Experiment*17*–*Hydroboration-Oxidation*of*Alkenes* * pg.*1*
17."Hydroboration-Oxidation)of)Alkenes!
and$Introduction$to$IR$Spectroscopy"
A.#Introduction#
1.#Hydroboration-Oxidation#of#Alkenes#
Alkenes can be oxidized to alcohols using a two-step method of hydroboration followed by
oxidation. The first step of this process, the hydroboration, utilizes borane (BH3), which is
available commercially as a borane-tetrahydrofuran complex (BH3!THF). In this complex, THF
acts as a Lewis base, stabilizing the electron deficient borane species. In the absence of THF,
borane exists as diborane, B2H6, which is a toxic and colorless gas.
In the hydroboration-oxidation process, three moles of alkene can be converted to three moles
of alcohol using only one mole of BH3. The reaction follows an anti-Markovnikov pathway where
a hydrogen is added to the more substituted carbon while the hydroxyl group is added to the
less substituted carbon. This regioselectivity is one of the major highlights of the hydroboration-
oxidation reaction.
The hydroboration-oxidation mechanism is shown in figure 1. The first step of the sequence,
hydroboration, involves addition of borane across the double bond. In this addition H and BH2
are added to the alkene carbons. The hydrogen goes to the more substituted carbon while the
BH2 goes to the less substituted carbon. The BH3 reagent is capable of reacting with three
equivalents of alkene to form a trialkylborane species; this is possible because BH3 has three
hydrogens that can be added over the course of three hydroboration steps. Next, NaOH and
H2O2 are added to oxidize the trialkylborane to three molecules of alcohol. The active oxidizing
agent, HOO－, is formed upon mixing sodium hydroxide and hydrogen peroxide. The
nucleophilic HOO－ reacts with the electron deficient boron to form a negatively charged boron
species. An alkyl shift from the boron to the oxygen with simultaneous loss of HO－ results in the
formation of a borate ester R3B-OR. Two more rounds of oxidation, results in the trialkyl borate
ester B(OR)3. Reaction of the trialkyl borate ester with NaOH and H2O hydrolyzes the three B-O
bonds, releasing three molecules of alcohol product. The byproduct is boric acid B(OH)3, which
can further react with NaOH to provide sodium borate Na3BO3.
!
H B
H
H
O
BH3•THF
B
HH
H
B
H
H H
B2H6 (borane dimer)
R
1. BH3•THF
OH
R
33 2. NaOH, H2O2
+ B(OH)3
