Configurational and conformational analysis of silyl ketene thioacetals by NMR spectroscopy and computational studies

The configuration and conformation of silyl ketene thioacetals derived from 2-pyridyl thioesters were studied by combined NMR and computational analysis. The results revealed that the (E)-silyl ketene thioacetal is always the thermodynamically favoured diastereoisomer; the equilibrium ratio is reversed /E/Z>2:98/ for steric reasons when two O-silyl substituents are present on the C=C double bond, The preferred conformational minima for the E and Z isomers are proposed on the basis of Monte Carlo conformational searches performed on selected model compounds; the computational results are in excellent agreement with the NMR data.