The effects of substituents in oxazoles on their13C,14N, and1H NMR spectra

Abstract

1. In oxazoles electron-donor substituants in position 2 shield the14N nucleus of the ring nitrogen atom and the13C nucleus of the C5 carbon but have little effect on C4. Electron-donor substituents in position 5 shield, and electron-acceptor substituants deshielded the13C nuclei of the C4 and C2 carbons and have a weak effect onδ14N. The following linear correlations were found:\(\delta _{C^2 } - \delta _{C^4 } ,\delta _{C^5 } \) electronegativity of substituent R5,\(\delta _{C^4 } - \delta _{C^5 } ,\delta _{C^2 } - \delta _{2 - H} ,\delta _{C^4 } - \delta _{4 - H} \) andδ13C for 4,5-disubstituted oxazoles with o-disubstituted benzenes. 2. Regression analysis has established the additivity of the chemical shifts of the13C carbons of the oxazole ring. Correlations of the theoretical values with the experimental values were obtained for C2 with r=0.996, C4 with r=0.995, and C5 with r=0.999. The possibility has been demonstrated of calculatingδ13C from three parameters ΔC2, ΔC4, and ΔC5), which characterize the contributions of the substituents in shielding the C2, C4, and C5 carbons. 3. The effect of substituents at C5 on the constants\(^1 J_{C^4 } \), 4−H and\(^1 J_{C^2 } \), 2−H,\(^1 J_{C^4 } \),4−H was found to increase in the series 5-Me < 5-NMePH < 5-OBu-n.