Monohydroboration 1,3-hexadiene

Unsymmetrical, nonconjugated dienes are generally easier to monohydroborate because of intrinsic differences between the two double bonds. Both 9-BBN-H and disiamylborane favor attachment to a terminal double bond rather than an internal double bond (e.g. equation 32). On the other hand, 2-methyl-1, S-hexadiene reacts predominantly at the 1-position with 9-BBN-H and almost exclusively at the 6-position with disiamylborane. The products of dihydroboration of a,o>-dienes with 9-BBN-H can be redistributed with borane-dimethyl sulfide to give boracyclanes. In this way, some of the problems sometimes associated with direct hydroboration of dienes with borane (see Section 3.10.2.1) may be overcome. 

BMB is superior to diborane for dihydroboration of terminal dienes to give, after oxidation, a.w-diols.- Thus 1,5-hexadiene is converted into 1,6-hexanediol in 93% yield, whereas with diborane the yield is 69% and the remainder is a mixture of isomeric diols. With disiamylborane, Zweifel, Nagase, and Brown were able to eifect monohydroboration of cyclic dienes in reasonable yield. This finding was used to advantage by a Merck group for selective conversion of the diene (I) to the primary alcohol (2). 

The hydroboration of symmetrical nonconjugated dienes, such as 1,5-hexa-diene with 9-BBN, in a 1 1 molar ratio proceeds in an essentially statistical manner, giving approximately 25% residual diene, 50% monohydroboration product, and 25% dihydroboration product. On the other hand, conjugated dienes, for example, 1,3-butadiene, behaves differently and affords equal amounts of residual diene and 1,4-dihydroboration product. Conjugation markedly decreases the reactivity of diene toward hydroboration. Consequently, nonconjugated dienes like 1,4-hexadiene are selectively hydroborated with 9-BBN in the presence of 1,3-pentadiene (Scheme 5.6) [3]. 

Thus, 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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