Organic Chemistry

164 CHAPTER 4 Reactions of Alkenes

and borane is a Lewis acid. So the reagent actually used as the source of for the

first step of hydroboration–oxidation is a borane–THF complex.

To understand why the hydroboration–oxidation of propene forms 1-propanol, we

must look at the mechanism of the reaction. The boron atom of borane is electron de-

ficient, so borane is the electrophile that reacts with the nucleophilic alkene. As boron

accepts the electrons and forms a bond with one carbon, it donates a hydride ion

to the other carbon. In all the addition reactions that we have seen up to this point,

the electrophile adds to the alkene in the first step and the nucleophile adds to the pos-

itively charged intermediate in the second step. In contrast, the addition of the elec-

trophilic boron and the nucleophilic hydride ion to the alkene take place in one step.

Therefore, an intermediate is not formed.

The addition of borane to an alkene is an example of a concertedreaction. A

concerted reactionis a reaction in which all the bond-making and bond-breaking

processes occur in a single step. The addition of borane to an alkene is also an example

of a pericyclicreaction. (Pericyclicmeans “around the circle.”) A pericyclic reactionis

a concerted reaction that takes place as the result of a cyclic rearrangement of electrons.

The electrophilic boron adds to the carbon that is bonded to the greater number

of hydrogens. The electrophiles that we have looked at previously (e.g., ) also added

to the carbon bonded to the greater number of hydrogens, in order to form the most

stable carbocation intermediate. Given that an intermediate is not formed in this con-

certed reaction, how can we explain the regioselectivity of the reaction. Why does

boron add preferentially to the carbon bonded to the greater number of hydrogens?

If we examine the two possible transition states for the addition of borane, we see

that the C B bond has formed to a greater extent than has the C H bond. Conse-

quently, the carbon that does not become attached to boron has a partial positive

charge. The partial positive charge is on a secondary carbon if boron adds to the

carbon bonded to the greater number of hydrogens. The partial positive charge is on a

primary carbon if boron adds to the other carbon. So, even though a carbocation

intermediate is not formed, a carbocation-like transition state is formed. Thus, the ad-

dition of borane and the addition of an electrophile such as H+take place at the same

sp^2

sp^2

sp^2

¬ ¬

sp^2

sp^2

H+

sp^2

CH 2

H BH 2

CH 3 CH CH 3 CH CH 2

H BH 2

an alkylborane

sp^2

p sp^2

B−

HHH

2

O

O+

diborane

tetrahydrofuran

THF

HH

HH

H

H

BB

BH 3

3-D Molecule:

Diborane

Movie:

Borane–THF complex

BORANE AND DIBORANE

Borane exists primarily as a colorless gas called

diborane. Diborane is a dimer—a molecule

formed by joining two identical molecules. Because boron is

surrounded by only six electrons, it has a strong tendency to ac-

quire an additional electron pair. Two boron atoms, therefore,

share the two electrons in a hydrogen–boron bond in unusual

half-bonds. The hydrogen–boron bonds in diborane are shown

as dotted lines to indicate that the bond is made up of fewer

than the normal two electrons.

2 B H

H

H

borane

BB

H

H

H

H

H

H

diborane