Transformation reactions cationic-radical

Ionization of 1,5-hexadiene in fluorochloroalkane matrix (Scheme 2.43) represents cation-radical monomolecular reactions. The initially formed cation-radical collapses to the cyclohexane cation-radical, that is, spontaneous cyclization takes place (Williams 1994). Zhu et al. (1998) pointed out that the ring formation from the excited valence isomer in the center of Scheme 2.43 is easier than in the corresponding ground-state dienes. Notably, tandem mass spectrometry revealed the same transformation of 1,5-hexadiene in the gas phase too. This provides ns with a hint that mass spectrometry can serve as a method to express predictions of monomolecnlar transformation of cation-radicals in the condensed phase. A review by Lobodin and Lebedev (2005) discnsses this possibility in more detail. 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

Steenken, S. (1989). Purine bases, nucleosides and nucleotides aqueous solution redox chemistry and transformation reactions of their radical cations and e" and OH adducts. Chem. Rev. 89, 503-520. 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

EPR study of electrochemical properties of nitrones and registration of resulting radical cation (RC) or radical anion (RA), such as, in the nitrone transformation into nitroxyl radicals, allows us to get direct answers to the questions concerning mechanisms of nitrone group reactions. The following schemes A-E can be realized depending on conditions as below (Scheme 2.77) ... 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

Cuprous and ferrous salts are preferable. Sometimes, a transition metal salt is deliberately added to a mixtnre of a snbstrate and a persnlfate salt (Dobson et al. 1986). The free or metal-coordinated sulfate anion-radical reacts with an organic snbstrate, giving rise to a snbstrate cation-radical (Minisci et al. 1983, Itahara et al. 1988, Telo and Vieira 1997). The substrate cation-radical is often able to expel a proton and transform into the corresponding radical. The latter regenerates the initial metallic ion. The whole reaction becomes a catalytic one with respect to the metal. 

The ion-radical mechanism is characteristic in cases of substrates, which are ready for one-electron oxidation and capable to give stable cation-radicals in appropriate solvents. As the cited examples show, such a mechanism can really be revealed. However, very rapid transformations of aromatic cation-radicals can mask the ion-radical nature of many other reactions and create an illusion of their nonradical character. At the same time, the ion-radical mechanism demands its own approaches for farther optimization of commercially important cases of nitration. This mechanism deserves onr continned attention. 

A striking example of the steric restriction considered is given by Sibert et al. (2006). This example concerns the reaction of mercury(2-l-) with para or ortho isomer of Wuerster azathiacrown ether. Although the para isomer transforms into the analog of Wuerster blue (into the cation-radical), the ortho isomer acts as a host for the mercury cation, giving rise to the inclusion complex (Scheme 6.7). 

This section is devoted to cyclizations and cycloadditions of ion-radicals. It is common knowledge that cyclization is an intramolecular reaction in which one new bond is generated. Cycloaddition consists of the generation of two new bonds and can proceed either intra- or intermolecularly. For instance, the transformation of 1,5-hexadiene cation-radical into 1,4-cyclohexadienyl cation-radical (Guo et al. 1988) is a cyclization reaction, whereas Diels-Alder reaction is a cycloaddition reaction. In line with the consideration within this book, ring closure reactions are divided according to their cation- or anion-radical mechanisms. 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

Scheme 7.26 reflects the final result of the reaction. The initial step of this reaction consists of one-electron oxidation of the substrate. The resulting cation-radical of o-diethynylbenzene transforms into a fulvenyl intermediate, which further reacts with the neutral substrate to yield fulvenyl... 

Methylated aromatic heterocycles (HetCHj) form cation-radicals that are typical n acids and expel a proton. Methylene radicals are formed. These radicals give rise to the corresponding carbocations if an oxidant was taken in excess. Nucleophiles attack the ions, completing the reaction. If water is the reaction medium (the hydroxyl anion is a nucleophile), an alcohol is formed. The alcohol rapidly transforms into an aldehyde on the action of the same oxidant. 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Electrochemical oxidation of hydrazidoyl halides (330) also affords 1,4-dihydro-1,2,4,5-tetrazines (104). A nitrilimine intermediate is not suggested for this reaction. The main process is the dehydrodimerization of the initially formed hydrazonyl radical, while a concurrent side-reaction leads to the l,4-dihydro-l,2,4,5-tetrazines (104), which are transformed into the corresponding cation radicals (336) on further oxidation (77IZV393, b-75MI22102). 

From the retrosynthetic point of view, electrochemical redox reactions are an easy way to accomplish the principle of redox-umpolung (polarity reversal) [2]. As can be seen in Scheme 22.1, oxidation of an electron-rich neutral compound will lead to an electrophilic cation radical, or starting with a nucleophilic anion, anodic oxidation may lead to an electrophilic cation. In the mirror image reductions, an electron-poor neutral compound is transformed to a nucleophilic anion radical, or an electrophilic cation will end up as a nucleophilic anion. 

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