Aryl halides, radical anion reactions

When aryl halides were applied in catalytic coupling reactions, the mechanistic evidence points to initial SET reduction by low-valent nickel phosphine species (selected investigations in [23, 24]). The competition of cage collapse to ArNi(PR3)2X vs. dissociation of the aryl halide radical anion to a free radical and Ni(I) complexes determines the cross-coupling manifolds. Thus, Ni(0)-Ni(II) and Ni(I)-Ni(III) catalytic cycles can occur interwoven with each other and a distinction may be difficult. Common to both is that the coupling process with aryl halides is likely to occur by a two-electron oxidative addition/reductive elimination pathway. 

Arrhenius activation parameters for cleavage reactions of aryl halide radical anions"... 

Aryl radicals electrochemically generated from the cleavage of aryl halide radical anions have been observed to react with nucleophiles other than iodide (Pinson and Saveant, 1974, 1978 Saveant, 1980), a reaction known as the SrjjI reaction (Bunnett, 1978). The most commonly used nucleophiles are thiophenolate, mercaptides, and cyanide ion. The reactions observed are... 

Radical anions of haloaromatic compounds are proposed to be intermediates in different type of reactions. Their fragmentation rates, determined electrochemically [300] or by pulse radiolysis [301] range from lO " s for phenyl halides to 10 s for some halonitrobenzenes. The rate of the reaction for some aryl hahde radical anions is too high to be measured electrochemically, the fragmentation of more stable radical anions such as those of 1-bromo- and 1-iodoanthraquinone [302], p-[303] and m-bromo- [304] and p- [303] and w-chloronitrobenzenes [304] occurs at considerably lower rates and the reaction is favored from their photoexcited state. Aryl halide radical anions may present a-n orbital isomerism depending on the orbital symmetry of their singly occupied molecular orbital [305], a proposal derived from theoretical and experimental evidences [306]. The isomerism is possible... 

The radical clock experiments as well as the stereochemical outcome of the reaction along with the reactivity profiles observed pointed to an ET process as the operating mechanism. Linear-free energy relationships were also consistent with this mechanistic pathway (see succeeding text). ET may proceed in two ways, usually referred to as inner-sphere and outer-sphere ET, which can be contemplated as the two extremes of a continuous mechanism [204]. Both processes are dissociative in nature for alkyl halides and presumably do not involve a discrete radical anion, RX" [205]. The situation may, however, be different for aryl halides. Radical anions do exist, and aryl halides probably undergo a stepwise reaction with an electron donor to give rise to RX [206]. 

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... 

The factors affecting the relative reactivity of aryl halides in SrnI reactions have been analysed and compared645. Competition experiments of pairs of substrates, in photo-stimulated reactions with pinacolone enolate ion in liquid ammonia, reveal a spread of reactivity exceeding three powers of ten. The ease of formation of the radical anion of the substrate appears to dominate the overall reactivity. The rate of dehalogenation of the radical anion may become important when its stability exceeds a certain threshold. When the fragmentation rate of the radical anion intermediate is fairly slow, the overall reactivity diminishes. 

An electron-transfer reduction of RX can he dissociative (no intermediate anion-radical RX ) or noil-dissociative (intermediate RX ). Where the noil-dissociative pathway is well documented (e.g.. aryl halides), only two reactions of the intermediate RX- arc generally posited, dissociation and electron-transfer oxidation [I7.18. However, product evidence suggests that R-X may undergo other reactions, including reduction. 

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>. 

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) ... 

Generation of aryl radicals by reduction of aryl halides in the presence of some nucleophiles, particularly alkyl or aryl sulphide ions and cyanide ions, leads to bond formation with the generation of a new radical-anion. Overall, a reaction between the initial aryl halide and a nucleophile is triggered at the cathode and is an equivalent of the Sr I process. It proceeds in stages according to Scheme 4.6 [156] and requires only a catalytic concentration of radical-anion. The reaction can... 

Mechanistic information from these reactions points to the initial formation of a radical anion of the aromatic compound, followed by loss of halide ion (3.15) subsequent attack by a second enolateanion and electron transfer to a second molecule of aryl halide provides the substitution product, and the reaction is propagated. The operation of a chain mechanism is indicated by the observation that quantum... 

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