SATURATED HETEROCYCLES .251. DETERMINATION OF SIGMA(-SUBSTITUTED PHENYL GROUPS VIA RING-CHAIN TAUTOMERISM() VALUES OF HETEROARYL)

The Hammett-Brown electrophilic substituent constants sigma(+) for fiv e heteroaryl-substituted phenyl groups were estimated via the ring-cha in tautomeric equilibria of three sets of oxazolidines and 1,3-oxazine s in deuteriochloroform solution. For oxazolidines derived from noreph edrine and norpseudoephedrine, the af values were also determined via the gas-phase ring-chain equilibria, by means of mass spectrometry. Th e results show that the heteroaromatic substituents studied have a sli ght electron-withdrawing character, with sigma(+) in the range -0.03 t o 0.59. The gas-phase sigma(+) values are significantly higher than th e solution values.