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.Ron Walker, Rosemary M. Conrad, and Robert H. Grubbs (2009): "The living ROMP of ''trans''-cyclooctene". ''Macromolecules'', volume 42, issue 3, pages 599–605.

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.Arthur C. Cope, C. R. Ganellin, H. W. Johnson, T. V. Van Auken, and Hans J. S. Winkler (193): "Molecular asymmetry of olefins. I. Resolution of ''trans''-cyclooctene". ''Journal of the American Chemical Association'', volume 85, issue 20, pages 3276–3279. Arthur C. Cope and Anil S. Mehta (1964): "Molecular asymmetry of olefins. II. The absolute configuration of ''trans''-cyclooctene". ''Journal of the American Chemical Association'', volume 86, issue 24, pages 5626–5630. 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.

Preparation

''trans''-Cyclooctene was first synthesized on a preparatory scale by Arthur C. Cope with a Hofmann elimination reaction of 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 () and with methyl iodide . (Similar procedures can give ' isomers of 1,4-cyclooctadiene and 1,5-cyclooctadiene).Edwin Vedejs, Karel A. J. Snoble, and Philip L. Fuchs (1973): "Phosphorus betaines derived from cycloheptene and cyclooctene oxides. Inversion of cyclooctenes". ''Journal of Organic Chemistry'', volume 38, issue 6, pages 1178–1183.

In addition, a photochemical method exists for the direct ''cis''–''trans'' isomerisation. Although this equilibrium strongly favours the more stable ''cis'' form, the reaction can be driven towards the ''trans'' form by trapping with silver ions.John S. Swenton (1969): "Photoisomerization of ''cis''-cyclooctene to ''trans''-cyclooctene". ''Journal of Organic Chemistry'', volume 34, issue 10, pages 3217–3218.

Reactions

Because of the higher internal strain on the double bond, the ''trans'' isomer is more reactive than the ''cis'' isomer and of typical unsaturated hydrocarbons. For instance, its double bond will rapidly add tetrazine and its derivatives.Ramajeyam Selvaraj, Joseph M Fox "''trans''-Cyclooctene — a stable, voracious dienophile for bioorthogonal labeling". ''Current Opinion in Chemical Biology'', volume 17, issue 5, pages 753-760 The compound also readily polymerizes with a ruthenium-based initiator.

References

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Cycloalkenes
Foul-smelling chemicals