Amides aromatic, radical anions

Reduction of the aromatic nucleus in AjjV-dimethylbenza-mide occurs by an initial single electron transfer to give a radical anion. Protonation of the radical anion generates a radical and a second electron transfer gives the amide enolate 1. Protonation of the cross-conjugated trienolate moiety in 1 occurs carbonyl group to give the cyclohexa-1,4-diene 2. ... 

A variation on the aryne mechanism for nucleophilic aromatic substitution (discussed above, Scheme 2.8) is the SrnI mechanism (see also Chapter 10). Product analysis, with or without radical initiation or radical inhibition, played a crucial role in establishing a radical anion mechanism [21]. The four isomeric bromo- and chloro-trimethylbenzenes (23-X and 25-X, Scheme 2.9) reacted with potassium amide in liquid ammonia, as expected for the benzyne mechanism, giving the same product ratio of 25-NH2/23-NH2 = 1.46. As the benzyne intermediate (24) is unsymmetrical, a 1 1 product ratio is not observed. 

E.s.r. Spectra. E.s.r. spectroscopic studies of the radical anions of NN-dialkylthiobenzamides have shown that these species (374) exhibit a twisted conformation, the aromatic nucleus and the thiocarbamoyl group being rotated out of the plane. Similar studies of the radical anions of thio-amides of the types (375) and (376) revealed a twisted conformation... 

Early on, it was demonstrated that aromatic nitro compounds may form radical anions in alkaline solutions [4], with the possibilities of photochemical reactions [5]. There followed the development of the radical chain mechanism [6]. An interesting early danonstration of reaction by this mechanism was in the reaction of ort/io-halogenoanisoles with potassium amide in liquid ammonia [7]. Reaction by the benzyne mechanism gives predominantly the uiera-substituted product due to the electronic influence of the methoxy group. Howeva, with an access of potassium metal, which promotes electron transfer, the pathway predominates yielding ort/io-anisidine, as shown in Scheme 6.4. The mechanism now forms an important synthetic pathway, and this and other homolytic processes are covered in Chapters 9 and 10. 

Carboxylic AcidPerivatives. Detailed investigations into the electrolytic reduction of carboxylic acid derivatives (esters, anhydrides, amides) in nonaqueous solutions and the procedures for producing the respective radical anions have been described in the report by iDyasov and his co-workers f39]. They obtained radical anions from esters of aromatic carboxylic acids (benzoates, phtha-lates, isophthalates) and from phthalic anhydride and analyzed their EPR spectra. The production of radical anions of acrylates and methacrylates by electrochemical generation and the effect of proton donors on their stability was also described [11]. 

In conclusion, structures containing polyiodide anions, with cationic aromatic ligands as counter parts of formulae [(L)(HL+)] (I ) are known to be synthesized by the treatment of the appropriate amide with HI [26-28], In contrast, the complexes with PYOH, in the present case, were formed by the direct reaction of 2-hydroxypyridine with di-iodine in a molar ratio of 2 1 and 1 2. This is a redox reaction, where 2-hydroxy-pyridine firstly is oxidized to pyridinone-2 radical cation. In the case of 2-hydroxy-pyridine however, peroxide structures are not formed like disulphides in the case of PYSH. Polyiodide anions are simultaneously produced in this case This should be a consequence of redox differences between -SH and OH groups and may be proven a useful pathway for the synthesis of polyiodide materials. 

The feasibility of some of these radical pathways has been examined using Marcus theory to obtain rate constants for comparison with the experimental data (Eberson, 1984). For some relevant anions, including hydroxide, methoxide, t-butoxide, the anion of benzaldehyde hydrate and di-2-propyl-amide, the necessary E°(RO-/RO) values are available or can be estimated with sufficient accuracy. For the reaction of t-butoxide with benzophenone in THF, or the benzaldehyde hydrate anion with benzaldehyde in aqueous dioxan, direct electron transfers between the anion and the neutral are not feasible the calculated rate constants are orders of magnitude too low to be compatible with the observed reduction rates. Any radicals observed in these reactions must arise by some other more complex mechanism. The behaviour of an aromatic aldehyde hydrate dianion has not been examined in this way, but MNDO calculation (Rzepa and Miller, 1985) suggests that such a species could easily transfer either a single electron or a hydrogen atom to an accepting aldehyde. 

Both aliphatic and aromatic esters may be converted to amides by electrochemical reduction of the ester in the presence of an amine in a divided cell as in Eq. (23) [95]. This reaction is not formally a reduction, but the reaction does not occur without passage of current. The mechanism is likely formation of an anion radical with abstracts a proton from ammonia to form amide ion. The amide ion subsequently displaces alkoxide in a chain reaction to form carboxamide. 

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