Illustrated Glossary of Organic Chemistry

Disiamylborane: IUPAC name bis(1,2-dimethylpropyl)borane. A sterically-hindered dialkylborane useful for regioselective and chemoselective hydroboration reactions.

Disiamylborane is prepared by hydroboration of 2-methyl-2-butene with one-half equivalent of borane.

That disiamylborane's steric hindrance allows it to be chemoselective for a terminal alkene in the presence of internal alkenes is illustrated in this hydroboration-oxidation reaction.