1-(3′-carboxyphenyl)-pyrazole-4-carboxaldehyde

meta-Hydrazinobenzoic acid 1.552g (10 mmol) was suspended in concentrated HCl 50mL. Arnold hexafluorophosphate salt 7.10g (15 mmol) was added, followed by acetic acid 100mL. The mixture was stirred for 10 min, then placed on a 100C oil bath and stirred under reflux for 3h 30 min. The reaction mixture was cooled to room temperature, filtered (the insoluble fraction was washed with water) and the combined filtrates were evaporated to dryness. The residue was suspended in water 100mL and stirred on ice bath for 1 hour, the precipitate was collected by filtration, washed with ice-cold water and dried by suction. The crude product was dissolved in a mixture of ethanol 45mL and water 140mL at reflux, charcoal 0.5g was added and refluxed for 5 min, then filtered while warm, the charcoal was washed with 3:1 (v/v) water-ethanol mixture (2x15mL) and the combined filtrates were allowed to cool to ambient temperature, filtered again from a small amount of oil, and the filtrates were stirred vigorously on ice bath for 1 hour. The precipitated product was collected by filtration, and re-crystallized for the second time (from a mixture of ethanol 100mL and water 200mL, 0C, fridge overnight) to provide 1.176g of a 95% pure product as a yellow-white solid (54.5% th)

LC/MS(+ESI): 217(M+1); 1H(d6-DMSO, 400MHz): 13.340(br s, 1H), 9.927(s, 1H), 9.370(s, 1H), 8.437(br t, 1.8Hz, 1H), 8.308(s, 1H), 8.176(ddd, 8.1Hz, 2.3Hz, 1.0Hz, 1H), 7.959(dt, d:7.8Hz, t:1.3Hz, 1H), 7.690(t, 7.9Hz, 1H)

Note: In the absence of conc HCl, the main product is the m-carboxyphenyl hydrazone of the pyrrole aldehyde (= 2:1 condenzation product of arylhydrazine with triformyl methane). HCl slows down the hydrolysis of the iminium groups. Arnold salt preparation was posted here on Feb 24, 2008.