Azine groups

Theoretically, two different possibilities of intramolecular criss-cross addition exist, as shown in Scheme 42. Apparently, the distance between the azine group and the multiple bond, as well as the thermodynamic stability of the cyclic product, determines whether a lateral or central type of cyclization is preferred. 

In order to estimate the excited state basicity of the azine group, the absorption and fluorescence spectra have been measured in HCIOa/CHjOH (Table 5). A calculation using eq. 6 showed that pK(j-pKg = -8.4 this corresponds to a greatly enhanced basicity in the excited state of benzaldazine and 2,2 dimethoxy-benzalda-zlne. In sallcyl aldazlne the bathochromlc shift upon acidification is smaller (2800 cm ) due to the already H-bond-lowered... 

Here also the acid radical is combined with the azine group, and the amido-groups only form salts in presence of an excess of acid, the change being accompanied by a characteristic change of colour through blue to green. 

The azine group, with the four adjacent carbon atoms, forms a new ring, containing six atoms, so that phenazine may be regarded as containing three rings, like, anthracene. 

Although the basic character of the azines is increased by the introduction of amido-groups, the azine group plays the principal part in the formation of salts. If acetyl groups are introduced into the amido-groups the basic properties of the compound are decreased, but not entirely destroyed. 

Another point in this connection is that the amidoazines dye fibres the colour of the monoacid, and not of the polyacid salts, thus showing that the azine group effects the combination with the fibre. 

The colour-reactions exhibited by the eurhodols with acids of different concentrations can only be explained on the assumption that mono- and diacid salts exist, in which, according to circumstances, one or both nitrogen atoms of the azine group exert basic functions. 

The well-marked basic properties of the saffranines are doubtless functions of the azine group, although two amido-groups are also present. The hydrogen atoms of the latter may be replaced by alcohol or acid radicals and from the fact that the diacetyl derivatives are still mono-acid bases, it is evident that the strongly basic properties of the azine group remain unaffected. 

The symmetrical structure of the azine group generally produces less complex spectra than groups such as the Hydrazones. Azine derivatives are usually produced from only one type of aromatic aldehyde so that the two protons of the azine group are equivalent. In corn-pounds synthesized utilizing two different aromatic aldehydes, the azine protons will appear as separate resonances at low field. 

As with other members of the azine group, the lactam carbonyl or the tautomeric hydroxy functions in the pyridine as well as in the pyrimidine moiety of pyrido[4,3-d]pyrimidinones have been converted to chloro substituents, in pyrido[4.3-rf]pyrimidin-5(6//)-ones by treatment with phosphoryl chloride alone 527,532 in the case of a pyrido[4,3-t/]pyrimidin-4(3//)-one with additional phosphorus pentachloride.508... 

Azine-linked COFs with pyrene columns inside show high luminescence properties. The unique stacking of pyrene and supramolecular bonding sites provided by azine groups is responsible for the good sensitivity towards chemosensing and the selective detection of explosives like 2,4,6-trini-trophenol (Figure 10.13). 

The existence of a conical intersection of the lowest n-a state with the ground state is a general property of planar aromatic systems containing hydroxy or azine groups. The H-detachment intersection has been documented along the N-H stretch coordinate in (c) pyrrole, and (d) 9H tautomer of adenine (Fig. 19). ° A similar conical intersection mediating 0-H cleavage has been identified in phenol [Fig. 19(e)]. 

A large variety of newer poly(ether imide)s has been described. Included among these are perfluorinated polymers (96), poly(ester ether imide)s (97), poly(ether imide)s derived from A/,Ar-diamino-l,4,5,8-naphthalenetetracarboxyHcbisimide (98), and poly(arylene ether imide ketone)s (99). In addition, many other heterocyHc groups have been introduced into polyether systems, eg, poly(pyrazole ether)s (100) and poly(aryl ether phenylquinoxaLine)s (101) poly(aryl ether oxazole)s with trifluoromethyl groups (102) and polyethers with other heterolinkages, eg, poly(arylether azine)s (103). 

Mauveine is in a group of azines termed safranine dyes ie, it is a A/-phenyl-phenazonium chloride. Although the stmctures of these dyes are often written to show a positive charge on a particular hetero atom, the charge is in fact distributed through resonance throughout the molecule, thus accounting for their deep color. 

An unusual 1,4-migration of a trifluoromethyl group was observed when azomethine imines were synthesized from hexafluoroacetone azine and alkoxy-acetylenes The rearrangement, which occurs at temperatures as low as 0 "C, results in the formation of A-(perfluoro-ferf-butyl)pyrazoles [207] (equation 46)... 

This group was developed as part of a scheme to prepare fluorescent tags to be used in DNA sequencing. Deprotection is accomplished by irradiation at 360 nm to release the NVOC group, which then sets up the system to form a diketopiper-azine while releasing the alcohol. ... 

R. Pybidines and Azines in Which a Fused Benzene Ring OR Phenyl Group Carries a Hydroxyl Group... 

A large number of nucleophilic substitution reactions involving interconversions of pyridopyrimidines have been reported, the majority of which involve substituents in the pyrimidine ring. This subject has been reviewed previously in an earlier volume in this series which dealt with the theoretical aspects of nucleophilic re-activiti in azines, and so only a summary of the nucelophilic displacements of the substituent groups will be given here. In general, nucleophilic substitutions occur most readily at the 4-position of pyrido-... 

The third volume of this series covers three specific groups of compounds the carbolines (reviewed by R. A. Abramovitch and I. D. Spenser), the thiatriazoles (K. A. Jensen and C. Pedersen), and the pentazoles (I. Ugi). The remaining four chapters deal with topics of general chemical interest from the heterocyclic viewpoint the quaternization of heterocyclics (G. F. Duffin), carbene reactions (C. W. Rees and C. E. Smithen), applications of the Hammett equation (H. H. Jaffe and H. Lloyd Jones), and some aspects of the nucleophilic substitution of heterocyclic azines (G. Rluminati). 

The scope of heteroaryne or elimination-addition type of substitution in aromatic azines seems likely to be limited by its requirement for a relatively unactivated leaving group, for an adjacent ionizable substituent or hydrogen atom, and for a very strong base. However, reaction via the heteroaryne mechanism may occur more frequently than is presently appreciated. For example, it has been recently shown that in the reaction of 4-chloropyridine with lithium piperidide, at least a small amount of aryne substitution accompanies direct displacement. The ratio of 4- to 3-substitution was 996 4 and, therefore, there was 0.8% or more pyridyne participation. Heteroarynes are undoubtedly subject to orientation and steric effects which frequently lead to the overwhelming predominance of... 

The catalytic effect of aromatic nitro groups in the substrate and product or in an added inert nitro compoimd (e.g., w-dinitrobenzene in 18) has been observed in the reaction of 2,4-dinitrochlorobenzene with an amine in chloroform. Hydrogen bonding to benzil or to dimethyl sulfone and sulfoxide also provided catalysis. It is clear that the type of catalysis of proton transfer shown in structure 18 will be more effective when hydrogen bonding is to an azine-nitrogen. 

In compounds bearing several different groups there wiU be a complex interaction of activation by the azine-nitrogen with activation or deactivation by the substituents (Section II, E). The complexity of the interaction is emphasized by the realization that the effects of two identical substituents in an azine (e.g., in 2,4-dichloropyrimidine) are not the same on each other (Section II,B,2,a). 

Steric hindrance to activation by carboaromatic nitro groups is well-known, but there seems to be no analogy in the chemistry of azines. The lone-pair of azine-nitrogen has a steric effect comparable " to, somewhat greater than, or somewhat less " than a hydrogen atom. It is not certain whether bulky groups such as i-butyl produce a steric distortion of the lone-pair orbital and whether activation or deactivation results. 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

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