Sodium borohydride reduction of aldehydes and ketones in the recyclable ionic liquid [bmim]PF6

Article abstract of DOI:10.1081/SCC-100105664

In our exploration of the new ionic liquid solvents as possible replacements for classical organic solvents, we have found that the reduction of aldehydes and ketones with NaBH₄ in the ionic liquid [bmim]PF₆ can be achieved. The ionic liquid can be recycled, and in some cases the product alcohol may be distilled directly from the ionic liquid eliminating classical organic solvents entirely.

Full text of DOI:10.1081/SCC-100105664

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SODIUM BOROHYDRIDE REDUCTION OF ALDEHYDES
AND KETONES IN THE RECYCLABLE IONIC LIQUID
[BMIM]PF6
J. Howarth a , P. James a & R. Ryan a
a School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
Version of record first published: 16 Aug 2006
To cite this article: J. Howarth, P. James & R. Ryan (2001): SODIUM BOROHYDRIDE REDUCTION OF ALDEHYDES AND
KETONES IN THE RECYCLABLE IONIC LIQUID [BMIM]PF6 , Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 31:19, 2935-2938
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SYNTHETIC COMMUNICATIONS, 31(19), 2935–2938 (2001)
SODIUM BOROHYDRIDE REDUCTION
OF ALDEHYDES AND KETONES IN
THE RECYCLABLE IONIC
LIQUID [BMIM]PF6
J. Howarth,* P. James,and R. Ryan
School of Chemical Sciences, Dublin City
University, Dublin 9, Ireland
ABSTRACT
In our exploration of the new ionic liquid solvents as possible
replacements for classical organic solvents, we have found
that the reduction of aldehydes and ketones with NaBH4 in
the ionic liquid [bmim]PF6 can be achieved. The ionic liquid
can be recycled, and in some cases the product alcohol may be
distilled directly from the ionic liquid eliminating classical
organic solvents entirely.
The ionic liquids [emim]PF6 and [emim]BF4, where [emim]þ is the
1-ethyl-3-methylimidazolium cation, were first discovered in 1994 and
1992 respectively.1 The analogous [bmim]PF6 1 and [bmim]BF4 ionic
liquids, where [bmim]þ is the 1-butyl-3-methylimidazolium cation, followed
shortly after.2 These liquids have several very interesting properties; they
*Corresponding author.
2935
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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HOWARTH,JAMES,AND RYAN
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 difficult 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-naphthol,4 hydrogenations,5 hydroformyla-
tion,6 dimerization of olefins,7 oxidation of aromatic aldehydes,8 and the
9
Heckreaction.
Herein we report, as far as we are aware, the results of the first sodium
borohydride reduction to be carried out in the new and versatile solvent
[bmim]PF6, a further, important example of the general application of this
type of solvent. The sodium borohydride reduction has many precedents in
synthetic chemistry,10 its widespread use makes it an ideal reaction to test the
versatility of the new [bmim]PF6 ionic liquid solvent, as a replacement for
classical organic solvents. As such we tooksix common aldehydes and
ketones 2a–f, and using the procedure given below we reduced them to the
corresponding alcohols 3a–f. The results of these reductions are given in
Scheme 1.
General procedure for the reduction of aldehydes and ketones in
[bmim]PF6,using benzaldehyde as an example.
Benzaldehyde (3.07 g,
0.03 mol) was dissolved in [bmim]PF6 (10 mL).11 Sodium borohydride
(3.00 g, 0.08 mol) was added slowly, with stirring, over a period of 0.5 h.
The reaction was left to stir for 1 h and then ice cold water was added
(10 mL). After 10 min the [bmim]PF6 layer was separated from the aqueous
layer and extracted with diethyl ether (2 ꢀ 50 mL). The combined extracts
were dried (MgSO4), filtered, and the solvent removed in vacuo, to yield the
crude benzyl alcohol as a yellow liquid. After purification by short path
distillation under vacuum pure benzyl alcohol 3b was obtained (2.82 g,
90% yield).12
Alternative procedure for volatile product alcohols. As above, but after
the [bmim]PF6 has been separated from the aqueous layer the [bmim]PF6

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IONIC LIQUID [BMIM]PF6
2937
Scheme 1.
was heated under high vacuum and the benzyl alcohol 3b was distilled
(75–80ꢁC, 0.2 mmHg) directly from the [bmim]PF6 in 70% yield using
short path distillation apparatus. Cyclohexanol 3f was distilled directly
from the [bmim]PF6 (50–54ꢁC, 0.9 mmHg) in a similar manner, 78% yield.
The yields obtained using this procedure are better than with extrac-
tion, and this procedure eliminates the use of all classical organic solvents
from the reaction if volatile alcohols are produced from the aldehydes or
ketones being reduced.
Compounds 3a, benzhydrol, and 3e, m-nitrobenzyl alcohol, were
purified by recrystallisation with melting points of 66–67ꢁC (Lit. mp13
65–67ꢁC) and 30–33ꢁC (Lit. mp14 30–32ꢁC) respectively. Compounds 3c,d
were purified by silica gel flash chromatography.
REFERENCES
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Received in the UK August 31, 2000

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