Regioselectivity aromatic compound nitration

Smith and coworkers have screened the solid catalysts for aromatic nitration, and found that zeolite (3 gives the best result. Simple aromatic compounds such as benzene, alkylbenzenes, halogenobenzenes, and certain disubstituted benzenes are nitrated in excellent yields with high regioselectivity under mild conditions using zeolite (3 as a catalyst and a stoichiometric quantity of nitric acid and acetic anhydride.11 For example, nitration of toluene gives a quantitative yield of mononitrotoluenes, of which 79% is 4-nitrotoluene. Nitration of fluorobenzene under the same conditions gives p-fluoronitrobenzene exclusively (Eqs. 2.1 and 2.2)... 

The reported improvement in yields and selectivities in both mono- and poly-nitration of aromatic compounds using Claycop with acetic anhydride (and if necessary nitric acid) in tetrachloromethane has been investigated.28 The reagent system is found to be modestly catalytic and regioselective in the mononitration of toluene but is neither catalytic nor regioselective in the nitration of 2-nitrotoluene. 

Zeolites have been used before in the vapour phase nitration of aromatic compounds using nitrogen dioxide.20 However, the conditions were harsh and there was no regioselectivity. Initially, therefore, we attempted to reproduce the mild conditions of Suzuki for nitration of chlorobenzene. Liquid N2O4 (approx. 10 ml) was condensed into a trap at -78 °C and was then warmed to 0 °C. Fe(acac)3 (0.355 g) and chlorobenzene... 

Laszlo and coworkers [13] developed a one-pot nitration method based on the use of claycop , cupric nitrate-impregnated acidic montmorillonite clay K-10. Nitration was conducted in the liquid phase under Menke conditions in the presence of acetic anhydride as water-trapping agent regioselectivity was high for p-NT. In a further development, the one-pot polynitration of mainly activated aromatic compounds and in particular toluene was examined using the same catalyst... 

Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 474 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 476 Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents 480 Substituent Effects in Electrophilic Aromatic Substitution Halogens 482 Multiple Substituent Effects 484 Retrosynthetic Analysis and the Synthesis of Substituted Benzenes 486 Substitution in Naphthalene 488 Substitution in Heterocyclic Aromatic Compounds 489... 

Mononitration of 1-hydroxynaphthalene could be obtained by use of ammonium hexanitra-tocerate(IV) supported on silica gel in acetonitrile (Chawla and Mittal, 1985) (scheme 39). The same reaction with CAN in acetic acid give the 2,4- and 4,6-dinitro derivatives. By the same method, 2-hydroxy naphthalene is converted into l-nitro-2-hydroxy naphthalene and 4-nitro-2-hydroxynaphthalene (scheme 40). The 1-alkoxynaphthalenes can be mononitrated regioselectively in the 4-position (scheme 41). The lower reactivity of CAN supported on silica gel is also evident from the fact that naphthalene derivatives are not oxidized into 1,4-naphthoquinones. The authors noticed that no reaction could be observed when ammonium hexanitratocerate(IV) was replaced by a mixture of ammonium nitrate and ammonium cerium(IV) nitrate. Silica supported CAN has also been used for the nitration of methoxyben-zenes (Grenier et al., 1999) and less electron rich aromatic compounds like benzene, toluene, chlorobenzene and bromobenzene (Mellor et al., 2000) (scheme 42). For the less electron rich aromatics, dichloromethane is used as solvent instead of acetonitrile. In most cases, the yields are good to excellent. 

The latter compounds 88 are unstable and readily hydrolyzed to isocou-marin 89 and deoxybenzoin 90 by analogy with /3-diketonic enol ethers (cf. Section III,F,2,d). The only example of electrophilic substitution in an aromatic ring conjugated with the cationic ring was described for the nitration regioselectively yielding the meta isomer 91 (84MI1). 

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