Conformational Studies of cis-Cyclodecene and cis-Cyclodecene Oxide by Dynamic NMR Spectroscopy and Computational Methods

Each of the five 13C signals of cis-cyclodecene (1) splits into two peaks of equal intensity at lower temperatures, with a free-energy barrier of 6.64 kcal/mol for the process at −139.7 °C. The 1H NMR spectrum of the allylic protons shows decoalescence at −36.4 °C, and a free-energy barrier of 10.9 kcal/mol was determined at this temperature. The MM3 strain-energy calculations of Saunders and Jimenez-Vazquez were reproduced to obtain drawings of these conformations, and relative free energies for 1 at −152.7 °C were calculated using this program. A single conformation of C1 symmetry resembling boat−chair−boat (BCB) cyclodecane was predicted by these calculations and by ab initio calculations at the HF/6-311G* level. 13C chemical shifts for the preferred conformation of 1 were calculated at the HF/6-311G* level using the GIAO method. cis-Cyclodecene oxide (2) also shows decoalescence in the 13C spectrum, with ΔG⧧ = 7.4 kcal/mol at −125.4 °C. The epoxide has a time-averaged plane of symmetry at room temperature, which no longer exists at −152.6 °C. Similarly, the lower barrier of 6.64 kcal/mol for 1 is suggested to be associated with a process which gives the compound a time-averaged plane of symmetry when it is rapid on the NMR time scale.