1.4- Hydroboration of 1,3-dienes

The rate of hydroboration coupled with relative reactivities of alkenes toward hydroboration with 9-BBN are used directly to predict hydroboration of un-symmetrical nonconjugated dienes. 1-Hexene is about 100 times more reactive than is ds-2-pentene and 116 times more reactive than is 2-methyl-2-bu-tene. Similarly, 2-methyl-1-pentene is 194 times more reactive toward 9-BBN than ds-2-pentene is. The data, thus, reveal that selective monohydroboration of terminal C=C of nonconjugated dienes can be achieved, as are illustrated in Chart 5.19 [1-3]. 

Monohydroboration achieved in other dienes is shown in Chart 5.20. 

The conjugation causes the deactivation toward the first hydroboration step. However, the formation of a homoallylic double bond in the hydroboration of 1,3-butadiene produces B-(3-buten-l-yl)-9-BBN, whose homoallylic double bond has reactivity comparable to that of 1-butene and second hydroboration, is much faster and produces 1,4-dibora product (Eq. 5.32) [3], 

On the other hand, in cases where the two olefinic centers of the diene possess structures that differ considerably in reactivity toward 9-BBN, clean mono-hydroboration [3] is achieved. 

the hydroboration of 2,5-dimethyl-2,4-hexadiene with 9-BBN proceeds rapidly to the monohydroboration product. The geminal methyl groups direct the boron atom to the allylic position in contrast to homoallylic position. 

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