Conformations of silicon-containing rings. Part 3. Relative conformational energies of alkylated 1,3,5-trisilacyclohexanes as calculated from quantum chemical and molecular mechanics methods

Abstract

The relative energies of the basic conformations for five series of Si-alkylated derivatives of 1,3,5-trisilacyclohexane 1 with methyl, ethyl, i-propyl and t-butyl alkyl groups have been calculated using quantum chemical (QC) methods (HF and RI-DFT) and the MM3 force field. The results were compared to those from previous NMR investigations. It was found that the QC methods predict for all 1-mono-, cis-1,3-di and cis–cis-1,3,5-tri-alkylated 1 the chair(eq) to be the lowest energy conformation. The QC methods also predict a preference for twisted conformation in the cases of trans-1,3-di- and cis–trans-1,3,5-tri- t-butylated 1. The QC results confirm earlier predictions from the NMR data. In contrast, MM3 fails to calculate relative conformational energies properly. The reason for its failure is discussed.