CONFORMATIONAL BARRIERS IN MEDIUM-SIZED RINGS. I. TRANS-CYCLODECENE-1,2,4,4,9,9-D6.

The peculiar conformational situation in transcycloalkenes of medium rign 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 trans-cyclodecene gave no detectable optical activity. In the present investigation, the n.m.r. spectrum of trans-cyclodecene1,2,4,4,0-d6 was studied as a function of temperature. Two processes which are slow on the n.m.r. time scale at low temperatures were 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 of 10.7 0.3 kcalmole and a frequency factor of 10 to the 11.7 0.3 power cyclessec. The other process for which no accurate rates could be determined, appears to involve restricted rotation of the C6H7 molecular segment in the cyclodecene ring. Author