¹³ C NMR spectroscopy of heterocycles: 1-phenyl-3-aryl/ <i>t</i> -butyl-5-arylpyrazoles

Abstract

Abstract A series of chalcones 1–12 were converted to pyrazolines ( 1Pi–12Pi ) by reaction with phenylhydrazine followed by DDQ oxidation to produce the corresponding pyrazoles ( 1Pz–12Pz ). Three 1-phenyl-3- t -butyl-5-arylpyrazoles ( 13Pz–15Pz ) were synthesized using an analogous approach. Molecular modeling studies predicted the 5-aryl group of the pyrazoles for both series to have a torsion angle of 52°–54° whereas the 1-phenyl group was predicted to have 35°–37° torsion angles. The 3-aryl group was predicted to be essentially coplanar (−3°) with the pyrazole system in the first series. 13 C NMR data for both series, 1Pz–12Pz and 13Pz–15Pz , were collected in DMSO- d 6 at 50°C. A plot of the C4 chemical shifts for 1Pz–12Pz versus Hammet constants for 5-aryl substituents yielded a very good linear correlation (R 2 =0.96) with a slope of 1.5. The chemical shift data for C4 showed little or no dependence on 3-aryl substituents. The result for 13Pz–15Pz , despite only three points, was consistent with the first series results and yielded a ρ value of 2.0. Distal transmission of substituent effects (5-aryl groups) to C4 of the pyrazole system was reduced by roughly 50–60% of that of the analogous planar isoxazole system, but are not consistent with results for the similarly twisted 4-bromoisoxazoles.