Page 372 - Advanced Organic Chemistry Part B - Reactions & Synthesis

344 Migration of boron to terminal positions is observed under much milder conditions
in the presence of transition metal catalysts. For example, hydroboration of 2-methyl-3-
CHAPTER 4 hexene by pinacolborane in the presence of Rh PPh Cl leads to the terminal boronate
3 3
Electrophilic Additions ester.
to Carbon-Carbon
Multiple Bonds CH
3
pinacol- O CH
(CH ) CHCH CHCH CH 3 borane (CH 3 ) 2 CH(CH 2 ) 4 B 3
2
3 2
O CH
Rh(PPh ) Cl CH 3 3
3 3
Ref. 179

4.5.2. Reactions of Organoboranes

The organoboranes have proven to be very useful intermediates in organic
synthesis. In this section we discuss methods by which the boron atom can be replaced
by hydroxy, carbonyl, amino, or halogen groups. There are also important processes
that use alkylboranes in the formation of new carbon-carbon bonds. These reactions
are discussed in Section 9.1.
The most widely used reaction of organoboranes is the oxidation to alcohols, and
alkaline hydrogen peroxide is the reagent usually employed to effect the oxidation.
The mechanism, which is outlined below, involves a series of B to O migrations of
the alkyl groups. The R−O−B bonds are hydrolyzed in the alkaline aqueous solution,
generating the alcohol.

R R
R B + HOO – R B – O OH R B OR OH+ –
3
R
R O RO
–
R BOR + HOO – R B – O O H R B + OH
2
R RO

–
(RO) BR + HOO – (RO) 2 B – O O H (RO) B + OH
3
2
R
(RO) B + 3 H O 3 ROH + B(OH) 3
2
3
The stereochemical outcome is replacement of the C−B bond by a C−O bond with
retention of configuration. In combination with stereospecific syn hydroboration, this
allows the structure and stereochemistry of the alcohols to be predicted with confidence.
The preference for hydroboration at the least-substituted carbon of a double bond
results in the alcohol being formed with regiochemistry that is complementary to that
observed by direct hydration or oxymercuration, that is, anti-Markovnikov.
Several other oxidants can be used to effect the borane to alcohol conversion.
.
.
®
Oxone 2K SO KHSO K SO has been recommended for oxidations done on a
5
2
4
4
2
179
S. Pereira and M. Srebnik, J. Am. Chem. Soc., 118, 909 (1996); S. Pereira and M. Srebnik, Tetrahedron
Lett., 37, 3283 (1996).