Methylcyclopentene hydration

Hydroboration-oxidation converts 1-methylcyclopentene to the desired alcohol by anti-Markovnikov syn hydration of the double bond. The resulting alcohol is then converted to its p-toluenesulfonate ester and treated with acetate ion as in part (a) to give m-2-methyl-cyclopentyl acetate. 

Problem 9.22 What products are formed from hydration of 4-methylcyclopentene What can you say about the relative amounts of the products ... 

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... 

Because the oxidation step in the hydroboration-oxidation synthesis of alcohols takes place with retention of configuration, the hydroxyl group replaces the boron atom where it stands in the alkylboron compound. The net result of the two steps (hydroboration and oxidation) is the syn addition of —H and —OH. We can review the anti-Markovnikov and syn aspects of hydroboration-oxidation by considering the hydration of 1-methylcyclopentene, as shown in Fig. 8.3. 

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

Second, the reaction is regiospecific, and the regiospecificity is opposite to that obtained by acid-catalyzed hydration (or oxymercuration). If we add water to 1-methylcyclopentene in the presence of acid, we obtain 1-methylcyclopentanol, via the tertiary carbocation (Figure 11.59). 

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See also in sourсe #XX -- [ ]

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