10.4 Boron Containing Compounds
Boron containing compounds find wide applications in organic synthesis. Borane (BH3) is commercially available in the form of complexes generally with THF, Et3O and Me2S. It can also be prepared in situ by the reaction of NaBH4 with BF3 ∙OEt2 complex (Scheme 1):
Scheme 1
Organoboranes are synthesized by the addition reaction of borane to alkenes and alkynes.
10.4.1 Hydroboration of Alkenes
The reaction of borane with alkenes gives alkylboranes that readily proceed oxidation in the presence of alkaline hydrogen peroxide to yield alcohols (Scheme 2). T he conversion of C-B bond into a C-OH takes place with retention of stereochemistry.
Scheme 2
In case of sterically hindered alkenes such as trisubstituted ones, it is more difficult to add three alkenes to borane. This becomes the basis for the development of a variety of borane derivatives. For some examples, see:
Mechanism
The hydroboration reactions proceed by cis addition of hydrogen and boron to alkenes, probably via a four-centred cyclic transition state (Scheme 3).
Scheme 3