Cyclooctene, hydration

Since reactivity of alkenes increases with increasing alkyl substitution, hydration is best applied in the synthesis of tertiary alcohols. Of the isomeric alkenes, cis compounds are usually more reactive than the corresponding trans isomers, but strained cyclic isomeric olefins may exhibit opposite behavior. Thus, for example, frans-cyclooctene is hydrated 2500 times faster than cw-cyclooctene.6 Similar large reactivity differences were observed in the addition of alcohols to strained trans cycloalkenes compared with the cis isomers. frans-Cycloheptene, an extremely unstable compound, for instance, reacts with methanol 109 faster at —78°C than does the cis compound.7... 

Furthermore, cis isomers are more easily hydrated than their trans counterparts, although exceptions are known. Thus, the hydration of cis-1,2-dicyclopropylethylene is 2.5 times faster than its trans isomer.282 Strain introduced into a ring by the incorporation of a trans double bond reverses the trend, thereby making the hydration of franr-cyclooctene more rapid than the cis compound.283 Smaller ring alkenes are also rather sluggish towards hydration.284... 

The stereoselectivity of this hydration process has been studied on a number of alkenes. Acyclic alkenes and most cyclic alkenes give exclusively trans addition (equations 214 and 2 1 5).331,332 However, some strained cyclic alkenes, such as rrans-cyclooctene and trans-cyclononene, give cis adducts.332... 

In a 100-ml., three-necked, round-bottomed flask, 2 g. (7.7 mmoles) of rhodium(III) chloride 3-hydrate is dissolved in an oxygen-free mixture of 40 ml. of 2-propanol and 10 ml. of water. Cyclooctene (6 ml.) is added. The solution is stirred for about 15 minutes under nitrogen. The flask is then closed and allowed to stand at room temperature for 5 days. The resulting reddish-brown crystals are collected on a filter, washed with ethanol, dried under vacuum, and stored under nitrogen at —5°C. The yield is 2.0 g. (74%). Anal. Calcd. for RhC16H28Cl Rh, 28.72 C, 53.56 H, 7.81 Cl, 9.91. Found Rh, 28.55 C, 53.76 H, 7.89 Cl, 9.76. 

Materials anhydrous TBHP was made by extracting a 70% TBHP solution (in water) in chlorobenzene and drying over 3A molecular sieves, to result in a 5 M solution. Cyclohexene and cyclooctene were distilled and were passed through a column of basic alumina before use. Vanadyl(IV) sulphate hydrate, Salen (N,N -bis(salicylidene)-ethylenediimine), 1,2,-dichloro-ethane (DCE), dichloromethane (DCM), acetone, and acetonitrile were reagent grade and used as received. (-) Carveol was purchased as a 1.48 1 mixture of trans- and cis-carveol and used as such. Cumylhydroperoxide was purchased as a 80% solution in cumene and used as received. 

In contrast to the protonation of the disubstituted alkenes mentioned above, high ktranJkcis ratios of 9 x 108 and 3 x 103, respectively have been observed in the acid-catalyzed addition of methanol to the trans- and cis-isomers of cycloheptene (39a and 38a) and cyclooctene (37a and 36a) (114a). The rate constants reflect partially the release of strain in the transformation of the cyclic olefins to the appropriate cycloalkyl cations. Comparison of the relative activation energies for these addition reactions with the difference of strain release leads to the estimate that the response to strain effects is about 60%. In a more recent study of the acid-catalyzed hydration of cis- (36b) and trans- 1-methylcyclooctene (37c), it was concluded that two conformationally different 1-methyl-carbocationic intermediates are... 

General acid catalysis was observed in the hydration of both trans-cyclooctene and 2,3-dimethyl-2-butene, which helped to establish the Ase2 mechanism for hydration of alkenes. Furthermore, 1,1-dicyclopropyl-ethene was foimd to react much faster than either cis- or trans-l,2-dicyclo-propylethene, which indicates that substituent location (and not just the total electron-donating ability of the substituents) affects the rate constant for hydration reactions. It appears that the approaching proton is undergoing bond formation to one of the olefinic carbon atoms in the transition structure but not to the other, so the regiochemistry of the hydration reaction is determined by the approach that leads to the development of the more stable carbocation. ... 

Strained alkenes show enhanced reactivity toward acid-catalyzed hydration. trans-Cyclooctene is about 2500 times as reactive as the cis isomer. This reflects the higher ground state energy of the strained alkene. 

Closely related to this, other olefins were tested in epoxidation reactions. The heterogeneous epoxidation of cyclooctene with H2O2 in scC02 using a manganese porphyrinate catalyst in the presence of hexafluoroacetone hydrate as the co-catalyst at 40" C and 200 bar, leads to complete transformation into cyclooctene oxide. The co-catalyst forms perhydrates and thereby helps to stabilise the porphyrine catalyst and to solubilise H2O2 in SCCO2. 

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