Chemistry Reference
In-Depth Information
Ar
CHO
Ar
CH
2
OH
HCl
N
BH
3
N
Fig.
8
.20
Borane reduction of carbonyls
into alcohols.
NaBH
4
CH
2
NMe
3
+
BH
4
-
Ar CH
2
OH
Ar CHO
T < 25
°
C , t < 5 h
CH
2
NMe
3
+
CNBH
3
-
Ar CH =CH
C
O
R
Ar CH
=
CH
CH
OH
R
Fig.
8
.21
Selective reduction of
a
-
b
-
unsaturated aldehydes.
(but no alane) derivatives have been used on poly-
mers in the form of neutral or ionic species. The
Lewis complex given by BH
3
with polymeric amines
such as cross-linked polyvinylpyridine is able to
reduce aliphatic and aromatic aldehydes and ketones
(Fig. 8.20): substituted benzaldehydes are reduced in
hot benzene [76].
The reactivity is increased significantly in acidic
conditions, so aliphatic and alicyclic aldehydes and
ketones are reduced. The polymeric hydride then
can be regenerated easily [77]. Such reductions are
performed more easily with ionic borane-derived
reagents BH
4
-
system [81]; this was not the case with Zn(BH
4
)
2
immobilised on cross-linked poly(4-vinylpyridine)
[82], presumably because the electrophilic assistance
provided by the Zn cation is buffered by the nitro-
gen atom of the pyridine ring. Anyway, this reagent
allows the reduction of only aldehydes, even in the
presence of ketones.
Interestingly, the primary but not the secondary
alcohols are obtained by regioselective hydro-
genolysis of epoxides with chloroaluminium tetrahy-
droborate on polyvinylpyridere [83] (Fig. 8.22); the
reagent is obtained by treating polyvinylpyridine first
with AlCl
3
and then by an exchange reaction with
LiBH
4
.
The stereochemical inversion of secondary alco-
hols is performed by a Mitsunobu-like reaction using
a polymer-bound triphenylphosphine (Fig. 8.23)
[84]; the ester that formed in the presence of a
benzoic acid and diethyl azodicarboxylate (DEAD) is
hydrolysed with a base.
and CNBH
3
-
when immobilised on
anion-exchange resins.
The ionic borohydrides were found to be interest-
ing for the reduction of several functional groups
and the selectivity in the reduction of carbonyls:
aldehydes are more reactive than ketones; unhin-
dered ketones and electrophilic carbonyls are
reduced more easily than the respective hindered
and aliphatic counterparts [78]; and a-b-unsaturated
aldehydes and ketones are reduced very selectively
to the b-enol, contrary to NaBH
4
that gives mixtures
of alcohols [79] (Fig. 8.21).
The reactivity/selectivity display can be tuned
further in the presence of nickel or lanthanide salts
[80] and the reactivity in the reduction of both alde-
hyde and ketones is increased with the BH
4
-
/CuSO
4
3.3 Aldehydes and ketones
The conventional oxidations of alcohols to
aldehydes and ketones can be performed with
permanganate, chromates and chlorochromates on
polyvinylpyridinium and anion-exchanged resins
(Fig. 8.24). No over-oxidation of aldehydes to acids
