Nucleophilic coupling aryl radicals

Pyrrole anion is unreactive in liquid ammonia under irradiation with PhBr or 1-chloro-naphthalene. However, the reactions of aryl chlorides (p-chlorobenzonitrile, 3- and 4-chloropyridines and 4-chlorodiphenyl sulphone) with 2,5-dimethylpyrrole anion under electrochemical inducement in the presence of a redox mediator gave the C3-substituted product in moderate yields (35-40%) (equation 120)225. The rate constant of the coupling reaction between this nucleophile and aryl radicals is about 5-8 x 109 M"1 s 1 determined by electrochemical methods225. 

The coupling of cyanides ions with aryl radicals is an interesting example where quantitative kinetic data are available.30 The forming bond is strong, but this favorable factor is counteracted, in terms of driving force, by the fact that °./x- [second term in equation (3.24)] is very positive (in other words, CN is a hard nucleophile). In addition, the large value of Drx r +x is unfavorable in terms of the intrinsic barrier. Overall, the presence of electron-withdrawing substituents is necessary to allow the... 

COUPLING OF ARYL RADICALS WITH NUCLEOPHILES AS A CONCERTED ELECTRON-TRANSFER-BOND-FORMING REACTION... 

A large number of these values are close to the diffusion limit. This is not actually very surprising since the coupling of the aryl radical with the nucleophile has to compete with quite rapid side-reactions, if only its electron-transfer reduction, for the substitution to be effective. When taking place homogeneously, the latter reaction itself at the diffusion limit and the parameter that governs the competition is Anu[Nu ]/Ad[RX]. This is the reason why a discussion of structure-reactivity relationships is necessarily restricted to a rather narrow experimental basis. 

Termination steps in SrnI reactions compete with the propagation steps and, although these processes have aroused considerable mechanistic and theoretical speculation (see Chapter 9 in ref. 18), their effects, with several important exceptions, are not significant. For example the self-coupling of aryl radicals (equation 11) does not appear to occur under the conditions used for the SrnI reaction. One potentially disruptive termination step is reduction of the intermediate aryl radical (equation 6). The source of the reducing electron can be a dissolving metal (or solvated electron), one of the radical anion intermediates in the reaction (ArNu- or ArX- ), an electrode, or the nucleophile itself. These termination... 

In designing multicomponent coupling reactions, the nature of the individual components is obviously a key factor. Generally speaking, carbon radical species, such as alkyl radicals, aryl radicals, vinyl radicals, and acyl radicals are all classified as nucleophilic radicals, which exhibit high reactivity toward electron-deficient alkenes [2]. To give readers some ideas about this, kinetic results on the addition of tert-butyl and pivaloyl radicals are shown in Scheme 6.2. These radicals add to acrylonitrile with rate constants of 2.4 x 106 M-1 s 1 and 5 x 105 M-1 s-1 at... 

R. K. Norris, Nucleophilic Coupling with Aryl Radicals, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 4, 451, Pergamon Press, Oxford, 1991. 

Radical arylations of phenols differ in some respects from those of phenolates (Scheme 37). First, the decreased nucleophilicity of the phenol, such as 100, allows the use of unmasked aryl diazonium chlorides 101 as radical sources. Given that an efficient reductant is present in the reaction mixture and that the diazonium salt is added slowly, biphenyl alcohols 102 can be prepared in moderate to good yields [153,154]. In this way, the concentration of the salt 101 is kept low at any time and homocoupling reactions (addition of the aryl radical to diazonium ions) as well as azo coupling to the phenol 100 can be successfully overcome. 

Norris RK. Nucleophilic coupling with aryl radicals. In Trost BM, ed. Comprehensive Organic Synthesis. Vol. 4. Pergamon Press, 1991 451-482. 

When an aryl radical couples with a nucleophile, it forms a radical anion (ArNu)", and if E° [ArN u/( ArNu) ] °[ArX/(ArX) ], the ET reaction of equation 112 is practically irreversible from left to right, with a rate constant close to the diffusion limit (catalytic system),... 

The regiochemistry of the coupling products between aryl radicals and ambident nucleophiles was at the center of the recent work of Pierini et al. [116] (see Table 4). Changing the heteroatom of the nucleophile into a softer one by going down in the periodic table (from naphtholate to naphthylthiolate for instance) leads to an increase in heteroatom substitution C substitution increases when the nucleophile heteroatom is more electronegative or if the aryl moiety of the nucleophile is a more delocalized one. 

Scope of the reaction 70 Qualitative assessment of the mechanism 75 Quantitative approach to mechanisms and reactivity in the case of aromatic substitution. Termination steps 79 Coupling of aryl radicals with nucleophiles as a concerted electron-transfer-bond-forming reaction 91 Additional remarks 93... 

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