Лако-красочные материалы — производство

4.3.1. 3-Heterocyclic Substituted Phthalides

A different approach for the modification of the basic Malachite Green lactone structure has been the replacement of one 4-dimethylaminophenyl group by electron-rich heterocycles. The most thoroughly investigated heterocycle has been the 3-indolyl residue, which may be introduced by two different routes as shown in Scheme 7.

The first compounds of this class46 have been obtained via Route A. The initial condensation of phthalic anhydride with dimethylaniline requires a Friedel-Crafts catalyst, while condensation of the resulting benzophenone with the indole requires acetic anhydride. For Route B preparation of the intermediate 1,2-dimethyl-3-(2-carboxybenzoyl)indole

has also been described47 by condensation of the two components in the presence of aluminum chloride. However, in our experience, aluminum chloride is, in this case, unnecessary, thus rendering this route the method of choice.

Scheme 7

These indolylphenylaminophthalides are blue color formers with gen­erally good light stability. Due to their ease of synthesis, it is not surprising that a large number of variations have been reported. For example, the phthalide ring has been substituted by dimethylamino groups,48 nitro — groups,49 or chlorine atoms50 to produce slightly varying shades, while long-chain alkyl groups have been attached to the indole nitrogen atom to improve solubility.51

In addition to indolylphthalides, other heterocyclic phthalides such as pyrrol-2-yl — and carbazol-3-yldialkylaminophenyl phthalides have also been prepared by analogous synthetic procedures.48,52 Depending on the other substituents in the molecule and the developer used, the pyrolyl color formers exhibit red to blue shades, while the carbazolyl compounds can vary from violet to green. Replacing the indole by 2-dimethylamino-4-methyl — thiazole in Route A leads to color formers such as 10.53 Compound 10 forms a green image in combination with a clay developer.