Triarylmethane Phthalides

Since the introduction of carbonless copying papers, CVL has undoubtedly found the most widespread use as a phthalide color former. Hence, it is appropriate to describe in detail the various approaches to its synthesis, particularly as they are representative of the preparations of various other phthalides described herein. 

However, despite the commercial importance of CVL as a color former, the product possesses a number of disadvantages, notably poor lightfastness especially in combination with clay developers, and low solubility in organic solvents. Hence, it is not surprising that a vast number of variations on the basic structure have been described in the patent literature, typical examples of which are structures 3-5 (see also for example Ref. 26). 

Phthalic anhydride condenses with the aniline derivative in the presence of zinc or aluminum chlorides to yield the intermediate benzoyl-benzoic acid, which subsequently reacts with l,3-bis-V,V-dimethylaniline in acetic anhydride to yield the phthalide. The above compound gives a violet-gray image when applied to a clay developer. Clearly this synthesis is also very flexible and variations in shades of color formers have been obtained by varying the aniline components and also by using phthalic anhydrides substituted, for example, by nitro groups or chlorine atoms. Such products have excellent properties as color formers and have been used commercially. Furthermore, this synthetic route is of great importance for the preparation of heterocyclic substituted phthalides, as will be seen later. 

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Fusion of the two arylamino groups of a triarylmethane phthalide color former results in the formation of spirofluorene phthalides. Due to the increased planarity of this system, a bathochromic shift results leading also to color formers showing infrared absorption when developed. This was first exemplified in 1983102 by preparation of phthalide 26 as shown in Scheme... 

One further example of this principle of bridging the aryl groups of a triarylmethane phthalide was reported in 1986.108 Thus, treatment of phthalide 27 with aluminum chloride results in the formation of the spirobenzanthracene 28 as shown in Scheme 11. These color formers also exhibit absorption in the near infrared spectral region, but no further reports of such compounds have since been published. 

Introduction of an ethylene bridge between the maso-carbon atom and one of the diaminophenyl groups of a triarylmethane-type phthalide results in a considerable bathochromic shift, thus producing color formers exhibiting absorption in the near infrared region of the electromagnetic spectrum. 

In general, the triarylmethane leuco skeleton can be represented by structures 1-4. Traditional leuco di- and triphenylmethane dyes frequently include compounds of type 1 and 3. The closely related compounds 2 and 4 are derived from 1 and 3. Another closely related type is the lactone or phthalide 5 (see Chapter 4). In all of these leuco dyes, one or more of the phenyl rings can be replaced by a hetaryl ring or by a fused aromatic ring such as a naphthalene. 

Colorless triarylmethane leuco materials 8 can be converted to carbon-ium ion (9)-colored materials, either by hydride abstraction or by chemical or photooxidation. In addition, some leuco compounds such as 11 can be converted to colored materials by treatment with an acid. The latter case is similar to the chemistry observed for fluoran (see Chapter 6) or phthalide (see Chapter 4) leuco compounds (Scheme 1). 

The chemistry of these phthalides has much in common with that of the compounds reviewed in the next chapter, i.e., the feMco-triarylmethanes, and in fact, the parent dyes of the two classes share the same basic chromophoric system. However, the latter are true redox systems, rather than pH indicators, and consequently have a different range of technical applications. The situation is complicated further in that the triarylmethane cationic dyes can also bleach at high pH, giving a hydroxide addition... 

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