Nucleophilic addition radical cations

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

Thus, polymerization will always occur when kp > k<, -E k ([solv] + pC ]). In the case of highly electron-donating substituents the stability of the radical cations may be so great that k > kp + k ([solv] -E PC ]) and most of the ions diffuse into the solution. By contrast, if — given electron-withdrawing subsituents and high oxidation potential — k ([solv] + PC ]) becomes greater than kp -E k, then the nucleophilic addition will dominate and the polymerization will be suppressed. 

Addition of alkyllithium to cyclobutanones and transmetallation with VO(OEt)Cl2 is considered to give a similar alkoxide intermediates, which are converted to either the y-chloroketones 239 or the olefinic ketone 240 depending on the substituent of cyclobutanones. Deprotonation of the cationic species, formed by further oxidation of the radical intermediate, leads to 240. The oxovanadium compound also induces tandem nucleophilic addition of silyl enol ethers and oxidative ring-opening transformation to produce 6-chloro-l,3-diketones and 2-tetrahydrofurylidene ketones. (Scheme 95)... 

Oxidative Alkoxylation of Nitrones to a-Alkoxy Nitrones and a-Alkoxy Substituted Nitroxyl Radicals The first direct experimental evidence of the possibility to carry out radical cation nucleophilic addition to nitrones with the formation of nitroxyl radicals has been cited in Section 2.4. Further, such a reaction route was referred to as inverted spin trapping this route is an alternative to a conventional spin trapping (508-512). Realization of either mechanism depends on the reaction conditions namely, on the strength of both nucleophile and oxidant. The use of strong oxidants in weak nucleophilic media tends to favour the radical cation mechanism... 

Owing to the existence of two centers for nucleophilic attack (at C2 and C5) in radical cations (220) obtained from the oxidation of 4-H -imidazole-1,3-dioxides (219), the formation of two products of methoxy group addition was observed, namely NNR (221) and NR of 3-imidazoline-3-oxide (222). The ratio of the products depends on the electronic nature of substitutes R1 and R2. Both, the donor character of R1 and acceptor character of R2 facilitate the formation of nitroxyl radicals (222) with the yield of (221) increasing with the inverted effect of the substituents. As was mentioned in Section 2.4, the results of preparative electrochemical oxidative methoxylation of 4H -imidazole-1,3-dioxides are similar to the results of chemical oxidation. 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

The homopolymerization ofl consists of a room-temperature reaction of the monomer dissolved in nitrobenzene in the presence of anhydrous ferric chloride. Polymerizations were carried out under a stream of dry nitrogen. As depicted in Scheme 2, the homopolymerization of 1 to form 6FNE takes place by means of the Scholl reaction. The mechanism of the Scholl reaction was assumed to proceed through a radical-cation intermediate derived from the single-electron oxidation of the monomer and its subsequent electrophilic addition to the nucleophilic monomer. The reaction releases two hydrogens, both as protons, to form the... 

The regiochemistry of nucleophilic addition to alkene radical cations is a function of the nucleophile and of the reaction conditions. Thus, water adds to the methoxyethene radical cation predominantly at the unsubstituted carbon (Scheme 3) to give the ff-hydroxy-a-methoxyethyl radical. This kinetic adduct is rearranged to the thermodynamic regioisomer under conditions of reversible addition [33]. The addition of alcohols, like that of water, is complicated by the reversible nature of the addition, unless the product dis-tonic radical cation is rapidly deprotonated. This feature of the addition of protic nucleophiles has been studied and discussed by Arnold [5] and Newcomb [84,86] and their coworkers. 

Table 5 Rate constants for nucleophilic addition to alkene radical cations ...

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

Selective generation of 2-aza-l,5-diradicals 146 through nucleophilic addition to the less substituted positions of the cation radicals 145 arising by intramolecular electron transfer was attributed to the cyclization regiochemistry (equation 66). 

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. 

Electron transfer from the alkene leads to a radical cation that can undergo coupling (Scheme la). The radical cation can also react with the nucleophilic heteroatom of a reagent to afford addition or substitution products (Scheme lb). Adducts can be likewise obtained by oxidation of the nucleophile to a radical that undergoes radical addition. Reactions between alkenes and nucleophiles can be realized too with chemical oxidants that are regenerated at the anode (mediators) (see Chapter 15). Finally, cycloadditions between alkenes can be initiated by a catalytic anodic electron transfer. These principal reaction modes are subsequently illustrated by selected conversions. 

Anodic addition of nucleophiles to olefins can be achieved via oxidation of the alkene to a radical cation.This means that a nucleophile can be added to a nucleophilic alkene by reversing its polarity to an electrophilic radical cation... 

Radical cations resulting from oxidation of olefins, aromatic compounds, amino groups, and so on, can react by electrophilic addition to a nucleophilic center as shown, for example, in Scheme 1 [2, 3]. The double bond activated by an electron-donating substituent is first oxidized leading to a radical cation that attacks the nucleophilic center. The global reaction is a two-electron process corresponding to an ECEC mechanism. 

Addition In anodic addition, either the olefin is oxidized to a radical cation that reacts with the nucleophihc species in the electrolyte or the nucleophile is oxidized to a radical that adds to the double bond. 

Electron transfer sensitization allows either the radical cation or the radical anion of an aromatic alkene to form as desired, which finally results in nucleophile addition with Markovnikov and anti-Markovnikov regiochemistry. In an apolar solvent, the tight radical ion pair undergoes a stereoselective reaction when the electron-accepting sensitizer is chiral (Figure 3.10). ... 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

It is important to differentiate between the effects of a nonnucleophilic salt such as Mg(C104)2 on one hand, and a weak nucleophilic salt such as Et4NOAc on the other. The effect of nonnucleophilic salts on photo-oxygenation via electron transfer can be understood as the stabilization of ion-radicals by coulombic interaction, resulting in the suppression of a back electron transfer between ion-radicals. The weak nucleophilic salts cause unusual effects. The addition of the anionic nucleophile to the cation-radical and complexation of the weak nucleophilic salt with the ion-radicals bring about these effects. 

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