Polymer-supported nitroxyl radical catalyst for selective aerobic oxidation of primary alcohols to aldehydes

Article abstract of DOI:10.1039/b415902k

PS-TEMPO, a polymer-supported 2,2,6,6-tetramethyl-piperidine-N-oxyl (TEMPO), was successfully applied as a recyclable, active and selective catalyst for the oxidation of primary aliphatic and benzylic alcohols to aldehydes by molecular oxygen in the presence of Co(NO₃)₂ and Mn(NO₃)₂ as co-catalysts. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415902k

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COMMUNICATION
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Polymer-supported nitroxyl radical catalyst for selective aerobic
oxidation of primary alcohols to aldehydes
Michelle Gilhespy, Martin Lok and Xavier Baucherel*
Received (in Cambridge, UK) 14th October 2004, Accepted 24th November 2004
First published as an Advance Article on the web 12th January 2005
DOI: 10.1039/b415902k
with full conversion and 99% selectivity after 2.5 hours of reaction
(Run 1).
PS-TEMPO,
a
polymer-supported 2,2,6,6-tetramethyl-
piperidine-N-oxyl (TEMPO), was successfully applied as a
recyclable, active and selective catalyst for the oxidation of
primary aliphatic and benzylic alcohols to aldehydes by
molecular oxygen in the presence of Co(NO3)2 and
Mn(NO3)2 as co-catalysts.
Primary aliphatic and benzylic alcohols were readily oxidised
with complete conversion and high selectivity to the corresponding
aldehydes (Runs 1–4). To our knowledge this represents the first
example of a truly heterogeneous TEMPO catalyst active and
selective for the difficult conversion of unreactive aliphatic primary
alcohols into aldehydes using molecular oxygen as terminal
oxidant.
The oxidation of alcohols is one of the most important
transformations in organic chemistry both on the laboratory and
on the industrial scale.1 Especially the catalytic conversion of
unactivated primary alcohols to aldehydes remains a challenging
process for academic research and is crucial for the synthesis of
fine chemicals such as fragrances or food additives.2 The systems
involving both 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)
and sodium hypochlorite3 or molecular oxygen in the presence
of a co-catalyst4 are of particular interest in this area. The
immobilisation of TEMPO on various supports has been reported
in the literature.5,6 However, very few catalytic systems6 combine
the advantages of using oxygen as terminal oxidant under non-
chlorinated conditions with the possibility to recover and recycle
the N-oxyl radical catalyst.
The oxidation of secondary aliphatic alcohols proved more
challenging for this system. A longer reaction time was required for
the oxidation of 2-octanol (24 h, 99% conversion at 95% selectivity,
Run 5, Table 1). In a competitive experiment, with a lower catalyst
loading, PS-TEMPO showed a marked preference for primary
We report herein the use of a polymer supported TEMPO for
the aerobic oxidation of alcohols. The catalyst was prepared by
coupling 4-hydroxy-TEMPO onto a carboxylic acid functionalised
polymer, FibreCat2.7 FibreCat2 consists of a polymer fibre
functionalised via graft copolymerisation, which results in a high
density of active functional sites being generated on the polymer
(6.5 mmol g21). Furthermore, the open structure of the polymer
and its good functional group accessibility are the reason it
achieves high catalyst loading (2.3 mmol g21 of nitroxyl radical).
This new immobilised TEMPO, subsequently referred to as PS-
TEMPO, proved to be an efficient heterogeneous catalyst for the
oxidation of alcohols using molecular oxygen under the conditions
developed by Minisci.4 1-Octanol was converted into octanal
Fig. 1 Competitive oxidation of 1-octanol and 2-octanol. Substrate
10 mmol, PS-TEMPO 200 mg, AcOH 10 ml, Mn(NO3)2?6H2O 58 mg
(2 mol%), Co(NO3)2?6H2O 58 mg (2 mol%), 40 uC, O2 1 atm. Conversion
and selectivity determined by GLC using p-methoxyacetophenone as
internal standard.
*xavier.baucherel@matthey.com
Table 1 PS-TEMPO oxidation of alcoholsa
Run
Substrate
Product
Time (h)
Conv. (%)
Sel. (%)
1
2
3
4
5
a
1-octanol
octanal
undecanal
3-phenylpropionaldehyde
benzaldehyde
2-octanone
2.5
3
2.5
1
. 99
. 99
95
. 99
. 99
. 99
. 99
94
. 99
95
1-undecanol
3-phenyl-1-propanol
benzyl alcohol
2-octanol
24
Substrate 10 mmol, PS-TEMPO 500 mg, AcOH 10 ml, Mn(NO3)2?6H2O 58 mg (2 mol%), Co(NO3)2?6H2O 58 mg (2 mol%), 40 uC, O2 1 atm.
Conversion and selectivity determined by GLC using p-methoxyacetophenone as internal standard.
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PS-TEMPO as catalyst was carried out in a two-necked round bottomed
flask equipped with a condenser attached to a gas burette. The flask was
charged with PS-TEMPO 500 mg (1.15 mmol based on 2.3 mmol g21
loading), Mn(NO3)2?6H2O 58 mg (2 mol%), Co(NO3)2?6H2O 58 mg
(2 mol%) and p-methoxyacetophenone 500 mg as internal standard. Glacial
acetic acid (10 ml) was added followed by the alcohol (10 mmol). The
reaction mixture was stirred at 40 uC under an atmosphere of molecular
oxygen. Samples of the reaction mixture were taken out regularly to
monitor the reaction by GLC. The products of the reaction were
determined by comparison with the commercially available carbonyl
compounds.
{ Procedure for the preparation of PS-TEMPO. The technique used for
immobilisation of 4-hydroxy-TEMPO involved suspending Smopex-102pp
(30 g) in anhydrous dichloromethane (300 ml) before addition of
diisopropylcarbodiimide (65 ml) and dimethylaminopyridine (2.48 g).
4-Hydroxy-TEMPO (35 g) dissolved in anhydrous dichloromethane (60 ml)
was then added followed by triethylamine (57 ml). The reaction mixture
was stirred at room temperature under an atmosphere of nitrogen for
16 hours. The solid catalyst was collected by filtration and successively
washed using dimethylformamide, methanol and dichloromethane. The
solid was air-dried before being soxhlet-extracted using dichloromethane to
remove any non-reacted 4-hydroxy-TEMPO. Finally the polymer-
supported TEMPO was dried under vacuum (40 uC) for 16 hours. This
method achieved an average TEMPO loading of 2.3 mmol g21 (as
determined by recovered 4-hydroxy-TEMPO).
Fig. 2 Conversion and yields obtained in the oxidation of 1-octanol.
Substrate 10 mmol, PS-TEMPO 500 mg, AcOH 10 ml, Mn(NO3)2?6H2O
58 mg (2 mol%), Co(NO3)2?6H2O 58 mg (2 mol%), 40 uC, 3 hours, O2
1 atm. Conversion and selectivity determined by GLC using p-methoxy-
acetophenone as internal standard.
alcohols (Fig. 1). While over 90% of 1-octanol was selectively
converted into octanal, less than 10% of 2-octanol was consumed
to give 2-octanone, confirming the marked preference of the
catalyst for the oxidation of primary hydroxyl groups.
1 M. Hudlicky, Oxidations in Organic Chemistry; American Chemical
Society: Washington, DC, 1990.
2 R. A. Sheldon, I. W. C. E. Arends and A. Dijksman, Catal. Today,
2000, 57, 157–166; T. Mallat and A. Baiker, Catal. Today, 1994, 19,
247–283.
3 A. E. J. de Nooy, A. C. Besemer and H. van Bekkum, Synthesis, 1996,
1153–1174; P. L. Anelli, C. Biffi, F. Montanari and S. Quici, J. Org.
Chem., 1987, 52, 2559–2562.
4 R. Liu, X. Liang, C. Dong and S. Hu, J. Am. Chem. Soc., 2004,
126, 4112–4113; P. Gamez, I. W. C. E. Arends, J. Reedijk and
R. A. Sheldon, Chem. Commun., 2003, 2414–2415; R. Ben-Daniel,
P. Alsters and R. Neumann, J. Org. Chem., 2001, 66, 8650–8653;
A. Dijksman, A. Marino-Gonzalez, A. Mairata i Payeras, I. W. C.
E. Arends and R. A. Sheldon, J. Am. Chem. Soc., 2001, 123, 6826–
6833; A. Cecchetto, F. Fontana, F. Minisci and F. Recupero,
Tetrahedron Lett., 2001, 42, 6651–6653; M. F. Semmelhack,
C. R. Schmid, D. A. Corte´s and C. S. Chou, J. Am. Chem. Soc., 1984,
106, 3374–3376.
Finally, we investigated the recyclability of the catalyst. PS-
TEMPO was recovered by filtration after 3 hours of reaction and
re-used, without any further reactivation process, in a sequence of
oxidations using 1-octanol as substrate. Fresh Mn(NO3)2 and
Co(NO3)2 were added to each successive run. The results presented
in Fig. 2 show that even after 5 subsequent runs the catalyst
activity and selectivity remained high, proving the high efficiency
of the catalyst recovery. The slight decrease in the conversion of
1-octanol observed during the successive runs could be due to loss
of some catalyst during filtration of the small amount of catalyst;
no addition of any fresh PS-TEMPO catalyst to keep the amount
of catalyst used constant was performed.
In conclusion we have demonstrated that PS-TEMPO, a
polymer-supported TEMPO, is an excellent catalyst for the
oxidation of alcohols. Especially it shows high activity and
selectivity for the more difficult conversion of non-activated
primary aliphatic alcohols affording the corresponding aldehydes
in high yields. The catalyst was also shown to be easy to recycle by
filtration without significant loss of activity.{{
5 G. Pozzi, M. Cavazzini, S. Quici, M. Benaglia and G. Dell’Anna, Org.
Lett., 2004, 6, 441–443; P. Ferreira, W. Hayes, E. Phillips, D. Rippon and
S. Chi Tsang, Green Chem., 2004, 6, 310–312; C. Tanyeli and A. Gu¨mu¨s,
Tetrahedron Lett., 2003, 44, 1639–1642; R. Ciriminna, C. Bolm, T. Fey
and M. Pagliaro, Adv. Synth. Catal., 2002, 344, 159–163; T. Fey,
H. Fischer, S. Bachmann, K. Albert and C. Bolm, J. Org. Chem., 2001,
66, 8154–8159; A. Dijksman, I. W. C. E. Arends and R. A. Sheldon,
Chem. Commun., 2000, 271–272; C. Bolm and T. Fey, Chem. Commun.,
1999, 1795–1796.
6 F. Minisci, F. Recupero, M. Rodino`, M. Sala and A. Schneider, Org.
Proc. Res. Dev., 2003, 7, 794–798; D. Brunel, F. Fajula, J. B. Nagy,
B. Deroide, M. J. Verhoef, L. Veum, J. A. Peters and H. van Bekkum,
Appl. Catal. A: Gen., 2001, 213, 73–82.
Michelle Gilhespy, Martin Lok and Xavier Baucherel*
Johnson Matthey Catalysts, PO Box 1, Belasis Avenue, Billingham,
Cleveland, UK TS23 1LB. E-mail: xavier.baucherel@matthey.com
7 S. Collard, C. F. J. Barnard, S. Bennett, S. H. Elgafi, G. R. Henderson,
G. Sweeney and M. Sundell, Catalysis for Organic Reactions,
(Proceedings 19th ORCS Conference), Marcel Dekker, New York,
2002, pp. 49–60; S. Buckley, Speciality Chemicals, October 2002, pp. 12–
13; J.-A. A. Nasman and R. T. Peltonen, EP629441.
Notes and references
{ General procedure. All alcohols and solvents were used as received
without any further purification. The oxidation of alcohols using
1086 | Chem. Commun., 2005, 1085–1086
This journal is ß The Royal Society of Chemistry 2005