Efficient Mn-Cu and Mn-Co-TEMPO-catalysed oxidation of alcohols into aldehydes and ketones by oxygen under mild conditions

Article abstract of DOI:10.1016/S0040-4039(01)01245-X

A catalytic amount of Mn(II)-Co(II) or Mn(II)-Cu(II) nitrates in combination with TEMPO allows the selective oxidation of primary and secondary alcohols to aldehydes and ketones by oxygen under very mild conditions. Mechanistic aspects are discussed. The catalyst can be considered the cheapest and the most effective for the selective aerobic oxidation of alcohols.

Full text of DOI:10.1016/S0040-4039(01)01245-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6651–6653
Efficient Mn–Cu and Mn–Co–TEMPO-catalysed oxidation of
alcohols into aldehydes and ketones by oxygen under
mild conditions
a
b
a,
a
Andrea Cecchetto, Francesca Fontana, Francesco Minisci * and Francesco Recupero
a
Dipartimento di Chimica del Politecnico, via Mancinelli 7, 20131 Milan Mi, Italy
b
Universit a` di Bergamo, Dipartimento di Ingegneria, viale Marconi 5, 24044 Dalmine BG, Italy
Received 24 May 2001; accepted 11 July 2001
Abstract—A catalytic amount of Mn(II)–Co(II) or Mn(II)–Cu(II) nitrates in combination with TEMPO allows the selective
oxidation of primary and secondary alcohols to aldehydes and ketones by oxygen under very mild conditions. Mechanistic aspects
are discussed. The catalyst can be considered the cheapest and the most effective for the selective aerobic oxidation of alcohols.
©
2001 Elsevier Science Ltd. All rights reserved.
1
Recently we have shown how Mn(II)–Co(II) or
Mn(II)–Cu(II) nitrates catalyse the oxidation of cyclo-
hexanone to adipic acid by oxygen under very mild
conditions (atmospheric pressure and room tempera-
ture) with very high selectivity. Cyclohexanol is com-
pletely inert under the same conditions; this could
appear paradoxical as cyclohexanol is extremely more
reactive than cyclohexanone towards uncatalysed
autoxidation, but it is well explained by a redox chain
involving an electron-transfer oxidation of the enol
form of the ketone (Eqs. (1) and (3)).
Adipic acid is one of the most important intermediates
for the production of nylon and it is prepared by
autoxidation of cyclohexane to a mixture of cyclohex-
anol and cyclohexanone at low conversion, in order to
have a good selectivity. This mixture is then selectively
oxidised to adipic acid by HNO .
3
We have considered the possibility of combining this
1
metal salt catalytic system with 2,2%,6,6%-tetra-
methylpiperidine-N-oxyl (TEMPO), which is known to
be an efficient catalyst for the oxidation of alcohols to
2
aldehydes and ketones by a variety of oxidants, in
order to oxidise the cyclohexanol–cyclohexanone mix-
ture to adipic acid by oxygen. TEMPO, however, is
also an inhibitor of free-radical processes. Thus, by
oxidising a mixture of cyclohexanol and cyclohexanone
by O in the presence of TEMPO and of the couples
(
(
1)
2)
2
Mn(II)–Co(II) or Mn(II)–Cu(II) nitrates, the only
result was the selective oxidation of cyclohexanol to
cyclohexanone under very mild conditions. Cyclohex-
anone, under the same conditions but in the absence of
1
TEMPO, is selectively oxidised to adipic acid.
This result suggested to us that the combination of
TEMPO with Mn(II)–Co(II) or Mn(II)–Cu(II) nitrates
could be a very effective catalyst for the aerobic oxida-
tion of alcohols to aldehydes or ketones. Actually this
catalytic system proved to be particularly effective for
the aerobic oxidation of alcohols under mild condi-
tions, as shown by the results in Table 1.
(
3)
*
0
Corresponding author. Fax: +39-02-23993080; e-mail: francesco.
minisci@polimi.it
The catalytic use of TEMPO for the selective oxidation
2
of alcohols by a variety of oxidants is well-known: a
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01245-X

6
652
A. Cecchetto et al. / Tetrahedron Letters 42 (2001) 6651–6653
particularly convenient method, involving a two-phase
combination of Mn(II) nitrate either with Co(II) or
Cu(II) nitrate considerably increases the catalytic
effectiveness, as the results in Table 2 show.
system (CH Cl /H O), hypochlorite as oxidant and bro-
2
2
2
mide ion as co-catalyst, was reported by Montanari and
3
–5
co-workers.
A variety of metal salt complexes was
2. An acidic medium is necessary to make the catalytic
system effective; the oxidation takes place selectively
in acetic acid solution, but no substantial oxidation
occurs in acetonitrile solution under the same condi-
also utilised in combination with TEMPO for the aero-
6–8
bic oxidation of alcohols. Copper salts were effective
with benzylic and allylic alcohols, while they proved6
less effective with primary and secondary alcohols.
2
tions. Since it is well-established that the actual
7
oxidant of the alcohol is the oxo-ammonium salt
(Eq. (4)), we explain our results by the fact that the
oxoammonium salt is formed by disproportionation
RuCl (PPh ) afforded an efficient catalyst at 100°C
2
3 3
8
and 10 bar of pressure. Recently, a more complex
catalyst, involving CuBr·Me S and perfluoroalkylated
2
9
of TEMPO catalysed by the acidic medium (Eq.
bipyridine and a perfluorooctane–chlorobenzene bipha-
sic solvent system was utilised at 90°C.
(
5)) and that TEMPO is regenerated by oxygen and
the metal salt catalytic system (Eq. (6)).
Our catalyst (Mn(II) and Co(II) or Cu(II) nitrates) in
combination with TEMPO appears, to the best of our
knowledge, the cheapest and the most effective (in the
most favourable cases the oxidation takes place with air
at atmospheric pressure and room temperature) and
therefore particularly convenient for practical
applications.
(
(
(
4)
5)
6)
Two aspects of this catalytic system can be emphasised:
1
. Mn(II) nitrate is more effective than Co(II) or
Cu(II) nitrates, when utilised separately, but the
a
Table 1. Conversion of alcohols to aldehydes or ketones by oxygen
Entry
Alcohol
Product
Time (h)
T (°C)
Yield (%)b
1
1-Heptanol
Benzyl alcohol
1-Heptanal
Benzaldehyde
6
10
6
4
6
6
3
5
9
9
6
6
6
40
20
20
20
20
20
40
40
20
20
20
20
20
20
97
98
99
98
97
98
99
100
96
98
97
98
96
99
c
2
3
c
4-CH -Benzyl alcohol
4-CH -Benzaldehyde
3
3
4
5
6
2-OCH -Benzyl alcohol
2-OCH -Benzaldehyde
3
3
2-NO -Benzyl alcohol
2-NO -Benzaldehyde
2
2
d
2-NO -Benzyl alcohol
2-NO -Benzaldehyde
2
2
7
8
9
1
1
1
1
1
Cinnamyl alcohol
2-Nonanol
Cyclohexanol
Cinnamaldehyde
2-Nonanone
Cyclohexanone
Cyclohexanone
2-Adamantanone
Acetophenone
4-Cl-Acetophenone
Acetophenone
0d
1
Cyclohexanol
2-Adamantanol
1-Phenylethanol
1-(4-Cl-Phenyl)ethanol
1-Phenylethanol
c
2
3
4
c
d
6
a
According to the standard procedure: 12.5 mmol alcohol, 1.25 mmol TEMPO, 0.25 mmol Mn(NO ) , 0.25 mmol Co(NO ) in AcOH (12.5 mL)
3
2
3 2
and O2 at atmospheric pressure.
Determined by GLC analysis of the crude product.
Air at atmospheric pressure instead of O2.
b
c
d
0
.25 mmol Cu(NO3)2 instead of Co(NO3)2.
a
Table 2. Effect of the metal salt catalyst and of the solvent in the oxidation of cyclohexanol by oxygen in AcOH as solvent
Catalyst
Time (h)
Conv. (%)
Catalyst
Time (h)
Conv. (%)
–
4
2
4
2
4
2
4
Traces
14
16
16
18
Mn(NO ) , Co(NO )
3 2
2
4
9
4
2
4
9
29
42
100
B2
30
3
2
Co(NO3)2
Co(NO3)2
Cu(NO3)2
Cu(NO3)2
Mn(NO3)2
Mn(NO3)2
Mn(NO ) , Co(NO )
3 2 3 2
Mn(NO ) , Co(NO )
3 2
3 2
b
Mn(NO ) , Co(NO )
3 2
3 2
Mn(NO ) , Cu(NO )
3 2
3 2
19
26
Mn(NO ) , Cu(NO )
47
100
3 2
3 2
Mn(NO ) , Cu(NO )
3 2
3 2
a
The standard procedure of entry 9, Table 1 was utilised.
b
CH CN as solvent instead of AcOH.
3

A. Cecchetto et al. / Tetrahedron Letters 42 (2001) 6651–6653
6653
In this catalytic cycle TEMPO has two important func-
pure 2-nitrobenzaldehyde (mp 44°C, 93% yield).
tions: it generates the oxo-ammonium salt (Eq. (5)),
which is responsible for the alcohol oxidation (Eq. (4)),
and it inhibits further oxidation of aldehydes and
ketones, which occurs by free-radical processes under
the same conditions (Eqs. (1)–(3)) in the absence of
TEMPO.
References
1. Minisci, F.; Fumagalli, C.; Pirola, R. Ital. Pat. 2000, no.
008303505, June 6/2000 Lonza s.p.a.; Eur. Pat. PCT/EP 01
(
05 400).
The very mild conditions are related to Eq. (6); in
non-acidic medium, where disproportionation of Eq.
2. de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H.
Synthesis 1996, 1153.
(
5) does not occur, more drastic oxidation conditions or
3. Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. J. Org.
Chem. 1987, 52, 2559.
4. Anelli, P. L.; Banfi, S.; Montanari, F.; Quici, S. J. Org.
more effective oxidants are required in order to oxidise
TEMPO to the oxo-ammonium salt (Eq. (7)).
Chem. 1989, 54, 2970.
5
6
. Anelli, P. L.; Montanari, F.; Quici, S. Org. Synth. 1990,
9, 212.
. Semmelhack, M. F.; Schmid, C. R.; Cort e´ s, D. A.; Chou,
(
7)
6
A typical example on a relatively large preparative
scale: a solution of 7.5 g of 2-nitrobenzyl alcohol, 2% of
TEMPO, 2% of Mn(NO ) , 2% of Co(NO ) , in 39 mL
C. S. J. Am. Chem. Soc. 1984, 106, 3374.
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Chem. Soc., Chem. Commun. 1999, 1591.
3
2
3 2
of CH COOH was stirred at 40°C during 6 h under O2
8. Betzemeier, B.; Cavazzini, M.; Quici, S.; Knochel, P. Tet-
3
at atmospheric pressure. GLC revealed a 99% yield.
Acetic acid was evaporated and the residue dissolved in
CH Cl . Chromatography on silica gel yielded 6.9 g of
rahedron Lett. 2000, 41, 4343.
9. Schiff, W.; Stefaniak, L.; Skolimowski, J.; Webb, G. A. J.
Mol. Struct. 1997, 407, 1.
2
2