Radical cations disproportionation

The anodic oxidation of alkanes in anhydrous hydrogen fluoride has been studied at various acidity levels from basic medium (KF) to acidic medium (SbFs) to establish optimum conditions for the formation of carbenium ions . The oxidation potential depends on the structure of the hydrocarbon methane is oxidized at 2.0 V, isopentane at 1.25 V vs Ag/Ag. Three cases of oxidation can be distinguished. In basic medium, direct oxidation of the alkane to its radical cation occurs. In a slightly acidic medium, the first-formed radical cation disproportionates to cation, proton and alkane. The oxidation is, however, complicated by simultaneous isomerization and condensation reactions of the alkane. In strongly acidic medium, protonation of the alkane and its dissociation into a carbenium ion and molecular hydrogen occurs. In acidic medium n-pentane behaves like a tertiary alkane, which is attributed to its isomerization to isopentane. The controlled potential electrolysis in basic medium yields polymeric species. 

The final products of oxidation of diarylselenides and tellurides (and sulfides as well) in the presence of nucleophiles are the corresponding chalcogen (IV) compounds. In the presence of water, the selenoxide or telluroxide (or the corresponding dihydroxy selenane or tellurane) is the final product. This still leaves several possible pathways, leveraged from early mechanistic studies done using electrochemical techniques on diaryl sulfides and outlined by Engman (Fig. 32). In these pathways, the initial radical cation can react with a nucleophile present in solution, or the dication resulting from further oxidation or disproportionation can do so. 

The absence of dimer radical cation formation by diphenyl selenide under the pulse radiolysis conditions is in contrast to bimolecular reactions believed to occur under electrochemical conditions/ In these experiments, a rotating disk electrode was used in combination with commutative voltammetry under anhydrous conditions. The results led to the conclusion that reversible one-electron oxidation is followed by disproportionation, then reaction of the resulting dication with diphenyl selenide or an external nucleophile, with the likely intermediacy of the dimer dication (Fig. 33). As expected, the dihydroxy selenane is formed when water is present. Based on the kinetics of the electrochemical reaction, the authors believe the diselenide dication, not the radical cation, to be the intermediate that reacts with the nucleophile. 

Specific reviews of the electrochemistry of mononuclear carbonyls have not appeared. The primary oxidation of the mononuclear carbonyls leads to the formation of 17-electron radical cations with half-lives in the order of seconds or less in MeCN electrolytes [14, 15]. Decay may take place by disproportionation, CO loss, and/or nucleophilic attack. Electrogeneration in solvents of low nucleophilicity such as trifluoroacetic acid can enhance the stability of the cations and indicates that nucleophilic attack is a major pathway for decay. This is concordant with the stability order [Cr(CO)g]+ > [Fe(CO)5]+ [Ni(CO)4]+, where the lower coordination numbers favor nucleophilic attack and... 

The adsorbed cationic species are next transformed to the adsorbed radical species by the electron transfer. Finally, the Mg deposition may occur via the adsorbed radicals that disproportionate laterally on the electrode surface to form Mg metal and solution species, as represented, for example, by equations 35 ". 

Cyclic voltammetry (Section II,A) and other electroanalytical techniques9 may give answers to the following questions (i) in which stage does the cyclization occur, e.g., in the stage of a radical-cation or dication, (ii) which is the rate-determining step, (iii) what is the sequence of the reactions in the overall conversion (RH2 - cR) The second electron is very commonly transferred through a disproportionation of the intermediate radical or radical-cation. 

The substituted 1,4-dihydropyridines 156 in dry acetonitrile, containing Bu4NC104, are oxidized in a one-electron step leading presumably to a radical-cation (157).235 The final product obtained in dry acetonitrile is a substituted pyridine (158) or pyridinium derivative (159), a two-electron product. This indicates a disproportionation of the initially formed radical-cation. By adding water to act as a base, the wave doubled in height indicating a deprotonation of the radical cation 157 to a radical that is oxidizable... 

The electrochemical oxidation of 4-dimethylaminoantipyrine (4-dimethyl-amino-2,3-dimethyl-l-phenyl-A3-pyrazolin-5-one) has been investigated in CH3CN-NaC104 at a glassy carbon electrode.421 The first step is a quasi-reversible electron transfer from the lone-pair electrons on the 4-dimethyl-amino nitrogen to form the radical-cation. The second-order disappearance of the radical-cation is presumably due to a disproportionation reaction. The oxidation at the potential of the plateau of the first wave gave the protonated 4-dimethylaminoantipyrine in 60% yield, but other products were not identified. 

X-ray ionization of o-vinylbenzaldehyde (136) in argon matrices leads to the quinoketene (137) via die radical cation, detected by IR spectroscopy.298 The product identity was confirmed by the independent preparation of (137) and (137+-) by the photo-stimulated ring-opening of 2-methylbenzocyclobutenone (138) (Scheme 21). The reactions of benzaldehyde, acetophenone, and benzophenone with OH, 0 and SC>4 have been studied by pulse radiolysis in aqueous solution.299 The addition of OH to the carbonyl moiety of benzaldehyde predominates over addition to the aromatic ring, whereas ring addition is predominant in the case of acetophenone. Disproportionation of the exocyclic OH adduct is proposed to explain the formation of benzoic acid, which is a major product in the reaction of benzaldehyde and OH or SO4T Rate constants for each reaction have been calculated. 

The structure and energy of a series of ions generated from penta-cyclo[3.3.1.13,7.01 3.05 7]decane (7) has been explored by using HF, MP2 and DFT methods to estimate enthalpy changes of isodesmic disproportionation reactions and by considering the reorganization of frontier orbitals as a consequence of addition or removal of electrons from the neutral molecule.8 The dication (72+), which is considered to be Three-dimensionally homoaromatic , is stable relative to a localized structure with similar features but is highly unstable compared to the radical cation (7+i)- hi contrast, the dianion (72 ) is unstable relative to the radical anion (T) and shows no evidence of electron delocalization. 

Other evidence was reported by Tsukahara and collaborators from the disproportionation reaction of the radical cation of the optically active bis-viologens, (S)-1 -(1 -naphthyl ethyl)carbamoyln methyl-1 -(4-(4-( 1 -naphthyl ethyl)carbamoyl methyl-pyridinio)pyridinio-4,4 -bi pyridinium tetra chloride, NBVPR2+, 1-phenyl analog, PBVPR2 +, and 1-cyclohexylethyl analog CHBVPR2+ (see Scheme 28) [70]. 

In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemically and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-1,4-dioxin, and derivatives 22a - c [90, 111, 57-159]. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-p-dioxino[2,3-b]-p-dioxin 68 [158] ... 

In fact, the disproportionation of dioxene radical cations can be substantiated by cyclic voltammetric studies, since they show a second irreversible oxidation wave, with a peak potential 0.2-0.5 V more positive than the half-wave potentials. In addition to the chemical proofs [see Eqs. (28, 29)], further evidence supporting the disproportionation of dioxene radical cations were achieved by NMR and... 

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