low temperature (barriers to rotation have been determined for some examples) and some mono-ortho-substituted radicals demonstrate marked conformational preferences; analysis shows that the largest proton-splitting (ca. 0.2 mT) derives from the meta-proton anti to the orbital of the unpaired electron on sulphur. INDO calculations yield results in good agreement with these findings.

E.s.r. evidence is described for three modes of photolytic decomposition of sulphinate esters [XS(O)OR]. For some alkyl arenesulphinates (X =p-tolyl, R = alkyl) the arenesulphinyl radical (XSO·) is detected, evidently as a result of S–O bond fission; for some alkyl alkanesulphinates, photolysis apparently cleaves the O–R bond and alkane-sulphonyl radicals (XSO2·;) are formed; for methyl phenylmethanesulphinate (X = PhCH2, R = Me) the detection of the benzyl radical and MeOSO· suggests that the C–S bond has been broken.It is suggested that the reaction between ButO·(from di-t-butyl peroxide) and sulphinates usually involves attack at sulphur to give an intermediate sulphuranyloxyl radical XṠ(O)(OR)OBut. Evidence that this can undergo dealkylation derives from the detection of the benzyl radical in the reaction of PhCH2S(O)OMe. On the other hand, the production of sulphonyl radicals (XSO2· X = alkyl, aryl) from this intermediate may involve dealkoxylation and further reaction of the resulting t-butyl sulphinate ester.Line-width variation in the e.s.r. spectrum of MeSO2· at low temperature is attributed to the occurrence of restricted rotation about the C–S bond in a radical of pyramidal type.

The e.s.r. spectra of a variety of photochemically generated thiophene-2-sulphonyl radicals are described. Their spin distribution is typical of σ-radicals; radicals without substituents at position 3 exhibit relatively rapid rotation about the C–S bond at all accessible temperatures while the 3-bromo-substituted ones demonstrate a marked conformational preference which has been interpreted in terms of a π-type conjugated structure. These findings are substantiated also by the results of INDO calculations carried out for the unsubstituted thiophene-2-sulphonyl radical with different relative arrangements of the SO2 and heteroaromatic moieties.

A conformational analysis is presented for a series of aliphatic sulphonyl radicals RSO2·; this is based on the magnitudes of hyperfine splittings from α- and β-protons (Cβ–Cα–SO2·), on the obervation of line broadening resulting from restricted rotation about both the Cα–S and the Cβ–Cα bonds, and on the results of molecular orbital calculations employing the INDO (Intermediate Neglect of Differential Overlap) method. The spectra and structure of the novel alkenesulphonyl radicals (e.g. CH2CHSO2·) are also described.

The use of liposomes has been crucial for investigations in biomimetic chemical biology as a membrane model and in medicinal chemistry for drug delivery. Liposomes are made of phospholipids whose biophysical characteristics strongly depend on the type of fatty acid moiety, where natural unsaturated lipids always have the double bond geometry in the cis configuration. The influence of lipid double bond configuration had not been considered so far with respect to the competence of liposomes in delivery. We were interested in evaluating possible changes in the molecular properties induced by the conversion of the double bond from cis to trans geometry. Here we report on the effects of the addition of trans-phospholipids supplied in different amounts to other liposome constituents (cholesterol, neutral phospholipids and cationic surfactants), on the size, fl-potential and stability of liposomal formulations and on their ability to encapsulate two dyes such as rhodamine B and fluorescein. From a biotechnological point of view, trans-containing liposomes proved to have different characteristics from those containing the cis analogues, and to influence the incorporation and release of the dyes. These results open new perspectives in the use of the unnatural lipid geometry, for the purpose of changing liposome behavior and/or of obtaining molecular interferences, also in view of synergic effects of cell toxicity, especially in antitumoral strategies.