Substituent effects on the gas-phase ring-chain tautomerism of 3,4,5,6-tetrahydro-2H-1,3-oxazines.

The electron ionization mass spectra of five series of seven 2-aryl,4-R-substituted (R = Me, Et, i-Pr, t-Bu or Ph) 3,4,5,6-tetrahydro-2H-1,3-oxazines were recorded at 14 and 70 eV in order to study the ring-chain tautomeric equilibria in the gas phase. Certain fragment ions were associated with the ring or with the open-chain forms of the compounds. As in chloroform solution, the electron-withdrawing effect of the aryl substituent (p-NO(2), m-Br, p-Cl, H, p-Me, p-OMe and p-NMe(2)) shifts the equilibrium towards the ring form. The correlation of ring-chain equilibria (log K = [ring]/[chain]) with the Hammett sigma+ constants of the aryl substituents was in general good or satisfactory although in some cases the p-NMe(2) did not fit these correlations.