Hydroboration. VI. A convenient synthesis of the alkane and cycloalkane boronic and borinic esters and acids

The addition of I-pentene or cyclopentene to a solution of one-half the molar quantity of diborane in tetrahydroftiran solution at 0 degrees results in the formation of 60 to 65% yields of di-n-pentylborane and dicyclopentylborane, readily converted to the corresponding borinic methyl esters by treatment with methanol. Equilibration of the initial reaction mixture with a deficiency of diborane (2 olefin/1 BH3) leads to the preferred formation of di-n-pentylborane in the case of I-pentene, but to the preferred formation of monocyclopentylborane (and tricyclopentylborane) in the case of the cyclic olefin, cyclopentene. Equilibration of the initial reaction mixtures at 25-50 degrees with excess diborane leads to the formation of mono-n-pentylborane and manocyclopentylborane in yields of 65 and 73170, respectively. These boranes are readily converted to the methyl esters by treatment with methanol, and the esters are readily converted to the correspondin g boronic acids by hydrolysis. Consequently, hydroboration of olefins provides a convenient synthetic route to the alkane and cycloalkane boronic and borinic acids and their derivatives.