Aromatic cation radical, coupling with

Generally, in non-nucleophilic solvents (yet in the presence of non-nucleophilic organic bases like lutidine or inorganic bases like sodium carbonate) two ways of couplings may concern electrogenerated aromatic cation radicals. Thus, with an aromatic substrate such as ArH, radical coupling [7] may compete with the addition cation radical/substrate. 

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... 

The main emphasis of this chapter will be on our attempts to find evidence for radical coupling in reactions of the thianthrene cation radical (Th +) with nucleophiles, that is, for the occurrence of steps such as equations 7 and 8. However, because discussions of reactions of cation radicals with radicals are relatively sparse, we will refer first to other works that principally deal with aromatic nitration. 

Thus, the literature taught us that particular aromatic cation radicals would couple with N02, whereas our own work gave evidence, but only circumstantial, for the coupling of phenyl and possibly ethyl radicals with Th +. We turned then to a twofold plan. First to trap radicals with Th + after generating them in situ by reaction of Th + with Grignard agents (equations 24 and 25) ... 

The direct nucleophilic substitution of electron-rich phenol ethers using hypervalent iodine oxidants in the presence of Lewis acid or fluorinated alcohols and involving aromatic cation-radical intermediates was originally developed by Kita and coworkers in 1994 [362], Since then this procedure with some variations has been extensively applied by Kita and other researchers for various oxidative transformations. In the intermolecular mode, this reaction (Scheme 3.122) has been utilized for the preparation of the products 298 from N3, AcO , ArS, SCN , 3-dicarbony 1 compounds and other external nucleophiles [320]. The oxidative coupling reaction in the intramolecular mode provides a powerful synthetic tool for the preparation of various... 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

RBSctions of Radical Anions With Radicals. The coupling of arene or alkene radical anions with radicals is an important reaction, and one that has significant synthetic potential. For example, radicals formed by nucleophilic capture of radical cations couple with the acceptor radical anion, resulting in (net) aromatic substitution. Thus, the l-methoxy-3-phenylpropyl radical (113 R = H) couples with dicyanobenzene radical anion loss of cyanide ion then generates the substitution product 132.2 + ... 

The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by the solvent to produce a methoxyalkyl radical. Coupling of these radicals with the aromatic radical anion produces acyclic adducts. As an alternative, the anion radical can be protonated, ultimately giving reduction product. Thus, the degree of charge separation within the excited state complex dramatically influences the observable chemistry. 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Previously, Ohashi and his co-workers reported the photosubstitution of 1,2,4,5-tetracyanobenzene (TCNB) with toluene via the excitation of the charge-transfer complex between TCNB and toluene [409], The formation of substitution product is explained by the proton transfer from the radical cation of toluene to the radical anion of TCNB followed by the radical coupling and the dehydrocyanation. This type of photosubstitution has been well investigated and a variety of examples are reported. Arnold reported the photoreaction of p-dicyanobenzene (p-DCB) with 2,3-dimethyl-2-butene in the presence of phenanthrene in acetonitrile to give l-(4-cyanophenyl)-2,3-dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyl-l-butene [410,411], The addition of methanol into this reaction system affords a methanol-incorporated product. This photoreaction was named the photo-NO-CAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) by Arnold. However, a large number of nucleophile-incorporated photoreactions have been reported as three-component addition reactions via photoinduced electron transfer [19,40,113,114,201,410-425], Some examples are shown in Scheme 120. 

In some cases the nucleophilic capture of a radical cation is followed by coupling with the radical anion (or possibly with the neutral acceptor), resulting ultimately in an aromatic substitution reaction. Thus, irradiation of 1,4-dicyanobenzene in acetonitrile-methanol (3 1) solution containing 2,3-dimethylbutene or several other olefins leads to capture of the olefin radical cation by methanol, followed by coupling of the resulting radical with the sensitizer radical anion. Loss of cyanide ion completes the net substitution reaction [144]. This photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction has shown synthetic utility (in spite of its awkward acronym). 

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