E-Cyclononene

E -cyclooctene is subject to thermal racemization. The molecular motion allows the double bond to slip through the ring, giving the enantiomer. The larger and more flexible the ring, the easier the process. The rates of racemization have been measured for E-cyclooctene, Zf-cyclononene, and Zi-cyclodecene. For E-cyclooctene the half-life is Ih at 183.9° C. The activation energy is 35.6 kcal/mol. E-cyclononene, racemizes much more rapidly. The half-life is 4 min at 0° C, with an activation energy of about 20 kcal/mol. F-cyclodecene racemizes immediately on release from the chiral platinum complex used for its preparation. ... 

E-Cyclooctene is also significantly straine4 but less so than -cycloheptene. As the ring size is increased, the amount of strain decreases. The. E-isomers of both cyclononene and cyclodecene are less stable than the corresponding Z-isomers, but for cycloundecene and cyclododecene, the E-isomers are the more stable. Table 3.10 gives data concerning the relative stability of the C7 through C12 cycloalkenes. 

In many instances, however, solvolysis of a halocyclopropane is deliberately accomplished in order to install an essential vinyl halide or hindered olefin. In 2000, Murphy and coworkers performed a silver ion-mediated ring expansion of gem-dibromocyclopropane 18 in wet acetone to afford the allylic alcohol 19 in 82% yield (Scheme 4.5).16 Under these conditions the desired cyclononene product was obtained as an inseparable mixture of E- and Z-isomers (7 93). Interestingly, two sets of peaks observed in the1H NMR spectrum indicated that the Z-isomer existed as two separate conformers at room temperature. This intermediate was subsequently used in Murphy s approach to the radical-based preparation of tricyclic indoles. 

In contrast to the normal Cope rearrangement, the irreversible oxy-Cope rearrangement allows cyclononene derivatives to be synthesized from 1.2-divinylcyclopentanes, e.g., formation of ( >5922 and 7923. 

Gyanogen chloride in dioxane added in a slow stream with stirring and ice-saltcooling at 3-8° to a soln. of l-(N-pyrrolidyl)cyclononene and a little more than 1 mole triethylamine in the same solvent, kept at 3-6° for an additional 2 hrs. and at room temp, overnight 2-cyanocyclononanone. Y 77%, F. cyclic -cyanoketones s. M. E. Kuehne, Am. Soc. 81, 5400 (1959). 

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