Oxidation of aromatic aldehydes in the ionic liquid [bmim]PF6

Article abstract of DOI:10.1016/S0040-4039(00)01037-6

Several aromatic aldehydes have been oxidised using the catalyst [Ni(acac)₂] and dioxygen at atmospheric pressure, as the oxidant, in the ionic liquid [bmim]PF₆. The catalyst and ionic liquid could be recycled after extraction of the carboxylic acid product. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01037-6

Tetrahedron Letters 41 (2000) 6627±6629
Oxidation of aromatic aldehydes in the ionic liquid
[bmim]PF6
Joshua Howarth*
School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
Received 28 April 2000; revised 13 June 2000; accepted 20 June 2000
Abstract
Several aromatic aldehydes have been oxidised using the catalyst [Ni(acac)2] and dioxygen at atmo-
spheric pressure, as the oxidant, in the ionic liquid [bmim]PF6. The catalyst and ionic liquid could be
recycled after extraction of the carboxylic acid product. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: ionic liquid; [bmim]PF6; oxidation.
The ionic liquids [emim]PF6 and [emim]BF4, where [emim]+ is the 1-ethyl-3-methylimidazolium
cation, were ®rst discovered in 1994 and 1992, respectively.1 The analogous [bmim]PF6 and
[bmim]BF4 ionic liquids, where [bmim]+ is the 1-butyl-3-methylimidazolium cation, followed
shortly afterwards.2 These liquids have several very interesting properties; they can solvate a wide
range of organic and inorganic materials, they are highly polar, yet non-coordinating, they are
immiscible with a wide range of organic solvents, and they have a nonvolatile nature. It is also
possible to vary their immiscibility with water and other solvents by varying the counter ion,
allowing for the possibility of biphasic reactions. Thus, for example [bmim]PF6 is water
immiscible and [bmim]BF4 is water miscible.
One of the prime concerns of industry and academia is the search for replacements to the
environmentally damaging solvents used on a large scale, especially those that are volatile and
dicult to contain. Moisture stable ionic liquids, with their unique properties, provide the
promise of a credible alternative. As such a rapid growth in the investigation of the [bmim]PF6
and [bmim]BF4 type ionic liquids as substitutes for classical solvents is underway.
To date some of the more important reactions that have been carried out and investigated in
[bmim]PF6 and [bmim]BF4 are a simple Diels±Alder reaction between cyclopentadiene and
methyl methacrylate,3 N-alkylation of indole and O-alkylation of 2-napthol,4 hydrogenations,5
hydroformylation,6 dimerization of ole®ns,7 and the Heck reaction.8 Herein we report, as far as
we are aware, the results of the ®rst oxidation reactions of aromatic aldehydes 1a±g using the
* Corresponding author. Tel: +353 1 704 5312; fax: +353 1 704 5503; e-mail: howarthj@ccmail.dcu.ie
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01037-6

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nickel(II)acetylacetonate, [Ni(acac)2], catalyst 2 and dioxygen at atmospheric pressure, as the
oxidant, to be carried out in the new and versatile solvent [bmim]PF6, a further, and industrially
important example of the general application of this type of solvent.
The [Ni(acac)2] catalyst and dioxygen type system has previously been employed to oxidise
aromatic aldehydes to the corresponding carboxylic acids by Knochel9 using the ingenious
biphasic to monophasic and back to biphasic reaction conditions applicable to per¯uorinated
solvents, and ®rst used for synthetic application by Horvath.10 In Knochel's work with per¯uorinated
solvents, modi®cation of the 1,3-diketone component of the Ni catalyst to incorporate long
per¯uorinated chains (to generate the corresponding Ni catalyst) was necessary. This modi®ca-
tion increased the solubility of the Ni catalyst, and reduced leaching. These expensive types of
modi®cation to catalysts for use in per¯uorinated solvents represent a major drawback in the use
of these solvents.11 Using the generally more cost e€ective and simpler [bmim]PF6/[Ni(acac)2],
with dioxygen at atmospheric pressure, oxidation system, we were able to eciently oxidise the
seven aldehydes given in Scheme 1 to the corresponding acids, with moderate to good yields.
Scheme 1.
The general procedure for the oxidation of aldehydes 1a±g using benzaldehyde 1a as an
example: Benzaldehyde, 1a (0.72 g, 6.7 mmol) was added to a Schlenk tube containing the solvent
[bmim]PF6 (2.5 ml)12 (dried at 50ꢀC under high vacuum for 48 h and saturated with O2) to which
[Ni(acac)2] (0.032 g, 0.02 mmol, 3 mol%) had been added. Warmed to 60ꢀC the solution was
agitated vigorously for 12 h under a blanket of O2 at atmospheric pressure, after which it was
extracted with ethyl acetate (4Â10 ml) to give, after puri®cation by recrystallisation, benzoic acid
3a (0.54 g, 66% yield).13
When the reactions were carried out under identical conditions but in the absence of the Ni
catalyst, we found that although some carboxylic acid was formed the yield was signi®cantly
lower, below 5%. We also found that the reactions could be repeated a further three times using
the same [bmim]PF6/[Ni(acac)2] system without leaching of the catalyst, and that the yield of
product for these subsequent runs were similar to those obtained in the initial run.
In relation to Knochel's work9 our yield of carboxylic acids obtained after oxidation are lower.
This could possibly be accounted for by the high solubility of dioxygen in per¯uorinated
solvents,11 and hence more ecient catalysis. However, there are currently no data available for
dioxygen solubility in [bmim]PF6 in order to make a direct comparison.

6629
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1
references therein. The H and 13C NMR spectra for compounds 3a±g corresponded exactly to those cited in the
literature, or obtained from the commercially available carboxylic acid, as did their melting points.