Hydroboration

Hydroboration

Aalkenes react rapidly with diborane in, e.g., ether at room temperature to form trialkylborances. Terminal alkenes give primary alkylboranes which, on oxidation with alkaline hydrogen peroxide, produce primary alcohols (Brown et al., 1957). Also, the overall hydration of the alkene has been shown to occur by cis-addition.

The anti-Markovnikov direction of addition to form the primary trialkylborane may be explained by the fact that borane is an electron-deficient molecule and so behaves as an electrophile, tending to attack at the point of highest electron density. To explain the overall cis-hydration, however, is not so simple. A possible mechanism is that hydroboronation is stereospecifically cis-addition, and this would be explained by the reaction proceeding stepwise through an intermediate cyclic T.S.

The oxidation must therefore occur by conversion of the C-B bond into C-OH without inversion. A possibility is one which involves a 1,2-shift.

Since R migrates with its bonding pair, the overall hydration is cis.

The Mirkovnikov direction of hydration may be carried out by the oxymercuration-demercuration method.

If the trialkylborane is treated with propionic acid, the corresponding alkane is obtained by protolysis:

Thus the final product is formed by reduction of alkene by a non-catalytic method.

In the same way, alkynes undergo monohydroboronation and protolysis to give almost pure cis-alkene:

Addition of diborane to a non-terminal double bond is at the least hindered carbon atom, and the nature of the product depends on the amount of hindering at the double bond e.g.,

These s- and t-alkylboranes undergo isomerisation to primary alkylboranes when heated, e.g., 

Bis-3-methyl-2-butylborane (disiamylborane) is particularly useful for selective hydroboronation; this borane is often written as Six₂BH. Thus;