Anion radicals dimerisation

Both sodium and nickel bring about oxidative dimerisations, despite the apparently reducing conditions, the former giving 4,4 -bipyridine and the latter 2,2 -bipyridine. Each reaction is considered to involve the same anion-radical resulting from transfer of an electron from metal to heterocycle, and the species has been observed by ESR spectroscopy, when generated by single electron transfer (SET) from lithium diiso-propylamide. In the case of nickel, the 2,2 -mode of dimerisation may be favoured by chelation to the metal surface. Bipyridyls are important for the preparation of Paraquat-type weedkillers. 

An alternative preparation method is sodium doping employed for noncarotenoid polyenes, resulting in radical anions, 53, as a charge transfer complex with one sodium ion [153], Fig. 29. It was shown that the charge storage depended upon the length of the polyene chain. Dimerisation of anion radicals was treated. Practical details concerning this method are available [154,155],... 

In general, the chemical reaction may be a homogeneous process occurring as the product of electron transfer (R in the case of a reduction) is transported away from the surface, or a heterogeneous process occurring while the species R is adsorbed on the surface. Reaction (1.10) is another example where such following chemical reactions are important the initial electron transfer produces the anion radical of acrylonitrile, and to form adiponitrile, a dimerisation, two protonations, and a further electron transfer must take place. 

Table 4.2 — Rate constants for the dimerisation of the anion radical in DMF/(C4H9)4N1 determined with a Pt/Pt ring disc electrode.

The first example [15] shows how cyclic voltammetry may be used to demonstrate the rapid isomerisation of the anion radical of diethylmaleate (DEM). Fig. 6.17 shows the voltammograms for DEM and diethylfumarate (DEF), the CIS and trans isomers respecively. The reduction of DEF shows a well formed reduction peak, Ep = —1.41 V vs SCE, and a coupled anodic peak above 0.1 Vs the curve has all the properties of a reversible le process. Below this scan rate, there is evidence for a slow chemical reaction, and more detailed investigations showed this to be dimerisation. The reduction peak for DEM, below 5 Vs is much broader, Ep = —1.61V, with no anodic peak corresponding to the reverse process. There is, however, an anodic peak at —1.35 V which is the same potential required for the oxidation of DEF. Also on the second and subsequent cycles a new reduction peak is seen to grow at —1.41V. On increasing the potential scan rate above 50Vs a small anodic peak is seen where the oxidation of DEM is expected and the anodic peak at —1.35 V becomes relatively smaller (i.e. when corrected for scan rate effects). Finally, the current functions, for the main reduction peaks for DEF and DEM are almost independent of v and correspond to reasonable values for a reversible and irreversible le reduction... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

Almost certainly, the acetylide anion—formed in the basic solution—is oxidised by Cu(u) (another one-electron oxidising agent) to the corresponding radical, which then undergoes dimerisation. 

Radicals, (34), that subsequently dimerise, are also obtained through the anodic oxidation of carboxylate anions, RCO20, in the Kolbe electrolytic synthesis of hydrocarbons ... 

Conversely, electrolysis of ketones, (35), results in their cathodic reduction to radical anions (36), which dimerise to the dianions of pinacols (37) ... 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

Increasing the current density should favour the dimerisation of the radical anion (route 2) while increasing the C02 concentration should favour route 3. If oxalate formation occurs via route 3, then changing the COz concentration and the current density should affect both the CO and C20yields equally whereas if oxalate formation is via route 2, the effect of changing these same two conditions with respect to the CO yield will be opposite to that observed in the yield of oxalate. 

When NH4PF6 was employed as the supporting electrolyte, no CO was produced but near-quantitative formation of H2 was observed. During the electrolysis an air-stable, green, sparingly soluble material was produced, which was isolated and characterised as the dimer, Bipy)Re[CO]3 2. It was fairly reasonable to assume that this was formed via dimerisation of the radical (Bipy)Re[CO]3. The authors postulated that more dimerisation occurs in the presence of a non-coordinating anion such as PF , rather than coordinating anions such as Cl- or CIO , due to the labilisation (and loss) of Cl and thus the exposure of the sixth coordination site and subsequent dimerisation. 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

The equilibrium between radical-anion and dimer for pyridine and quinoline has been examined in a number of aprotic solvents. Radical-anions of pyridine dimer-ise rapidly in liquid ammonia in tire presence of alkali metal ions [15] In hex-amethylphosphoramide with alkali metal counter ions, the monomer is detectable in an equlibrium concentration [16], The monomeric species can be stabilised by substituents and 2- or 4-cyanopyridines give radical-anions which persist in liquid ammonia while 3-cyanopyridine radical-anion dimerises with a rate constant of 2 x 10 [17], Quinoline radical-anion is stable in hexamelhylphosphoramide [16] but in liquid ammonia it dimerises irreversibly [18]. 

Boujlel et al. <2003SC1675> have prepared oxazolooxazoles such as 90 by the rather more unusual method of electrochemical reduction of 91 which then undergoes dimerisation giving good yields of the symmetrical heterocycles. Various substituents have been included. Their proposed mechanism involves capture of an electron to give radical anion 92 followed by dimerisation and cyclization as outlined in Scheme 9. 

Those EGB s formed as a result of bond cleavage must always suffer the disadvantage that it is unlikely that the probases will be easily regenerated. Unless, therefore, the probases are very cheap, or they are converted into more valuable chemicals (as in the CCl to CHClj case), the alternative type of probase is to be preferred. These are compounds which may be reduced to radical anions or dianions ideally such species should have significant lifetimes in the absence of acids or electrophiles, e.g. dimerisation of the radical-anions should be slow. 

The results summarised in Table 4 show clearly that good yields of the 1 1 Michael adducts can be obtained in cathodically initiated reactions. It is not clear, however, why the reaction should fail for acrylonitrile even if dimerisation of the initially formed radical-anion is much faster thaq,protonation the resulting dimeric dianion should still be sufficiently basic to deprotonate dialkylmalonate esters. A feature of especial significance is the usefulness of esters of ethenetetracarboxylic acid. Apart from their use in EGB catalysed reactions they have been much used in stoichiometric amount. 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

Fig. 6.16 Data from derivative cyclic voltammetry for the dimerisation of the radical anions of (-)-bornyl cinnamate. The rate constant, kjim/ obtained by matching the two scales is 5.6 x 102 M-1 s 1. Reprinted with permission [38],...

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