Anion radical intermediate

The fact that both the thermal and the photochemical insertion reactions yield the same products via formation of charge-transfer complexes leads to the conclusion that the reactive ion-radical pair in equation (52) is the common intermediate for both activation processes. Such a conclusion is further verified by the direct observation of anion-radical intermediates from the thermal reaction of TCNE and DDQ with various metal hydrides.188... 

This nuance of the original Sr I mechanism may thus occur in quite a number of cases. Nomenclature purists may consider it necessary to find other symbols to name this mechanism and, presumably, to question the adequacy of the 1 in this case. Beyond symbols, if the Sr I mechanism is viewed as an outer sphere electron-transfer-induced nucleophilic substitution , a possible designation of the mechanism under discussion might be dissociative electron-transfer-induced nucleophilic substitution . The original designation of these reactions as nucleophilic reactions proceeding via anion radical intermediates (Komblum, 1975) would still apply to both nuances of the mechanism since, in the present case, RNu is an essential intermediate in the reaction, even if RX is not. 

Thus, there are three possible pathways for the radiation degradation of polymer molecules neutral radical, cation-radical and/or anion-radical intermediates. Interest in the formation of these three types of reaction intermediates has fluctuated over the years with the utilization of different techniques and with the particular interests of different investigators. It is likely that all three species will be produced, but their relative importance in the degradation mechanism will depend on the chemical structure of the polymer. Evidence for their involvement will depend on the experimental methods used and the temperature and time scale of observation. In this paper we illustrate our investigations of many of the fundamental aspects of the radiation degradation of polymers through studies of series of polymers and copolymers. 

Substituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition —only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction (see Exercise 22-35). 

Examples of intramolecular trapping of carbonyl ylide dipoles by alkenes have now been reported.These include, for example, the conversion of the oxirane (172) into the tetrahydrofuran (173). Carbonyl ylides have also been prepared by irradiation of 2,3-bis-(p-methoxyphenyl)oxirane in the presence of dicyanoanthracene as electron-transfer sensitizer direct or triplet-sensitized irradiation, however, leads mainly to rearrangement via carbon-oxygen bond cleavage. In contrast, cyclohexene oxide and styrene oxide, on naphthalene-sensitized irradiation in alcohols, undergo solvolysis via oxide anion-radical intermediates. ... 

These cyclizations both involve the reductive intramolecular addition of an electron deficient alkene function to an aldehyde carbonyl function, and both are effected in ca 90 % yields. The mechanism of this latter type of electrochemically induced cyclizations of carbon-carbon double bonds to carbonyl double bonds have been studied rather extensively, with especial attention to the fundamental mechanistic question of whether the cyclization step involves an anion radical, radical, or anionic mechanism [122]. The latter two mechanisms would involve the protonation of the initially formed anion radical intermediate to form a radical, which could then cyclize or, alternatively, be further reduced to an anion, which could then cyclize. Extensive and elegant electrochemical and chemical studies have led to the formulation of these reactions as involving anionic cyclization (Scheme 74). 

Scheme 74. The proposed anionic mechanism for cyclizations involving anion radical intermediates.

It is known that group 14 metals facilitate the electron transfer from the neighboring jt-system by virtue of so called interaction [196]. The LUMO level is decreased by such interaction, which in turn favors the reduction of the system. The silyl group also stabilizes the anion radical intermediate formed by a one-electron reduction [197,198]. These effects can also be explained in terms of the orbital interaction with a low-lying a orbital of the sily group instead of the vacant d orbital. 

Some fundamental studies were made earlier. It was found that the hydrodimerization of an O, j6-unsaturated ketone (enone) was at least partially stereospecific, with the d, I dimer formed from the /ranj-enone and mainly the meso dimer formed from the cw-enone. Kanetsuna and Nonaka [50] also examined both the stereoselectivity and stereospecificity of the hydrodimerization in detail. It was indicated that either the reaction mechanism or the stereochemical course is changed by the nature (protic or aprotic) of the reaction media. Particularly, the latter is greatly influenced by the manner of adsorption of an anion radical intermediate on the cathode. It was also postulated to explain the experimental results that the radical anion is adsorbed at the )6-position, where the radical p electron is delocalized and the adsorption is affected by the nature of the media. 

Irradiation of aromatic compounds in the presence of a good electron donor (nucleophile) may promote electron transfer from this species to the excited aromatic substrate in order to form an anion-radical intermediate, which releases the leaving group (Sr-nIAt, where R = radical, = anion Scheme 6.93).836 838 840 852 In contrast, electron transfer from the excited aromatic substrate to good electron acceptors, followed by the reaction of the cation-radical intermediate thereby formed with a nucleophile, is possible when an electron-donating group is present on the aromatic substrate (Sr+n1 Ar, where + = cation not shown).836... 

Electrochemical reductions of aryl halides occur through anion-radical intermediates RX-. which... 

Anion-Radical Intermediates in Reductions of Organic Halides... 

The a criterion lor anion-radical intermediates RX- (o < 0.3) may he useful, but it is not rigorous. There are indications that il may fail iTn reductions of I-halo I methyl-2.2-diphcnylcvclo propancs,... 

Anion-radical intermediates in reductions of omanie li.itiih-s 23.X-47 Amhiacone 407 0... 

For the anion radicals intermediates and final products were identified. They correspond at least to four reaction types, namely a) dimerization of the diazo anion... 

In a study of electron-transfer reactions. House has examined the stabilities of the anion radicals derived from a number of y-cyclopropyl-ap-unsaturated ketones. It would appear that for the conversion of substrates containing the unit depicted by (240) into (241) a rearrangement rate in excess of 10 s is necessary in order to detect the anion radical intermediate present in metal-ammonia reductions. A rate of ca. 10 s is likewise required in lithium dimethylcuprate reactions. ... 

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