Malononitrile, a Synthetic Tool for New Chromophoric Systems
New near-IR-absorbing dyes were synthesized by condensation of the dicyanovinyl derivative of 1-acetyl-3-oxo-2,3-dihydroindole with nitroso compounds. The resulting products were found to be mixtures (ca. 9:1) of the (E)- and (Z)-isomers. The more bathochromic (E)-form could be converted into the (Z)-form by heating in methanol. At higher temperature new cyclization products were obtained. A mechanism of the isomerisation and cyclization is proposed.
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