Pyrolytic syntheses of 6,5,5 heterocyclic systems as novel magenta dye couplers

• Main Author: Clemence, Nathan C.
• Published: University of Edinburgh 2005
• Subjects: 547.59
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.643258

Heterocyclic ring systems of the 6,5,5 azoloindole type have been found to have potential applications in colour photography as magenta dye couplers. The work described here has involved using flash vacuum pyrolysis (FVP) as a novel synthetic route to these tricyclic systems and structurally related tetracyclics. With new disconnections involving a key FVP stage, synthesis of complex ring systems can be reduced to as little as two or three steps, giving a significant advantage over more conventional synthetic routes. In this case FVP of substituted 2-(azolo)nitrobenzene and related heterocyclic derivatives gave the potential dye couplers, by means of a phenyl radical translocation/cyclisation reaction. A range of pyrolysis precursors was synthesised by nucleophilic substitution reactions of an activated aromatic (or heterocyclic) compound with an appropriate azole, typically with potassium carbonate as base and dimethylformamide as solvent; however, less vigorous reaction conditions were often employed for some of the more activated species. An alternative route to phenyl radicals, utilising an allyl ester leaving group instead of the nitro group, led to an isomeric azoloindole system. Flash vacuum pyrolysis of the precursors was carried out with a furnace temperature of 850°C and a pressure range of 10^-2-10^-1 Torr; the extreme heat caused radical cleavage of the nitro group while the low pressure permits only intramolecular reactions. In all, seven different heterocyclic ring systems were successfully synthesised (a variety of azacyclopentaindenes and indolobenzimidazoles) two of which were also produced with amino or cyano substituents. Due to difficulties caused by the reactive nitrous gases which are co-products of the pyrolyses, the yields were relatively low, ranging from 20-50% on a 500 mg scale. The potential couplers produced were reacted with a phenylenediamine developer, base and oxidising agent, giving azamethine dyes with hues ranging reds to intense magentas.