Bisazomethines

Bisazomethines derived from suitable dialdehydes were condensed on the surface of wide-pore silica gels for testing the distance selectivity. The most promising substances for the condensation were of the type RSi(CH3)(OCH3)2 (entry c). The residual free silanol groups on the surface were then blocked by hexamethyldisila-zane to prevent non-specific adsorption. After cleavage of the template, two amino groups were left behind at a definite distance apart (Fig. 4.4). 

Although the mechanism of this reaction is uncertain it seems likely that the bisazomethine tautomer 20 is again involved, nucleophilic attack on the bromine atom by base leading to an anion which, in turn, extracts a proton from the solvent or from another diazepine molecule [Eq. (7) ].36... 

None of the many mono- or bisazomethine pigments described in the literature, e.g. have gained commercial importance, mainly because of their insufficient resistance to solvents and migration, low colour strength or for economic reasons. 

Yet another type of polycyclic azomethine isoindolinones, where the isoindolinone benzene ring is either linearly or angularly fused with other benzene rings, have also been reported. Yellow to orange bisazomethine pigments of structure 20 were synthesized from the corresponding naphthalene dicarboxylic acids and diamines in the presence of phosphorous pentachloride. ... 

The bisazomethine could only be obtained when the macrocyclic ring was at least 12-membered. Otherwise, hydride shift occurred. F. e. s. A. P. Bindra and J. A. Elix, Tetrahedron 26, 3749 (1970) ring closures with pyrrole-3,4-dicarbox-aldehydes s. R. Kreher and B. Vogt, Ang. Ch. 82, 958 (1970). 

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