Syntheses of formazans under phase-transfer conditions

Abstract

Several 1,3,5-triarylformazans were synthesized (42-77%) using a new methodology. Azo-coupling of aryldiazonium salts with arylaldehyde arylhydrazones under mild basic conditions in two-phase liquid-liquid media is efficiently promoted by phase-transfer catalysts (onium salts or dicyclohexano-18-crown-6) at 5-25°C. The condensation of benzaldehyde with phenylhydrazine followed by phase-transfer catalyzed azo-coupling with phenyldiazonium chloride (one-pot procedure) gave 1,3,5-triphenylformazan in a 54% yield without isolation of the intermediate benzaldehyde phenylhydrazone. A double azo-coupling reaction of phenyldiazonium chloride with 9 different CH-active compounds afforded corresponding formazan only in the case of phenylpyruvic acid. Reaction in malonamide gave 3-carbamoyl-1,5-diphenylformazan instead of the expected 1,5-diphenylformazan.