Substituent effects in heterocyclic systems by carbon‐13 nuclear magnetic resonance. Isoxazoles

The carbon‐13 nmr spectra of a series of 3‐aryl‐5‐phenylisoxazoles (I) and 3‐phenyl‐5‐arylisoxazoles (II) have been recorded and the signals assigned. Carbon‐13 data for series I show little effect of substituent on the chemical shift of the isoxazole ring carbons. However, a plot of the carbon‐13 chemical shift of carbon‐5 in the isoxazole system I versus the chemical shift of carbon‐3 in the 3‐(4′‐aryl)‐1‐phenylpropenones gives a straight line (r = .989) with a slope of 0.35. In series II, the chemical shifts of both carbon‐4 and ‐5 are relatively sensitive to substituent effects. Fair correlations between Hammett sigma values and the chemical shifts of these two carbons are found; dual substituent parameter treatment improves the correlations. The results obtained from correlations with carbon‐4 in series II are similar to those obtained from β‐carbons of a number of styrene systems. The data show that carbon‐4 in series II is approximately 20% less sensitive to substituent effects than the previously reported data for carbon‐3 of 2‐arylfurans. Transmission of substituent effects in the isoxazole system compare well with those of the benzothiazole system.