Towards radical anion

A subtle approach198 to the stepwise removal of one member of a family of protecting groups is illustrated by the reductive cleavage of the (1-naphthyldiphenylmethyl) group from 3 -0-(p-anisyldi-phenylmethyl)-5 -0-(l-naphthyldiphenylmethyl)thymidine on treatment with the anthracene radical-anion in oxolane. 3 -0-(p-Anisyldi-phenylmethyl)thymidine was isolated in 87% yield, and the yield of thymidine was only 3%. It is noteworthy that the relative reactivity of 5 -0-(l-naphthyldiphenylmethyl) and 5 -0-(p-anisyldiphenylmethyl) derivatives of thymidine towards radical anions is the reverse of that towards acid. 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

The simplest method for obtaining selective fluonnation is to conduct reactions under conditions that invigorate the electrophilicity of fluorine In practice this method entails the creation of anionic or strongly nucleophilic reactive centers on substrate molecules while suppressing or reducing the tendency toward radical attack Numerous examples of seleetive fluorine attack on carbanionic, amido and carboxylato species are documented Especially abundant is alpha fluonnation of nitroalkanes in polar solvents [42 43, 44, 45 46] (equations 10-14)... 

The electrogenerated radical anions of aromatic hydrocarbons, e.g. DPA, rubrene, fluorene, can also act as reductants towards electro-chemically obtained radical cations which are derivatives of other aromatic compounds such as N,N-dimethyl-/>-phenylenediamine (Wurster s red) 150> (see Section VIII. B.). When a mixture of DPA and a halide such as 99 (DPACI2) or 100 is electrolysed, a bright chemiluminescence is observed the quantum yields are about two orders of magnitude higher than that of the DPA radical anion-radical cation reaction 153>. 

The general trend of nitrones toward radical reactions can be explained by a variety of reasons (a) their readiness to be transformed into stable nitroxyl radicals as a result of the so-called spin trapping (b) one-electron oxidation into radical cations and (c) one-electron reduction into radical anions (Scheme 2.77, routes C,D and E). Depending on the reaction conditions either route has been... 

Typical acceptors in Michael additions are reducible and their radical anions often undergo dimerization (hydrodimerizations). Either the radical anions or more likely the dimer dianions can act as EGBs toward the donor in the Michael addition. Since the reaction is catalytic in base when the product anion is more basic than the donor anion, the Michael addition can take place by reduction of a small fraction (2-10%) of the acceptor [129]. The reaction takes place in 20 to 77% yield... 

Hydroxymethylation of 2- or 4-nitroalkyl-benzenes, (44), takes place by passing a catalytic amount of charge through a solution of (44) and (CH20) Scheme 40 [140]. (44) is reduced at the electrode and electron transfer from (44) to CH2O is envisaged [140]. The resulting formyl radical anion then acts as an EGB toward... 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

Around pH 6-8, two polarographic waves are seen and die sum of the two wave heights corresponds to a one-electron process. The first wave is due to the two reactions above and decreases in height because protons are in low concentration and do not diffuse sufficiently fast to the electrode surface. The second wave is due to formation of the radical-anion followed by proton transfer from a general acid present as a component of the buffer. In alkaline solution, the concentration of acid component in the buffer decreases and this wave moves towards more negative potentials. Finally, E>/. becomes independent of pH in very alkaline solution where... 

Experimental data (Figure 4.2) for the dissociative electron transfer between radical anions and the carbon-halogen bond in alkyl halides indicates a linear relationship between log(k ) and Ed over a wide range of reaction rates [5, 9]. Very fast reactions become controlled by the rate of diffusion of two species towards each other, when every close encounter gives rise to electron transfer. A parabolic... 

Aromatic radical-anions react as nucleophiles towards alkyl halides and carbon dioxide. With alkyl halides, the rate-determining step is an outer sphere one-electron transfer to generate the alkyl radical and halide ion. The radical then adds... 

The radical-anion intermediates derived from aromatic imines behave as nucleophiles towards carbon dioxide, as with 48 [190,191]. Ibis nudeophic character is enhanced by reduction in the presence of chlorotrimethylsilane. A carbanion... 

Concerning the reactivity of the dianion 2 compared to that of the radical-anion 1, the first one behaves as a typical lithiating agent toward alkyl chlorides displaying an outer-sphere electron transfer reactivity profile no significant kinetic differences are found in the reaction of dianion 2 (as well as the radical-anion 1) with primary, secondary, tertiary and phenyl chlorides. On the other hand, a notable difference has been found in the reactivity of both species with several substrates and solvents . 

Lithium phenyl is not a t5 ical ionic compoimd because of the considerable deformation of the jr-electron system of the phenyl radical anion by the lithium ion (77). However the phenyl radical anion is stabilized by boron triphenyl, which functions as EPA towards the carbanion ... 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

Alkenes may be activated toward electrochemical reduction by electron-withdrawing sucstituents. Thus acrylonitrile and acrylate esters are easily reduced and, depending among other factors on the proton availability of the medium, they undergo either hydrogenation or hydrodimerisation. The basic character of the radical-anions of such substrates has been put to use in EGB promoted Michael additions of the type outlined in Scheme 15 the case where the probase is azobenzene has already been discussed. 

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