Dedicated to Professor Dr. K.H. Btiechel on the occasion of his 60th birthday. Summary : The very special dvity of the amino subabituent in malenoniuik. dimer (1) has been used to develop a range of new dyes covering the full speuml range. Under the spproprLte reaction conditiot~s. the amino substituent in dyes derived from 1 can met with ekctmphik~ such as acyl halii. resulting in a large bathochrmic shift. The amino group can thereby be transformed into a good leaving group suitable for nuckcphilic sub6tiMion.
Malononitrile dimer or 2-amino-1.1.3~tricyano-1-propene (1) is a very versatile building block in organic synthesis and has found a lot of applications. l Its attmfdon lies in its reactive methylene group which can be easily condensed with aldehydes2, ketones3. aromatic diaxonium salts4 and aromatic nitroso compounds*, to a variety of yellow and magenta dyes. In our search for new dyes we made use of the as yet unexplored and very special reactivity of the amino substituent of 1 to develop a range of new dyes with high molar extinction coefftcients, covering the fug spectra) range.
Condensation of 1 with aromatic N,Ndialkyl4nitroso-anilines, e.g. 2, results in the formation of magenta dyes, e.g. 45 (Scheme 1). The amino substituent of 4 has been found to undergo electrophilic substitution with carboxylic acid chlorides and anhydrides, and sulfonic acid chlorides (Scheme 1). Bisacylation of the amino group in 4 results in a large bathochromic shift of the absorption maximum (80-120 MI), yielding bright cyan dyes with very high molar extinction coefficients.
The synthesis of this new class of cyan dyes is described in Scheme 1. The outcome of the acylation reaction of 4 or 5 is highly dependent upon the base that is used. Acylations with chloroformates in the presence of pyridine run smoothly affording the bisacylated products in high yields (e.g. 6-7). but in the presence of tiethylamine the reaction proceeds sluggishly! However, with anhydrides. such as succinic anhydride and citraconic anhydride, the reaction is surprisingly fast (15 min) and straightforward at 20°C in the presence of triethylamine, but there is no reaction in the presence of pyridine! During the formation of the succinimides g-14 the presence of small amounts of the intermediate non-cyclized products has been shown by thin layer c hmmamgmphy. These cyclixe very rapidly to the final products. Cam has to be taken to avoid water in the reaction mixture because the cyan dyes g-10 axe not stable under wet reaction conditions. The reaction of 5 with ptoluenesulfonyl chloride can be stopped at the sulfonamide stage 11 when triethylamine is used as the base. Further acylation with a different acylating agent, e.g. in the preparation of 12, only takes place in the presence of pyridine! Introduction of an acetylamino group into the aromatic ring greatly improves the light stability of the dyes (e.g. 7, 9, lo), probably due to an intramolecular hydrogen bridge (9; scheme 2)6. and shifts the absorption maximum to still longer wavelengths (compare 8, 9 and 10 in Note 7). indicating a higher degree of conjugation. Increasing the steric strain in the acylamino substituent results in a large reduction in the extinction coefficient (9 vs. 13; Note 7). The huge difference in extinction