Chemistry Reference
In-Depth Information
R
1
R
2
R
1
R
2
OH
O
+
AlCl
3
LiBH
4
N
BH
4
-
N AlCl
2
Fig.
8
.22
Regioselective
hydrogenolysis of epoxides.
P
OH
OH
H
H
R
1
R
2
R
2
R
1
Fig.
8
.23
Stereochemical inversion of
secondary alcohols.
R
3
CO
2
H, DEAD, THF, rt
CH
2
NMe
3
+
A
-
A
-
=
HCrO
4
-
, ClCrO
3
-
or Cr
2
O
7
--
A
-
= Cr
2
O
7
--
primary allylic and benzylic alcohols
aldehydes
A
-
= ClCrO
3
-
, CF
3
CO
2
CrO
3
-
primary or secondary aliphatic,
allylic and benzylic alcohols
aldehydes or ketones
Fig.
8
.24
Oxidations with chromates.
is observed [85-87] but some esters may form via a
different sequence: the aldehyde and the starting
alcohol give a hemi-acetal, which is oxidised further
to the ester. Although all these supported metallates
can be regenerated, the spent metal is collected in
the acid or basic washings; the use of such heavy-
metal-based reagents is not so clean in terms of
environmental aspects.
Silver dichromate on polyethyleneimine [88] and
pyrazolium chromate, chlorochromate or perman-
ganate [89-91] immobilised on copolymers of
methylmethacrylate with EGDMA or DVB as cross-
linking agents are stable, mild and efficient oxidising
reagents for the oxidation of hydroxy compounds to
the corresponding carbonyl compounds (Fig. 8.25).
More interesting are the N-halogenated poly-
amides for the oxidation of alcohols to aldehydes or
ketones; they are obtained very simply from pow-
dered commercially available Nylon 6-6, Nylon 6 or
Nylon 3 after treatment with
t
-butyl hypochlorite
(Fig. 8.26).
Among the different polyamides examined, only
Nylon 6-6 is chlorinated at the higher level regard-
less of the chlorinating agent being used (
t
-BuOCl,
Cl
2
O, or HClO) [92,93]. Recycling appears to be
possible.
