The tautomeric equilibrium and stereochemistry of β-sulfonyl enamines

Abstract

Compounds of three groups of parent secondary aliphatic β-sulfonyl enamines, unsubstituted or with an alkyl substituent in the α or β positions, have been studied. The β-sulfonyl enamines are found in equilibria between the E and Z enamine and imine forms in varying amounts in CDCl₃ solution. ¹H, ¹³C and ¹⁵N data were acquired and used for the assignment of the three species in solution. Considerable variations in the amounts of enamine E and Z and imine forms are found, depending on the substitution pattern. The conformational space of the equilibrated tautomers have been investigated using steady-state NOE experiments, dynamic NMR and ⁿJ(H,H) and ⁿJ(C,H) long-range spin-spin coupling constants, as well as ab initio HF and DFT calculations for structure determination and relative energy evaluations. Based on NOE, saturation transfer experiments and conformational analysis, an equilibrium scheme is proposed showing the mechanism and the involvement of water in the transformation of β-sulfonyl imines into the corresponding enamines.