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following the reaction with 1H NMR. As expected, the spectra of both the nitrito and nitro compounds are observed. The equilibrium can therefore be approached from either side. The reaction mixture is unstable, losing oxides of nitrogen; after several hours the major product visible has a spectrum corresponding to the arylnitromethane 50. More recently Eberson, Hartshorn and coworkers36 have observed such nitrito compounds spectroscopically, during the photochemical nitration of 1,4,5,8-tetramethylnaphthalene with tetranitromethane. These were observed during the first hour of the reaction, subsequently disappearing.
V. DISPLACEMENT OF ipso-SUBSTITUTED GROUPS
Many examples may be found in the literature1 of the displacement of substituents other than hydrogen during nitration with concurrent formation of aromatic nitro compounds. Often these displacements are from highly activated substrates, and one frequently suspects that the dominating mechanism of formation of the final nitro product is nitrous acid catalysed nitration, the product-determining stage being attack of NO2 radical rather than nitronium ion on the ipso-position. An interesting use of ipso-displacement of a tert-butyl group by nitro is exemplified by the approach of Verboom and collaborators64 to the functionalizing of calixarenes. Calix[4]arenes are often functionalized at the upper rim by nitration of free positions para to the hydroxyl group on the lower rim or by ipso nitration of p-sulphonate groups. Thus nitration of tetra-t-butyl-tetramethoxycalix[4]arene, where the t-butyl groups are para to the methoxyl groups, resulted in a good yield of the corresponding tetra-nitro compound. Dialkylated calix[4]arenes reacted much more quickly than the tetra-alkylated species to give regiospecific products where nitro-de- t-butylation occurs para to hydroxyl not methoxyl; this product, under more forcing conditions, then gives the expected tetra-nitro derivative. In this way, by control of the oxygenated function at the lower rim, a variety of usefully functionalized calixarenes may be prepared.
Another interesting system that has been investigated by Yamato and coworkers65 comprises a series of 1,n-bis(5-t-butyl-2-methoxy-3-methylphenyl)alkanes where n ranges from 1 to 4, together with the corresponding biphenyl. Even in the absence of the 3- methyl group, the para-directing influence of the methoxyl group was sufficient to give significant amounts (13%) of the mono-5-nitro compound (n D 2). With the 3-methyl substituent blocking the other activated position, both monoand di-nitro-de-t-butylation occurred with yields ranging up to 93%. The authors suggest that the high yields may be explained by one aromatic ring stabilizing the other areneonium ring arising from the ipso-attack of the nitronium ion.
Displacement of groups undergoing ipso-attack in indoles under both nitrating conditions and nitrosating conditions has been investigated by Colonna, Greci and Poloni66. Indoles are reactive species to electrophilic attack which occurs mainly at the 3-position. If this position is substituted by the N2C6H5, CH2OH, COCH3 or CHO groups, all could be displaced by treatment with 70% nitric acid mixed with twice its volume of acetic acid, to give the corresponding 3-nitro compound, often in addition to the 3,4- and 3,6- dinitro species. They failed to isolate any ipso-adduct, but deduced its presence from the deep colour observed on addition of the nitric/acetic acid solution. The reaction of the same substrates with sodium nitrite in acetic acid also gave the 3-nitro compounds; since the authentic 3-nitroso derivative could be oxidized by nitrous acid through to the corresponding 3-nitro compound, they deduced that the mechanism was one involving the 3-nitroso compound as an intermediate. However, the 3-nitrosoindole gives both the 3-nitro- and 3,5-dinitroindoles under these conditions; this was ascribed to the possibility of the ipso-cation undergoing both direct nitration in the 5-position as well as nitro-de- nitrosation. When carrying out the reactions in the cavity of an ESR spectrometer, a signal
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ascribed to the NO2 radical was sometimes observed. In these cases an electron transfer mechanism to generate the substrate cation radical, followed by radical pair collapse, was suggested. A sequence of leaving abilities of the various groups in the 3-position, based on yields and reaction times, is also given. In a later paper67, this work is extended to 4-substituted N,N-dimethylanilines and indolizines: further confirmation of the radical process was obtained.
As a final example in this section, we may consider the ipso-displacement of the nitro group itself. Liu and Zhao68 have investigated the substitution of the nitro group in a series of 4-nitrobenzoate esters by the phenylthiolate anion. Here the process again involves radical species, but now it is the radical anions of the nitro compounds which are observed as well as the thiophenoxyl radical which could be trapped.
VI. CONCLUSION
Of necessity, only a few of the many papers published in this area have been examined in detail in this review. Clearly, the area of aromatic nitration is still full of surprises38 and ipso-attack of nitrating species on substituted aromatics is proving useful in elucidating many of the subtle mechanistic details37.
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