Trimethines

In this last case, the main skeleton considered could be the cationic trimethine cyanine and the third ring with its independent chain could be named as a substituent at the a or j3 position. When this substituent is of an ionic nature, it could be named after the corresponding ring, for... 

In a third step, the anhydrobase reacts on the carbon which is the most sensitive to nucleophilic attack. The unstable intermediate 23 splits into aminothiophenol and trimethine thiazolocyanine 124). 

When a quinaldinium salt and 2-unsubstituted thiazolium are condensed together in the presence of a basic agent, the resulting bis-(methylquinoline-2)trimethine cyanine is issued from the cleavage of the thiazolium ring of the anhydrobase (25). It is induced by the -CHj attack of quinaldinium according to a process already described (Scheme 28). 

Due to the structure of the symmetrical anhydrobase. every time a 2-methylthiazolium undergoes the attack of a base, theoretically it can result in two trimethine thiazolocyanines the mesomethylsubstituted one and the unsubstituted one. For an unexplained reason, it seems that when position 5 of the starting molecule is substituted, only the mesomethyl dye is produced according to the absorption spectrum, 530 nm for the methylmeso and 569 nm for the 4-phenyl substituted derivative (Scheme 29). ... 

As an illustration of the real complexity of Mill s reaction, when two molecules of heterocycloammoniums of different nature, one of them being thiazolium (2-substituted or not), are put together in a basic medium, nine dyes theoretically can be produced (depending on the nature of the substituent in the ring) three thiazolomonomethine cyanines (two symmetrical, one asymmetrical) and six trimethine cyanines (two symmetrical, two symmetrical mesosubstituted. one unsymmetrical, one unsymmetrical mesosubstituted). One cannot separate such a mixture by usual chromatographic means. 

A mesophenyl-substituted trimethine cyanine has been identified as a by-product in syntheses leading to styryl dyes it is also obtained when the anhydrobase reacts with the styryl dye (26). 

The reason for this relative lack of reactivity of 2-methylthiazoIium is probably due to the too-weak nucleophilic character of its carbon-2. For example, any /S-alkoxyalcene (29) derivatives resulting from the condensation of o-ester could never have been isolated, whereas they constitute the essential intermediate step in trimethine syntheses for rings of acidic character (64). However, even if a negative 5-substituent such as ethoxy-carbonyl increases the yield (61) by promoting independently the possible formation of the methylene base, it may be stressed that the presence of this base is not the essential condition of the reaction, since the isolated anhydrobase itself is not reactive toward the o-ester (Scheme 41). 

Other Access Routes for Symmetrical and Unsymmetrical Thiazolo Ds-es. Other functionally symmetrical reagents such as diethoxymethylacetate (method C), ethoxymethylenediethylmalonate (method D), diphenyl formamidine, and ethylisoformanilide condense with 2-methylthiazoliura to give the trimethine thiazolo dyes. 

Anilino vinyl derivatives of thiazolium (30, R = H) or acetanilido (30, R = C0CH3), as well as formyl methylene 30b (methods E-G), give asymmetrical dyes when condensed with a methyl reactive group of another species (Scheme 42). Mesosubstituted symmetrical or unsymmet-rical thiazolocyanines are obtainable via /S-alkylmercaptovinyl thiazolium derivatives (32) (methods H and I) (Scheme 43). a or /S carbon atoms of the trimethine chain can be substituted by acetyl when a dye is treated with acetic anhydride (method L). The hydrolysis of neocyanines lead to trimethine cyanine by fractional elimination of a composant chain (method K). 

Two steps take place in the synthesis, as in the case of trimethine dyes. The first intermediate is a 2-anilinobutadieny] thiazolium, which then reacts with a second mole of 2-methylthiazolium salt or another molecule of a different ring according to the desired product either a symmetrical or asymmetrical dye (method B). 

The methine chain is obtained by reacting ethyl o-formate (method A ) or ethylisoformanilide (method B) with a bis quaternary salt of bis-(2-thiazolyllbutane. Concerning dyes with fused thiazolo rings pyrrolo[2. lb]thiazoIe. thiazolo[2.3a]indole. thiazolo[2.3c]1.4-benzox-azine. the a carbon directly linked to the carbon 2 of the thiazoJe ring is also responsible for the classical syntheses giving trimethine or penta-methine dyes. 

The electronic structure of a trimethine asymmetrical cyanine, controls the attack of a ketomethylene (Scheme 54). There is a condensation of the nucleophilic carbon on the electrophilic central carbon atom of the methine chain, leading to a neutrodimethine cyanine and simultaneously elimination of the more basic nucleus. 

For example, when N-ethylrhodanine is condensed with (quino)ine-4)(4-methylthiazole-2)trimethine cyanine or (pyridine-2)(4-methylthia-zole-2)trimethine cyanine, the yields obtained for neutrothiazolo-dimethine dye are 25 and 75%, respectively (53). 

The easier elimination of pyridine compared to quinoline-4 may be related to the pK value of 4-methylthiazole, which is between those of lepidine and 2-picoline (25. 55). This reaction explains also why a neutrodimethine cyanine is obtained with such good yields when reacting together a quaternary salt, ketomethylene, and o-ester in a basic medium. As the reaction proceeds, the trimethine cyanine is attacked by the ketomethylene. The resulting 2-methyl quaternary salt is transformed into trimethine cyanine, consuming the totality of the ketomethylene (1, p. 512 661). The mesosubstituted neutrodimethine cyanine is practically pure. 

The cleavage of thiazolopentamethine cyanine and also neocyanine results in a tetramethine neutrocyanine together with neutrodimethine cyanine (26). 2-Methylthiazolium reacts with a trimethine oxonol dye and gives a neutrotetramethine dye (Scheme 56) (26). 

This derivative condenses either on itself (64) or on the anhydrobase, giving the trimethine dye. Indeed, the nucleophilic a-carbon of the dye—the proton is labile and can be replaced (70, 71)—is liable to add onto the electrophilic /3-carbon of the alcene derivative. The neocyanine results from elimination of a molecule of ethanol. 

Whatever their nature may be, phenyl or alkyl, the substituents of the thiazole ring in position 4 or 5 give a bathochromic shift (110, 111) of the absorption of a symmetrical trimethine thiazolocyanine compared to an unsubslituted dye. For a given substituent, this shift is greater for position 5 than for 4 (112). 

Until now, no final conclusion seems to have been reached as to the origin and amplitude of the phenomenon. The nature of the alkyl group fixed on the nitrogen atoms does not influence the position of but substituents of different kinds produce, for trimethine thiazolocyanine as well as styryl derivatives (116), a bathochromic shift in the order ... 

The of mesomethyl-substituted trimethine thiazolo dyes is shifted by 30 nm toward shorter wavelength. The shift is only of 20 to 25 nm for an ethyl group, whereas phenyl has no effect. 

A rigidized molecule obtained when the two a-carbons of the trimethine chain are linked by a dimethylene bridge cannot be planar. According to the resonance concept, its stability increases as a bathochromic effect of 41 nm is observed (122). The of the bistyryl dye obtained by the substitution of the -proton in the chain of a styryl dye by a dialkylamino group is nearly the same as for the parent styryl dye (123). 

A great number of monoaza or polyaza. either symmetrica] or unsym-metrical, mono trimethine thiazolocyainines have been synthesized in order to verify or to obtain semiempirical rules, more or less based on the resonance theory, concerning the relation between the color of a thiazolo dye and the number and place of nitrogen atoms in the chromophoric chain. For example. Forster s rule applies to ionic dyes and stipulates that the will increase with the decreasing tendency of chromophoric atoms lying between the two auxochromes to take up the characteristic charges (90). 

TABLE 2I21A. SYMMETRICAL UNSUBSTITUTED TRIMETHINE CYANINES... 

In certain carbocyanines. used as photographic sensitizers in silver halide emulsions, selenazoline rings linked bv trimethine bridges are found (56, 72). 

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