Nitroso group, structure

The nitroso group, — N = Op is one of the few nonhalogens that is an ortho- and para-directing deactivator. Explain by drawing resonance structures of the carbocation intermediates in ortho, mela, and para electrophilic reaction on nitrosobenzene, C<3Hs N = 0. 

Sliver. Frepd by addn of Ag nitrate soin to a soln of the Na salt (see above) (Refs 3 10) ign point 181° (Ref 12). When treated with alkyl iodides it gives alkyl esters. From the reaction with Me iodide a colorless and a yellow dimethyl ester were isolated (Ref 3). The UV spectra of the colorless (called a) and the yellow (called 3) esters are given in Ref 9 it is concluded that the color of the / ester is due to a nitroso group, hence its structure is probably (CH301 Q2CH2... 

As already outlined in the previous review1, the nitroso group is of great interest from the theoretical point of view but also because of the facility with which these molecules dimerize only a few monomeric structures have been noted in the literature in the period under examination. 

In Table 5 structural parameters for nitroso groups are presented. 

The large difference between the nitrogen shieldings in nitroso groups and those molecules that are isomeric, tautomeric or related in any way to nitroso structures facilitates their spectral distinction111. 

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). 

In modern medicinal chemistry, the creation of diversity on a structural framework is important. In principle, diversity at positions 2, 4, 6, 7, and 8 of pteridines can be achieved using such solid-phase chemistry. This prototype solid-phase synthesis involved nitrosation of the resin-bound pyrimidine, reduction of nitroso group with sodium dithionite, and subsequent cyclization with biacetyl to afford pteridines 114 and 115. Cleavage from the resin by nucleophilic substitution of the oxidized sulfur linker using w-chloroperbenzoic acid or DMDO led to the pteridine products 116 and 117 (Scheme 23). 

In this reaction the source of the azoxy oxygen appears to be the nitroso group [6]. The preparation of t-butyl-OAW-azoxymethane (iV-methyl-A -t-butyldiazine jV -oxide) is an example of preparation of an unsymmetrical azoxy compound which is quite generally applicable. The structure assignment is based on NMR data. 

A kinetic study of structural effects on the A-nitrosation of amino acids by nitrite in aqueous solution has established that the dominant term in the rate equation corresponds to nitrosation by dinitrogen trioxide.189 Nitrosation by intramolecular migration of the nitroso group from an initially nitrosated carboxylate group can compete when the transition state has a five- or six-membered ring structure. Nitrosation of A-methyl-4-tolylsulfonylguanidine involves rapid nitrosation of the N-... 

This structure was based upon the IR spectrum, which shows a normal carbonyl stretching band at 1640 cm-1,60 and it was confirmed by an X-ray study.118 The interesting fact is that the nitroso oxygen atom prevails over the carbonyl for close contact with sulfur. A nitro group is less attracted than the nitroso group by sulfur.119,120... 

The venom of the fire ant, Solenopsis punctaticeps, contains several 2,5-dialkyl-pyrrolidines and -pyrrolines their structures have been settled by combined g.c-m.s., and confirmed by syntheses employing the Hofmann-Loffler reaction on the corresponding primary amines.7 A new synthesis of 2,5-dialkyl-pyrrolidines via lithiated iV-nitrosopyrrolidine and two stages of alkylation, followed by removal of the nitroso-group, should be applicable to the synthesis of the venom components. The major products are trans in stereochemistry.8... 

The nitroso compound is unstable because it can tautomerize with the transfer of a proton from carbon to the oxygen of the nitroso group. This process is exactly like enolization but uses an N=0 instead of a C-O group. It gives a more familiar functional group from Chapter 14, the oxime, as the stable enol . The second structure shows how the oxime s O-H can form an intramolecular hydrogen bond with the ketone carbonyl group. Hydrolysis of the oxime reveals the second ketone. 

Reactive free radicals also react with the nitrogen of nitroso groups, forming a nitroxide one atom closer to the trapped radical than is the case with nitrone spin traps. This results in ESR spectra containing more chemical structural information. While nitroso spin traps provide radical identification, the resultant adducts are often less stable than those derived from nitrone traps. In particular, nitroso traps are unreliable for oxygen-centered radicals even in vitro. 

FIGURE 16.3 Chemical structures of chloramphenicol and thiamphenicol. Thiamphenicol, in which the nitroso group of chloramphenicol is replaced by a methylsulfone group, retains antibiotic activity, but does not cause the aplastic anemia that is a major concern with chloramphenicol therapy. 

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