Cation radicals coupled with neutral products

The mechanism given in equation 47 has been proposed for this reaction. The initially formed cation radical reacts with molecular oxygen to generate an intermediate, which may couple with a neutral cyclic silane to form species A. The intermediate A decomposes to the final product B and its cation radical B+", which could also be generated by direct anodic oxidation of the siloxane B. A further oxygen insertion step could take place via intermediate C+. ... 

THIS CHAPTER IS CONCERNED WITH A REACTION of aromatic and hetero-cyclic cation radicals about which only little is so far known their ability to react with neutral radicals. The reaction is expressed simply for the coupling of an aromatic cation radical (ArH +) with a radical (R-) in equation 1. This simple equation, presently only poorly documented, is nevertheless part of current thinking in two reactions of wide scope electrophilic aromatic substitution and reactions of cation radicals with nucleophiles. The product of equation 1 is a a complex, (ArHR)+, which is structurally the same as that... 

Figure 9 shows plots of Hammett fr+ values versus E j2 for the 8-p-X-Ph-dG adducts. In Fig. 9A, the OH (—0.92 ) fr+ value was used and the regression deviated from linearity. However, Fig. 9B shows that the regression is improved to almost unity when the O (2.30 ) fr+ value is used. These results suggested that the oxidation of 8-p-PhOH-dG may be coupled with phenol deprotonation. As shown in Scheme 12, resonance structures for the radical cation of 8-p-PhOH-dG create a p-substituted phenol radical cation, which possess negative pAa values (pifa for phenol radical cation ). Phenolic radical cations undergo deprotonation rapidly in the presence of water (0.6-6 x to yield neutral phenolic radicals. In the anhydrous DMF solvent used for electrochemical measurements, an N-7 adduct atom or adventitious water in the solvent could serve as base to facilitate phenolic radical production. 

An important application of combined electrochemistry and ESR spectroscopy is the characterization and identification of intermediates and products of electrode reactions [334,336,379-391]. For instance, the ESR technique is particularly useful to measure the degree of protonation under conditions where the radical ions take part in acid-base equilibria [380,381]. Such information may be obtained only with difficulty by other methods, but the coupling pattern of the ESR spectrum may often give the answer directly. An illustrative example is found in the anodic oxidation of 2,4,6-tri-rert-butylaniline, which, as expected, gives the radical cation as the initial electrode product [380]. In an aprotic solvent like MeCN or CH3NO2 the radical cation is stable and the ESR spectrum observed is in accordance with the reversible one-electron transfer indicated by CV. However, when the electrolysis is carried out in the presence of diphenylguanidine as a base, the ESR spectrum changes drastically and can be attributed to the presence of the neutral free radical formed by deprotonation of the radical cation. 

Another example of a coupling reaction initiated by reaction of the radical anion with an electrophile is the reductive coupling of substituted 4/f-pyran-4-thiones (34) in the presence of alkyl halides (Scheme 8) [110,111]. The neutral radical formed by alkylation at sulfur is apparently not reduced at the potential of the electrolysis but undergoes dimerization. If the substrate is methylated prior to reduction, the sulfonium cation is reduced more easily than the neutral substrate to give the same dimeric product. The initially formed dimer (35) eliminates disulfide in an oxidatively induced process during the electrolysis, yielding the final bipyranilidene (36) [110]. 

One-electron oxidation of an aryl ether, for example at the anode, gives rise to a radical cation whose fate may be radical coupling (shown as dimerization) or substitution into a neutral phenol ether both paths are shown in Scheme 6 and converge to a biaryl product. Other products are possible if coupling at a quaternary center takes place. Mechanisms of this type must operate for the important couplings of phenol ethers with phenol ethers, and phenols with phenol ethers possibly they should not be neglected for phenol-phenol coupling also, in certain cases. 

The [4 + 2]-cycloaddition of 02( Ag) to rubrene has been shown to be a simple method for the determination of oxygen concentrations in organic solvents.Irradiation of benzene solutions of tropone (86) in the presence of 9,10-dicyanoanthracene leads to the formation of four products, (87), (88), (89) and (90), and in acetonitrile-dichloromethane there is also an [8 + 4]tc adduct (91) produced. It has been suggested that this latter compound arises by coupling of the radical cation of (86) with the radical anion of the dicyanoanthracene (Scheme 2). Solvent-dependent quenching of the lowest excited state of 9,10-dibromoanthracene by 2,5-dimethylhexa-2,4-diene has been studied and appears to proceed by an exciplex. Flash photolysis investigations have shown that a neutral radical species is an intermediate in the formation of the [4 + 2] adduct which is a dibenzobicyclo[2.2.2]octadiene-type compound. Irradiation of 9,10-dicyanophenanthrene (DCA) in the presence of buta-1,3-diene gives a mixture of the product of [3 + 2]-photo-... 

Usually, as the formation of a radical-cation from a neutral substrate is associated with an increase in its acidity, facile deprotonation can take place [6, 7]. In a majority of instances, proton transfer takes place between radical-cation/radical-anion pairs, with the net result being the formation of two radicals and consequently a bimolecular coupling product (Scheme 3). This process is encountered in benzyl radical-cations, olefin radical-cations, and amine radical-cations. 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

A variation of the reaction involved the use of the alkene itself as nucleophile. In this case, a radical cation dimer was formed by attack of the alkene radical cation by the neutral alkene, forming a distonic radical cation (Scheme 14.9, left part). With a-methylstyrene (17) as the alkene, a cychzation took place and the neutral radical resulting from the ensuing deprotonation coupled with the radical anion of the acceptor (in this case TCB), leading to the NOCAS adduct 18 as a diastereo-isomeric mixture in overall 90% yield [55]. The irradiation of aromatic nitriles in the presence of aUcenes may lead to different products, particularly when carried out in an apolar medium. As an example, 1,4-dicyanobenzene gave isoquinohnes by a [4-1-2]-cycloaddition with a cyano group through irradiation in the presence of diphenylethylenes in benzene via a polar exciplex [56]. 

In neutral aqueous solutions, the ultimate product of one-electron abstraction from guanine is the guanine neutral radical. In DNA, this radical is formed via the deprotonation of the guanine radical cation arising, e.g., from hole localization or directly via proton-coupled electron transfer from guanine to an appropriate electron acceptor. The G(-H) radicals do not exhibit observable reactivities with molecular oxygen (fc<10 s ) [93]. 

In each case, the mechanism involves generation of an aryl radical from a covalent azo compound. In acid solution, diazonium salts are ionic and their reactions are polar. When they cleave, the product is an aryl cation (see p. 856). However, in neutral or basic solution, diazonium ions are converted to covalent compounds, and these cleave to give free radicals (Ar and Z"). Note that radical reactions are presented in Chapter 14, but the coupling of an aromatic ring with an aromatic compound containing a leaving group prompted its placement here. Note the similarity to the Suzuki reaction in 13-12. 

The major reaction pathway in all cases consists of reduction to the neutral radical followed by coupling, primarily or exclusively in the 4-position with respect to the ring nitrogen. Like the dimeric dianions formed by dimerization of, for example 91 , the neutral dimeric products obtained from positively charged A-heteroaromatic systems are in many cases difficult to isolate since they are easily reoxidized back to the substrate cations. 

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