Dimerization/radical addition/nucleophilic

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

Electrolysis of carboxylates RCO2 in the presence of olefins (XXVIII) may afford the radical addition products XXX, XXXI, XXXII, and XXXIII. Plausible reaction pathways are illustrated in Eq. (18). The radical R generated by electrodecarboxylation of RCO2 first combines with the olefin (XXVIII) to give the radical intermediates (XXIXa), which may provide the dimers (XXX) or the radical coupling products (XXXI). Further one-electron oxidation of XXIXa may provide cations (XXIXb), which subsequently react with the nucleophiles Nu or liberate to give substituted products (XXXII) or olefins (XXXIII). 

Pure radical chemistry was observed when styrene or various substituted styrenes where irradiated with 250 nm light in methanol in the presence of Cu(II) or Fe(III) [33]. As given in Scheme 13 all products derive from either double methanol addition to the radical cation or from radical-cation dimerization with final nucleophilic quenching by MeO . Again the effect of the metal ion is to direct the reaction course from polymerization towards the formation of dimers and ethers by accepting one electron from the excited... 

Kolbe radicals can be added to olefins that are present in the electrolyte. The primary adduct, a new radical, can further react by coupling with the Kolbe radical to an additive monomer I (Eq. 9, path a), it can dimerize to an additive dimer II (path b), it can be further oxidized to a cation, that reacts with a nucleophile to III (path c), or it can disproportionate (path d). 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

Radical cations can dimerize in a radical-radical coupling reaction (Scheme la) to afford dimer dications. An alternative pathway to form the dimer dication is a radical-substrate coupling in an electrophilic addition of the radical cation to the nucleophilic substrate. The dimer dication can lose two protons to form a bis-dehydro dimer or react with two nucleophiles to yield a disubstituted dimer. 

If anions R are oxidized in the presence of olefins, additive dimers (24) and substituted monomers (26) are obtained (Scheme 5, Table 8, and Ref. [94]). The products can be rationalized by the following pathway the radical R obtained by a le-oxidation from the anion R adds to the alkene to give the primary adduct (25), which dimerizes to afford the additive dimer (24) with regiospeciflc head-to-head connection of the two olefins, or couples with R to form the additive monomer (26). If the substituent Y in the olefin can stabilize a carbenium ion, (25) is oxidized to the cation (27), which reacts intra- or inter-molecularly with nucleophiles to give (28) or (29). 

Combination of an Ri, radical with an Ra radical yields the single p-qninone methide dimer (V). Here the quinone methide cannot become stabilized by an intramolecnlar addition reaction. Instead, nucleophilic attack of its y-carbon atom occurs by a hydroxyl ion from the medium, for example aromatization and protonation of the phenoxido ion thus formed give rise to guaiacylglycerol- 3-coniferyl ether (VI), again in racemic form dc-spite its two asymmetric carbon atoms. Since attack by the extraneous hydroxyl ion can occur on either side of C-y of the p-quinone methide (V), complete equilibration of the specific hydrogens from the original conifcryl alcohol moiety in the lower half of (V) presumably occurs (sec formulae on p. 131). 

Most of the reactions of silicon or germanium organic compounds proceed photochemically via a radical mechanism or via a cycloaddition mechanism (Chapters 4 and 6). There are few examples of nucleophilic addition of RSi or RGe to Cgo [118,119]. Reaction of silyUithium derivatives RjSili or germyllithium derivatives RjGeLi with different alkyl- and aryl-substituents R yields mainly the 1,2-adduct 28 or the 1,16-adduct 29 1,4-addition and dimerization of two fullerene-units was also found as a minor pathway. One example is given in Scheme 3.15. 

The electrophilic primary and secondary quinones undergo addition of nucleophiles, including flavonoids. For instance, nucleophilic addition of catechin to its enzymatically generated quinone yielded a catechin dimer in which the catechin moieties are linked through a C6 C8 biphenyl linkage. This B-type dehydrodicatechin further oxidized to yellow pigments. Additional dehydrodicatechins arise from radical coupling of the catechin semi-quinones formed by retro-disproportionation, in which the catechin moieties are linked... 

With ions or dipolar substrates, radical ions undergo nucleophilic or electrophilic capture. Nucleophilic capture is a general reaction for many alkene and strained-ring radical cations and may completely suppress (unimolecular) rearrangements or dimer formation. The regio- and stereochemistry of these additions are of major interest. The experimental evidence supports several guiding principles. 

Dimer formation can be quenched by conducting the experiment in a nucleophilic solvent, and the product obtained is characteristic of radical cation trapping. The anti-Markovnikov addition of acetone across the C-C single bond of the methylated analogue, eq. 41 (116,117),... 

When electron transfer reactions of olefins are carried out in nucleophilic solvents (alcohols) or in the presence of an ionic nucleophile (KCN/acetonitrile/2,2,2-trifluoroethanol), the major products formed are derived by anti-Markovnikov addition of the nucleophile to the olefin. In several cases, nucleophilic capture completely suppresses dimer formation [122, 143]. It is important to realize that the observed mode of addition reflects the formation of the more stable (allylic) intermediate and cannot be interpreted as evidence for the charge density distribution in the radical cation. 

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