Methine

In this simplified example of phenylalanine, in the first iteration the methyl groups arc given a value of I in the first classification step because they contain a primaiy C-atom, The methylene group obtains a value of 2, and the methine carbon atom a value of 3. In the second step, the carbon atom of the methyl group on the left-hand side obtains an extended connectivity (EC) value of 2 because its neighboring atom had a value of 2 in the first classification step. 

The methine carbon atom also obtains an EC value of 4 (= 1 -h 1 + 2) in the second iteration. This process is repeated iteratively until the number of different EC values (c) is lower than or equal to the number of EC values in the previous iteration. Then the relaxation procc.ss is terminated. Next, the EC numbers of the previous iteration are taken for a canonical numbering and for the determination of constitutional symmetry (Figure 2-44),... 

Naturally occurring porphyrins are usually symmetrically substituted about the 15-methine bridge. These porphyrins can be synthesized by the condensation of two dipyrroiic intermediates. Typical dipyrroiic intermediates in current use arc the dipyrromethanes and the dipyrromethenes. Both methods will shortly be described. This again is a highly specialized... 

Active methylene or methine compounds, to which two EWGs such as carbonyl, alko.xycarbonyl, formyl, cyano, nitro, and sulfonyl groups are attached, react with butadiene smoothly and their acidic hydrogens are displaced with the 2,7-octadienyl group to give mono- and disubstituted compounds[59]. 3-Substituted 1,7-octadienes are obtained as minor products. The reaction is earned out with a /3-keto ester, /9-diketone, malonate, Q-formyl ketones, a-cyano and Q-nitro esters, cya noacetamide, and phenylsulfonylacetate. Di(octadienyl)malonate (61) obtained by this reaction is converted into an... 

C. Nitrogen Atoms Replacing Methine Groups of the Chain. 78... 

Hamer s book in this series (1). which reviewed the synthesis of sensitizing dyes, their physical characteristics, and general photographic properties up to 1958, remains the basic encyclopedic source for the study of methine dyes. 

Another objective is to discuss briefly recent and major trends in the field of methine dyes color. Indeed, because of its relatively simple structure, the thiazole ring has been chosen in the past for studying color-structure relations. Using Brooker s basicity concepts (5), numerous valuable attempts in different countries succeeded in establishing semiempirical rules for explaining the effects of structural changes on color. 

Applying this rule, the preceding dyes are written (3-methyl-4-phenyl-thiazole-2)(3-methylthiazole-2)methine cyanine iodide (3a) and (3-methyl-4-phenylthiazole-2)( l-methylquinoline-2)methine cyanine iodide (4), respectively. Any substituent in the chain is named and its position designated by a, ft or y, for example, (3-methyl-4-phenylthia2ole-2)(3-methylthiazole-2)-/S-methyltrimethine cyanine iodide (3b). 

Merocyanines belong to the class of nonionic methine dyes combining two nuclei, one of which is a ketomethylene of acidic nature such as pyrazolone, rhodanine, oxazolone, thiohydantoin,. 

The linking methine cheiin includes an even number of methine groups (0,2,4). They are commonly named as derivatives of the ketomethylene ring, for example. 3-ethyl-5-(3-ethyl-4,5-diphenylthiazolin-2-ylidene)-rhodanine (5) and 4-[4- 3-methyl-4,5-diphenylthiazolin-2-ylidene)-2-butenylidene]-3-methyl-l-p-sulfophenyl-2-pvrazolin-5-one (6) (Scheme 4). 

German or Japanese authors name these dyes (3-ethyl-4,5-diphenylthiazole-2)(3-ethylrhodanine-5)-0-methine neutrocyanine and (3-methyl-4,5-diphenyl thiazole-2)(3-methyl-l-p-sulfophenyl-2-pyrazol-5-one-4)tetramethineneutrocyanine, respectively. 

Rhodacyanines possess two chromophoric systems. They are at the same time neutrocyanine derivatives, which involves position 5 of the ketomethylene, and methine cyanine, which involves position 2. Following lUPAC s standard nomenclature rules, structure 7 is named 3-ethyl-4-phenyl-2- 4-oxo-3-ethyl-5-[2-(3-ethy]-2,3-dihydro-benzo-l,3-thiazo-lylidene)ethylidene]-tetrahydro-l,3-thiazolylidene-methyl -1.3-thiazolium iodide (Scheme 5). It implies that the 4-phenyl thiazole ring having the... 

In styryl compounds, a part of the methine chain is replaced by a phenyl group. Their name is based on the nuclei from which they are issued 3-ethyl-5-methyl-2-(p-dimethylaminostyryl)thiazolium iodide (Scheme 7). 

In 1923, Mills introduced thiazole for the first time in the synthesis of methine dyes through a somewhat indirect route. In order to demonstrate the 2,4 -cyanine mechanism of formation by quinoline and quinaldine quaternary salts reacting together, Mills used other pairs of quaternary salts as 2-methylthiazolium with either quinolinium or benzothiazolium (42, 43). 

The three methine carbons of the chain can be provided by 1.3.3-triethoxypropene (method C) or /S-anilinoacroleinanil, vinylog of diphenyl formamidine issued from the condensation of aniline on tetraal-koxypropane (method A). 

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. 

These dyes possess two independent chromophoric chains of even methine (neutro) and uneven methine (cyanine) fixed on a central ketometbylene nucleus. The methylene reactive group is first used for the neutrocyanine synthesis in position 5. the, quaternization of which can ensure a subsequent polymethine synthesis in position 2 of a cationic dye by ordinary means (Scheme 58). As indicated, this quaternized neutrocyanine (37) may as well give another neutrocyanine. 

Among the authors whose contribution has been important in the past, either by their experimental work or by their theoretical approach, the names of Koenig. Hamer, Fischer. Brooker, Kiprianov, Sklar, Schwarzen-bach, Eistert, Dimroth, and Platt are well familiar to scientists having an interest in the field of methine dyes, as well as more recently those of Kuhn, Wizinger, Scheibe, Hiinig, Van Dormael, Dahne, and Fabian. 

Most of the qualitative relationships between color and structure of methine dyes based on the resonance theory were established independently during the 1940 s by Brooker and coworkers (16, 72-74) and by Kiprianov (75-78), and specific application to thiazolo dyes appeared later with the studies of Knott (79) and Rout (80-84). In this approach, the absorptions of dyes belonging to amidinium ionic system are conveyed by a group of contributing structures resulting from the different ways of localization of the 2n rr electrons on the 2n l atoms of the chromophoric cationic chain, rather than by a single formula ... 

As a consequence of the alternative distribution of an even number (2n) TT electrons on an odd number (2n - 1) carbon atoms, centers of the methine chain susceptible to nucleophilic attack are effectively the even carbons atoms starting from nitrogen, as it has been proven experimentally (103), particularly with a ketomethyiene giving a neutrocyanine compound (53, 67). 

An alkyl group in the chain of a monoraethine thiazolocyanine prevents the molecule from being planar. It gives a bathochromic shift of 40 nm and a decrease of the oscillator strength (26), as is the case for other methine dyes. 

Any electron-attracting group, -NO2, -CN, -Ac, or -COjEt, on the a-carbon of the methine chain of a thiazole dye gives a bathochromic shift (120). A reverse effect is observed for an alkoxy group. 

Knott s rule concerns the importance of the place of the nitrogen atom replacing a methine carbon in the conjugated chain when the atom is separated from the active auxochromic atoms by an odd number of conjugated atoms, the shift is bathochromic. It is hypsochromic when there is an even number, Tne importance of the shift could establish a measure of M effect of various heterocyclic nuclei (79. 124). Many papers have been published, and examples have been given to verify these rules (79-84). 

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