Nitrobenzene para substitution

The usual directive influences are not operative in this and similar reactions for ortho - para substitution occurs (this may be modified by steric hindrance) irrespective of the nature of R in the aromatic liquid CsHjR, e.g. phenyldlazo hydroxide and nitrobenzene yield 4-nitrodiphenyl this supports the assumption that neutral free radicals are formed. 

Nitrobenzene reacts with the O-trimethylsilyl ketene acetal 663 in the presence of tris(dimethylamino)sulfur(trimefhylsilyl)difluoride (Me2N)3S(Me3SiF2) (TASF) to give the O-silylated adduct 1007 a, which can be oxidized in situ, e. g. by bromine, to give the 4-substituted nitrobenzene 1008 in an overall yield of 79% [87] (Scheme 7.28). With less hindered ketene-acetals, however, mixtures of ortho- and para-substituted nitrobenzenes are obtained. Yet, on reaction of 4-fluoronitroben-zene with the cyclic O-trimethylsilyl ketene acetal 1009 the ortho-substitution product 1010 is obtained in 79% yield [87]. 

In the presence of KOH, /m(benzotriazol-l-yl)methane 729 reacts with nitrobenzenes to produce />-(/fc (bcnzo-triazol-lyl)methyl]nitrobenzenes 730 (Scheme 114) <1996TL347>. This vicarious nucleophilic substitution of hydrogen <1991S103> can be considered as a convenient way to />-nitrobenzaldehydes 731. Meta and para substituted nitrobenzenes do not react with compound 729 under these conditions, probably due to steric reasons, but 1-nitronaphthalene reacts producing a naphthalene analog of derivative 730. 

However, it should be emphasized that the OH loss also occurs from meta- and para-substituted nitrobenzenes provided a side-chain of more than one carbon atom and at least one a-hydrogen atom is present. This loss of OH shows up to a minor, but nevertheless significant, extent. 

Lipkowitz54 studied the 170 shifts of some meta- and para-substituted nitrobenzenes. However, the high concentrations and small range of substituents investigated make these data of limited use. 

Substituent effects on the 170 chemical shifts in meta- and para-substituted nitrobenzenes are presented in Table 13, showing that these shifts are quite sensitive to the nature of the substituent, and range over nearly 40 ppm. Examination of Table 13 shows that the direction of substituent effects is different for 15N and 170 shifts, with electron-withdrawing substituents (such as NO2, CN or CF3) causing upheld 15N and... 

The 6-yl radicals produced by (the selective) [23, 24] addition of OH to C(5) of the C(5)/(6) double bond of naturally occurring pyrimidine bases, nucleosides and nucleotides or those formed by H-abstraction from C(6) of 5,6-dihydro-pyrimidines [25] react with para-substituted nitrobenzenes by addition (k 6xl0 to 2 X s ) to yield nitroxyl-type radicals which were... 

In nitration with nitronium salts in sulpholan, nitrobenzene was substituted in the following proportions 8% ortho, 90% meta and 2% para j40" under the same conditions benzylidyne trifluoride yielded 8 %, 88 % and 4 % of 0-, m- and p-nitro compound respectively.40" Both of these aromatic compounds were stated to be io-3-io-4 times less reactive than benzene.40"... 

In support of the free radical mechanism is the observation that the usual directive influences are not operative in these reactions. Substitution usually takes place para and ortho to the substituent in the benzene ring, irrespective of the nature of the groups. Even with nitrobenzene, para and ortho derivatives are formed. Thus, N-nitrosoacetanilide and nitrobenzene give 2- and 4-nitrobiphenyl. 

Gough, K.M. and Kaiser, K.L.E., QSAR of the acute toxicity of para-substituted nitrobenzene and aniline derivatives to Photobacterium phosphoreum, in QSAR 88. Proceedings of the Third International Workshop on Quantitative Structure-Activity Relationships in Environmental Toxicology, Turner, J.E., England, M.W., Schultz, T.W., and Kwaak, N.J., Eds., National Technical Information Service, U.S. Department of Commerce, Springfield, VA, 1988, pp. 111-121. 

The shifts of protons ortho, meta, or para to a substituent on an aromatic ring are correlated with electron densities and with the effects of electrophilic reagents (Appendix Chart D.l). For example, the ortho and para protons of phenol are shielded because of the higher electron density that also accounts for the predominance of ortho and para substitution by electrophilic reagents. Conversely, the ortho and para protons of nitrobenzene are deshielded, the ortho protons more so (see Figure 3.23). 

Scheme 9.19 Formation of ortho- and para-substituted anilines in the hydrogenation of nitrobenzene in an acidic medium.

Therefore the electron substituent effects in radical dianions of 2-substituted 5(6)-nitrobenzimidazoles are transmitted with approximately equal contributions of inductive and resonance components. A similar picture is observed for para-substituted nitrobenzene radical anions also [689, 883], Actually, the aN(N02) correlation of 2-substituted 5(6)-nitrobenzimidazoles RDA (RDA BI) with the aN(N02) of para-substituted nitrobenzene RA (RA Bz) indicates the same mechanism of substituent effects transmission but with different intensity, as shown in equation (3.4) ... 

In nitrobenzene, ortho,para substitution is thus slower than meta substitution because electron withdrawal by —NO2 is more effective during attack at the positions ortho and para to it. 

Nitroarenes are reduced to azoxybenzenes by Bi KOH or Bi NaBH in alcoholic solvents (Scheme 14.113) [239]. Under microwave irradiation the reduction proceeds much more rapidly to give azobenzenes (Scheme 14.114) [239 b]. para-Substituted nitrobenzenes are reduced to para-substituted anilines by Bi-(NH4)2SO4 (Scheme 14.115) [240]. The combined use of BiClj and a reducing agent converts nitroarenes into azoxybenzenes, N-hydroxyanilines, or anilines, depending on the reagents and reaction conditions employed [241-244]. 

Hashimoto S, Kano K. 1970. Photochemical reduction of para-substituted nitrobenzenes in isopropanol. Tetrahedron Lett 40 3509-3512. 

The greater Lewis acidity of the Ln(dfhd)3 shift reagents is also demonstrated by the fact that adducts are formed and relatively large shifts are induced even with weak Lewis bases such as nitrobenzene derivatives. The induced shifts are perhaps best illustrated by the para-substituted derivatives such as p-chloronitrobenzene and p-nitrotoluene. The data for induced shifts with Eu(dfhd)3 are summarized in Table IV. 

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