Nitrobenzyl halide radical-anions

To gain additional information on these fragmentation reactions, the reactivity of a wide variety of structurally modified nitrobenzyl halide radical anions has been investipted (Table 13) [283, 290, 294],... 

The absolute rate of dissociation of the /j-nitrobenzyl chloride radical anion has been measured as 4 x 10 s The m-nitro isomer does not undergo a corresponding reaction, owing to the fact that the m-nitro group does not provide any resonance stabilization of the benzylic radical. The kinetics of the overall reaction supports a concerted electron transfer involving dissociation at the stage of electron transfer from the nitronate to the benzylic halide. ... 

The fact that the anion radical is an intermediate in this case falls in line with the observation that it is also an intermediate in the reduction of the same substrates by homogeneous or heterogeneous outer sphere electron donors and also that nitrobenzyl halides are quite easy to reduce (see Section 2, p. 66). In the other cases, the generation of the R radical has been assumed to proceed by halogen-atom transfer (158). It should, however, be noted that an outer sphere, dissociative electron-transfer reaction (163) would also... 

Other examples of the use of electron acceptors whose ion radicals are unstable with respect to fragmentation to an anion and a radical capable of initiation of polymerization were provided by Eaton (63,101,102). It was shown that -nitrobenzyl halides could be used in dye-sensitized compositions of semiconductor pigments such as Ti02 and CdS to induce polymerization of vinyl monomers using visible light. The sequence of events is outlined in eqs. 46-49 and Scheme 6 ... 

Nitrobenzyl halides are reduced in a 1 e-process to radical anions, which rapidly lose halide ion to form the neutral nitrobenzyl radical. The rates for this dissociation were calculated from cyclic voltammetry data to be k = 2,5 for m-nitrobenzyl chloride and k = 2 109 secfor o-nitrobenzyl bromide. The nitrobenzyl radicals predominantly dimerize (90%), whereas a small amount yields nitrotoluenes (< 10%) by hydrogen abstraction 4541. From a series of substituted benzyl bromides those with the more positive reduction potential form bibenzyl in 25—74% yield, whereas from the less easily reducible ones dibenzyl-mercury derivatives are obtained (50—60%) 485). Reduction of benzyl chloride at the plateau of the first wave yields dibenzylmercury 4 By reduction of diphenyliodonium hydroxide at -1,6 V 51% diphenylmercury is obtained 488 ... 

The radical anions of / -nitrobenzyl chloride, bromide and iodide203, of p-nitrocumyl bromide203, of 28, X = Br, I204 and of 29, X = Cl, Br205 postulated as reactive intermediates in the S l reactions of these compounds, have been observed by ESR spectroscopy, and so have the radicals formed by C—Br bond cleavage of the radical anions of / -nitrocumyl bromide and 29, X = Br. The radical anions of the primary homocyclic benzyl halides were stable under the reaction conditions used to generate them (y-irradiation, 77 K, THF or MeOH matrix) and so were the radical anions of 28 and 29, X = Cl. 

The initially formed chlorides PyX ionize to a pyridinium halide while PyR represents mixed 1,4- and 1,2-dihydropyridines.233,234 4-Nitrobenzyl halides react by a different mechanism the initial step here is electron transfer from 66 to the halide to give a nitro anion-radical which subsequently loses halide. Since this reaction involves ionic intermediates it is much more susceptible to solvent effects than the atom transfer reaction.235,236... 

P. Neta and D. Behar, Intramolecular electron transfer in the anion radicals of nitrobenzyl halides, J. Am. Chem. Soc., 102 (1980) 4798. 

In the mass spectrometer, one can observe the following reaction of the corresponding radical anion of (30) (this reaction could also occur by dissociative resonance capture, i.e., direct formation of the nitrobenzyl anion and a halide radical) ... 

The results of a recent investigation of the dependence of the cleavage rate constant upon the solvent of two similar anion radicals, those of 3-nitrobenzyl chloride and 3-chloroacetophenone,85 may likewise be interpreted as the outcome of a competition between the Lewis acid solvation of the developing halide ion and of the negatively charged oxygen atoms in the initial state. 

There is good evidence that some nucleophilic substitution reactions do involve a single electron transfer, but the best established use a slightly different mechanism. These are the SrnI reactions, with the subscript RN standing for radical nucleophilic. Examples are the reaction of the nitronate anion 4.14 with p-nitrobenzyl chloride 4.15, 251 and the reaction of the pinacolone enolate 4.16 with bromobenzene.252 The former might have been a straightforward SN2 reaction, but actually takes the S l pathway because the nitro groups make the electron transfer exceptionally easy. The latter cannot take place by a conventional Sn2 reaction, because aryl (and vinyl) halides are not susceptible to direct displacement, and the S l pathway overcomes this difficulty. 

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