Substituent Influences on the Stability of the Ring and Chain Tautomers in 1,3-O,N-Heterocyclic Systems: Characterization by13C NMR Chemical Shifts, PM3 Charge Densities, and Isodesmic Reactions

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Abstract
Substituent effects on the stabilities of the ring and chain forms in a tautomeric equilibrium of five series of 2-phenyloxazolidines or -perhydro-1,3-oxazines possessing nine different substitutions at the phenyl moiety have been studied with the aid of 13C NMR spectroscopy and PM3 charge density and energy calculations. Reaction energies of the isodesmic reactions, obtained from the calculated energies of formation, show that electron-donating substituents stabilize both the chain and ring tautomers but the effect is stronger on the stability of the chain form than on that of the ring form. The 13C chemical shift changes induced by the phenyl substituents (SCS) were analyzed by several different single and dual substituent parameter approaches. The best correlations were obtained by equation SCS = ρFσF + ρRσR . In all cases the ρF values and in most cases also the ρR values were negative at both the CN and C-2 carbons, indicating a reverse behavior of the electron density. This concept could be verified by the charge density calculations. The 13C chemical shifts of the CN and C-2 carbons show a normal dependence on the charge density (qtot), but the charge density shows a reverse dependence on substitution. Correlation analysis of the 13C chemical shifts, solvent effect (CDCl3 vs DMSO-d6) on the NMR behavior as well as the effect of substituents on the electron densities and on the stabilities of the ring and chain tautomers show that the substituent dependence of the relative stability of the ring and chain tautomers in equilibrium is governed by several different electronic effects. At least intramolecular hydrogen bonding between the imine nitrogen and the hydroxyl group as well as polarization of the CN bond seem to contribute in the chain form. Stereoelectronic and electrostatic effects are possible to explain the increase in stability of the ring form by electron-donating substituents.