_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The first three periodic groups
(c) ANIONIC COMPLEXES. Triphenylborane reacts with phenyllithium in
ether to give lithium tetraphenylborate. This dissolves in water to form the ions
Li+ and BPh;. The sodium salt is obtained by adding sodium chloride to the
product of the reaction between BF 3 • 0Et 2 and a slight excess of phenylmagnesium
bromide (after removal of magnesium as carbonate). Being soluble in
water and some polar organic solvents it is a useful precipitating agent for large
cations including K+, Rb+ and cs+, Tl+, quaternary ammonium ions, Ph 4 P+,
and Co(C 5 H 5 );. At one time potassium was often determined quantitatively by
gravimetric analysis ofKBPh 4 , although nowadays this laborious technique has
largely been superseded by instrumental methods. Ammonium tetraphenylborate
decomposes on strong heating, providing a route to triphenylborane.
Tetraalkylborates, e.g. LiBMe 4 , are prepared similarly from LiR and R 3 B,
conveniently in benzene, which can be used for recrystallization (see p. 38 for
structure). Trialkyl- and -aryl-boranes also react quantitatively with LiH or NaH
to give trialkyl(aryl)hydridoborates.
UH+Et,B
THF,65'C
30min
LiBEt,H
NaH+Bu~B
THF, 65'C
3h
NaBBu~H
The products are useful reducing agents in organic chemistry. Those which
contain bulky alkyl groups allow excellent steric control of the reductions, for
example of cyclic ketones. Approach of the reagent, which adds H- to carbon,
takes place from the less sterically congested side of the molecule.
11 _H
H
vi.
\o
H
~OH
v-+Me
H
cis- 2- methyl cyclohexonol
(>99%)
NOT
vi:
OH
H
trons-2- methylcyclohexonol
3.8.3 Boron-oxygen compounds
Alkyl or aryl boron chlorides RBCl 2 and R 2 BCl react readily with water to give
boronic (RB(OH) 2 ) or borinic (R,BOH) acids respectively. The alkylboronic acids
74