Saturated heterocycles. 251 . Determination of σ+ values of heteroaryl-substituted phenyl groups via ring-chain tautomerism

The Hammett-Brown electrophilic substituent constants σ⁺ for five heteroaryl-substituted phenyl groups were estimated via the ring-chain tautomeric equilibria of three sets of oxazolidines and 1,3-oxazines in deuteriochloroform solution. For oxazolidines derived from norephedrine and norpseudoephedrine, the σ⁺ values were also determined via the gas-phase ring-chain equilibria, by means of mass spectrometry. The results show that the heteroaromatic substituents studied have a slight electron-withdrawing character, with σ⁺ in the range -0.03 to 0.59. The gas-phase σ⁺ values are significantly higher than the solution values.