Electrophilic Aromatic Reactivities via Ring--Chain Tautomerism of Tetrahydro-1,3-Oxazines and 1,3-Oxazolidines.

Brown's electrophilic σ + -constants have been estimated for 2-, 3- and 4-pyridyl substituents with the aid of the ring-chain tautomeric equilibria of several sets of 2-aryl substituted 1,3-oxazolidines and 1,3-oxazines at ambient temperature. The σ + -values so obtained were best comparable to those determined earlier by solvolytic methods except that for 2-pyridyl which was approximately double. This deviation is due to extra stabilization of the ring from (by ca. 2-3 kJ mol −1 ) caused by intramolecular hydrogen bonding as indicated by FT-IR measurements