Radical relay mechanism

Figure 5. Radical relay mechanism in the pyrolysis of silica-attached 1, 3-diphenylpropanc.

Breslow s template-directed remote oxidation of steroids utilizes an aryl iodide as a template to direct the oxidation of steroid tertiary carbons by the radical relay mechanism, in which a chlorine radical is transferred from a [9-1-2] [PhICl] radical to the iodine atom of the template and then relayed to a geometrically accessible hydrogen atom. This method allows a highly regioselective functionalization of nonactivated carbon atoms of steroids [Eq. (78)] [137,138]. 

Two novel variations on remote oxidation involve radical relay mechanisms. Chlorine radicals generated by photolysis of iodobenzene dichloride are carried by the iodine atom of a suitable iodo-aryl ester of 5a-cholestan-3a-ol to permit hydrogen abstractions from C-9 or C-14, depending upon the ester employed.237 The m-iodobenzoate (293) afforded the 9a-chloro- and thence the cholest-9(ll)-ene derivative (294), whereas the p-iodophenylacetate similarly gave a 14-ene. 

Free-radical Reactions Directed by Tethered Templates -the Radical Relay Mechanism... 

Some variants on the simple template-directed chlorination were also developed. For example, a steroid carrying a tethered iodophenyl group was chlorinated by electrolysis of a solution carrying chloride ion [54]. In this case, the electrolysis furnished CI2 in solution to carry a radical relay process and electrolysis also initiated the radical process by one-electron oxidation of the iodophenyl group. As another variant, the radical relay mechanism requires that it be a chlorine atom that attaches to the iodine or pyridine or sulfur to abstract hydrogen, since a complexed bromine atom is not reactive enough, but the new bond to the substrate does not have to be a carbon-chlorine link. That bond is formed by untemplated attack of the substrate carbon radical on a reagent in solution and, with an appropriate sequence of tandem reactions, other atoms can be linked to the substrate. 

A study [58] on the use of non-tethered templates to direct the radical relay mechanism will be described in a later section of this chapter. 

We showed that radical relay in the presence of phenyliodine dichloride but with an excess of CBr4 afforded the bromosteroid, with bromine at the point that is normally chlorinated [55]. When (SCN)2 was the additive, rather than CBr4, an SCN group was formed [55]. The mechanism shown (Scheme 6-8) involves a bromine atom transfer to... 

Scheme 6-8 Mechanism of formation of a bromosteroid by radical relay functionalization.

Taking into account the high viscosity of PTFE, one may presume that, during its thermal degradation, the recombination of the macroradicals involves the monomer as a chain-transfer agent in a relay mechanism [22], which is due to migration of the radical centre in a chain process ... 

Some of the materials highlighted in this review offer novel redox-active cavities, which are candidates for studies on chemistry within cavities, especially processes which involve molecular recognition by donor-acceptor ii-Jt interactions, or by electron transfer mechanisms, e.g. coordination of a lone pair to a metal center, or formation of radical cation/radical anion pairs by charge transfer. The attachment of redox-active dendrimers to electrode surfaces (by chemical bonding, physical deposition, or screen printing) to form modified electrodes should provide interesting novel electron relay systems. 

It was this widespread, but misplaced, belief which caused the controversy over the interpretation of the observed rapid intra-molecular ET rates (= 107 s-1) in the radical anion of the semi-flexible traw.v-dinaphthyl-cyclohexane system 3(7), where the number in parentheses refers to the number of C-C bonds in one of the relays of the hydrocarbon bridge connecting the pair of chromo-phores.50 The observed rapid ET rate was tacitly attributed to a TS mechanism, rather than to a TB mechanism, even though the two naphthalene rings are about 7 A apart (edge-to-edge). A TS mechanism was likewise advanced to account for the observed rapid intra-molecular ET occurring in the radical anion of 4(6).51... 

Under circumstances where the sensitizer is efficiently reduced by the donor in the excited state, then one can obtain photoreduction of MV2+ via a reductive cycle. Proflavine and other acridine dyes sensitized reduction of MV2+ as well as Zn-Porphyrin sensitized reduction under high donor concentrations have been shown111,319 to follow such a mechanism. In both cases, the photogenerated sensitizer anion-radicals being good reductants themselves, also can play the role of the acceptor (relay) molecules. With efficient Pt-PVA catalysts, it has been shown118,333 to be the case for proflavine and Ru(bipy)3. 

Kovacic P, Pozos RS (2006) Cell signaling (mechanism and reproductive toxicity) redox chains, radicals, electrons, relays, conduit, electrochemistry and other medical implications. Birth Defects Res Part C Embryo Today Rev 78 333-344 Kraemer SA, Arthur KA, Denison MS, Smith WL, DeWitt DL (1996) Regulation of prostaglandin endoperoxide H synthase-2 expression by 2, 3, 7, 8,-tetrachlorodibenzo-p-dioxin. Arch Biochem Biophys 330 319-328... 

Thus, the detection of the direct and selective influence of the paramagnetic lipophilic electron relay on the H-NMR spectra of the surrounding lipid molecules, which constitute the bilayer lipid membranes, appears to be direct evidence for the localization of the paramagnetic moiety of lipophilic cetylviologen cation radicals just near the surface of the vesicle membrane. Evidently, this coincides with the above data from molecular mechanics calculations. 

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