Benzenes nitroso groups

The nitroso-group in the p-position has a remarkable influence in making possible the hydrolytic removal of the dimethylamino group from the benzene ring. The reaction is used technically for the preparation of secondary amines. (Trimethylamine is obtained by heating ammonium chloride with formaldehyde.)... 

The anion-radicals from aromatic nitro compounds preserve the second-order axis of symmetry. The analysis of superfine structure of the ESR spectrum of the nitrobenzene anion-radical reveals equivalency of the ortho and meta protons (Ludwig et al. 1964, Levy and Myers 1965). With the anion-radical of nitrosobenzene, the situation is quite different. This was evidenced from the ESR data (Levy and Myers 1965, Geels et al. 1965). Following electron transfer, the bent nitroso group fixes in the plane of the benzene ring to a certain extent. This produces five different types of protons, since both meta and ortho protons become nonequivalent. The nonequivalence of the ortho and meta protons has also been established for the anion-radicals of acetophenone (Dehl and Fraenkel 1963) and 5-methylthiobenzoate (Debacher et al. 1982 Scheme 6.17). 

For a review of this and closely related reactions, see Morrison, in Feuer The Chemistry of the Nitro and Nitroso Groups, pt. 1 Wiley New York, 1969, pp. 165-213, 185-191. For a review of photochemical rearrangements of benzene derivatives, see Kaupp Angew. Chem. Int. Ed. Engl. 1980, 19, 243-275 [Angew. Chem. 92, 245-276). See also Yip Sharma Res. Chem. Intermed. 1989, 11, 109. 

There are some exceptions to this general similarity of mechanism between thiophene and benzene substrates in electrophilic substitution. One of them is the mercuration of thiophene by mercury(II) acetate in acetic acid. It is very probable that the reason for the divergence is that a preliminary coordination of mercury with the sulfur atom is involved. Another reaction which deviates from the pattern in the benzenoid series is nitration in this case the complication may be due to the principal route being nitrosation, followed by oxidation of the nitroso group. 

Interesting solvent scales based on NMR measurements have been proposed by Taft et al. [90] and by Gutmann, Mayer et al [91]. A solvent polarity parameter, designated as P, has been defined by Taft et al [90] as the F chemical shift (in ppm) of 4-fluoro-nitrosobenzene in a given solvent, relative to the same quantity in the reference solvent cyclohexane cf. Table 6-6 and the discussion in Section 6.5.1). These parameters define a scale ranging from P = 0.0 in cyclohexane to P = 2.7 in sulfolane, and can easily be measured in a wide variety of solvents. The P values appear to be related to the ability of the solvents to form specific 1 1 complexes with the nitroso group of the standard compound. A compilation of P values can be found in reference [92], In addition, chemical shifts of (trifiuoromethyl)benzene and phenylsulfur pentafiuoride have been used by Taft et al. to study nonspecific dipolar interactions with HBD solvents and utilized to define n values of solvent dipolarity/polarizability for protic solvents [249]. 

Nitrosamine and para Nitroso Methyl Aniline.—The reaction with nitrous acid is characteristic of secondary amines and yields phenyl methyl nitrosamine, the nitroso group entering the amino radical. This, however, undergoes rearrangement with the transference of the nitroso group to the ring yielding a nitroso benzene compound. 

Thermal disproportionation of 3,5-di-t-butyl-2-nitrosotoluene in boiling benzene produces a mixture of 5,7-di-r-butylanthranil and the amino-toluene.159 The corresponding di-t-butylnitrotoluene remains unchanged even under harsh conditions (in methanol, sealed tube at 110°C for 9 days). Cyclization of the nitroso compounds becomes more rapid as the nitroso group becomes more sterically crowded. For example, the o-nitrosoethyl analog cyclizes to the anthranil at 5°C, whereas the isopropyl derivative is unstable. 

Butadiene reacts with 4-nitrobenzofuroxan to give a 1 1 adduct, 247, and with the 4,6- and 5,6-dinitro compounds it forms 2 1 adducts 248. Isoprene and 2,3-dimethylbutadiene react similarly. The authors pointed out that no products of addition to the nitroso groups of the ring-opened o-dinitroso-benzene derivative could be found.340... 

By far the most stable nitroso compounds are those of the aromatic series. The smoothest C-nitrosation is achieved with phenols and tertiary amines having a free para-position. In benzene derivatives the nitroso group enters almost exclusively284 the para-position to the hydroxyl or amino group. In the naphthalene series the reaction is not always unidirectional 1-nitroso-2-naphthol is obtained almost quantitatively from 2-naphthol,285 but 1-naph-thol gives a mixture of about equal parts of 2- and 4-nitroso-l-naphthol.286... 

Cam8 87Cam8 benzene ring exo to nitroso group... 

An important feature of the electronic structure of the Jt-complex XLIXa is that at the stage of its formation there occurs the complete electron transfer from the aromatic molecule to the electrophile. The total charge of the nitroso group in XLIXa equals—0.03e (MINDO/3), i.e., this structure corresponds to the pair of benzene cation radical-nitrogen oxide. 

---