Hydroboration of Dienes and Heteroatom-Containing Alkenes

Catecholborane (34) reacts with alkynes but requires reaction temperatures that are higher than those used for reaction of alkynes and alkylboranes, as noted earlier. Phenylethyne, for example, reacts with 34 to give excellent yields of vinyl borane, but the reaction must be done in refluxing THF. Alkenylborinates derived from catecholborane are stable and can be isolated by distillation or recrystallization, 

Internal alkynes react with horanes to form the cis-alkenylborane. This, of course, is due to the four-center transition state leading to the vinylborane product. The reaction is selective for the less hindered of the two 

An alternative route to vinylborinates adds the dimethyl sulfide complex of HBBr2 to 1-bromo-l-alkynes. Reaction of 1-bromo-l-hexyne (52) gave 53 in 87% yield, after treatment with an alcohol (2-propanol in this case).Subsequent reduction of the bromide with potassium triisopropoxyborohydride [KBH(0/-Pr)3] generated cis-vinylborane (54) in 89% yield. 

---