Immobilized baker's yeast reduction of ketones in an ionic liquid, [bmim]PF6 and water mix

Article abstract of DOI:10.1016/S0040-4039(01)01601-X

The bioreduction with immobilized baker's yeast of several ketones was carried out in a 10:1 [bmim]PF₆ ionic liquid/water mix. The reductions produced alcohols with comparable enantioselectivities to baker's yeast reductions in alternative media.

Full text of DOI:10.1016/S0040-4039(01)01601-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7517–7519
Immobilized baker’s yeast reduction of ketones in an ionic liquid,
[bmim]PF6 and water mix
Joshua Howarth,* Paraic James and Jifeng Dai
School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
Received 6 July 2001; revised 10 August 2001; accepted 24 August 2001
Abstract—The bioreduction with immobilized baker’s yeast of several ketones was carried out in a 10:1 [bmim]PF6 ionic
liquid/water mix. The reductions produced alcohols with comparable enantioselectivities to baker’s yeast reductions in alternative
media. © 2001 Elsevier Science Ltd. All rights reserved.
Baker’s yeast has been used to carry out a variety of
transformations in synthetic organic chemistry. How-
ever, its use in this area has been limited by the
necessity to employ aqueous solvent systems. For such
bioreagents/catalysts to become more widely applicable
in synthetic chemistry they need to operate and retain
their selectivity in solvents more compatible with
organic compounds. There have been several reports
discussing alternative organic solvents such as liquefied
petroleum gas,1 hexane,2 benzene,3 toluene,4 carbon
tetrachloride,4 and petroleum ether.5 The main advan-
tage of employing such solvents is the ease with which
the product can be isolated. The drawbacks associated
with the use of organic solvents are their potential
toxicity to the bioreagent/catalyst, and problems associ-
ated with their disposal.
whole-cell biotransformations such as yeast mediated
reductions of ketones in an ionic liquid. This would
combine the advantages of whole cell bioreagents with
the advantages of ionic liquids, principally their recy-
clable nature. For the purposes of the investigation we
chose the readily available ketones 1–7 for reduction. It
has been shown that the inactivation of enzymes in
organic solvents can be avoided if the enzyme is sur-
rounded by a layer of water.12 Thus, we decided to add
a quantity of water to the ionic liquid. The ionic liquid
we chose to use was the moisture stable 1-butyl-3-
methylimidazolium
phosphorous
pentafluoride,
[bmim]PF6, because like previous organic solvents
explored1–5 it is essentially immiscible with water. We
also decided to employ immobilized baker’s yeast (IBY)
in order to simplify the work-up procedure whereby the
IBY could be removed by filtration. The method of
yeast immobilization that we chose was encapsulation
in calcium alginate beads,13 as this is a cost effective
and facile method of immobilization. We also added a
quantity of methanol as an energy source2 to the reac-
tion system, Scheme 1, (see Ref. 14 for details of
reaction procedure).
Concurrent research on ionic liquids has shown that
they support a large and diverse range of organic
reactions, these include amongst many others oxida-
tions,6 coupling reactions,7 nucleophilic displacements,8
reductions,9 and alkylations.10 There have also been
reports of purified enzyme reactions in ionic liquids.11
We speculated that it may be possible to carry out
It should be noted that, whilst in an aqueous system the
coenzyme, NADPH, necessary for reduction is, through
various metabolic pathways within the yeast, continu-
ously recycled, this does not occur in non aqueous
systems. Thus, the extent of the reaction is limited by
the initial concentration of the coenzyme within the
yeast.5
O
OH
[bmim]PF6 : H2O ( 10:1 )
R2
R1
R2
R1
MeOH, IBY
Scheme 1.
The results from our investigation are given in Table 1.
We found that the yield of product varied over the
range of substrates, some giving poor yields whilst
Keywords: baker’s yeast; ionic liquid; reduction.
* Corresponding author. Tel.: +353 1 7005312; fax: +353 1 7005503;
e-mail: joshua.howarth@dcu.ie
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01601-X

7518
J. Howarth et al. / Tetrahedron Letters 42 (2001) 7517–7519
Table 1.
Entry
1
Ketone substrate
Entry
Alcohol product
Yield % Lit. ( )
35
Ee % Lit. ( )
[h]2D0 Lit. ( )
1a
–
–
O
OH
2
2a
20
–
–
O
OH
3
4
5
3a
4a
5a
40 (18)15
22 (90)17
70 (78)5
79 (82)15
95 (74)17
95 (97)5
+9.3
(+11.7)16
O
OH
+38.2
(+40.0)17
O
O
O
O
OH
OH
O
O
+41.0
(+43.0)18
OEt
6a
OEt
6
75 (66)19
84 (99)19
+12.3
(+14.7)19
O
OH
O
O
OEt
OEt
7
7a
60 (43)20
76 (91)20
−7.1
(−9.4)21
O
O
OEt
OEt
O
OH
for yeast reductions carried out in alternative media. In
general the enantiomeric excesses obtained in an ionic
liquid medium were comparable to those obtained in
other media, although entry 4a was significantly higher
and entry 7a significantly lower.
others gave good yields. When we attempted to reduce
the two aromatic ketones, 4-bromoacetophenone and
4-methoxyacetophenone, we observed no reduction and
only starting material was recovered. The literature
values for enantiomeric excesses given in Table 1 are

J. Howarth et al. / Tetrahedron Letters 42 (2001) 7517–7519
7519
We also found that under high vacuum that it was
possible to distil the alcohols 1a and 2a directly from
the [bmim]PF6. This negated the extraction with
organic solvents. The ionic liquid [bmim]PF6 was recy-
cled after use in the reactions. We also noted that whilst
these reactions may be carried out in the absence of
water the yields and enantiomeric excesses were
extremely poor, probably because of the inactivation of
the enzyme within the yeast responsible for the reduc-
tion as stated above.
10. Earle, M. J.; McCormac, P. B.; Seddon, K. R. Chem.
Commun. 1998, 2245.
11. (a) Cull, S. G.; Holbrey, J. D.; Vargas-Mora, V.; Seddon,
K. R.; Lye, G. J. Biotechnol. Bioeng. 2000, 69, 226; (b)
Madeira Lau, R.; Van Rantwijk, F.; Seddon, K. R.;
Sheldon, R. A. Org. Lett. 2000, 2, 4189; (c) Erbeldinger,
M.; Mesiano, A. J.; Russell, A. J. Biotechnol. Prog. 2000,
16, 1129.
12. Katyar, S. S.; De Tapas, K. Biochem. Ind. 1990, 20, 1127.
13. Takeda, A.; Sakai, T.; Nakamura, T.; Fukuda, K.;
Amano, E.; Utaka, M. Bull. Chem. Soc. Jpn. 1986, 59,
3185
As far as we are aware this is first example of the use of
a whole-cell biotransformation in a moisture stable
ionic liquid, in this case [bmim]PF6, and it clearly
expands the potential and possibilities for moisture
stable ionic liquids as environmentally sound solvents
to support a broad range of synthetic organic transfor-
mations. Furthermore it points to further development
for the role of bioreagents, in particular yeast, in this
field of synthetic chemistry.
Preparation of immobilized baker’s yeast: Sodium alginate
(5 g) was added to water (200 mL) and the mixture was
stirred until the sodium alginate was completely dis-
solved. Baker’s yeast (20 g) was added to water (80 mL)
and the mixture was stirred for 2 h to produce a homoge-
nous suspension. This suspension was then transferred to
the sodium alginate solution. The combined mixture was
stirred for a further 2 h. After this time the mixture was
transferred to a dropping funnel with a 2 mm outlet. The
sodium alginate and yeast mixture was added dropwise to
aqueous calcium chloride (3% solution, 800 mL). The
resultant beads that were formed were filtered and
washed several times with water. They were then stored
in a refrigerator until required.
Acknowledgements
The authors would like to acknowledge the contribu-
tion to this research by Enterprise Ireland.
14. Bioreduction reaction procedure: The ionic liquid
[bmim]PF6 (100 mL) and water (10 mL) were mixed
together and warmed to a temperature of 33°C. Subse-
quently calcium alginate beads containing yeast (10 g)
were added to the solution that was then stirred. After 10
min methanol (2 mL) was added and the whole system
was stirred for a further 1 h. The ketone (10 mmol) was
then added and the reaction was stirred at 33°C for 72 h.
The beads were then filtered and the remaining filtrate
was extracted with diethyl ether (5×100 mL). The extracts
were combined and dried (anhyd. Na2SO4), filtered and
the solvent was removed in vacuo. The resulting oil was
purified by flash chromatography or short path distilla-
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