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

Author:

Hockstedler Amy N.1,Edjah Beatrice A.1,Azhar Saajid Z.1,Mendoza Hadrian1,Brown Nicole A.1,Arrowood Hayley B.1,Clay Andrew C.1,Shah Anand B.1,Duffek Glenda M.1,Cui Jianmei1,Baumstark Alfons L.2

Affiliation:

1. 1Department of Chemistry, Georgia State University, Atlanta, GA 30302-4098, USA

2. 2Department of Chemistry, Center for Biotech and Drug Design, Georgia State University, Atlanta, GA 30302-4098, USA

Abstract

AbstractA 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. 13C NMR data for both series, 1Pz–12Pz and 13Pz–15Pz, were collected in DMSO-d6 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 (R2=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.

Publisher

Walter de Gruyter GmbH

Subject

Organic Chemistry

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