Radical 1,4-aryl transfer

Intramolecular dissociative electron transfer in aryl halide radical-anions involves an interaction between the n-aroinatic orbital and the o-type carbon-halogen bond. These orbitals are orthogonal, but bending of the carbon-halogen bond allows the necessary interaction. Qualitatively, the influence of several factors on the bond cleavage rate can be discerned [18],... 

The addition of aryl radicals, generated by chemical reduction of aryldiazonium salts, onto arenes in the Gomberg-Hey reaction is well established [163]. The addition of these radicals to alkenes in the Meerwein reaction is also well known [164], Aryl o-radicals generated by electrochemical reduction of aryl halides take part in similar reactions. Good yields of the products are obtained when the intermediate phenyl radical can react in an intramolecular manner. The addition step is then fast and competes successfully with further electron transfer to form the phenyl car-banion, followed by protonation. 

Competition between cyclization of the intermediate a-radical or further reduction is illustrated with the benzanilide substrates 50 [165], In all, four types of product are formed. Cyclization of the phenyl radical to a six-ring radical intermediate leads to the cyclization product 51. Cyclization of the phenyl radical to a five-ring radical intermediate leads to the diphenyl 52 after a further electron transfer step. Reduction of the aryl o-radical before cyclization gives 51, X = H. Cleavage of the carbonyl-nitrogen bond in the radical-anion affords a trace of the aniline S3. 

These reactions are related to the reaction of aryl diazonium salts with iodide yielding iodoaryls, the mechanism of which seems to be a one-electron transfer (radical) reaction and not a nucleophilic displacement. Just as iodide is easily oxi- zed to iodine by the aryl diazonium cation, 2.4.6-triphenyl-X -phosphorin is oxidized to the radical cation 58. 

The aryl transfer is thought to be initiated by addition of the silyl radical to the thiocarbonyl. The radical so formed adds to the alkene to generate the benzylic radical 7. This radical adds to the arene to give 8, which fragments to 5. 

A key step in a synthetic approach to 2-substituted 5-arylpiperidines has involved a radical 1,4-aryl transfer that proceeds by the mechanism depicted in Scheme 33.56... 

In this variant, the ipso substitution of 68 led to the cyclohexadienyl radical 69 which underwent C-P bond cleavage and expulsion of the tether to give biphenyl 70. A comparable mechanism is operative for the aryl transfer from silicon to carbon in which a comparable biphenyl 71 (c.f. 70) was synthesized from siloxane 72 (Scheme 26) [65, 133]. 

A photoexcited anion also plays a role in the photoelectrocatalytic reduction of 4-chlorobiphenyl using the anion radicals of anthracene and 9,10-diphenylanthracene409. The anion radicals were electrochemically generated and excited by means of visible light. Formation of the aryl anion radical then takes place either by direct electron transfer or by... 

However, as we will see later on, other modes of evolution of the primary intermediate radical ions can be suggested to explain some oxidation reactions mediated by electron-transfer processes. In fact, several exceptions to the Foote s BQ-controlled photooxygenation procedure have been reported during the last years on several electron-rich substrates. Thus, the involvement of superoxide ion, as an oxygen active species, in all of the DCA-sensitized photooxygenations, remains questionable [96,105,112,115,128]. Schaap and co-workers [98] recorded under inert atmosphere the characteristic ESR spectrum of the (DCA ) radical anion. On the other hand, the involvement of aryl-olefin radical cations and their reactions with superoxide ion was easily observed by quenching experiments with compounds exhibiting lower oxidation potentials than those of aryl-olefins [85, 95, 98],... 

The mechanism of the Sandmeyer reaction involves the transfer of electron from Cu" " to diazonium salt. Elimination of nitrogen generates aryl free radical, which reacts with CuXCl to give aryl halide via either path a or path b (Scheme 2.33). 

It is assumed that, in the reaction of arenes with aryllead triacetates, the arylation takes place via the corresponding cationic TC-complexes. In the reaction with lead tetraacetate, an electron transfer-radical cation mechanism was postulated. 1 3 jn agreement with this assumption, 4,6,8-trimethyl-... 

The reaction with aryl radicals of this kind has found an interesting application in the direct polymerization of vinylic monomers like styrene, methyl methycrylate (MMA), or hydroxyethyl methacrylate (HEMA) on the diamond. Furthermore, the covalent attachment of an initiator molecule allows for an atom transfer radical polymerization (ATRP) to take place immediately on the diamond surface and results in a covalently bound composite material (Figure 6.41). 

When the substrate molecule contains functional groups, quite different chemical reactions can occur following the initial radical cation formation. For example, benzyl alcohol derivatives can deprotonate at the a-hydrogen and then, after the second charge transfer, deprotonate once more substantial yields of the benzaldehyde derivatives are ob-tained. Aryl iodide radical cations attack a second substrate molecule ... 

During their studies of Ugi postcondensation reactions, Kaim et al. [68] discovered an intramolecular aryl transfer/oxidative cleavage/cyclization process (Scheme 5.44). Reaction of the Ugi product 201 with a radical obtained from malonate led to the radical intermediate 202, which underwent a 1,4 aryl transfer to give radical 203. Oxidative addition of AcOH followed by hydrolysis of the formed N,0-acetal led to 205. The final step in the domino process was an oxidative 5-exo-trig cyclization to form the indane motif206 in 59% yield. 

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