Electron transfer anionic radical reactions

Figure C3.2.10.(a) Dependence of electron transfer rate upon reaction free energy for ET between biphenyl radical anions and various organic acceptors. Experiments were perfonned with the donors and acceptors frozen into...

The ability of a nltro group in the substrate to bring about electron-transfer free radical chain nucleophilic subsdnidon fSpj li at a saniratedcarbon atom is well documented. Such electron transfer reacdons are one of the characterisdc feanires of nltro compounds. Komblum and Russell have established ihe Spj l reaction independently the details of the early history have been well reviewed by them. The reacdon of -nitrobenzyl chloride v/ith a salt of nitro ilkane is in sharp contrast to the general behavior of the ilkyladon of the carbanions derived from nitro ilkanes here, carbon ilkyladon is predominant. The carbon ilkyladon process proceeds via a chain reacdon involving anion radicals and free radicals, as shovmin Eq. 5.24 and Scheme 5.4 fSpj l reacdoni. 

It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. Photoinduced electron-transfer and photoelectrochemical reactions using the C o clusters have been extensively reported because of the interesting properties of C o clusters [25,26]. The M F Es on the decay of the radical pair between a Cgo cluster anion and a pyrene cation have been observed in a micellar system [63]. However, the MFEs on the photoinduced electron-transfer reactions using the Cgo cluster in mixed solvents have not yet been studied. 

Photoinduced single-electron transfer followed by fragmentation of the radical cation is an efficient method for generating carbon-centered radicals under exceptionally mild conditions. The fate of the thus formed radicals depends primarily on their interaction with the acceptor radical anions. Typically observed reactions are either back-electron transfer or radical coupling, but from the synthetic point of view, another most intriguing possibility is the trapping of the radical with suitable substrates such as olefins (Scheme 16). 

Experimental data (Figure 4.2) for the dissociative electron transfer between radical anions and the carbon-halogen bond in alkyl halides indicates a linear relationship between log(k ) and Ed over a wide range of reaction rates [5, 9]. Very fast reactions become controlled by the rate of diffusion of two species towards each other, when every close encounter gives rise to electron transfer. A parabolic... 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

Some of the fastest photochemical processes occur on the ps time-scale, for instance electron transfer reactions. In the case of intermolecular electron transfers the actual reaction rate constants cannot be obtained when the diffusion of the reactants is the limiting factor. High concentrations must be used to ensure that encounters are faster than the reactions. Figure 8.7 shows the ps transient absorption spectra of the electron transfer between benzo-phenone and DABCO in acetonitrile. The triplet excited state of benzophe-none is seen to decay at 525 nm while the radical anion grows at about 700 nm to reach a maximum concentration after 1 ns. The decay and growth kinetics are shown in (b) of the same Figure. 

Intramolecular photoinduced electron transfer reactions of homonaphthoquinones are also made possible by the presence of Mg(C104)2 in MeCN [212]. As shown in Scheme 27, the photoexcitation of 10 in the presence of Mg2 + results in intramolecular electron transfer due to the complexation of Mg2 + with the semiquinone anion moiety, which can accelerate the photoinduced electron transfer and at the same time may retard the back electron transfer [212], No reaction occurred in the absence of Mg2+ or in the dark at ordinary temperature [212], The generated radical ion I undergoes ring... 

A review has examined the use of radical anions in elucidation of the role of electron transfer in nucleophilic reactions through the determination of rates of electron-transfer reactions or obtaining reduction potentials of short-lived radical species.203 The control of conjugation and high-spin formation of radical anions of linear and ladder-type n-... 

The solvated C02 anion radical has been observed both in the gas phase and in condensed matter (Holroyd et al. 1997) and has been well characterized by ESR spectroscopy (Knight et al. 1996). Being solvated, C02 anion radicals form complexes that yield quasi-free electrons upon photoexcitation. Gas-phase studies (Saeki and others 1999) and ab initio calculations (Tsukuda et al. 1999) indicate that static ion-dipole interactions stabilize the [(C02)n m(R0H)m] type of small clusters. In supercritical carbon dioxide, monomers and dimers of water, acetonitrile, and alcohols also form metastable complexes (Shkrob Sauer 2001a,b). Such complexation should be taken into account in studies of electron-transfer kinetics in reactions with the participation of C02. ... 

The reaction of the ketyl radical anion with the oxidized rhenium complex is the energy-releasing electron transfer step. This reaction cannot be carried out separately. While ketyl radical anions are stable species, the oxidized complex is not stable and must be generated as short-lived intermediat. ... 

Muneer et al. [129] examined the photocatalytic oxidation of three pesticide derivatives, propham, propachlor and tebuthiuron in aqueous TiC>2 suspensions. The rates of degradation of each compound were found to be strongly affected by the type of TiC>2 used, pH, catalyst and substrate concentration. For each compound several intermediate products were identified using GCMS. This study indicated that the photocatalytic oxidation process proceeded by reactions involving electron transfer, hydroxyl radical and superoxide radical anions. Scheme 2 displays the proposed mechanism for the photocatalytic decomposition of propham. 

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

Another mechanism for the 2n -f 27t ) photocycloaddition of alkenes via electron transfer is the reaction that proceeds via a triplet state which is produced by a back-electron transfer from a radical anion of the electron acceptor to a radical cation of the electron donor. The triplet state alkenes generated by this way can undergo the cyclodimerization (Scheme 22). Farid showed that the DCA-sensitized (2n 3- 2n) photocyclodimerization of 1,2-diphenylcyclo-propene-3-carboxylate occurs via the triplet state of the cyclo-propene in acetonitrile [84]. In this photoreaction, two types of the (An -(- 27t) photocycloaddition reactions take place between DCA and the cyclopropene depending upon solvents. One type of the cycloadduct is produced in benzene via exciplex and the other type of the photocycloadduct is produced in... 

An unusually efficient example of an electron transfer catalyzed electrocyclic reaction proceeding via anion radicals has also been established [124]. The conversion of the bis(allene) shown in Scheme 78 to the corresponding cyclobutene derivative... 

The occurrence of intramolecular electron transfer in radical ion bond cleavage reactions is probably not straightforward and in this respect, two examples may help the reader—C-H deprotonation in the toluene radical cation and C-Cl bond cleavage in the 4-nitrobenzyl chloride radical anion (Scheme 25). 

The ability to trap alkyl radicals during the alkylation step is suggestive of a strong balance between electron transfer and substitution reaction. Historically, naphthalene anion, in fact, has been used explicitly to generate alkyl radicals from alkyl halides (16). The presence of alkyl radicals in the alkylation of coal can be expected to complicate interpretation of reaction pathways. The observation of alkylated but unreduced aromatic products led Stock to postulate the presence of alkyl radicals during alkylation (13), although aromatic carbanions could provide similar products through nonradical pathways (1). 

Miller, J. R., Calcaterra, L. T., Closs, G. L., Intramolecular Long distance Electron Transfer in Radical Anions. The Effect of Free Energy and Solvent on the Reaction Rates, J. Am. Chem. Soc., 1984, 106, 3047 3049. 

Figure 12.1.1 Schematic representation of possible reaction paths following reduction and oxidation of species RX. a) Reduction paths leading to (1) a stable reduced species, such as a radical anion (2) uptake of a second electron (EE) (3) rearrangement (EC) (4) dimerization (EC2) (5) reaction with an electrophile, E , to produce a radical followed by an additional electron transfer and further reaction (ECEC) (6) loss of X followed by dimerization (ECC2) (7) loss of X followed by a second electron transfer and protonation (ECEC) (8) reaction with an oxidized species. Ox, in solution (EC ), b) Oxidation paths leading to (1) a stable oxidized species, such as a radical cation (2) loss of a second electron (EE) (3) rearrangement (EC) (4) dimerization (EC2) (5) reaction with a nucleophile, Nu , followed by an additional electron transfer and further reaction (ECEC) (6) loss of X followed by dimerization (ECC2) (7) loss of X" followed by a second electron transfer and reaction with OH (ECEC) (8) reaction with a reduced species. Red, in solution (EC ). Note that charges shown on products, reactants, and intermediates are arbitrary. For example, the initial species could be RX, the attacking electrophile could be uncharged, etc.

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