4-nitrobenzyl chloride radical anion

The occurrence of intramolecular electron transfer in radical ion bond cleavage reactions is probably not straightforward and in this respect, two examples may help the reader—C-H deprotonation in the toluene radical cation and C-Cl bond cleavage in the 4-nitrobenzyl chloride radical anion (Scheme 25). 

In the toluene radical cation the electron hole is delocalized over the re-system and the deprotonation reaction is coupled with intramolecular electron transfer from the scissible bond to the aromatic ring, leading to the benzyl radical. In the 4-nitrobenzyl chloride radical anion, the unpaired electron resides in a re orbital that does not belong to the leaving group and C-Cl bond cleavage occurs simultaneously with an intramolecular electron transfer from this orbital to the a orbital of the scissible bond. 

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 ability of a nltro group in the substrate to bring about electron-transfer free radical chain nucleophilic subsdnidon fSpj li at a saniratedcarbon atom is well documented. Such electron transfer reacdons are one of the characterisdc feanires of nltro compounds. Komblum and Russell have established ihe Spj l reaction independently the details of the early history have been well reviewed by them. The reacdon of -nitrobenzyl chloride v/ith a salt of nitro ilkane is in sharp contrast to the general behavior of the ilkyladon of the carbanions derived from nitro ilkanes here, carbon ilkyladon is predominant. The carbon ilkyladon process proceeds via a chain reacdon involving anion radicals and free radicals, as shovmin Eq. 5.24 and Scheme 5.4 fSpj l reacdoni. 

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. 

Nitrobenzyl chlorides are also reduced by microsomes through one-electron reduction mechanism. Moreno et al. [47] suggested that p- and o-nitrobenzyl chlorides are reduced by rat hepatic microsomes to unstable radical anions, which are decomposed to form benzyl radicals under anaerobic conditions. However, in the presence of dioxygen the radical anions of these compounds participate in futile redox cycling yielding superoxide (Figure 24.2). In contrast to p- and o-nitrobenzyl chlorides, m-nitrobenzyl chloride was reduced by microsomes to a relatively stable m-nitrobenzyl radical anion. 

FIGURE 3.25. Potential energy profiles (from B3LYP/6-13G calculations) for the clevage of 3- and 4-nitrobenzyl chloride anion radicals (a and b, respectively) in the gas phase (top) and in a solvent (middle and bottom) (from COSMO solvation calculations with a dielectric constant of 36.6 and 78.4, respectively). Dotted and solid lines best-fitting Morse and dissociative Morse curves, respectively. Adapted from Figure 3 of reference 43, with permission from the American Chemical Society. 

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The RaPO anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate in single-electron transfer processes to produce carbon-centred radicals. ... 

The solvent effects on the C-Cl bond cleavage in the aromatic radical anions of 9-chloroanthracene, 3-nitrobenzyl chloride, and 3-chloroacetophenone were described by applying the Saveant model. The results showed that the bond dissociation energy is not strongly solvent dependent. 

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. 

Whereas m-nitrobenzyl chloride reacts with nucleophiles by an SN1 mechanism5, m-nitrocumyl chloride5 has been reported to react with nucleophiles by a mechanism involving radical anions, free radicals and cage collapse of radical pairs rather than by an S l process. 

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. 

The absolute rate of dissociation of the radical anion of p-nitrobenzyl chloride has been measured as 4 x 10 s". The m-nitro isomer does not undergo a corresponding reaction. 

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... 

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