Unusual conformational preferences of 1,3-dimethyl-3-isopropoxy-3-silapiperidine

The conformational analysis of the first representative of the Si‐alkoxy substituted six‐membered Si,N‐heterocycles, 1,3‐dimethyl‐3‐isopropoxy‐3‐silapiperidine, was performed by low‐temperature 1H and 13C NMR spectroscopy and DFT theoretical calculations. In contrast to the expectations from the conformational energies of methyl and alkoxy substituents, the Meaxi‐PrOeq conformer was found to predominate in the conformational equilibrium in the ratio Meaxi‐PrOeq : Meeqi‐PrOax of ca. 2 : 1 as from the 1H and 13C NMR study. The thermodynamic parameters obtained by the complete line shape analysis showed that the main contribution to the barrier to ring inversion originates from the entropy term of the free energy of activation. Copyright © 2012 John Wiley & Sons, Ltd. The conformational analysis of the title first representative of Si‐alkoxysilapiperidine was performed by low‐temperature 1H and 13C NMR spectroscopy and DFT theoretical calculations. In contrast to the expectations from the conformational energies of the substituents, the Meaxi‐PrОeq conformer predominates in the conformational equilibrium with the ratio of ca. 2 : 1.