TEMPO oxidations with a silica-supported catalyst

Article abstract of DOI:10.1039/a905683a

Silica-supported 1-hydroxy-2,2,6,6-tetramethylpiperidine (SG-TMP-OH) was successfully applied as a recyclable catalyst in the oxidation of various alcohols.

Full text of DOI:10.1039/a905683a

TEMPO oxidations with a silica-supported catalyst
Carsten Bolm* and Thomas Fey
Institut für Organische Chemie der RWTH Aachen, Professor-Pirlet-Straße 1, D-52056 Aachen, Germany.
E-mail: carsten.bolm@oc.rwth-aachen.de
Received (in Cambridge, UK) 14th July 1999, Accepted 29th July 1999
Silica-supported 1-hydroxy-2,2,6,6-tetramethylpiperidine
(SG-TMP-OH) was successfully applied as a recyclable
catalyst in the oxidation of various alcohols.
1).2a§ As shown by quantitative GC analysis, aldehydes and
ketones were formed in high yields from the corresponding
primary and secondary alcohols, respectively. Both benzylic
and aliphatic substrates react equally well.
Aminoxyl† radicals are well-established catalysts for the
oxidation of alcohols.1 Typically, such oxidations are carried
out in the presence of 1 mol% of catalyst and a stoichiometric
amount of a terminal oxidant such as bleach,2 sodium chlorite,3
sodium bromite,4 tert-butyl hypochlorite,5 N-chlorosuccini-
mide,6 MCPBA,7 [bis(acetoxy)iodo]benzene,8 or oxygen in
combination with a high-valent metal salt.9 Furthermore,
electrochemical reoxidation of the catalyst is possible.10
Especially for large scale oxidations, the TEMPO–bleach
protocol introduced by Anelli et al.2 appears most useful.11
Although usually a small quantity of the catalyst is sufficient for
an efficient transformation, it is still desirable to develop
improved work-up conditions which allow simple product
isolation as well as catalyst separation and recycling.12 We
envisaged that both requirements could be realised by using a
polymer-supported aminoxyl radical. Usually TEMPO-cata-
lysed reactions afford the desired carbonyl compounds in high
yields without the formation of significant amounts of by-
product, and thus catalyst separation and product isolation could
easily be accomplished by simple filtration and phase separation
followed by evaporation of the organic solvent (Scheme 1).
Although several polymer-bound aminoxyl radicals have
already been synthesised,13 only a very few of them were
applied as oxidation catalysts.13a–d To the best of our knowl-
edge, none of them has been studied under the conditions
mentioned above. Due to several publications concerning the
application of polymer-supported oxidation reagents and cata-
lysts,14 and based on our own expertise in the use of
heterogenised alkaloids in the Sharpless dihydroxylation re-
action,14g,15 we focused our attention on the attachment of the
TEMPO precursor 1-hydroxy-4-oxo-2,2,6,6-tetramethylpiper-
idine (2)16 onto silica. The synthesis of 3 (SG-TMP-OH) was
accomplished by reductive amination of 2 with commercially
available aminopropyl-functionalised support (Scheme 2).17‡
The resulting solid was carefully washed with hot MeOH using
a Soxhlet extractor for several days in order to ensure complete
removal of non-chemically bounded hydroxypiperidine deriva-
tives from the surface.
In order to study the selectivity profile in the oxidation of
primary and secondary alcohols, we performed competition
experiments using equimolar mixtures of both. Entries 8 and 9
of Table 1 show a high selectivity towards the oxidation of the
primary alcohols, which resembles the known behaviour of the
unsupported TEMPO catalyst.19 Thus, the corresponding
aldehydes of nonan-1-ol and benzyl alcohol were formed in 96
and 92% yield, respectively, whereas the oxidation of the
secondary alcohols nonan-2-ol and 1-phenylethanol occured in
low yield only (1 and 3%, respectively). In addition, as in
catalysed oxidations with TEMPO itself,2c no racemisation of
optically active compounds occured (entry 10).
We finally investigated the possibility of recycling the
catalyst. Therefore, the silica-bound TEMPO derivative was
recovered by filtration after the reaction and reapplied in a
sequence of oxidations using nonan-1-ol as substrate. The
results illustrated in Fig. 1 show that even after 10 subsequent
runs the catalyst activity remained high. In both substrate
conversion and product yield only a minor decrease was
observed, proving the high efficiency of the catalyst recovery.
In conclusion, we have demonstrated that silica-supported
TEMPO is an excellent catalyst for the oxidation of alcohols,
affording the corresponding carbonyl derivatives in high yields.
Primary alcohols are selectively oxidised in the presence of
Scheme 2
Table 1 TEMPO oxidation of various alcohols to the corresponding
aldehydes and ketones with silica-supported catalyst 3a
SG-TMP-OH 3 was then applied as catalyst in the oxidation
of various alcohols according to the Anelli protocol (Table
Entry
Substrate
Yield (%)b
1
2
Benzyl alcohol
Heptan-1-ol
92 (75)
90
3
Octan-1-ol
88
4
5
Nonan-1-ol
Dodecan-1-ol
90 (85)
81
6
Nonan-2-ol
65
7
8
9
10
1-Phenylethanol
Nonan-1-ol/nonan-2-ol
Benzyl alcohol/1-phenylethanol
(S)-2-Methylbutan-1-olc
91
96/1
92/3
60
a
b
Oxidation experiments conducted following the protocol in note §.
Yields were determined by quantitative GC analysis using decane or
dodecane as internal standard. Values in parentheses are isolated yields.
c The ee of 95% remained unchanged (ref. 18).
Scheme 1
Chem. Commun., 1999, 1795–1796
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Fig. 1 Yields [hatched (%)] and conversions [black (%)] in consecutive
oxidation reactions of nonan-1-ol when using silica-supported catalyst 3.
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This research was supported by the Katalyseverbund NRW
and the Fonds der Chemischen Industrie.
Notes and references
† CAS nomenclature = nitroxyl.
‡ Aminopropyl-functionalised silica was purchased from Aldrich. Accord-
ing to the distributor’s information the degree of functionalisation is
12 For a review on that topic: D. P. Curran, Angew. Chem., Int. Ed., 1998,
37, 1174.
approximately 1.0 mmol g21
.
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§ General procedure for the oxidation of alcohols with SG-TMP-OH 3 as
catalyst: The reaction was performed in a glass tube with a cooling mantle.
The bottom of the tube was equipped with a ceramic filter plate and a
stopcock allowing easy separation of the supported catalyst from the
reaction mixture by filtration. In this vessel was placed 9.2 mg of SG-TMP-
OH 3 (8 mmol based on complete functionalisation; degree of functionalisa-
tion = 0.87 mmol g21). Then a CH2Cl2 solution (2 ml) of the alcohol (0.4
M) and decane or dodecane (0.12 M; as internal standard) was added
followed by an aqueous solution (0.16 ml) of KBr (0.5 M). After the cooling
of the reaction mixture to 0 °C, 2.7 ml of aq. NaOCl (diluted to a final
concentration of 0.37 M and buffered by the addition of NaHCO3 to a pH
of 9.1) was added. Then, the reaction mixture was vigorously shaken for 30
min. After filtration, the organic phase was separated, dried over MgSO4,
and the product composition was analysed by means of GC [ultra-2-column
(Hewlett Packard)]. The filtered solid was washed five times with water,
MeOH and CH2Cl2 (2 ml each), air-dried, and reused as such. Preparative
runs were carried out on a five-fold scale, and the reaction time was
extended to 1 h. After filtration the organic layer was separated, and the
aqueous phase was extracted with CH2Cl2 once. The combined organic
phases were dried over MgSO4, the solvent was removed in vacuo, and the
crude product purified by column chromatography on silica gel with
CH2Cl2 as eluent.
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Communication 9/05683A
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Chem. Commun., 1999, 1795–1796