Arenes radical-anions

Scheme 1 Epoxide opening with arene radical anions under ET conditions...

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

Since different reactivity is observed for both the stoichiometric and the catalytic version of the arene-promoted lithiation, different species should be involved in the electron-transfer process from the metal to the organic substrate. It has been well-established that in the case of the stoichiometric version an arene-radical anion [lithium naph-thalenide (LiCioHg) or lithium di-ferf-butylbiphenylide (LiDTBB) for using naphthalene or 4,4 -di-ferf-butylbiphenyl (DTBB) as arenes, respectively] is responsible for the reduction of the substrate, for instance for the transformation of an alkyl halide into an alkyllithium . For the catalytic process, using naphthalene as the arene, an arene-dianion 2 has been proposed which is formed by overreduction of the corresponding radical-anion 1 (Scheme 1). Actually, the dianionic species 2 has been prepared by a completely different approach, namely by double deprotonation of 1,4-dihydronaphthalene, and its X-ray structure determined as its complex with two molecules of N,N,N N tetramethylethylenediamine (TMEDA). ... 

Nonphotochemical Generation of Radical Anions of Aromatic Halides. The second method involved using an arene radical anion as a convenient electron donor. In order to avoid side reactions, discovered when lithium naphthalenide was employed, presumably arising from coupling reactions between the donor and radical derived from acceptor radical anion, lithium p,p -d -tert-butylbiphenylide (LiDBB) [46] was used as donor. The presence of the cert-butyl groups is known to prevent the side reactions encountered with naphthalene [46]. Treatment of 1 with LiDBB in THF gave the three isomers of tetrachloro-benzene as products as shown in Eq. 17. 

PET generated arene radical cations and arene radical anions have found significant applications in organic synthesis. We will summarize the important examples of both types in this section. 

The corresponding hydrido/alkyl (and aryl) complexes v-[RuHR(L-L), ] (L-L = dppe, dppm, dmpe R = Me, Et, Ph) are readily prepared from m-[RuClR(L-L)2] and Li[AlH4]1659 whereas treatment of cis- or tvans-[RuCl2 (dmpe)2 ] with arene radical anions affords d.v-[RuH(f 1-aryl)(dmpe)2] (aryl = phenyl, 2-naphthyl, anthryl, phenanthryl).1389 In solution, these compounds are in tautomeric equilibria with significant concentrations of Ru° complexes (e.g. equation 148) although X-ray analysis for aryl = 2-naphthyl confirms the presence of the six-coordinate Ru" species (373) in the solid state.2459 Some reactions of (373) with various substrates to produce other hydrido complexes are shown in Scheme 74.44>24m Note that the compound of empirical formula [ Ru(dmpe)2 ] obtained by pyrolysis of [RuH(2-np)(dmpe)2] (reaction (iv) Scheme 74) is a binuclear Ru" hydrido complex, resulting from intermolecular oxidative addition of methyl groups to ruthenium.1390... 

Bimolecular reactions of radical anions are largely restricted to arene aocqRors owing to dteir generally more persistent character. The ambivalence of arene radical anions generally relates to the reactivity towards acids, electrophiles and electron acceptors. 

Another versatile chemical reductant which has not been widely applied in electron-transfer chemistry is sodium naphthalide. This arene radical anion is easy to generate and capable of reducing carbonyl compounds to the corresponding ketyl... 

Reduction of a Metal Halide by an Arene Radical Anion... 

Reduction of /rcMr-RuCl2[Me2PCH2)2]2 by arene radical anions such as sodium naphthalide has been shown to lead to a series of hydridoaryl complexes of the type Ru(H)(aryl)[(Me2PCH2)2]2 63). The hydride structure of the napthalene derivative has been shown in solution (63) and in the solid state 232). However, on the basis of the chemistry of the complexes, the following equilibrium was proposed 63), between a hydride and a small amount of a dihapto-coordinated arene-Ru(O) complex ... 

S. by Action of Lithium Arene Radical Anions and Dianlons. 

Lithium arene radical anions and dianions, [ArH2] Li and [ArH] 2 Li", can react with acidic hydrocarbons , alkyl halides or alkyl sulfides to form organolithiums via electron transfers. 

Table 1. Formation of Organolithiums via Reaction of Organic Halides with Lithium Arene Radical Anions in THF...

Alkali metal arene radical anion complexes are useful sources of solvated electrons for reductive desulfonylation reactions.14 Aromatic compounds such... 

Use of Lithium Naphthalenide. Lithium arene radical anion complexes are mild and highly effective reagents for the reductive desulfonylation process of functionalized sulfones. These reagents have only rarely been used with vinylic and allylic sulfones. In addition to high yields and their operational simplicity, metal arene radical anion complexes demonstrate high chemoselectivity (Eq. 67).123... 

From the experimental point of view, reductive desulfonylations with alkali metal arene radical anion complexes require a large excess of the radical anion, very short reaction times at low temperatures, and must be run under an inert atmosphere. Sodium or lithium naphthalenides in tetrahydrofuran at —78° or lower temperatures are typical reaction conditions. Tetrahydrofuran solutions of lithium naphthalenide are dark green. This color is lost when the substrate is added and restored once the reaction is finished. Upon completion, the excess reagent is quenched with a saturated aqueous solution of ammonium chloride or low molecular alcohols such as methanol or ethanol. 

Irradiation of haloarenes in the presence of amines in polar solvents promotes electron transfer from the eunine to the excited arene this can be followed by loss of halide from the resulting arene radical anion. Ultimately dehalogenated arenes are obtained. Two new reports of this process have been published, one for 9,10-dlchloroanthracene and the other for hexabromobenzene. ... 

Epoxides can be reduced under Birch conditions by solvated electrons [34] and by arene radical anions [35] without the presence of low-valent metal complexes. In both cases )5-lithiumoxy organolithium compounds are formed after further reduction with a second equivalent of the electron transfer reagent. These species are stable enough to be trapped by electrophiles at low temperatures. They do not show the typical reactivity patterns of radicals. Thus, these transformations will not be dealt with here in detail. 

Arene radical anions, particularly from polynuclear aromatic hydrocarbons (e.g., phenanthrene, anthracene, and pyrene), generated by ET using amines as the electron donor has been shown [165] to undergo carboxylation reaction (e.g., Phen->189) by the electrophilic addition of CO2, followed by the termination of the resultant radical species by H-abstraction from the solvent (Scheme 39). A laser flash photolysis study [166] has recently confirmed the involvement of arene radical anions in this reaction. 

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