Radical-cations from arenes

Guldi D M and Asmus K-D 1997 Electron transfer from Cjg D2) and Cjg C2 ) to radical cations of various arenes evidence for the Marcus inverted region J. Am. Chem. See. 119 5744-5... 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

It seems that no general mechanistic description fits all these experiments. Some of the reactions proceed via an addition-elimination mechanism, while in others the primary step is electron transfer from the arene with formation of a radical cation. This second mechanism is then very similar to the electrochemical anodic substitution/addition sequence. 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

Pandey and co-workers have generated arene radical cations by PET from electron-rich aromatic rings [119]. The photoreaction is apparently initiated by single-electron transfer from the excited state of the arene to ground state 1,4-dicyanonaphthalene (DCN) in an aerated aqueous solution of acetonitrile. Intramolecular reaction with nucleophiles leads to anellated products regio-specifically. The author explains the regiospecifidty of the cyclization step from... 

In addition to the former example, Pandey et al. achieved efficient a-aryla-tion of ketones by the reaction of silyl enol ethers with arene radical cations generated by photoinduced electron transfer from 1,4-dicyanonaphthalene. Using this strategy various five-, six-, seven-, and eight-membered benzannulated compounds are accessible in yields in the range 60-70% [39],... 

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

Radical cations of n donors are derived typically from substrates containing one or more N, O, or S atoms they are substituted frequently with alkene or arene moieties. Among these systems, we mention only a few examples, including two radical ions derived from l,4-diazabicyclo[2.2.2]octane (2) and the tricyclic tetraaza compound (3). For both ions, ESR as well as OS/PES data were measured. The bicyclic system (an = 1-696 mT, 2N ah = 0-734 mT, 12H) ° shows... 

In a first successful approach, Kochi, Renzepis, and co-workers [41] chose EDA complexes of 9-cyanoanthracene (14) and tetracyanoethene (TCNE, 15) since their charge transfer (CT) absorption bands are well separated from the absorption bands of the monomers. Excitation with a 25 ps laser pulse produced two transient absorption bands near 460 and 750 nm, which decayed simultaneously within ca. 60 ps. As was shown in the chloranil-enolether system 9—10, cf. Fig. 6), the transients can be identified with the arene radical cation (14a+ ) and the olefin radical anion (/5- ), respectively (Scheme 5). 

Aryl radical cations generated by electron-transfer processes from methoxy substituted arenes to DCN, tethered by oxygen, nitrogen as well as carbon nucleophile leads to intramolecular cyclizations (Scheme 8.67). The synthetic potentials of... 

A stoichiometric example of the combination of two reagents has recently been reported by O Connor in the context of the Bergman cycloaromatization. The generation of the aromatic diradical can be induced by [Cp Ru(CH3CN)3]PF6. Radical reduction occurs through HAT from CpW(CO)3H to provide a cationic Ru-arene complex as shown in Scheme 15 [35]. 

A versatile strategy for efficient intramolecular oc-arylation of ketones was achieved by the reaction of silyle enol ethers with PET-generated arene radical cations. This strategy involved one-electron transfer from the excited methoxy-substituted arenes to ground-state DCN [42]. Pandey et al. reported the construction of five- to eight-membered benzannulated as well as benzospiroannulated compounds using this approach (Sch. 20) [42a]. The course of the reaction can be controlled via the silyl enol ether obtained... 

As mentioned in Sec. 15.2.3, benzylic radicals are obtained also from the cleavage of a nucleofugal group from the radical anion. This may lead again to benzylation, and it has been shown that irradiation of 1,4-dimethoxynaphthalene in the presence of substituted benzyl halides leads to benzylated naphthalenes (mainly in position 2) via benzyl radical/arene radical cation combination, which is analogous to the benzyl radical/radical... 

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