potassium permanganate ...

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1
POTASSIUM PERMANGANATE
Potassium Permanganate
1
−
4
duces an effective heterogeneous oxidant, has further expanded
its usefulness.
The general features of the reactions of permanganate dissolved
in aqueous solutions, or in organic solvents with the aid of a phase-
transfer agent, and as a heterogeneous oxidant will be briefly de-
scribed, followed by specific examples.
KMnO
4
[7722-64-7]
KMnO
4
(MW 158.04)
InChI = 1/K.Mn.4O/q+1;;;;;-1/rK.MnO4/c;2-1(3,4)5/q+1;-1
InChIKey = VZJVWSHVAAUDKD-QPPHZJHPAS
Aqueous Permanganate Oxidations.
Potassium permanga-
nate is a general, but relatively nonselective, oxidant when used
in aqueous solutions. When an organic compound contains only
one site at which oxidation can readily occur, this reagent is a
highly efficient and effective oxidant. For example, oleic acid is
converted into dihydroxystearic acid in quantitative yield when
oxidized in a dilute aqueous solution of KMnO
4
at 0–10
◦
C.
5
If the aqueous solution is made acidic by addition of mineral
acid, the rate of reaction increases, most probably because of for-
mation of permanganic acid
1
which is known to be a very strong
oxidant.
6
The rate of the reaction is also accelerated by addition
of sodium or potassium hydroxide. It has been proposed that this
acceleration may be due to ionization of the organic reductant; for
example, conversion of an alcohol into an alkoxide ion.
1
How-
ever, similar observations for the oxidation of compounds such as
sulfides, which lack acidic hydrogens, suggests that other factors
may be involved.
7
Under acidic conditions, permanganate is reduced to soluble
manganese(II) or -(III) salts, thus allowing for a relatively easy
workup. However, under basic conditions the reduction product
is a gelatinous solid, consisting primarily of manganese dioxide,
that is difficult to separate from the product. As a consequence, for
laboratory scale preparations the reaction product is not isolated
until after the MnO
2
has been reduced by addition of HCl and
sodium bisulfite. For large scale (industrial) processes, MnO
2
is
removed either by filtration or by centrifugation.
(oxidant; conversion of arenes into carboxylic acids,
10
,
11
α
-ketones,
12
−
15
or
α
-alcohols;
14
degradation of aromatic rings;
3
preparation of diols,
17
,
18
ketols,
5
,
19
,
20
,
22
and
α
-diketones
22
−
24
from nonterminal alkenes; preparation of carboxylic acids,
27
aldehydes
26
and 1,2-diols
28
from terminal alkenes; oxidation of
alkynes to
α
-diones;
29
,
30
oxidation of enones to 1,4-diones;
31
conversion of 1,5-dienes into substituted tetrahydrofurans
32
,
33
or
lactones;
34
conversion of primary and secondary alcohols into
carboxylic acids
1
,
2
,
37
and ketones,
1
−
4
,
9
,
35
respectively; oxida-
tion of allylic alcohols to
α
,
β
-unsaturated ketones
35
and other
unsaturated alcohols and
α
,
ω
-diols to lactones;
36
,
37
oxidation of
aliphatic thiols to disulfides and aromatic thiols to sulfonic acids;
4
oxidation of sulfides and sulfoxides to sulfones,
4
,
38
−
40
sulfinic
acids to sulfonic acids,
43
sulfites to sulfates,
44
and thiones to
ketones;
45
preparation of tertiary nitroalkanes from the corres-
ponding amines;
47
oxidation of tertiary amines to amides or
lactams;
48
−
50
allylic oxidations when used in conjunction with
t
-butyl hydroperoxide;
51
preparation of iodoaromatic compounds
when used with I
2
and sulfuric acid;
52
oxidation of nucleic acids
to the corresponding diols and ketols;
53
oxidation of guaiol and
related compounds to rearranged ketols;
54
oxidation of poly(vinyl
alcohol) to poly(vinyl ketone);
56
oxidation of nitroalkanes to
aldehydes or ketones; oxidation of imines to nitrones)
Alternate Name:
potassium manganate(VII).
Physical Data: d
2.70 g cm
−
3
; decomposition 237
◦
C.
Solubility:
water (at 20
◦
C) 63.8 g L
−
1
; sol acetone, methanol.
Form Supplied in:
purple solid; commercially available.
Handling, Storage, and Precautions:
stable at or below rt.
Because it is a strong oxidant it should be stored in glass, steel,
or polyethylene vessels. Sulfuric acid should never be added to
permanganate or vice versa. Permanganate acid, an explosive
compound, is formed under highly acidic conditions.
Phase-transfer Assisted Permanganate Oxidations.
2
KMnO
4
may be dissolved in nonpolar solvents such as benzene
or CH
2
Cl
2
by complexing the potassium ion with a crown ether
or by replacing it with a quaternary ammonium or phosphonium
ion. Although most reactions observed are similar to those found
in aqueous solutions, the ability to dissolve permanganate in
nonpolar solvents has greatly increased the range of compounds
that can be oxidized.
The first example of a phase-transfer assisted permanganate
oxidation involved the complexing of the potassium ion by a crown
ether in benzene;
8
however, it was later found that the use of
quaternary ammonium or phosphonium salts was less expensive
and just as efficient.
2
Phase transfer into a nonpolar solvent can occur either from
an aqueous solution or from solid KMnO
4
. Evaluation of vari-
ous phase-transfer agents for these purposes has indicated that
benzyltributylammonium chloride is highly efficient for trans-
fer from aqueous solutions while alkyltriphenylphosphonium
halides, tetrabutylammonium halides, and benzyltriethylammo-
nium halides are all effective for the transfer from solid KMnO
4
.
2
Adogen 464, an inexpensive quaternary ammonium chloride com-
mercially available, is usually satisfactory for both purposes.
Quaternary ammonium and phosphonium permanganates can
also be used as stoichiometric oxidants. For descriptions of their
Original Commentary
Donald G. Lee
University of Regina, Regina, Saskatchewan, Canada
Introduction.
Permanganate is an inexpensive oxidant that
has been widely used in organic syntheses. Its most common salt,
KMnO
4
, is soluble in water and as a consequence oxidations have
traditionally been carried out in aqueous solutions or in mixtures
of water and miscible organic solvents such as acetone, acetic acid,
acetonitrile, benzonitrile, tributyl phosphate, or pyridine. The dis-
covery that KMnO
4
can, with the aid of phase-transfer agents,
be readily dissolved in nonpolar solvents such as CH
2
Cl
2
, and
the recent observation that is adsorption onto a solid support pro-
Avoid Skin Contact with All Reagents
2
POTASSIUM PERMANGANATE
O
properties, refer to the separate articles on
Methyltriphenyl-
phosphonium Permanganate
and
Benzyltriethylammonium Per-
manganate
.
KMnO
4
, alumina
(7)
ClCH
2
CH
2
Cl, ∆
86%
Heterogeneous Permanganate Oxidations.
The use of
permanganate, activated by adsorption on a solid support, as a
heterogenous oxidant has further increased the scope of these re-
actions. CH
2
Cl
2
or 1,2-dichloroethane (if a high reflux tempera-
ture is required) are the preferred solvents and
Alumina
, silica, or
hydrated
Copper(II) Sulfate
are the most commonly used solid
supports. The selectivity of the oxidant is dramatically altered by
use of a solid support. For example, although carbon–carbon dou-
ble bonds are very easily cleaved in homogeneous permanganate
solutions, secondary allylic alcohols can be cleanly oxidized to
the corresponding
α
,
β
-unsaturated ketones without disruption of
the double bond under heterogeneous conditions.
9
In addition to increased selectivity, the use of permanganate
under heterogeneous conditions allows for easy product isolation.
It is necessary only to remove spent oxidant by filtration followed
by flash evaporation or distillation of the solvent. Products iso-
lated in this way are often sufficiently pure to permit direct use in
subsequent synthetic procedures.
OH
KMnO
4
, alumina
(8)
ClCH
2
CH
2
Cl, ∆
79%
Oxidation of Aromatic Rings.
Permanganate will oxida-
tively degrade aromatic rings under both acidic and basic
conditions.
3
The effect of acid and base on the reaction has been
demonstrated by the oxidation of 2-phenylpyridine; under basic
conditions the product is benzoic acid (presumably because the
oxidant attacks the site of greatest electron density) (eq 9), while
under acidic conditions (where the nitrogen would be protonated)
the product is picolinic acid (eq 10).
3
KMnO
4
OH
–
N
(9)
CO
2
H
Benzylic Oxidations.
Permanganate oxidizes side chains of
aromatic compounds at the benzylic position.
3
In aqueous solu-
tion, carboxylic acids are usually obtained (eqs 1 and 2).
10
,
11
H
KMnO
4
N
N
+
H
+
+
KMnO
4
N
CO
2
H
(1)
Me
CO
2
H
(10)
H
2
O, py
71%
MeO
MeO
Polycyclic aromatic compounds are also oxidatively degraded
to a single-ring polycarboxylic acid (eq 11).
16
t
-Bu
t
-Bu
CO
2
H
KMnO
4
(2)
CO
2
H
KMnO
4
OH
–
The oxidation of alkylbenzenes proceeds through the corre-
sponding
α
-ketones, which can occasionally be isolated (eqs 3
and 4).
12
,
13
CO
2
H
CO
2
H
HO
2
C
+
(11)
CO
2
H
HO
2
C
CO
2
H
(3)
CO
2
H
O
Oxidation of Nonterminal Alkenes.
Nonterminal alkenes
can be converted into 1,2-diols, ketols, or diketones by choice
of appropriate conditions. The reaction, which proceeds by
syn
addition of permanganate to the double bond as indicated, gives
the corresponding
cis
-diol under aqueous alkaline conditions
(eq 12).
17
N
N
(4)
80%
O
Under heterogeneous conditions where alumina (acid, Brock-
man, activity 1)
14
or copper sulfate pentahydrate
15
is used as the
solid support,
α
-ketones and alcohols are obtained with little or
no carbon–carbon cleavage (eqs 5–8).
OH
–
O
O
O
–
+
MnO
4
–
Mn
O
H
2
O
45%
KMnO
4
, alumina
(5)
ClCH
2
CH
2
Cl, ∆
69%
OH
OH
(12)
O
KMnO
4
, CuSO
4
•5H
2
O
Syn
addition can also be achieved in nonaqueous solvents with
the aid of a phase-transfer agent (PTA). Subsequent treatment with
aqueous base gives 1,2-diols in good yields
2
(6)
CH
2
Cl
2
88%
O
(eq 13).
18
Equally
A list of General Abbreviations appears on the front Endpapers
3
POTASSIUM PERMANGANATE
good results were reported when the reaction was carried out in
aqueous
t
-butyl alcohol.
18
of the Lemieux–von Rudloff reagent (aqueous potassium perio-
date containing catalytic amounts of permanganate).
3
,
25
Under
heterogeneous conditions, either aldehydes or carboxylic acids
are obtained, depending on the conditions used (eqs 20 and 21).
26
OH
1. KMnO
4
, PTA, CH
2
Cl
2
(13)
2. 10% NaOH
OH
O
86%
KMnO
4
, alumina, H
2
O
H
(20)
H
Under neutral conditions the product obtained from the oxi-
dation of alkenes is the corresponding ketol.
5
Good yields are
obtained when aqueous acetone containing a small amount of
acetic acid (2–5%) is used as the solvent. The function of acetic
acid is to neutralize hydroxide ions produced during the reduc-
tion of permanganate. The oxidations of 5-decene and methyl
2-methylcrotonate provide typical examples (eqs 14 and 15).
19
,
20
CH
2
Cl
2
75%
O
KMnO
4
, silica
CO
2
H
(21)
HO
2
C
C
6
H
6
74%
Oxidation of Terminal Alkenes.
Although oxidation of ter-
minal alkenes by permanganate usually results in cleavage of the
carbon–carbon double bond to give either a carboxylic acid
27
or
an aldehyde,
26
1,2-diols can be obtained through use of a phase-
transfer assisted reaction (eqs 22–24).
2
,
28
KMnO
4
MeCOMe, H
2
O, MeCO
2
H
74%
OH
(14)
KMnO
4
, PTA
O
CO
2
H
( )
17
(22)
( )
17
CH
2
Cl
2
, H
2
O
75%
O
O
KMnO
4
MeCOMe, H
2
O, H
+
80%
CO
2
Me
(15)
OMe
KMnO
4
, alumina, H
2
O
CHO
OH
(23)
( )
12
( )
12
CH
2
Cl
2
55%
Heterogeneous oxidations of alkenes with a small amount of
t
-
butyl alcohol and water present to provide an ‘omega phase’
21
results in the formation of
α
-ketols in modest to good yields
(eqs 16 and 17).
22
OH
1. KMnO
4
, CH
2
Cl
2
, PTA
(24)
( )
5
OH
( )
5
2. 3% NaOH
80%
O
OH
KMnO
4
, CuSO
4
•5H
2
O
(16)
Oxidation of Alkynes.
Oxidation of nonterminal alkynes
results in the formation of
α
-diones. Good yields are obtained
when aqueous acetone containing NaHCO
3
and MgSO
4
,
29
or
CH
2
Cl
2
containing about 5% acetic acid,
30
is used as the solvent
(eqs 25–27). A phase-transfer agent to assist in dissolving KMnO
4
must be used when CH
2
Cl
2
is the solvent. Terminal alkynes are
oxidatively cleaved, yielding carboxylic acids containing one
carbon less than the parent alkyne.
CH
2
Cl
2
,
t
-BuOH, H
2
O
55%
O
O
O
KMnO
4
, CuSO
4
•5H
2
O
CH
2
Cl
2
,
t
-BuOH, H
2
O
79%
OH
(17)
Under anhydrous conditions, 1,2-diones are formed in good
yields when alkenes are oxidized by permanganate. Appropriate
conditions can be achieved by using acetic anhydride solutions
(eq 18)
23
or by dissolving permanganate in CH
2
Cl
2
with the aid
of a phase-transfer agent (eq 19).
24
KMnO
4
, PTA
CO
2
H
(25)
( )
3
( )
3
CH
2
Cl
2
, AcOH
61%
O
KMnO
4
, PTA
(26)
O
KMnO
4
CH
2
Cl
2
, AcOH
80%
O
(18)
Ac
2
O
66%
O
O
KMnO
4
, acetone, H
2
O
O
(27)
6
KMnO
4
, PTA
6
6
NaHCO
3
, MgSO
4
81%
(19)
O
6
CH
2
Cl
2
, MeCO
2
H, H
2
O
69%
O
Similar yields are obtained under heterogeneous conditions,
where workup procedures are much easier.
22
The carbon–carbon double bonds of alkenes can also be
oxidatively cleaved to give carboxylic acids in good yield by use
Oxidation of Enones to 1,4-Diones.
Enones react with
nitroalkanes (Michael addition) to form
γ
-nitro ketones that can
be oxidized in good yield to 1,4-diones under heterogeneous con-
ditions (eq 28).
31
Avoid Skin Contact with All Reagents
4
POTASSIUM PERMANGANATE
O
O
OH
O
F
–
KMnO
4
, silica
KMnO
4
, CuSO
4
•5H
2
O
+
MeCH
2
NO
2
(34)
( )
4
( )
4
MeCN
C
6
H
6
80%
CH
2
Cl
2
89%
NO
2
O
(28)
KMnO
4
, CuSO
4
•5H
2
O
O
CH
2
Cl
2
HO
no product
(35)
Oxidation of 1,5-Dienes.
The oxidation of 1,5-dienes results
in the formation of 2,5-bis(hydroxymethyl)tetrahydrofurans with
the indicated stereochemistry (eq 29).
32
When R
6
in (eq 29) is
chiral, a nonracemic product is obtained.
33
Use of heterogeneous
conditions results in the formation of lactones (eq 30).
34
KMnO
4
, CuSO
4
•5H
2
O
O
(36)
CH
2
Cl
2
, H
2
O
56%
O
OH
Good yields of carboxylic acids are obtained from primary
alcohols under heterogeneous conditions (KMnO
4
/CuSO
4
5H
2
O)
only when a base such as KOH or Cu(OH)
2
CuCO
3
is intermixed
with the solid support.
37
Under these conditions the reagent has
also been reported to be selective for primary alcohols.
37
The oxidation of
α
,
ω
-diols under heterogenous conditions
results in the formation of lactones. A good example is found
in the preparation of 3-hydroxy-
p
-menthan-10-oic acid lactone
(eq 37).
37
KMnO
4
, CO
2
MeCOMe, H
2
O
R
1
R
6
R
3
R
4
(29)
R
1
R
2
R
6
R
5
O
R
2
R
5
–20 °C
60–70%
R
3
R
4
OH
HO
H
KMnO
4
, CuSO
4
•5H
2
O
CH
2
Cl
2
CH
2
OAc
H
OAc
OH
OAc
O
+
(30)
O
O
O
O
KMnO
4
, CuSO
4
•5H
2
O
(37)
62%
8%
HO
HO
O
CH
2
Cl
2
83%
O
Oxidation of Alcohols and Diols.
Primary and secondary
alcohols are converted to carboxylic acids and ketones, respec-
tively, when oxidized by aqueous permanganate under either
acidic or basic conditions (eq 31).
1
Similar results are obtained
with phase-transfer assisted oxidations in organic solvents such
as CH
2
Cl
2
(eq 32).
2
Oxidation of Organic Sulfur Compounds.
Aromatic thiols
are oxidized by permanganate to the corresponding sulfonic acids
while aliphatic thiols usually give disulfides, which are resistant
to further oxidation.
4
Sulfides and sulfoxides are easily oxidized
in CH
2
Cl
2
to the corresponding sulfones under both homoge-
neous
38
,
39
and heterogeneous conditions (eqs 38–42).
40
KMnO
4
CO
2
H
(31)
( )
4
( )
4
OH
H
2
O, H
2
SO
4
66%
SO
3
–
SH
KMnO
4
OH
–
, H
2
O
91%
N
N
(38)
N
O
N
O
KMnO
4
, Adogen
(32)
Me
Me
CH
2
Cl
2
, AcOH
92%
OH
O
KMnO
4
, PTA
(39)
S
S
O
CH
2
Cl
2
, H
2
O
90%
O
Heterogeneous oxidations are very effective with secondary
alcohols (eq 33)
35
and provide the added advantage that al-
lylic secondary alcohols can be converted to the corresponding
α
,
β
-unsaturated ketones without disruption of the double bond
(eq 34).
9
Unsaturated secondary alcohols in which the double
bond is not adjacent to the carbon bearing the hydroxy group are
resistant to oxidation (eq 35) unless an ‘omega phase’
21
is created
by adding a small amount of water (50
µ
L per g KMnO
4
). The
products are lactones under these conditions (eq 36).
36
O
O
S
S
KMnO
4
, PTA
(40)
O
CH
2
Cl
2
, H
2
O
91%
O
KMnO
4
, PTA
(41)
Bu
2
SO
Bu
2
SO
2
CH
2
Cl
2
, H
2
O
86%
OH
O
O
O
S
KMnO
4
, CuSO
4
•5H
2
O
KMnO
4
, CuSO
4
•5H
2
O
(33)
S
(42)
( )
7
( )
7
CH
2
Cl
2
100%
CH
2
Cl
2
96%
A list of General Abbreviations appears on the front Endpapers
5
POTASSIUM PERMANGANATE
NH
2
O
KMnO
4
Permanganate oxidizes sulfoxides more readily than sulfides,
as indicated by the products obtained from the oxidation of com-
pounds containing both sulfide and sulfoxide functional groups
(eqs 43 and 44).
41
,
42
(50)
t
-BuOH, H
2
O
70%
Amides (or lactams, if the amine is cyclic) are obtained from
the oxidation of tertiary amines (eqs 51 and 52).
48
−
50
O
O
O
KMnO
4
(43)
MeS
S
MeS
S
Me
Me
Et
Et
MeCOMe, H
2
O
97%
KMnO
4
N
N
O
(51)
MeCOMe, AcOH
70%
S
S
QMnO
4
(44)
S
S
F
F
CH
2
Cl
2
O
OO
Q = benzyltriethylammonium ion
O
KMnO
4
F
F
N
N
MeCOMe
70%
(52)
N
N
The greater ease of oxidation of sulfoxides is also responsible
for the observation that
gem
-disulfides are oxidized to monosul-
fones.
42
Monosulfoxides, although not isolated, are likely to be
intermediates in these reactions (eq 45).
Miscellaneous Oxidations.
Use of permanganate in conjunc-
tion with
tert-Butyl Hydroperoxide
results in allylic oxidation
(eq 53).
51
O
O
O
KMnO
4
KMnO
4
MeS
SMe
MeS
S
MeS
S
R
R
Me
(45)
MeCOMe
Me
70%
KMnO
4
,
t
-BuOOH
Oxidation of sulfinic acids results in the formation of sulfonic
acids,
43
(53)
C
6
H
6
, silica
while sulfites give sulfates (eqs 46 and 47).
44
AcO
AcO
O
SO
3
–
SO
2
H
KMnO
4
OH
–
, H
2
O
70%
Aromatic compounds are oxidized to aryl iodides when treated
with permanganate,
Iodine
, and
Sulfuric Acid
(eq 54).
52
(46)
( )
11
( )
11
KMnO
4
, I
2
(54)
I
O
O
KMnO
4
O
H
2
SO
4
70%
S
O
S
(47)
O
O
AcOH
45%
O
Chemical modification of nucleic acids by treatment with
permanganate results in oxidation of the
5
double bond to give
Cyclic
thiones
are
readily
oxidized
to
the
corresponding
either diols or ketols (eq 55).
53
ketones by permanganate (eq 48).
45
O
S
S
KMnO
4
KMnO
4
HN
OH
MeO
OMe
O
pH 8.6
36%
MeCOMe
64%
O
N
OH
R
HN
(55)
O
O
O
N
S
KMnO
4
R
HN
OH
MeO
OMe
(48)
pH 4.3
40%
O
N
O
R
Oxidation of Amines.
The synthetic usefulness of perman-
ganate as an oxidant for aliphatic amines is decreased by the fact
that a complex mixture of products is often obtained.
4
,
46
Good
yields of tertiary nitroalkanes can, however, be obtained from the
oxidation of the corresponding amines (eq 49).
47
Guaiol and related compounds can be oxidized to rearranged
ketols using aqueous glyme as the solvent (eq 56).
54
HO
KMnO
4
, pH 8
(56)
KMnO
4
glyme, H
2
O
70%
(49)
R
3
CNH
2
R
3
CNO
2
OH
O
MeCOMe, H
2
O
70–80%
OH
Primary and secondary amines react with permanganate in
buffered, aqueous
t
-butyl alcohol to give aldehydes and ketones
(eq 50).
46
cis
-2,5-Dihydro-2,5-dimethoxyfuran is oxidized to the corre-
sponding
α
-diol in preference to the
trans
compound (eqs 57
and 58).
55
Avoid Skin Contact with All Reagents
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