C/s-Cyclooctene

Normally, a trans alkene is more stable than its cis isomer. tra/75-Cyclooctene, however, is less stable than c/s-cyclooctene by 38.5 kj/mol. Explain. 

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

The ionization potential, 8.69 eV, is lower than in the case of c s-cyclooctene (8.98 eV) or cyclohexene (9.12 eV), as expected. The highly strained anti-Bredt olefin, 11-bromo-e tfo-9-chloro-7-ethoxybicyclo[5.3.1]undec-l(l l)ene has been synthesized and its struc-... 

The stereospecific conversion of c/s-cyclooctene epoxide to alcohol 51, first reported by Cope,87 has been examined in greater detail recently.88-90 Of importance is... 

In an investigation of a modified Hofmann degradation, Wittig and Polster found that HBr-elimination from trimethylcyclooctylammonium bromide with phenyllithium gave an olefin mixture containing 80% of c/s-cyclooctene but that with potassium amide in liquid ammonia trans-cyclooctene predominated. 

Subsequent work revealed that the ylid mechanism was induced by metal alkyls and aryls but not by metal alkoxides and amides ". Stereochemical evidence was cited, metal alkyls giving more c/s-cyclooctene and metal amides and alkoxides more trans-cyclooctene for elimination from cyclo-octyltrimethylammonium ion (49). Elimination from cyclooctyldimcthyl-(bromomethyl)ammonium ion with methyl lithium gave the same predominance of the c(i-cyclooctene. 

EpoxidatUm. Bach and Knight recommend this reagent of Payne s for large-scale epoxidations, since it is inexpensive and safer to handle than peracids. An example cited is the epoxidation of c/s-cyclooctene (4.4 moles) (equation 1). 

The cw-cyclooctenyl radical is formed preferentially, and ultimately goes on via reduction by a second electron to give c/s-cyclooctene. 

Table 7.4 c/s-Cyclooctene oxidation performance using homogeneous and heterogeneous Mn dimer catalysts at 273 K for 3 h [43]. 

In 2006, CoUman and Brauman employed ArIO in investigations of kinetics of the [(TPFPP)MnQ]-catalyzed epoxidation of c/s-cyclooctene, styrene, and 1-decene [57]. Their results established that the iodosylarene and metal complex rapidly react to form an active oxidant, which then reversibly forms an adduct with the substrate and then produces the epoxide product. The overall reaction follows saturation kinetics and can be fitted with a MichaeUs—Menten model. The identity of the active oxidant, however, could not be unambiguously established. 

Under the conditions of the reaction, c/s-cyclooctene (246) does not react. Evidently, therefore, the biradical that would be formed from (246) prefers to cleave into its components rather than cyclize to (245). The selectivity due to the tendency of the initially formed biradical to collapse immediately to form (244), rather than isomerize to the precursor of (245), is enhanced in the product by extensive (88 %) dissociation of the latter into (246) + F2C=CCl2. 

The two-phase permanganate oxidation of olefins generally affords products of oxidative cleavage. Weber and Shepherd found that when benzyltriethylammonium chloride was used as catalyst and the temperature was maintained near 0°C, internal olefins were oxidized by basic permanganate in dichloromethane to the corresponding czs-glycols in moderate yields (Eq. 11.3) [6]. Cyclohexene, c/s-cyclooctene and trans-cyclododecene were dihydroxylated by this method in 15%, 50%, and 50% yields... 

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