Trans-Cyclooctene, chiral forms

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

In these representations, the configuration about each double bond is cis. Is it possible to have a trans configuration in these and larger cycloalkenes To date, trans-cyclooctene is the smallest trans cycloalkene that has been prepared in pure form and is stable at room temperature. Yet, even in this trans cycloalkene, there is considerable angle strain the double bond s 2p orbitals make an angle of 44° to each other. Cis-cyclooctene is more stable than its trans isomer by 38 kj (9.1 kcal)/mol. Note that the trans isomer is chiral even though it has no chiral center. 

Alkenes or polyenes with isolated or coupled double bonds that are devoid of chiral atoms are not optically active. Such activity occurs in cyclic alkenes where double bonds occur in the trans form, such as in trans-cyclooctene (Figure 2.18). Another group of alkenes that includes representatives having optical activity is that of cumulenes. The name refers to cumulation of double bonds in such molecules. The best-characterized group of these compounds is that of allenes in which two double bonds occur next to each other [54]. Compounds of this type have a so-called chirality axis determined by cumulated double bonds. Besides allenes, higher optically active cumulenes are also known. An example of optically active cyclic allene is provided by 1,2-cyclononadiene, which was synthesized in 1972 [55]. 

Strained trans-cycloalkene derivatives such as t-cyclooctene are chiral molecules which can be isolated in pure enantiomeric form. Owing to its high reactivity, resolved t-24 should not normally be isolable. However, an interesting example of a kinetic resolution of t-24 was reported recently [63]. 

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