Disiamylborane

Disiamylborane (bis(1,2-dimethylpropyl)borane) is an organoborane with the formula [((CH3)2CHCH(CH3))2BH]2 (abbreviation: Sia2BH). It is a colorless waxy solid that is used in organic synthesis for hydroboration–oxidation reactions. Like most dialkyl boron hydrides, it has a dimeric structure with bridging hydrides.

Reactions
Disiamylborane is prepared by hydroboration of trimethylethylene with diborane. The reaction stops at the secondary borane due to steric hindrance.

Disiamylborane is relatively selective for terminal alkynes and alkenes vs internal alkynes and alkenes. Like most hydroboration, the addition proceeds in an anti-Markovnikov manner. It can be used to convert terminal alkynes, into aldehydes.

The hydroboration process proceeds via an initial dissociation of the dimer.

Related reagents

 * 9-Borabicyclo[3.3.1]nonane (9-BBN).
 * Thexylborane ((1,1,2-trimethylpropyl)borane, ThxBH2), a primary borane obtained by hydroboration of tetramethylethylene.

Naming
The prefix disiamyl is an abbreviation for "di-sec-isoamyl", where sec-isoamyl ("secondary isoamyl") is an archaic name for the 1,2-dimethylpropyl group (amyl being a obsolescent synonym of pentyl).