Dinitrobenzene, radical trapping

These additives can have essentially a reductive character such as the Zn powder or a radical trapping character such as p-dinitrobenzene, triphenylphosphine, p-diaminobenzene, di-tertiobutylphenol or both characters in the case of hydroquinone. 

The yields of reaction products from thermal nucleophilic substitution reactions in DMSO of 0- and p-nitrohalobenzenes (Zhang et al. 1993) or p-dinitrobenzene (Liu et al. 2002) with the sodium salt of ethyl a-cyanoacetate were found to be markedly diminished from the addition of small amounts of strong electron acceptors such as nitrobenzenes. At the same time, little or no diminution effects on the yields of the reaction products were observed from the addition of radical traps such as nitroxyls. These results are consistent with the conclusion that such reactions proceed via a nonchain radical nucleophilic substitution mechanism (Scheme 4.26). 

In an attempt to elucidate the mechanism, the reaction was performed in the presence of a radical trap (TEMPO=2,2,6,6-tetramethyl-l-piperidinoxyl), a reversible electron acceptor (dinitrobenzene), or a radical sensitizer (dimethoxybenzene). In all cases the proportion of coupled product was increased, suggesting that both radical and polar mechanisms contribute to the outcome of this reaction. 

Experimental evidence for the presence of radical intermediates is provided by the identification of expected products from radical rearrangements, by the use of appropriate radical probes and by direct detection by electron spin resonance (ESR). Other mechanistic evidence includes inhibition by radical traps, such as di-t-butylnitroxide (DTBN), TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), galvinoxyl and oxygen, and by radical anion scavengers such as p-dinitrobenzene (p-DNB). 

Inhibition by radical traps or radical anion scavengers has been extensively used in providing evidence for the mechanism with both aliphatic and aromatic substrates. The most commonly employed inhibitors are compounds that add irreversibly to radicals butylnitroxide (DTBN), 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO), galvinoxyl, etc.] and good reversible electron acceptors such as dinitrobenzenes (DNB) which intercept the radical anions38. 

Either p-dinitrobenzene or m-dinitrobenzene commonly is used as a radical trap in electron transfer reactions. The compound that forms the most stable radical anion is the better trap. Consider the radical anions formed when either of these starting materials adds an electron and predict which compound is commonly used. 

Once again, start by drawing the structure of the conjugated parent compound and then add an electron to form the radical anion. Only a few of the possible resonance forms are drawn. Nonetheless, it can be seen that the anion and radical can be delocalized onto both nitro groups simultaneously for the -dinitrobenzene, and this leads to more possible resonance forms. Because there is more delocalization in the intermediate from the para compound, it should be easier to transfer an electron to /7-dinitrobenzene, and hence, it should be a better radical trap. 

Scheme 6.6 illustrates a coupling reaction related to the third example of Scheme 6.4, but with diphenyl disulfide and m-dinitrobenzene [12]. The resulting imidoyl radical is trapped by m-dinitrobenzene to give the corresponding amide,... 

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