Intramolecular anion radicals

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

The addition of the nucleophile to the aryl radical is the reverse of the cleavage of substituted aromatic anion radicals that we have discussed in Section 2 in terms of an intramolecular concerted electron-transfer-bondbreaking process and illustrated with the example of aryl halides. The present reaction may thus be viewed conversely as an intramolecular concerted electron-transfer-bond-forming process. The driving force of the reaction can be divided into three terms as in (131). The first of these, the... 

Rathore et al. (2006) studied the intramolecular single-electron transfer in anion-radicals formed from fluorenylidene derivatives. The derivatives used for the reduction were Me— Flu—CH2—Flu—CH2—Flu—CH2—Flu—Me and its deuterated analog, Me—Flu—CH2—(Flu-d8)— CH2—(Flu-dj)—CH2—Flu—Me. Each parent compound initially gave an anion-radical in which an unpaired electron was tunneled between the two internal Flu nuclei and then occured within the outer Flu nuclei. In the outer part, coordinative solvation of the anion-radical by HMPA proceeded much more effectively because of ready space accessibility. Such a solvation provides a driving force for electron tunneling. As the solution electron affinities of perdeuterated aromatic hydrocarbons are less than those of perprotiated hydrocarbons, the electron tunneling was found to be at least an order of magnitude faster only in the case of [Me—Flu—CH,—(Flu-do)—CH,— (Flu-d8)-CH2-Flu-Me]-. ... 

Clark (1988) calculated the stabilities of diverse species with odd-electron o bonds. Cataldo et al. (2001) produced evidence for the existence of the anion-radical with an intramolecular one-electron bond between two phosphorus atoms in a macrocyclic structure of the metacyclophane type. Dutan et al. (2003) observed a similar situation for the anion-radical of a di (m-silylphenyl-enedisiloxane) analog. 

The sodium cation chelation by the bis(enone) anion-radicals shown in Scheme 3.52 controls their further transformations although they proceed at the expense of other reaction centers (Yang et al. 2004). This kind of intramolecular cyclobutanation is characterized with the pronounced cis-stereoselectivity. However, this stereoselectivity disappears if the reaction proceeds in the presence of the tetrabutylammonium cation, when such a chelation is impossible. 

Pressure provokes transition of the linear (extended) conformation into the bent (V-like) one. (The V-like form is more compact and occupies a smaller volume.) It is obvious that the V-like form is favorable in respect of intramolecular electron transfer from the donor (the aniline part) to the acceptor (the pyrene part). In the utmost level of the phenomenon, the donor part transforms into the cation-radical moiety, whereas the acceptor part passes into the anion-radical moiety. Such transformation is impossible in the case of the extended conformation because of the large distance between the donor and acceptor moieties. The spectral changes observed reflect this conformational transition at elevated pressures. 

It should also be mentioned that cases are possible when polarity of the solvent allows transforming charge-transfer intramolecular complexes into molecules containing the cation- and anion-radical... 

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 anion-radical mechanism for these syntheses is based on the following facts. The reactions require photo- or electrochemical initiation. Oxygen inhibits the reactions totally, even with photoirradiation. Indoles are formed from o-iodoaniline only the meta isomer does not give rise to indole. Hence, the alternative aryne mechanism (cine-substitution) is not valid. What remains as a question is the validity of the ion-radical mechanism exclusively to the substitution of the acetonyl group for the halogen atom in o-haloareneamine or also for intramolecular condensation. 

The photolysis of donor-acceptor systems provides unique synthetic opportunities. Direct irradiation of the donor-acceptor systems, such as systems containing arene and amine components, leads to intramolecular electron transfer, that is, to amine cation-radical and arene anion-radical moieties. After generation, these moieties undergo cyclization reactions providing efficient synthetic routes to fV-heterocycles with a variety of ring sizes. Thus, direct irradiation of secondary amino-ethyl and aminopropyl stilbenes leads to benzazepines in improved yields (Hintz et al. 1996). As known, benzazepines are used in medicine as antidepressants. Scheme 7.44 illustrates ion-radical cyclization with the formation of benzazepine derivative (65% yield). 

The ketone receives an electron in the antibonding n MO to form n anion-radical, which is transformed into a (-C-C1) moiety by an intramolecular electron transfer to the antibonding a fragmental orbital of the -C-Cl bond. The (-C-C1) moiety then fragments to form a chloride ion and the corresponding radical (Scheme 7.64). 

Specifically Netzel et al. ( - ), in studies of face-to-face , covalently-linked MgP-P dimers, found evidence for the formation within 6 psecs of a low-lying, relatively long-lived intramolecular CT state of the type MgP -P" in polarizable or highly polar solvents and in solvents where chloride ion coordinates with the magnesium ion of the MgP-macrocycle. These workers also observed the formation of benzoquinone anion radicals as stable photoproducts of the CT formation process when the experiments were carried out in the presence of benzoquinone ( ). This approach provides a more direct test for the formation of an intramolecular CT state, and the results are in sharp contrast to those typically observed when porphyrin Ktt, ) states are quenched in the presence of benzoquinone (23). 

On the other hand, since oxime ethers were electrochemically more inert than ketones under the electroreduction conditions, the electroreductive intra- and inter-molecular coupling of ketones with oxime ethers proceeded via anion radicals in good yields (equations 5 and 6) °4i. Moreover, cobaloxime-mediated intramolecular radical addition onto oxime functions in the electrolysis media proceeded to afford the cyclized aminoethers (equation 7). ... 

Cytochrome c552 from Euglena gracilis (also known as cytochrome / or c6) contains 87 amino acid residues, two hemes and one flavin per molecule.693 NMR studies706 indicate that the chirality of the axial methionine is similar to that of cytochrome c but different from cytochrome c5Sl. Rapid intramolecular transport has been demonstrated by the use of pulsed laser excitation, and the measurement of reduction kinetics. Both flavin and heme groups are reduced simultaneously on a multisecond time scale, with the transient formation of a protein-bound flavin anion radical.707... 

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