13C NMR spectroscopy of heterocycles: 3,5-diaryl-4-bromoisoxazoles

Abstract

AbstractA series of 3,5-diarylisoxazoles (1–9) underwent reaction with N-bromosuccinimide in acetic acid to yield the corresponding 3.5-diaryl-4-bromoisoxazoles (1Br–9Br) in low to moderate yield. The X-ray structure of 3-(4-chlorophenyl)-5-phenyl-4-bromoisoxazole showed torsion angles of ~40° and ~36° between the 3- and 5-aryl groups and the central ring system, respectively. Molecular modeling studies predicted the 3,5-phenylisoxazole ring system to be essentially coplanar and the 3,5-diphenyl groups of the 4-bromoisoxazole derivative to be twisted with torsional angles of 50° and 37°, respectively. Carbon-13 nuclear magnetic resonance (13C NMR) data of the 3,5-diaryl-4-bromoisoxazoles were obtained in dimethyl sulfoxide-d6 (DMSO-d6) at 50°C. Plots of the C4 chemical shift data for isoxazoles (1–9) vs. those for 4-bromoisoxazoles (1Br–9Br) showed a good linear correlation (r2 = 0.974) with a slope of 0.96. Substitution on the 3-aryl group had essentially no effect on the chemical shift for C4 of the 4-bromoisoxazole, whereas that on the 5-aryl group showed an excellent correlation with σ+ values. Despite the predicted torsion angle differences, distal transmission of substituent effects in the 4-bromo ring system was essentially analogous to that in the planar 3,5-diarylisoxazoles.