Acetylene radical anion

The potassium salt of the 2,2 -dipyridyl acetylene anion-radical represents another important example. In this case, the spin and charge are localized in the framework of N-C-C=C-C-N fragment. The atomic charge on each nitrogen atom is -0.447, that is, close to unity in total. The energy of this ion pair is minimal when the potassium counterion is located midway between the two rather close nitrogen lone pairs. Such a structure is consistent with the fact that the ESR spectrum of this species is almost insensitive to temperature. It means that the counterion does not hop between two remote sites of the anion-radical (Choua et al. 1999). 

The acetylene anion radical was successfully generated in a glassy MeTHF matrix via ionizing irradiation (60Co) at 77 K. Upon illumination with light 430 nm, the following isomerization takes place (Itagaki Shiotani 1999) ... 

The majority of the known heterocyclic dienes are pentadienes, largely as a result of the discovery that alkynes react with alkali metals and form dianions via dimer addition of the acetylene anion radical. A method that has become useful for the generation of larger cyclic dienes is the addition of an organometallic dihydride to a suitable dialkyne. Synthetic approaches to six-membered dienes tend to be unique for each system, and no general method has been developed. Some of the six-membered heterocyclic dienes have been converted to trienes for the study of systems that may be aromatic. 

The acetylene anion radical undergoes autodetachment of the electron, but the vinylidene anion can be generated easily [83]. Since the calculated isomerization barrier is -45 kcal/mol, the 2B2 ground-state vinylidene anion radical is predicted to be stable with respect to the 1,2-hydrogen shift [30, 84, 85]. As mentioned before, the vinylidene anion radical was used as the precursor for the generation of the singlet vinylidene in Lineberger s experimental studies. 

Dick turned up some interesting chemistry of caprolactam and its O-alkyl imino ethers. He and collaborators went on to explore the chemistry of allene, for example, its reactions with acetylene, carbon monoxide, and tetrafluo-roethylene. He did extensive work on the chemistry of cyclooctatetraene and of ferrocene. In the cyanocarbon area he collaborated on studies of the anion radical of tetracyanoethylene, that is, tetracyanoethylene bearing an extra electron. He was author or coauthor of 45 papers and 16 U.S. Patents that came out of the Central Research Department. 

Acetylenic carbanions or anion-radicals generated by a superbase system can serve as reducing agents (66JOC248 81UK248). 

Several anion radicals have been found to undergo protonation on carbon by water. Steady-state esr studies on electron adducts in water have shown that the adducts of acrylate and acetylene-dicarboxylate protonate on carbon rapidly whereas the adducts of fumarate and maleate do not (Neta and Fessenden, 1972). A more recent study by pulse techniques has shown that the differences between the various adducts are not qualitative but present differences in the rate of protonation. It has been found that the acid forms of the acrylate electron adduct protonate slowly on carbon whereas the basic form reacts much more rapidly [reaction (80)]... 

The electrolysis of diphenyl acetylene in the presence of CO gave both diphenylmaleic anhydride and diphenylfumaric acid and these products were cited as evidence for dianion formation. The results, however, can easily fit an anion radical mechanism (Scheme 4). 

In the pioneering papers of Wawzonek et al. [15,186] it was demonstrated that CO2 can be added to cathodically reduced hydrocarbons to yield dihydrodicarbonylates. Examples of this kind of reaction include naphthalene [186,187], anthracene [15], 9,10-diphenylanthra-cene [15], phenanthrene [186], butadiene [187], stilbene [15], and diphenyl acetylene [187]. The mechanism is assumed to be analogous to that for protonation, with the essential steps being nucleophilic addition by an anion radical and subsequently by an anion [188]. 1,4-Addition to naphthalene suggests that carboxylation is kinetically controlled formation of dihydromonocarboxylates indicates competition with protonation. Mechanisms involving CO2 have to be considered for those hydrocarbons that are reduced at more negative potentials than [189], and for the electroinactive norbornadiene, which... 

Among them, distannynes 60-62, tin analogs of acetylene, and their anion radicals 63-66 " and dianions 67-69 were synthesized by Power et al. and characterized by UV-vis, and NMR,... 

When the polarographic reduction of phenyl(4-methylphenyltelluro)acetylene was examined in MeCN in the presence of benzoic acid, the first reduction wave corresponding to the 2-electron electrochemical cleavage of the Csp—Te bond was progressively superseded by the 4-electron process of the hydrogenation of the triple bond. This reduction has been interpreted as involving an electron transfer to the adsorbed substrate yielding an anion radical, which after protonation and further reduction would form the saturated telluride (Scheme 1). 

Shono et al. have found that the 2-methylenecyclopentanol ring system (59), found amongst the gibberellins, can be generated by electroreductive cyclization of an appropriate y-acetylenic ketone (58) this method complements others using naphthalene anion radical or Li-NHa. 

Eisch, Behrooz and Galle196 give compelling evidence for the intervention of radical species in the desulphonylation of certain acetylenic or aryl sulphones with metal alkyls having a lower oxidation potential at the anionic carbon. The primary evidence presented by these workers is that the reaction of 5-hexenylmagnesium chloride outlined in equation (85) gives a mixture of desulphonylation products, in accord with the known behaviour of the 5-hexenyl radical, in which the cyclopentylmethyl radical is also formed. 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

Reduction of vinyl radicals by to the corresponding anion also has been observed (216). When purified acetylene is bubbled through Fenton s reagent, acetaldehyde is formed as a product, presumably via the following mechanism ... 

In addition to studying the behavior of benzoyl chloride. Cheek and Horine [72] have examined the reduction of benzoyl fluoride electrolysis of the latter compound affords benzyl benzoate, diphenyl-acetylene, stilbenol benzoate, and some polymers. Another feature of the reduction of benzoyl chloride is the possibility that both acyl radicals and acyl anions are involved as intermediates [71]. 

The exhaustive controlled-potential reduction of 6-chioro-l-phenylhex-l-yne at — 1.57 V in dimethylformamide containing tetrabutylammonium perchlorate gave a mixture of products. among which was ( >(2-phcnylvinyl)cyclobutane (9).11 It is probable that the mechanism involves initial isomerization of the acetylene to an allene 8 which is reduced at — 1.57 V to the radical anion. Protonation and further onc-clectron reduction then yield the allylic anion. An intramolecular nucleophilic substitution eventually gives the cyclobutane.11... 

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