Why is the borane–THF complex used for hydroboration?
The borane-THF complex (BTHF) is used for hydroboration for reasons of safety and convenience.
The active ingredient is borane,
Borane exists naturally as the dimer
Also, it ignites spontaneously in moist air at room temperature.
In a solution in THF, borane exists as a loose Lewis acid-base complex. This allows boron to have an octet and makes the reagent more stable.
The solution is commercially available in a 1 mol/L concentration in volumes from 25 to 800 mL.
It is much more convenient to work with the solution than with a gas. Even so, the solution must be stored at 2 to 8 °C, and it must have a stabilizer added.
Borane forms a more stable and more soluble Lewis acid-base complex with dimethyl sulfide:
It is available in concentrations of 2, 5, and 10 mol/L and in volumes from 25 mL to 18 L.
That should make it a more convenient reagent than the BTHF complex.
There is only one problem: It has the highly disagreeable smell of rotten cabbage!