Reactions
reactions that concern us, acts much as though it were BH 3. Indeed, in tetrahydrofuran,
one of the solvents used for hydroboration, the reagent exists as the monomer, in the form
of an acid-base complex with the solvent. Hydroboration involves addition to the double
bond of BH 3 (or, in following stages, BH 2 R and BHR 2 ), with hydrogen becoming attached
to one doubly-bonded carbon, and boron to the other. The alkylborane can then undergo
oxidation, in which the boron is replaced by -OH. Thus, the two-stage reaction process of
hydroboration-oxidation permits, in effect, the addition to the carbon-carbon double bond
of the elements of H-OH. Reaction is carried out in an ether, commonly tetrahydrofuran
or ”diglyme” (diethylene glycol methyl ether, CH 3 OCH 2 CH 2 OCH 2 CH 2 OCH 3 ). Diborane is
commercially available in tetrahydrofuran solution. The alkylboranes are not isolated, but
are simply treated in situ with alkaline hydrogen peroxide.
Stereochemistry and Orientation
Hydroboration-oxidation, then, converts alkenes into alcohols. Addition is highly re-
giospecific; the preferred product here, however, is exactly opposite to the one formed by
oxymercuration-demercuration or by direct acid-catalyzed hydration. The hydroboration-
oxtdation process gives products corresponding toanti-Markovnikovaddition of water
to the carbon-carbon double bond. The reaction of 3,3-dimethyl-l -butene illustrates a
particular advantage of the method. Rearrangement does not occur in hydroboration ev-
idently because carbonium ions are not intermediates and hence the method can be used
without the complications that often accompany other addition reactions. The reaction of
1,2-dimethylcyclopentene illustrates the stereochemistry of the synthesis: hydroboration-
oxidation involves overallsyn addition.
55.3.3 Oxymercuration/Reduction
6
Figure 156 Oxymercuration/Reduction of 1-propene
Alkenes react with mercuric acetate in the presence of water to give hydroxymercurial
compounds which on reduction yield alcohols. The first stage, oxymercuration, involves
addition to the carbon-carbon double bond of -OH and -HgOAc. Then, in reduction, the
-HgOAc is replaced by -H. The reaction sequence amounts to hydration of the alkene, but is
much more widely applicable than direct hydration. The two-stage process of oxymercura-
tion/reduction is fast and convenient, takes place under mild conditions, and gives excellent
yields often over 90%. The alkene is added at room temperature to an aqueous solution
of mercuric acetate diluted with the solvent tetrahydrofuran. Reaction is generally com-
plete within minutes. The organomercurial compound is not isolated but is simply reduced
in situ by sodium borohydride, NaBH 4. (The mercury is recovered as a ball of elemental
mercury.) Oxymercuration/reduction is highly regiospecific, and gives alcohols correspond-
ing to Markovnikov addition of water to the carbon-carbon doublen bond. Oxymercuration
involves electrophilic addition to the carbon-carbon double bond, with the mercuric ion act-
ing as electrophile. The absence of rearrangement and the high degree of stereospecificity
6 Organic Chemistry, John McMurry