Nitro group introduction

Some differences in the change of 13C NMR chemical shifts of nitrobenzothiazoles compared with nitrobenzimidazoles and nitrobenzoxazoles have been discussed. In Table 3.30 the 13C NMR chemical shifts of nitrobenzothiazoles are presented. The nitro group introduction into position 2 leads to a 10 ppm down field shift of the ipso-carbon resonance, whereas a similar effect of the ipso-substitution in the phenylene fragment of benzothiazoles is 20 ppm [778-781], The results of regression analysis of the 13C NMR chemical shifts of benzothiazoles in terms of the inductive and resonance constants of substituents (F and R, c, and cR, c, and aR°) provide evidence for the fact that the substituent effect transmission from positions 2-6 is approximately 30% weaker than in the opposite direction [779], As stated previously, an analogous picture is observed for benzimidazoles. 

Mononitration into the imidazole ring causes more significant changes in UV spectra (AA, 100 nm) than the nitro group introduction into the nitropyrazole cycle (A na 50 nm). In the spectra of nitroimidazoles there are usually two maxima, one... 

Naphthyridines are (just as pyridines) characterized as 7r-deficient systems. Introduction of an electron-withdrawing group such as the nitro group further depletes the ring of its 7r-electrons and lowers its electron density. On account of this low electron density, nitronaphthyridines show a high reactivity to nucleophilic reagents and low reactivity to electrophiles several characterictic examples of this behavior are shown in this chapter. 

Ultraviolet absorption spectra of tetrahy dro-1,3-oxazines do not show any maximum. Only after the introduction of a chromophoric group do bands appear. Thus 5-nitro derivatives show a strong maximum near 270 m/A, which is typical for a nitro group, and another one near 200 m(x which is probably also produced by the nitro group.In the instance of 5-nitro-5-hydroxymethyl derivatives, the absorption is much weakened this was explained by Urbanski in terms of a hydrogen bond between the hydroxyl and the nitro group. Other chromophores, such as C=0, C=NH, C—C, also cause the appearance of absorption maxima in the range 210-265 m/A and near 360... 

Nitration can be carried out either directly by introduction of the nitro group in place of a hydrogen atom or by adding it to a double bond, or else indirectly, by introducing into a compd a group which can readily be replaced by or converted to the nitro group... 

The following nitrating agents are most frequently used in industry for the direct introduction of the nitro group ... 

Tetranitro derivative 90 (z-TACOT Section 12.10.15.5) treated with methanolic sodium methoxide at ambient temperature does not lead to simple product of nucleophilic substitution of a nitro group but provides compound 92. Its formation can be rationalized by introduction of the methoxy group into the 1-position, followed by scission of the remote triazole ring of 91 to give the final product. Compound 90 subjected to the vicarious nucleophilic substitution (VNS) conditions using either hydroxylamine or trimethylhydrazinium iodide gives a very insoluble red solid, which was identified as l,3,7,9-tetraamino-2,4,8,10-tetranitrobenzotriazolo[2,l- ]benzotriazole 93 (Scheme 5) <1998JOC3352>. 

Benzyne generated in situ by diazotization of anthranilic acid adds readily to aminides 172 to provide cycloadducts 171. Introduction of a nitro group into para position of the phenyl ring on the nitrogen terminus of the 1,3-dipole (X2 = N02) stabilizes the system and results in higher yields of product 171 (70% vs. 50% forX2 = H). Electron-deficient imines react also with aminides 172, but the yields of isolated adducts 173 are relatively low (10-26%) (Scheme 19) <2003ARK(vii)110>. 

The ortho-ring junction that converts the triphenylmethyl structure into that of the ion LX increases the stability of the carbanion but decreases that of the carbonium ion. It will be recalled that this structural modification of the triphenylcarbonium ion had about the same effect as the introduction of one to two nitro groups. 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

Only one process was observed in these derivatives by low-temperature NMR (see, e.g., Ref. 417). As a rule, the barrier of this process was very high (133,203, 346) although, for unknown reasons, the introduction of two nitro groups at the C-3 atom leads to a noticeable decrease in the barrier height (see Scheme 3.166). 

The ease of converting a nitro group to a carbonyl or other functional group has significantly increased the synthetic potential of nitroalkane derivatives as reagents for the nucleophilic introduction of functionalized alkyl groups in the synthesis of natural products. 

Henry reaction of nitro sugar 11 with formaldehyde allowed the introduction of two hydroxymethyl groups at the carbon bearing the nitro group, and hence opened a specific route for the preparation of branched-chain imino sugar 57 and analogues (Scheme 20).44... 

Other types of ester have been studied (Fendler and Fendler, 1975 Bender and Komiyama, 1978 Szejtli, 1982), though in much less detail. Brass and Bender (1973) studied the cleavage of two diaryl carbonates and three diaryl methylphosphonates in basic buffers (Table A5.ll). For the carbonates, reacting with /1-CD, introduction of p-nitro groups increases the acceleration ratio and worsens substrate binding, so that KTs barely alters. More interesting are the results for the phosphonates in that the effects of nitro groups depend on their position and on the CD. 

In contrast to allenyl sulfones, allenyl phenyl sulfoxide failed to react with the Danishefsky s diene even at an elevated temperature [116]. Introduction of an electron-withdrawing nitro group on the aromatic ring, however, lowered the LUMO energy level and facilitated the cycloaddition, providing phenol 134. 

The chemical character of a compound is not fundamentally altered by the introduction of a nitro-group. Thus the ring-substituted nitro-derivatives of the hydrocarbons are neutral compounds like the hydrocarbons themselves. If, however, a nitro-group enters a substance having, for instance, an acid character, then this character is thereby intensified the nitrophenols, for example, are more acidic than phenol. Correspondingly, the strength of bases is decreased by nitration the nitranilines are less basic than aniline. 

The difficulty of nitration increases progressively with the number of nitro-groups introduced. Already the introduction of a second nitro-group into nitrobenzene requires much more powerful reagents than the nitration of benzene itself. Symmetrical trinitrobenzene is formed only after several days boiling of the dinitro-compound with fuming nitric add and even then only in poor yield. 

The introduction of halogen and of the nitro-group leads exclusively to the a-derivative. This is also the case with the sulphonic group. When naphthalene is sulphonated at a low temperature, such as that mentioned above, the a-sulphonic acid is produced it can thus be prepared also on a technical scale. The jS-sulphonic acid, on the other hand, is only formed at higher temperatures when the a-acid is, to a large extent, decomposed hydrolytically into naphthalene and sulphuric acid. The equilibrium between sulphonation and hydrolysis at the temperature (170°-180°) here used lies rather to the left in the case of the a-acid, and far to the right in that of the j8-acid. 

It is appropriate that the present Introduction should contain a specifically historical section, particularly in relation to the nitro group, whose electronic effect played a distinctive role in organic chemistry long before it was recognized as such. In this section and in later sections references are often given to classical papers and texts, whose importance has been overlaid by more recent work. 

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