Halides reaction with radical anions

For some halides, it is advantageous to use finely powdered lithium and a catalytic amount of an aromatic hydrocarbon, usually naphthalene or 4,4 -di- -bu(ylbiphcnyl (DTBB).28 These reaction conditions involve either radical anions or dianions generated by reduction of the aromatic ring (see Section 5.6.1.2), which then convert the halide to a radical anion. Several useful functionalized lithium reagents have been prepared by this method. In the third example below, the reagent is trapped in situ by reaction with benzaldehyde. 

Alkyl mercury halides participate in a photo-stimulated radical chain reaction of the anion of nitroalkanes (see Eq. 5.38) in which a 275-W sun lamp is used.56 c Primary, secondary, and tertiary alkyl radicals generated from alkyl mercury halides react with the anion of nitroalkanes to form new C-C bonds. 

Reactions of radical anions with halides are believed to occur via electron transfer and subsequent radical coupling according to Eq. (249) 56 5 ... 

The reactions of radical anions in the gas phase often parallel those of analogous anions in solution. For example, p-nitrocumyl halides [(30) X = halogen] in solution react with anions A" in the following way ... 

Figure 9.12 Marcus-type plot of established electron-uansfer processes of alkyl halides. Reactions with e , Co(CN)j, and radical anions. From the best fit, as shown, the derived value of X is 218 kJ mol. From L. Eber-son. Ref. [18, p. 120].

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

In the ethyl and isopropyl cases, the steepest descent pathway still connects the Sn2-TS to the SN2 products but the formation of ET products along the bifurcation in the indirect ET pathway is expected to increase. These trends are likely to be at the origin of the stereochemistry of the reaction of the anion radical of anthracene with optically active 2-octyl halides recalled at the beginning of this section. 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

The attack by the thiolate anion on the N-oxide oxygen of 62 produces the intermediate sulfenic acid derivative 65, which, in the presence of thiols, further reacts with the thiolate anion, to give the oxime 66, which has been isolated among the reaction products. By contrast, spontaneous loss of the halide anion from 65 affords the ni-troso intermediate 67 that, by losing NO and the thiyl radical directly, or through 68, produces the a-nitrosoolefm 69. By a Michael type reaction with water this last product immediately yields the final oxime 70, which has been isolated among the reaction products. 

In a reducing environment, conditions may allow for the same type of mechanism to occur, but with the radical anion of the spin trap as the intermediate. Actually, the possibility of radical ion-mediated spin trapping was first discussed in a study of a reductive system, namely in the search for radical intermediates in the reaction between alkanethiolates and alkyl halides conducted in the presence of TBN [2] (Crozet et al., 1975). TBN is known to trap primary radicals with formation of nitroxides (attack of R at N), and it was therefore anomalous to find alkoxyaminyl radicals (attack of R at O) in the above reaction. It was suggested that the alkanethiolate or some other reductant reduces TBN to its radical anion, which attacks the alkyl halide via oxygen in an SN2 fashion, as in equations (8) and (9) (see p. 129). 

Earlier, in Sect. 8.3.1, a generalized mechanistic scheme for the reduction of simple alkyl halides was presented. What distinguishes aryl halides (ArX) from alkyl halides (RX) is the finite lifetime of the initially electrogenerated anion radical (ArX ). Thus, although ArX exhibits the same kinds of reactions as RX, a key difference is that the transient anion radical (ArX ) can undergo a homogeneous electron-transfer reaction with the aryl radical (Ar) (Eq. 4) ... 

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