Intermediate formation of radical

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

As in reduction reactions, two possible mechanisms exist for substitution reactions (1) electron-radical, involving the intermediate formation of radicals and their reaction with nucleophiles X ... 

The addition of tosyl cyanide to cyclopentadiene leads to intermediate formation of radical 27, which then is trapped by tosyl cyanide by cyano group transfer. The trans-... 

Explanation of the observed de-oxygenation with the formation of the corresponding imines in the photochemical reaction of a-aryl-A-methylaldonitrones, confirms the intermediate formation of radical cations resulting from PET (441, 467). 

Electrochemical studies192 show that reduction of polynuclear systems involve intermediate formation of radical anions, e.g.94. ... 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

This gave rise to some objection concerning, in particular, the structure of the intermediates ([230], part 2, p. 73). It would be more logical to suppose the intermediate formation of radical cations according to the following Scheme ([230], part 2, p. 83) (Scheme 23). 

It has been shown voltammetrically that oxidation of 1,4-dioxin and of its 2,3,5,6-tetraphenyl, benzo, and dibenzo derivatives takes place at a rotating platinum electrode in acetonitrile in single-electron steps with the intermediate formation of radical cations.The simplest dioxin cation-radical for which an ESR spectrum has been obtained is 79, for which hyperfine splittings are indicated in gauss.The radical was prepared by treatment with sulfuric acid of the dimer which forms spontaneously on the storage of acetoin. 

Chromium(VI) oxo derivatives are stiong oxidants which are also used as catalysts in organic synthesis [63]. The reaction of alkanes with chromium(VI) 0X0 compounds apparently takes place with the intermediate formation of radicals ... 

The development of the chain theory has produced a simple explanation of these facts in terms of the intermediate formation of radicals. The latter react with molecules much more readily than stable hydroperoxide molecules. The formation of free radicals was further estabhshed for many cases of oxidation of metal ions and complexes. 

More recently, Kirmse and his coworkers found two other deamination reactions where product studies indicated the intermediate formation of radicals, namely the nitrous acid decompositions of 2-aminobutylnitrile (Bunse and Kirmse, 1993) and of the epimeric 2-aminonorbornyl-2-carbonitrile (Kirmse et al., 1993b). Like 2-amino-2-methylpropylnitrile (7.59), these compounds are characterized by a CN group at the same C-atom as the NH2 group. 

It is difficult to decide whether the above approaches generally applicable to the reactions of aromatic compotmds with electrophiles. It seems desirable to compare it further with the intermediate formation of radical cations of aromatic substrates (see Sect. IV.6) whose conversion into cr-complex is controlled by the spin density distribution and by the relative stability of [Pg.210]

Another postulates the intermediate formation of a free radical ... 

The stability of the alkali metal ozonides increases from Li to Cs alkaline-earth ozonides exhibit a similar stability pattern. Reaction of metal ozonides with water proceeds through the intermediate formation of hydroxyl radicals. 

Formation of Hydrogen Tetroxide. The reaction of hydrogen atoms withHquid ozone at — 196°C proceeds through the intermediate formation of hydroperoxyl radicals forming hydrogen tetroxide, which decomposes on warming to produce equimolar amounts of and O2 (53). 

The alternative route in the oxidation of metlrane, widr CaHe formation, follows a similar path with the intermediate formation of CH3 by thermal decomposition and CHO radicals before CO is formed. 

Xenon difluoride [4, 5, 7, 8,10] is a white crystalline material obtained through the combination of fluorine and xenon m the presence of light The reagent is commercially available and possesses a relatively long shelf-life when stored cold (freezer) Xenon difluoride is very effective for small-scale fluormation of alkenes and activated nucleophilic substrates. The reactions are usually conducted between 0 °C and room temperature in chloroform or methylene chloride solutions Hydrogen fluoride catalysis is sometimes helpful Xenon difluoride reacts in a manner that usually involves some complexation between the substrate and reagent followed by the formation of radical and radical cation intermediates... 

PhSSPh and N2 via the intermediate formation of the purple radical [(PhSlaN], which is readily detected by its five-line (1 2 3 2 1) EPR spectrum. This radical is also generated by the oxidation of (PhSfaNH with lead dioxide. ... 

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

It should be pointed out however that there is no proof for the intermediate formation of free radicals and other mechanisms may explain the product distribution equally well. Such an alternative may be an ionic mechanism, possibly catalyzed by impurities or the walls of the container. In the above experiments Pyrex tubes were used [64]. 

Evidence for the intermediate formation of a free radical comes from labeling experiments, which show that both reduction and isomerization, which sometimes proceeds concurrently, are nonstereospecific 94,10). 

The catalytic hydrogenation of alkyl halides (RX) probably also proceeds via the intermediate formation of free radicals, which are formed in this case by the abstraction of a halogen atom (see Section III,B). 

Although Ce(IV) oxidation of carboxylic acids is slow and incomplete under similar reaction conditions , the rate is greatly enhanced on addition of perchloric acid. No kinetics were obtained but product analysis of the oxidations of -butyric, isobutyric, pivalic and acetic acids indicates an identical oxidative decarboxylation to take place. Photochemical decomposition of Ce(IV) carbo-xylates is highly efficient unity) and Cu(ll) diverts the course of reaction in the same way as in the thermal oxidation by Co(IIl). Direct spectroscopic evidence for the intermediate formation of alkyl radicals was obtained by Greatorex and Kemp ° who photoirradiated several Ce(IV) carboxylates in a degassed perchloric acid glass at 77 °K in the cavity of an electron spin resonance spectro-... 

The mechanism of carbon dioxide reduction in aqueous and nonaqueous solutions was investigated by several authors. It is now generally accepted that the reduction of carbon dioxide to formate ions is a multistep reaction with the intermediate formation of free radicals CO2 and HCO2 either in the solution or adsorbed on the electrode ... 

The intermediate formation of both types of radicals was shown by spectroscopic methods. 

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