Organic Chemistry

Addition reactions

55.3 Addition reactions

55.3.1 Hydroboration

Hydroboration is a very useful reaction in Alkenes, not as an end product so much as an
intermediate product for further reactions. The primary one we’ll discuss below is the
Hydroboration/Oxidation reaction which is actually an a hydroboration reaction followed
by a completely separate oxidation reaction.

Figure 152 Hydroboration mechanism

The addition of BH 3 is a concerted reaction in that several bonds are broken and formed
at the same time. Hydroboration happens in what’s called syn-addition^4 because the boron
and one of its hydrogens attach to the same side of the alkene at the same time. As you
can see from the transition state in the center of the image, this produces a sort of box
between the two alkene carbons and the boron and its hydrogen. In the final step, the
boron, along with its other two hydrogens, remains attached to one carbon and the other
hydrogen attaches to the adjacent carbon.

This description is fairly adequate, however, the reaction actually continues to happen and
the -BH 2 continue to react with other alkenes giving an R 2 BH and then again, until you
end up with a complex of the boron atom attached to 3 alkyl groups, or R 3 B.

This trialkyl-boron complex is then used in other reactions to produce various products.

Figure 153 B 2 H 6 complex Figure 154 BH 3 -THF complex

Borane, in reality, is not stable as BH 3. Boron, in this configuration has only 6 electrons
and wants 8, so in its natural state it actually creates the B 2 H 6 complex shown on the left.

Furthermore, instead of using B 2 H 6 itself, BH 3 is often used in a complex with tetrahy-
drofuran (THF) as shown in the image on the right.In either situation, the result of the
reactions are the same.

55.3.2 Hydroboration/Oxidation

5

Figure 155 Hydroboration/Oxidation reaction

With the reagent diborane, (BH 3 ) 2 , alkenes undergo hydroboration to yield alkylboranes,
R 3 B, which on oxidation give alcohols.The reaction procedure is simple and convenient,
the yields are exceedingly high, and the products are ones difficult to obtain from alkenes
in anyother way. Diborane is the dimer of the hypothetical BH3 (borane) and, in the

4 Chapter131.18on page 433
5 Organic Chemistry, John McMurry