Cycloalkenes 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... 

Reactions between benzenetellurinyl trifluoroacetate and cycloalkenes, terminal olefins, or internal olefins in acetonitrile with boron trifluoride-etherate as catalyst produced fi-acetamidoalkyl or -cycloalkyl phenyl tellurium oxides. These oxides were reduced with hydrazine hydrate in ethanol2. 

In the photoaddition of water (equation 30) or acetic acid (equation 31) to alkynes it seems likely that ionic addition occurs by protonation of the alkyne excited state, particularly in view of the observation that the hydration reaction is speeded up by acid and retarded by base. The sensitized addition of acetic acid to medium-ring cycloalkynes to give enol acetates (equation 32) is strongly reminiscent of the analogous addition to cycloalkenes, which has been shown to go by way of protonation of the highly strained /ra/is-cycloalkene. 

General Preparation. The monomers were synthesized by addition of methylene to the double bond of the corresponding cycloalkenes or methylenecycloalkenes—Simmons and Smith reaction (14)—or by addition of a dihalocarbene followed by the reduction of the dihalocyclopro-pane group by a Na/hydrated methanol system (Equation 2) (15, 16) ... 

Experiments with cycloalkenes have shown that hydration of a double bond by addition of borane involves cw-addition to the double bond thus 85% of fraw.y-2-methylcyclopentanol is formed from 1-methylcyclopentene.77... 

The intermediacy of strained cycloalkenes in such reactions is further supported by the reaction of limonene. As shown in Figure 12.35, photohydration of limonene (16) produces an 89% yield of the alcohol 17 (plus a small amoimt of 18) as a result of hydration of the endocyclic double bond only. In contrast, acid-catalyzed hydration of 16 produces only the diol 19, in which both double bonds have been hydrated. ° ... 

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