Ethyl cyclopentylacetate

A dry 5(X)-mI flask equipped with a thermometer, pressure-equalizing dropping funnel, and magnetic stirrer is flushed with nitrogen and then maintained under a static pressure of the gas. The flask is charged with 50 ml of tetrahydrofuran and 13.3 ml (0.15 mole) of cyclopentene, and then is cooled in an ice bath. Conversion to tricyclo-pentylborane is achieved by dropwise addition of 25 ml of a 1 M solution of diborane (0.15 mole of hydride see Chapter 4, Section 1 for preparation) in tetrahydrofuran. The solution is stirred for 1 hour at 25° and again cooled in an ice bath, and 25 ml of dry t-butyl alcohol is added, followed by 5.5 ml (0.05 mole) of ethyl bromoacetate. Potassium t-butoxide in /-butyl alcohol (50 ml of a 1 M solution) is added over a period of 10 minutes. There is an immediate precipitation of potassium bromide. The reaction mixture is filtered from the potassium bromide and distilled. Ethyl cyclopentylacetate, bp 101730 mm, 1.4398, is obtained in about 75% yield. Similarly, the reaction can be applied to a variety of olefins including 2-butene, cyclohexene, and norbornene. 

The rates of hydrolysis in 80% aqueous ethanol at 25°C were 60 and 199 times faster than that of ethyl cyclohexylacetate and cyclopentylacetate. These rate enhancements were considered to indicate the occurrence of participation by the neighbouring y-keto group. Furthermore, unusually small enthalpies of activation were found for these esters, as shown in Table 1. 

Perchyonok also demonstrated that dimenthylphenyltin hydride 19 reacts with bromide 23 in toluene at -78° and in the presence of (5,5)-(+)-N,N -bis(3,5-di-terZ-butylsalicylidene)-l,2-cyclohexanediaminomanganese(III) chloride 24 to afford ethyl (.S )-2-plicnyl-2-cyclopentylacetate 25 with 96% ee (Scheme 5). To the best of our knowledge, this result represents the highest-ever reported enantioselectivity observed during any organic free-radical transformation. More extensive enantioselectivity data can be found in our recent publication. " ... 

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