1- Fischer’s aldehyde

The strategies used in the synthesis of polymethine dyes are illustrated for a series of indoline derivatives in Scheme 6.1. There is an even wider range of synthetic routes to polymethine dyes than is described here, but they are based for the most part on a similar set of principles. The starting material for the synthesis of this group of polymethine dyes is invariably 2-methylene-1,3,3-trimethylindolenine (121), known universally as Fischer s base. As illustrated in the scheme, compound 121 may be converted by formylation using phosphoryl chloride and dimethylformamide into compound 122, referred to as Fischer s aldehyde, which is also a useful starting material for this series of polymethine dyes. When compound 121 (2 mol) is heated with triethylorthoformate (1 mol) in the presence of a base such as pyridine, the symmetrical cyanine dye, C. I. Basic Red 12 109 is formed. The synthesis of some hemicyanines may be achieved by... 

Of some importance as textile dyes are aza analogues of polymethine (cyanine) dyes. Azacarbocyanines result when Fischer s aldehyde is heated with primary aromatic amines. Thus Cl Basic Yellow 11 (6.220) is obtained when Fischer s aldehyde is condensed with 2,4-dimethoxyaniline. The equivalent reaction with 2-methylindoline gives Cl Basic Yellow 21 (6.221), which has superior light fastness but has been classified by ETAD as toxic [73]. The tinctorially strong golden yellow diazacarbocyanine dye Cl Basic Yellow 28 (6.222) is prepared by coupling diazotised p-anisidine with Fischer s base (6.223), followed by quaternisation with dimethyl sulphate. Some triazacarbocyanine dyes are also used commercially. 

Several high production products for the dyeing of polyacrylonitrile textile fibres come from the azacarbocyanines class. Example in this class are the azacarbocyanine Basic Yellow 11 (2.27), synthesised from 2,4-dimethoxyaniline (2.26) and Fischer s aldehyde (2.25) the diazacarbocyanine Cl Basic Yellow 28 (2.30), synthesised from the diazonium derivative of (2.29) and Fischer s base (2.28) as shown in Figure 2.16. These dyes, although they are very bright, do tend to suffer from low fasteess to light. 

An important class of cationic dyes, particularly for yellow to bluish-red hues, is derived from Fischer s base by condensation with 4-formyl-substituted anilines, or from Fischer s aldehyde (which may be prepared from Fischer s base via Vilsmeier formylation (B-71MI11205)) and an aromatic amine (Scheme 3). There are many examples of resonating charge dyes of this type which include a variety of aromatic amines and aldehydes. Of heterocyclic interest are the dyes formed by condensation of Fischer s aldehyde with... 

Scheme 3 Synthesis of Astrazon dyes from Fischer s base and Fischer s aldehyde, respectively...

The second method corresponds to moving the formyl group from the aldehyde intermediate to the methylene base, and is a standard method for preparing a merocyanine (the open form of a spiropyran). This method is useful for the reaction of the easily obtained and stable Fischer s aldehydes (2-formylmethyleneindolines) (1) with ketomethylene compounds such as 2-hydroxybenzofuran (2) or 2-hydroxybenzothiophene where the corresponding hydroxyaldehyde is difficult to obtain (Scheme 4). These two methods include the several routes to various symmetrical and unsymmetrical spiro(dipyrans).6... 

Vilsmeier formylation of l-(2-hydroxyethyl)Fischer s base (in its cyclic form as trimethylindolino[l,2-6]oxazolidine) (11) gave l-(2-hydroxyethyl)Fischer s aldehyde in 82% yield when the reaction was performed at 35°C, but gave instead 66% of l-(2-chloroethyl)Fischer s aldehyde at 60°C.27... 

Fischer s aldehydes having a 2-hydroxyethyl or 2-chloroethyl substituent on the N atom and a 5-nitro substituent were prepared by formylation of the corresponding... 

In attempts to prepare 3-acyl-substituted BIPS (113), various salicylaldehydes were condensed with l,3,3-trimethyl-2-formylmethyleneindoline (Fischer s aldehyde). In alcohol, dimethylformamide, tetrahydrofuran, Y-mcthylpyrrolidonc. and hexamethylphosphoramide as the sole solvents, decarbonylation occurred and the only products were the BIPS compounds. However, in dimethyl sulfoxide, 5-nitro-and 3-bromo-5-nitrosalicylaldehydes gave 11 and 5.6% of the desired 3-formyl-BIPS, respectively. The structures were assigned from their IR and NMR spectra. 

Developed in the late 1930s for acetate, methine dyes derived from Fischer s Base (2,3 -dihydro-1,3,3-trimethyl-2-methylene-1H-indole) and Fischer s Aldehyde ([1,3-dihydro-1,3,3-trimethyl-2H-indol-2-xylidene]-acetaldehyde) later found use in dyeing acrylic fibers. They provide bright yellow to violet dyeing with acceptable lightfastness. Basic Red 14 and Basic Yellow 11 are examples of this type with a delocalized charge ... 

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