Coupling of radicals

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

It was subsequently shown that the polymers contain semi-quinonoid structures 47 proposed to arise from a-p coupling of radicals 46 as shown in Scheme 9.15., (M It was also suggested that 47 could be subject to radical-... 

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

Strong evidence for the radical cation-radical cation coupling mechanism shown in Scheme 1 has been obtained from double-step chronoam-perometry studies.66,67 However, an alternative polymerization mechanism, involving the coupling of radical cations with unoxidized molecules, has been claimed by a number of authors.55,68,69 Some of the... 

There has been considerable discussion of the mechanism of the acyloin condensation. One formulation of the reaction envisages coupling of radicals generated by one-electron transfer. 

Cleavage of anion radicals. Coupling of radicals with nucleophiles... 

Heterolytic Cleavages. Coupling of Radicals with Nucleophiles... 

Radical addition to the pyridine ring is another known facile process and coupling of radical intermediates from the reduction of ketones to pyridines in... 

Reduction of dicarbonyl compounds can be expected to lead to a cyclic 1,2-glycol formed by the intramolecular coupling of radical intermediates. 

The evidence for this mechanism is based on mass spectroscopy of the gas-phase radiolysis of isobutylene, which may not be applicable to the typical liquid-phase polymerization system. Initiation in condensed systems may follow the same course as electroinitiation— coupling of radical-cations to form dicarbocations. 

Carbon-carbon coupling of radicals observed in the photo-Kolbe reaction could also be observed with other surface generated radicals. Kisch and coworkers have shown, for example, that cyclic allylic ethers undergo alpha deprotonation under photoelectrochemical activation, producing radicals that can be oxygenated, Eq. (29). On colloidal zinc sulfide, hydrogen evolution accompanies the photocatalytic... 

The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt by preformed catalysts , or by particles supported within surfactant vesicles The idea of employing semiconductor surfaces as environments for the enhanced coupling of radicals had previous support in the photochemical coupling of cyclopentadienyl radicals formed by excitation of the corresponding anions at single crystal electrodes... 

Intermolecular anodic cyclizations often involve initial coupling of radical-cations followed by a chemical cyclization reaction. An alternative is cyclization by internal nucleophilic addition of some reactant to an intermediate derived by anodic oxidation. 

A beautiful extension of this reaction has recently been communicated by Nozaki, Oshima, and Utimo-to.184 These workers simply admixed f-butyl iodide (3 equiv.), benzaldehyde (1 equiv.), methyl vinyl ketone (1 equiv.) and triethylborane (1 equiv.) in benzene (Scheme 60). After 5 min at 25 C, the reaction was subjected to standard extractive work-up and the crude product was purified by chromatography to give (54) in 63% yield. If methanol is substituted for benzaldehyde, the protonated product (55) is isolated in 79% yield. Although enones are equivalents of synthon (56), such a direct coupling of radical and ionic reactions had not been achieved previously. 

The unexpected formation of the blue crystalline radical cation (97) from the reaction of triazinium salt (98) with tetracyanoethylene has been reported and the product identified by its EPR spectrum and by X-ray crystallography (Scheme 42).199 Carboxylic acids react with the photochemically produced excited state of N-t-a-phenynitrone (PBN) to furnish acyloxy spin adducts RCOOPBN. The reaction was assumed to proceed via ET oxidation of PBN to give the PBN radical cation followed by reaction with RCO2H.200 The mechanism of the protodiazoniation of 4-nitrobenzenediazonium fluoroborate to nitrobenzene in DMF has been studied.201 Trapping experiments were consistent with kinetic isotope effects calculated for the DMF radical cation. The effect of the coupling of radicals with different sulfur radical cations in diazadithiafulvalenes has been investigated.202... 

Following the study of the simple coupling of radicals derived from the salt of a single carboxylic acid, it was found that the electrolysis of a mixture of carboxylate anions or of the salts of half esters of dicarboxylic acids increased the synthetic value of the method. This arises from the possibility of the formation of symmetrical and unsymmetrical coupled products of the derived radicals. These anodic syntheses are illustrated in the synthesis of hexacosane (Expt 5.11), sebacic acid (decanedioic acid), octadecanedioic acid and myristic acid (tetra-decanoic acid), in Expt 5.131. 

Because the electronic distribution and nuclear configuration of the donor and the acceptor in the (Class II) successor complexes are similar to those of the free donor/acceptor product (i.e. radical pair), it is reasonable to suggest that products can originate directly from the successor complex (pathway Pi). Such a reaction, which includes an electron-transfer step, does not necessarily proceed via a pair of free ion radicals, and the effective activation energy can be even lower than that required by pathway P2. When the follow-up reaction involves the coupling of radicals, the reaction directly proceeding from the (ET) successor complex state can be kinetically favorable (since it excludes diffusional processes). 

The ESR spectra of the lowest excited triplet states of several naphthyridines in ethanol glasses and stretched polyvinyl alcohol films at 77 K have been determined <87JMR(7l)303>. A comparison of the calculated and experimental ESR spectral hyperfine coupling of radical anions has been reported <83ZN(A)415>. 

A number of coal-derived liquids were examined by cyclic-voltammetry and other electrochemical techniques and found to show some activity. At anodic potentials films form on glassy carbon electrodes. It is suggested that this film formation is caused by oxidative coupling of radical cationic species with neutral ring structures through a mechanism similar to that which causes charring and coking in coal conversion processes. 

Growth is terminated by coupling of radicals from two growing polymer molecules or by the reactive sites becoming buried in the polymer matrix. Polymers formed by the di-p-xylylene process have been shown (6, 7,8, 9,10, II, 13) to be living polymers and exhibit radical concentrations of 5-10 X 10 4 mole of free electrons per mole of p-xylylene. 

PET reactions where products arise by coupling of acceptor-donor pairs are summarised in this section. Usually these reactions do not involve direct coupling of radical ions, instead, recombination of radical pairs, formed by the... 

The best-known neutral ambident radicals are phenoxy radicals. Many substituted phenoxy radicals couple to give the polymers in wood, and dimerise or couple intramolecularly in the biosynthesis of a number of alkaloids, It is believed that many phenoxy radical couplings are actually the coupling of radical with radical and not of radical with neutral molecule, although the attack of a radical on a phenate ion may occasionally be an important pathway. A radical coupling with a radical is inherently a very fast process, since it is so exothermic, but only... 

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