Configuration and predominant conformations of 5-isopropyl-2,2,4-trimethyl-1,3-dioxa-2-silacyclohexane stereoisomers

The stereochemistry of 5-isopropyl-2,2,4-trimethyl-1,3-dioxa-2-silacyclohex ane (mixture of stereoisomers) and model 5-isopropyl-2,2-dimethyl-1,3-dioxa -2-silacyclohexane was studied by H-1 and C-13 NMR spectroscopy. The molecu les of the latter compound and of the trans isomer of the trimethyl derivat ive exist mainly in the chair conformation with the equatorial isopropyl gr oup, and the molecules of the cia isomer, in the 3,6-twist Term. The experi mental data were confirmed by AM1 and MM2 calculations of the optimal geome try.