A novel electro-organic synthesis of pyrrolopyrazole derivatives

Abstract

The polarographic reduction of two series of pyrazolone derivatives, namely ethyl 4-arylazo-5-oxo-4,5-dihydropyrazol-3-ylacetate derivatives (1a–d) and ethyl 4-arylazo-5-oxo-1-phenyl-4,5-dihydropyrazol-3-ylacetate derivatives (2a–d), is reported in detail for aqueous buffered media at a dropping mercury electrode (DME). Controlled-potential electrolysis (CPE) on four members showed that the products of electrolysis were ethyl 4-amino-5-oxo-4,5-dihydropyrazol-3-ylacetate (5a), 3,5-dioxo-2,3,3a,4,5,6-hexahydropyrrolo[3,2-c]pyrazole (6a), ethyl 4-amino-5-oxo-1-phenyl-4,5-dihydropyrazol-3-ylacetate (5b), and 3,5-dioxo-2-phenyl-2,3,3a,4,5,6-hexahydropyrrolo-[3,2-c]pyrazole (6b). These were isolated and identified through IR, ¹H NMR, and MS analyses. Also, the acid dissociation constants pK_a1 and pK_a2 were determined spectrophotometrically and a mechanism has been suggested and discussed for the electrode and ionization processes. Finally the results were shown to be predicted through Hammett correlations.