Polymer supported perruthenate (PSP): Clean oxidation of primary alcohols to carbonyl compounds using oxygen as cooxidant

Article abstract of DOI:10.1055/s-1998-2106

Molecular oxygen has been used as stoichiometric oxidant in polymer supported perruthenate (PSP) catalysed conversion of primary alcohols to carbonyl compounds affording pure products without the need for conventional workup procedures.

Full text of DOI:10.1055/s-1998-2106

SYNTHESIS
July 1998
977
Polymer Supported Perruthenate (PSP): Clean Oxidation of Primary Alcohols to
Carbonyl Compounds Using Oxygen as Cooxidant
Berthold Hinzen, Roman Lenz, Steven V. Ley*
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England
Received 10 December 1997
the presence of tetrapropylammonium perruthenate.4
Based on these results we now report the use of molecular
oxygen or air as a stoichiometric oxidant5 in PSP-cata-
lysed conversion of alcohols to aldehydes. This consti-
tutes a remarkable example of a clean technology process
where compounds are formed free from any contaminat-
ing byproducts.
Abstract: Molecular oxygen has been used as stoichiometric oxi-
dant in polymer supported perruthenate (PSP) catalysed conver-
sion of primary alcohols to carbonyl compounds affording pure
products without the need for conventional workup procedures.
Key words: clean technology, solid supported reagents, oxida-
tion, oxygen, alcohols.
The perruthenate ion has been shown to be a very effec-
tive agent for the mild conversion of alcohols to carbonyl
compounds.1 It can be used as its tetrapropylammonium
salt (TPAP),2 which is soluble in common organic sol-
vents, or as a polymer supported oxidant either in stoi-
chiometric amounts or as a catalyst in combination with
a cooxidant, usually N-methylmorpholine N-oxide
(NMO).2, 3 However, in parallel synthesis applications the
removal of the catalyst and the side products is tedious.
With a view to solving this problem we recently devel-
oped polymer supported perruthenate (PSP)3 and investi-
gated the oxidation of alcohols using molecular oxygen in
Initially, molecular oxygen was bubbled through a mix-
ture of the alcohol, PSP (10 mol%) and toluene or dichlo-
romethane at room temperature. However, best results
were obtained by heating a solution of the alcohol in tolu-
ene in the presence of PSP (0.02 mmol, 10 mol%) to 75–
85°C under an atmosphere of oxygen6 (Scheme). The
mixture was stirred at the temperature shown in Table 1
until GC analysis indicated the complete consumption of
the starting material. The reaction mixture was simply fil-
tered and the filtrate and washings were evaporated. The
yields were determined by GC (Table 1).
Table 1. PSP-Catalysed Oxidation of Alcohols to Carbonyl Compounds using Molecular Oxygen as Cooxidanta
Entry
1
Alcohol
Product
Time (h)
1
Temp (°C)
75
Yieldb (%)
>95
2
3
0.5
1
75
75
>95
>95
4
5
8
8
85
85
>91
>95
6
7
6
8
75
75
>90
56
a The oxidations were performed in toluene on a 0.2-mmol scale using 10% PSP under O2.
b Reaction yields were determined by GC.

SYNTHESIS
Short Papers
978
suitable for automated synthesis, therefore offering excit-
ing opportunities in parallel synthesis applications.
With the exception of (5S,9S)-5,9-bis(tert-butyldimethylsilyloxy)de-
can-1-ol (Table 1, entry 7)7 all alcohols used in this study are com-
mercially available and were used without further purification.
Toluene was freshly distilled from CaH2. Amberlyst A-26 was pur-
chased from Fluka. All oxidation products are known compounds and
Scheme
In the case of activated alcohols the starting materials
were consumed in less than 1 hour and only the corre-
sponding aldehydes were observed in the GC trace (Table
1, entries 1–3). Non-activated substrates required longer
reaction times and in certain cases elevated reaction tem-
peratures (Table 1, entries 4, 5). Even under these forced
conditions neither over-oxidation to the corresponding
carboxylic acid nor decomposition was observed. The
only products found were the expected carbonyl com-
pounds and, in the case where the reaction had not gone to
completion, the starting material was the only impurity
(Table 1, entry 7). However, in most cases excellent yields
were obtained after simple filtration and removal of the
solvent. Furthermore, a remarkable selectivity for primary
versus secondary alcohols was observed (Table 2). Using
a 1:1 mixture of benzyl alcohol and sec-phenethyl alcohol
the sole reaction product was benzaldehyde (Table 2, en-
try 1). The secondary alcohol was not oxidised under the
reaction conditions. Similar results were obtained for the
oxidation of octan-1-ol versus octan-2-ol. Whereas 83%
of the primary alcohol was consumed only 13% of the sec-
ondary alcohol was oxidised to the corresponding ketone.
The lower selectivity compared to the selective oxidation
of benzyl alcohol is probably due to the higher reaction
temperature and the longer reaction time required for the
oxidation of these non-activated substrates.
1
were identified by comparison of their GC retention times and H
NMR data with those of authentic samples. Yields refer to quantita-
tive GC analysis and were determined on a Hewlett Packard 5890 se-
ries II gas chromatograph (HP-1 crosslinked methyl silicone column)
1
coupled to a Hewlett Packard 3396 series III integrator. H NMR
spectra were recorded in CDCl on a Bruker DRX-200 MHz spec-
3
trometer using TMS as an internal standard.
Preparation of Polymer Supported Perruthenate (PSP):
Amberlyst A-26 (20.0 g) was thoroughly washed with water (500 mL),
CH2Cl2 (500 mL), acetone (500 mL), and toluene (200 mL) and dried
in vacuo. The resin was densely packed into a chromatographic col-
umn (Ø 4 cm). In a round-bottomed flask (500 mL) equipped with a
magnetic stirrer, KRuO4 (570 mg, 2.79 mmol) was solubilised in wa-
ter (500 mL) using ultrasonification for 10 min at r.t. The resulting
black solution was filtered through the freshly prepared ion exchange
column within approx. 15 min. Subsequently, the black polymer
beads were thoroughly washed with water (500 mL) and acetone (500
mL) and dried in vacuo.
To estimate the loading of the polymer a solution of benzyl alcohol
(150 mg, 1.41 mmol) in CH2Cl2 (5 mL) was stirred over the polymer
supported perruthenate (PSP) (6.30 g) for 72 h under N2. From the
amount of benzaldehyde determined by GC and NMR analysis (68%
conversion) the loading of the perruthenate anion on the polymer was
calculated to be 0.1 mmol per gram resin. This corresponds to the gain
in weight of the polymer measured upon the immobilisation of the
perruthenate ion.
Oxidation of Primary Alcohols with PSP/O2; General Procedure:
To a solution of alcohol (0.2 mmol) in toluene (2 mL), polymer sup-
ported perruthenate (200 mg, 0.02 mmol) was added and the mixture
was stirred at 75–85°C under O2 (O2 balloon) for 0.5–8 h. The
progress of the reaction was monitored by GC analysis. The mixture
was filtered through cotton wool and the residue washed with toluene.
The filtrate was evaporated in vacuo and the product directly analysed
by NMR spectroscopy without further purification.
The combination of polymer supported perruthenate and
molecular oxygen as cooxidant constitutes an excellent
example of a clean technology process for the conversion
of alcohols to aldehydes. The only workup required con-
sists of a filtration through cotton wool to remove the
polymer supported reagent. We believe this process to be
Table 2. Competitive PSP-Catalysed Oxidation of Primary vs. Secondary Alcohols using Molecular Oxygen as Cooxidanta
Entry
1
Alcohols
Products
Time (h)
Temp. (°C)
Conversionb (%)
>99
3
75
<1
83
13
2
6
85
a The oxidations were performed in toluene on a 0.2-mmol scale using 10 mol% PSP under O2.
b Determined by GC.

SYNTHESIS
July 1998
979
We gratefully acknowledge financial support from the Swiss National
Science Foundation (to B.H. and R.L.), the Ciba-Geigy Foundation
(to R.L.), the Novartis Research Fellowship and the BP endowment
(to S.V.L.). Further funds for this work were provided by a DuPont
Aid to Education grant.
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(5) Oxygen as stoichiometric oxidant for the oxidation of alcohols to
carbonyl compounds in the presence of various catalysts has been
reported previously:
(6) These solvent and temperature conditions were originally used by
T. Yamada and T. Mukaiyama for the oxidation of secondary al-
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