Thioether substituted radical

There are numerous reports on the chemical synthesis of models for the active site of galactose oxidase both in the reduced Cu(l) and the oxidized Cu(II) form. We mention only a selection in which EPR is at least used to characterize the complex either on the phenoxy radical or on the copper part, typically in conjunction with X-ray data.48,49 50 A review on structural, spectroscopic and redox aspects of galactose oxidase models is available.51 More important with respect to EPR is the report on the 3-tensor calculation of the thioether substituted tyrosyl radical by ab initio methods but this is borderline to the aspects treated in this review since the copper ion is not involved.52... 

This is markedly different from the behavior of a simple tyrosine phenoxyl, such as that found in ribonucleotide reductase, whose spectrum exhibits a strong rhombic splitting (Fig. 17, line c) but precisely the same as observed for the 0-methylthiocresyl model radical (Fig. 17, line b). This clearly identihes the Tyr-Cys side chain as the site of the oxidized apoGAOX radical and demonstrates that the electronic structure of the thioether-substituted phenoxyl is distinct from that of a simple phenoxyl radical. 

Babcock, G. T., El-Deeb, M. K., Sandusky, P. O., Whittaker, M. M., and Whittaker, J. W., 1992, Electron paramagnetic resonance and electron nuclear double resonance spectroscopies of the radical site in galactose oxidase and of thioether-substituted phenol model compounds, J. Am. Chem. Soc. 114 372793734. 

The reactions of the OH radicals with two model thioether compounds, S-ethylthioacetate (SETAc) (H3C-CH2-S-C(=0)-CH3) and S-ethylthioacetone (SETA) (H3C-CH2-S-CH2-C(=0)-CH3, containing electron withdrawing acetyl groups in the a and f3 positions, with respect to the sulfur atom have been studied by pulse radiolysis. The a-positioned acetyl group in SETAc destabilizes >S -OH radicals within the five-membered structure that leads to the formation of alkyl-substituted radicals H2C -CH2-S-C(=0)-CH3 and H3C-CH2-S-C(-0)- CH2. At high proton and SETAc concentration, the OH-induced oxidation does not lead to intermolecularly three-electron-bonded dimeric radical cations SETAc(S.. S), only the a-(alkylthio)alkyl radicals H3C- CH-S-C(=0)-CH3 were observed (Amax = 420 nm). These observations are rationalized in terms of the... 

The radical site prior to the initiating proton transfer is, moreover, proposed to be located at the axial tyrosine (Tyr495), rather than at the equatorial thioether substituted Tyr272, as previously suggested. This would bring consistence between GO and model experiments by Stack, where there are strong indications that the radical resides axially. The cysteine cross-link, finally, was shown to have a very small effect on energetics and spin properties of the system. 

A remarkable case of a Meerwein reaction of phenylacetylene was reported by Leardini et al. (1985) in a new synthetic route to benzothiophene derivatives. Aryldi-azonium salts with a thioether group in the 2-position add phenylacetylene and substituted phenylacetylenes in the presence of metallic copper or iodide ion in acetone, or of FeS04 in DMSO (Scheme 10-60). The radical 10.21 formed initially is attacked intramolecularly by the sulfur atom of the thioether group to give the benzothiophene 10.22 in high yields (55-95%) as shown in (Scheme 10-60). Lear-... 

As the final example in this section, a Li-mediated carboaddition/carbocycliza-tion process will be described. Thus, Cohen and coworkers observed a 5-e%o-trig-cy-clization by reaction of the lithium compound 2-349 and a-methyl styrene 2-350 to give 2-352 via 2-351 (Scheme 2.82). Quenching of 2-352 with methanol then led to the final product 2-353 [189]. In this process, 2-349 is obtained by a reductive lithia-tion of the corresponding phenyl thioether 2-348 with the radical anion lithium 1-(dimethylamino)naphthalenide (LDMAN) (2-354). Instead of the homoallylic substance 2-348, bishomoallylthioesters can also be used to provide substituted six-membered ring compounds. 

Chemoselective oxidation of 3-arylsul-fenylmethyl-A -cephem affords 3-meth-oxymethyl-A -cephem in an anodic substitution. From the two thioethers, the arylthio group is more easily oxididized to a radical cation, that then undergoes cleavage to a thiyl radical and a carbocation (Fig. 31) [152]. 

Phenolic derivatives were prepared and then converted into thioether analogs using ethanedithol followed by oxidation of this intermediate to the disulfide. Phenolic resins were prepared by electrophilic substitution of allyl phenol derivatives with formaldehyde and then flee radically copolymerizing with ethanedithol. Epoxidation was performed using epichlarohydrine. 

A second example for a sulfur-directed cyclization, in which even two thio-phenyl groups were present as substituents on the target alkene 36, is shown in Scheme 14 [85]. This substitution pattern is capable of reversing the usual regios-electivity observed for aryl radical additions to enamides. In the presence of a fivefold excess of tributyltin hydride, one sulfur group is removed immediately after the cyclization step. The resulting tricyclic thioether 37 was further converted to mappicine ketone 38. 

The radical polymerization behavior of captodative olefins such as acrylonitriles, acrylates, and acrylamides a-substituted by an electron-donating substituent is reviewed, including the initiated and spontaneous radical homo- and copolymerizations and the radical polymerizations in the presence of Lewis acids. The formation of low-molecular weight products under some experimental conditions is also reviewed. The reactivity of these olefins is analyzed in the context of the captodative theory. In spite of the unusual stabilization of the captodative radical, the reactivity pattern of these olefins in polymerization does not differ significantly from the pattern observed for other 1,1-disubstituted olefins. Classical explanations such as steric effects and aggregation of monomers are sufficient to rationalize the observations described in the literature. The spontaneous polymerization of acrylates a-substituted by an ether, a thioether, or an acylamido group can be rationalized by the Bond-Forming Initiation theory. 

H atoms (H ) tend to abstract hydrogen atoms from C-H bonds, particularly if these are activated by neighboring functionalities such as sulfur atoms. The sulfur atom in thioethers was not usually considered as the primary target of H atom despite some, mostly recent, reports showing that bimolecular homolytic substitutions with H atom might occur with thioethers. Because these sulfur atoms are hypervalent, they can form adducts with H atoms. Radical chemistry studies on 1,3,5-trithiane (1,3,5-TT), performed by pulse radiolysis,... 

The secondary reduction of the terminal radical by Sml2 generates samarium alkyl species which are suitable for classical organometallic reactions, e.g. protonation, acylation, reactions with carbon dioxide, disulfides, diselenides, or the Eschenmoser salt. A broad variety of products is available (hydroxy-substituted alkanes, esters, carboxylic acids, thioethers, selenoethers, tertiary amines) by use of the double-redox four-step (reduction-radical reaction-reduction-anion reaction) route (Scheme 20) [73]. 

The photochemical step Is initiation of radical anion formation by electron transfer from the nucleophile to the aryl halide, one of the two being In the excited state. Thus amino acid substituted diaryl thioethers have been prepared In high... 

Silyl substitution at a carbon adjacent to an oxygen causes a significant decrease in the oxidation potential of ethers and alcohols by stabilizing cations or cation radicals (y6-silicon effect Chapter 20) [210] in some cases a similar effect is found for thioethers. 

Hydroxy Alkyl Phenyl Ketones The basic structures, 2-hydroxy-2-methyl-l-phenyl-1-propanone HAP and 1-hydroxy-cyclohexyl-1-phenyl ketone HCAP, are also well known [17] and still largely used (10.10) and studied [2i,f,18]. The para substitution of the benzoyl group (with an amino, an ether, or a thioether group) drastically changes the electronic transitions but does not significantly affect the reactivity of the derived benzoyl initiating radical. 

Antioxidant Activity of Some Phenyl-Substituted Phenols (VII) in PP at 180°C (Oxygen-Uptake Test). In Table I the results obtained with some phenyl-substituted phenols are shown and compared with some well-known ferf-butyl-substituted phenols. A general conclusion from this table is the marked synergism between phenyl-substituted phenols and -activated thioethers compared with the ferf-butyl-substituted analog. Since the measurements are done in a closed tube, the influence of the volatility of the thiodipropionates is not reflected in the induction periods. An explanation for the anomalous behavior of phenyl-substituted phenols in synergistic mixtures can be found in the results of the model reactions, carried out with phenoxyl radicals and sulfides or sulfoxides (see "Model Reactions ). 

Carbon-silicon bond heterolysis of the radical-cation from trimethylsilyl-substituted ethers, thioethers, and amines, generated by PET to DCA, produces the corresponding methylene radical, which ultimately combines with reduced DCA to yield coupling products (Scheme 12) [22]. 

Thio-ene addition is another attractive method for the attachment of organic thioether groups to polysiloxane chains. It is based on the free-radical addition of a thiol group to a vinyl-substituted siloxane or the addition of a terminal alkene to a thiol-substituted siloxane (Eq. 2). " Boileau et al. used this reaction to graft different amino acids to the polymer backbone to induce potential interactions with polar surfaces. 

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