Study of the effect induced by the substituents on the ring-chain tautomerism of Schiff bases derived from norephedrine.

The solution behavior and the spectroscopic properties of the novel Schiff bases (1S,2R)-R(1)CH=NCH(Me)CH(OH)Ph [with R(1) = ferrocenyl-[Fc (2b)], 5- or 3-methylthienyl [hereafter referred to as 5-MeTf and 3-MeTf (2c and 2d), respectively]] are reported. NMR studies show the existence of a tautomeric equilibrium between these imine forms (2b-d) and 2-substituted 4-methyl-5-phenyloxazolidines. The comparison of the results reveals that the molar ratios imine/oxazolidines (K): (a) are solvent and temperature dependent, (b) are higher than that obtained for (1S,2R)-PhCH=NCH(Me)CH(OH)Ph (2e), (c) are strongly affected by the nature of the R(1) group, and (d) increase according to the sequence Ph < Fc < 5-MeTf < 3-MeTf. Density functional theory (DFT) calculations on the open forms (imines 2b-e) and on the diastereomers of the closed forms (2-substituted 4-methyl-5-phenyl oxazolidines) have also been carried out in order to rationalize the differences detected in the solution behavior of 2b-d and their analogue with R(1) = Ph (2e).