Azomethine group

Despite the inconveniences, a certain number of studies have been carried out, particularly concerning dyes containing azomethine groups. Such as hydrazones, pyrazolones, formazans, and selenazoles quinoids. Saturated heterocycles, that is, selenazolines and selenazolidines. have also been tackled. Selenium derivatives for pharmacological or physiological applications are little developed by comparison with their thiazole homologs. 

Aryl-2-phenyl-4,5-dihydropyridazin-3(2//)-ones react either with phenylmagnesium bromide or with phenyllithium to give 6-aryl-2,6-diphenyl-l,4,5,6-tetrahydropyridazin-3(2//)-ones (135) (products of 1,2-addition to the azomethine bond), while 2-methyl-6-phenyl-4,5-dihydropyridazine-3(2//)-one reacts with two equivalents of phenylmagnesium bromide at the carbonyl and azomethine group to produce 2-methyl-3,3,6,6-tetraphenyl-hexahydropyridazine (136) (Scheme 53) (80JPR617). 

Table 7-4 also includes two compounds with azomethine groups, i.e., the analogues of the phenylazo substituent (-N2-C6H5), in which one of the azo nitrogens is replaced by a methine group these are also called benzylideneanilines. 

Azolediazonium ions, isomeric 309 f. Azomethine group 154 Azoperoxides 35 f. 

In 1990, Baumeister et al. [127] described the crystal and molecular structure of 4-ethoxy-3 -(4-ethoxyphenyliminomethyl)-4 -(4-methoxy-benzoy-loxy)azobenzene. The molecules have a bifurcated shape. The phenyliminom-ethyl branch is bent markedly from the nearly linear three ring fragment, but is almost coplanar with the azobenzene moiety. They found that the molecular conformation is affected by an intramolecular interaction of the carboxylic and azomethine groups. The crystal packing was described in terms of a sheet structure with interdigitating rows of molecules. 

The nitrofuran group of drugs (Fig. 5.18) is based on the finding over 40 years ago that a nitro group in the 5 position of 2-substituted furans endowed these eompounds with antibacterial activity. Many hundreds of such compounds have been synthesized, but only a few are in current therapeutic use. In the most important nitrofurans, an azomethine group, —CH=N—, is attached at C-2 and a nitro group at C-5. Less "important nitrofurans have a vinyl group, —CH=CH—, at C-2. 

Replacement of amino groups at 2 -position with azomethine groups gives brown color. These include 6 -diethylamino-2 -ethylidenamino-fluoran (49 R = CH3),42 2 -(2-butenylidenamino)-6 -diethylaminofluoran (49 R = CH3CH=CH),42 2 -benzylidenamino-6 -diethylaminofluoran (49 R = CsHs),42 and 2 -cinnamylidenamino-6 -diethylaminofluoran (49 R = C6H5CH=CH).42... 

The selectivity in this process is governed by preliminary chelation of the RCu species by the azomethine group and the allylic double bond. The proposed chelates for the cases of (S)-(Z)-10 and (S)-( )-10 are shown in Fig. 8.1. 

Also, in one case bromination of a 5(7)-methyl group rather than substitution at the 6-position has been reported [69JCS(C)1449]. This took place when l,7-dimethyl-5-phenyldihydrodiazepine (15) was treated with bromine in methanol, and presumably results from the facts that, unlike N-unsubstituted dihydrodiazepines, this base has a fixed structure, incapable of tautomeric rearrangement, and in this structure the 7-methyl group is activated by conjugation with the azomethine group. 

This ready nucleophilic substitution at the 6-position is surprising since this position is electron-rich in both dihydrodiazepines and dihydrodiaze-pinium salts and is the site at which electrophilic substitution occurs. The likely explanation is that in the presence of base some prototropic rearrangement of the normal dihydrodiazepine base into a bis-imino form takes place. Although the equilibrium concentration of the bis-imine is likely to be very small (it has not been observed spectroscopically) it would be strongly electrophilic at the 6-position owing to the combined effects of the bromine atom and the two azomethine groups, and could well be the reactive species in the nucleophilic substitution of the bromine atom ... 

Nitrofuran antibacterials are synthetic compounds that are substitution products of the 5-nitrofuran nucleus, differing in the substituent at position 2. This substituent may be an azomethine group connected with other ring systems, or an alkyl, acyl, hydroxyalkyl, or carboxyl group, free or esterified. All these antibacterials are susceptible to photolysis, particularly by sunlight, and manipulations must be carried out under subdued light. 

Some complexes with tridentate 02N donor Schiff base ligands form from the reaction of OTi(C104)2 with H2L in aqueous methanol. A structure containing four oxygens in the plane of the octahedron and trans axial azomethine groups has been assigned (23). There is no evidence for the Ti=0 group (which is reported to occur in the IR spectrum at ca. 1280 cm-1).103... 

Indolones have both an azomethine and carbonyl group carbon atom within the same five-membered ring system. The azomethine group is the more reactive center and readily takes part in reactions typical of that group.120... 

R = Ph) is the most widely investigated of these compounds and serves as a suitable reference compound. Kalb and Bayer3 reported the methanol, ammonia, aniline, and sodium bisulfite adducts of this compound. The reactivity of the indolone increases when the electron density at the 2-position is reduced. In these cases (158 R = 4-nitrophenyl, 2-pyridyl, C02alkyl),49,61,62,91 the indolone adducts (175 Nu = OH, OEt) are stable and isolable the free indolones do not exist. The 2-alkylindolones (158 R = alkyl), in which the conjugation of the azomethine group is less extensive, are also very reactive. They too are only isolated as adducts (175 R = alkyl Nu = OH) with the exception of 158 (R = 1-Bu, Section IV,A,2). 

Phenylindolone (158 R = Ph) shows a similar shift when nucleophilic addition occurs at the azomethine group (175).140 The reported spectra50 of 158 (R = Ph) and the adducts (175 R = Ph Nu = OH, OEt) in carbon tetrachloride are very similar, indicating dissociation of the adducts in this solvent (Section V,A,1). 2-(2-Pyridyl)-indolone hydrate (175 R = 2-pyridyl Nu = OEt) fluoresces strongly at 530 nm (Aex 400 nm). 

Several examples of oxidative ring closures by attack on an azomethine group by an amine have been reported78 thus arylidene-o-phenylenedi-amines (32) have been oxidized to 2-arylbenzimidazoles (33) [Eq. (35)]. 

Anodic addition of an oxygen function to an azomethine group may be exemplified by the oxidative cyclization of o-arylidenaminophenols (63) to 2-arylbenzoxazole derivatives (64) in CH3CN-Et4NC104, using controlled potential electrolysis78,120 [Eq. (51)]. 

Unlike [3.3.3]cyclazine (230), the cyclopentacyclazines (285) are unequivocally diatropic compounds (see Table 11). The five-membered ring double bonds have been shown to be delocalized on the basis of the vicinal coupling constants of the corresponding protons. These observations suggest that the cyclazines (285) may be considered as [13]annulene anions weakly coupled to a localized azomethinium cross-link represented as in (286). The mutual interaction between the two closed shell systems may be small since the bonding MOs of the peripheral [13]annulene can interact only weakly with the antibonding MO of the azomethine group by reason of symmetry. 

The crystal and molecular structures of isomeric cluster complexes, HFe3(MeC=NH)(CO)9 and HFe3(N=CHMe)(CO)9, derived from reduction of acetonitrile have been reported.149,150 In addition to these, a wide variety of complexes with ligands containing azomethine groups (>C=N—) along with other functional groups (e.g. metal complexes of salicylaldiimine) are known and these will be discussed in Chapter 20. 

This appears also from NMR spectra (Table 11). The signal due to the azomethine group CH=N shows two or more sharp peaks, the more intense probably due to intramolecular hydrogen bonds, the less intense to intermolecular hydrogen bonds. Furthermore, the rather broad signal due to protons c in CDC13 has a chemical shift which slightly depends on concentration.07... 

Orlov et al. [112] concluded that the protonation took place on the azomethine group nitrogen atom on the basis of IR spectral data. In concentrated sulfuric acid, the formation of the only acidolysis product, 1,3-diarylpropan-l,2-one 107, was observed (Scheme 1.29). Orlov et al. [112] assumed that first protonation took place on the ketimine nitrogen atom (intermediate 106), but in strong acidic media the aziridine nitrogen atom is protonated as well, making the system unstable and promoting additional transformations. 

However, acylation reactions of N-unsubstituted pyrazolines with halogen anhydrides of acids are very often followed by the destruction of the heterocyclic ring owing to the ease of breaking the C(5)—Nq) bond located at the (3-position of the azomethine group [140, 141, 142, 166]. For example, pyrazoline 108 treated with halogen anhydrides of acids yields a,(3-unsaturated ketone 109 and diacyl-hydrazine 110 [166] (Scheme 2.29). 

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