Trans-Cyclooctene

trans-Cyclooctene is a cyclic hydrocarbon with the formula [–(CH2)6CH=CH–], where the two C–C single bonds adjacent to the double bond are on opposite sides of the latter's plane. It is a colorless liquid with a disagreeable odor. Cyclooctene is notable as the smallest cycloalkene that is readily isolated as its trans-isomer. The cis-isomer is much more stable; the ring-strain energies being 16.7 and 7.4 kcal/mol, respectively. A planar arrangement of the ring carbons would be too strained, and therefore the stable conformations of the trans form have a bent (non-planar) ring. Computations indicate that the most stable "crown" conformation has the carbon atoms alternately above and below the plane of the ring. A "half-chair" conformation, with about 6 kcal/mol higher energy, has carbons 2,3,5,6, and 8 on the same side of the plane of carbons 1,4, and 7. All conformations of trans-cyclooctene are chiral (specifically, what some call planar-chiral) and the enantiomers can be separated. In theory, conversion of between the enantiomers can be done, without breaking any bonds, by twisting the whole –CH=CH– group, rigidly, by 180 degrees. However, that entails passing one of its hydrogens through the crowded ring. trans-Cyclooctene was first synthesized on a preparatory scale by Arthur C. Cope with a Hofmann elimination reaction of N,N,N-trimethylcyclooctylammonium iodide. The reaction gives a mixture of cis and trans isomers, and the trans isomer is selectively trapped as a complex with silver nitrate. Other methods exist where the trans isomer is synthesized from the cis isomer in several synthetic steps. For instance, it can be prepared in almost 100% yield by converting the cis isomer to 1,2-epoxycyclooctane ("cyclooctene oxide") followed by reactions with lithium diphenylphosphide (LiPPh2) and with methyl iodide CH3I. (Similar procedures can give isomers of 1,4-cyclooctadiene and 1,5-cyclooctadiene). In addition, a photochemical method exists for the direct cis–trans isomerisation.