Conformational Barriers in Medium-Sized Rings. I. trans-Cyclodecene-1,2,4,4,9,9-d_6

The peculiar conformational situation in trans-cycloalkenes of medium ring size is discussed. It is concluded that one pair of optical isomers should exist in the odd-membered rings and two such pairs in the even-membered rings. For a given ring size, the various isomers are expected to be separated from one another by a substantial energy barrier. Cope and co-workers have shown previously that, for trans-cyclooctene, at least one of the barriers is high enough to allow isolation of two very stable optical isomers. Optically active trans-cyclononene was found to be much less stable and transcyclodecene gave no detectable optical activity. In the present investigation, the n.m.r. spectrum of transcyclodecene-1,2,4,4,9,9-d_6 has been studied as a function of temperature. Two processes which are slow on the n.m.r. time scale at low temperatures have been discovered. One of them, which involves rotation of the trans-alkene group through the loop formed by the methylene groups, was found to have an activation energy ΔE^* = 10.7 ± 0.3 kcal/mole and a frequency factor A = 10^(11.7±0.3) sec^(-1). The other process, for which no accurate rates could be determined, appears to involve restricted rotation of the C-6-C-7 molecular segment in the cyclodecene ring.