Hydrogen-transfer reduction of aromatic ketones in basic ionic liquids

Article abstract of DOI:10.1007/s00706-009-0161-3

Several achiral and chiral basic ionic liquids (ILs) were prepared and tested as the medium for Ru-catalyzed hydrogen-transfer reduction of different aromatic ketones. Hydrogen-transfer reduction of ketones proceeded well in achiral basic ILs using chiral catalysts. The interesting observation was made that raising the reaction temperature did not have a negative effect on enantioselectivity of the reaction. On the other hand no reaction was observed in chiral ILs.

Full text of DOI:10.1007/s00706-009-0161-3

Monatsh Chem (2009) 140:1189–1194
DOI 10.1007/s00706-009-0161-3
ORIGINAL PAPER
Hydrogen-transfer reduction of aromatic ketones in basic ionic
liquids
ˇ
Martin Hut’ka Æ Stefan Toma
Received: 21 October 2008 / Accepted: 26 January 2009 / Published online: 23 June 2009
Ó Springer-Verlag 2009
Abstract Several achiral and chiral basic ionic liquids
(ILs) were prepared and tested as the medium for Ru-cat-
alyzed hydrogen-transfer reduction of different aromatic
ketones. Hydrogen-transfer reduction of ketones proceeded
well in achiral basic ILs using chiral catalysts. The inter-
esting observation was made that raising the reaction
temperature did not have a negative effect on enantiose-
lectivity of the reaction. On the other hand no reaction was
observed in chiral ILs.
dendritic polymers [15–17], silica [18, 19], polystyrene
[20, 21], or, with hydrophilic function, for biphasic catal-
ysis [22].
Noyori’s ligand can be immobilized on an ionic liquid
moiety by introduction of imidazolium tags either at the
complexed arene [22] or at the tosyl group [23].
Ionic liquids (IL) are interesting alternative ‘‘green’’
solvents to the common organic solvents [24–30] which
have been used in many catalytic processes [31, 32]. New
generation ILs are examples of so-called task-specific ionic
liquids (TSILs) which have a desirable functional group
bonded to the imidazolium moiety. The most popular are
acidic ILs [33], which are either protonated alkylimidazole
salts [34, 35] or x-sulfonylalkylimidazolium salts [36].
Much less frequent are basic ILs, for example 1,3-dialky-
limidazolium hydroxide [37–39] or monoalkylammonium
salts of diamines which are prepared from tertiary diamines
by mono-quaternization [40, 41].
Keywords Basic ionic liquids Á Catalysis Á
Hydrogen-transfer reduction Á Stereoselective synthesis
Introduction
Asymmetric transfer hydrogenation of prochiral ketones
and imines is an effective method for preparation of opti-
cally active alcohols or amines [1–3]. The most famous
catalysts for asymmetric hydrogen-transfer reaction are
Ru(arene)(diamine) complexes with chiral ligands [4–8]. In
particular, a ruthenium catalyst with a chiral ligand such as
mono N-tosylated diphenylethylenediamine (TsDPEN;
Noyori’s catalyst) has been well studied on a variety of
substrates [9–11].
Although application of ILs in organic synthesis and
catalysis is a very active area of chemistry, few papers
report use of ILs for hydrogen-transfer reactions. Dyson
and Geldbach [22] described the modification of a ruthe-
nium complex by its immobilization to 1-butyl-2,3-
dimethylimidazolium tetrafluoroborate ionic liquid. This
complex in combination with (1S,2R)-2-amino-1,2-
diphenylethanol and (1R,2R)-N-tosyl-1,2-diphenylethyle-
nediamine was used in transfer hydrogenation of
acetophenone. Reactions were carried out at 40 °C for 24 h
and better results were obtained with the azeotropic mix-
ture HCOOH–Et3N than with NaOPri–propan-2-ol. They
proved that the ionic liquid containing the ruthenium
complex can be used several times in the same reaction.
Similar work was reported by Kawasaki et al. [23], who
described the synthesis of chiral TsDPEN immobilized on
an imidazolium ionic liquid moiety and its use for
Attempts to improve the handling and separation of the
catalyst from the reaction product, and its recycling, have
led to immobilization of this catalytic system on a variety
of supports, either inorganic or organic [12–14]. Noyori’s
catalyst has been immobilized on supports such as
ˇ
M. Hut’ka Á S. Toma (&)
Faculty of Natural Sciences, Comenius University Bratislava,
842 15 Bratislava, Slovakia
e-mail: toma@fns.uniba.sk
123

ˇ
M. Hut’ka, S. Toma
1190
Fig. 1 The structures of basic
ILs 1 and 2 used in hydrogen-
transfer reactions
O
N+
N+
N
N
-
N(SO2CF3)2
-
N(SO2CF3)2
2
1
hydrogen-transfer reduction of aryl ketones. Excess of
HCOOH–Et3N mixture was used as the solvent and the
reaction time, depending on the substrate, was 5–24 h. The
yield of 1-phenylethanol, the product of transfer hydroge-
nation of acetophenone, in the sixth recycle of the catalytic
system was 75% with 90% ee.
HCOOH in combination with a base can be used as a donor
of H- anions. For this reason we decided to synthesize
several types of basic ILs and try using these in the
hydrogen-transfer reactions of acetophenones without
addition of any external base. In this case, the only waste is
CO2, this is the main advantage of usage of a basic ionic
liquid. At the beginning of our work, two basic ILs 1 and 2
were synthesized (Fig. 1).
Joerger et al. recently tested several ILs for hydrogen-
transfer reduction of acetophenone [42]. They found that
[bmim]PF6 was the best solvent, but they were working
with six equivalents of a formic acid–triethylamine (5:2)
mixture. A disadvantage of this reaction is the relatively
long reaction time, which was at least 22 h.
Ionic liquid 1 was prepared by alkylation of DABCO
with 1-bromooctane and subsequent metathesis with
lithium bis(trifluoromethanesulfonyl)imide [43, 44]. IL 2
was prepared from N-(2-(2-(dimethylamino)ethoxy)ethyl)-
N-isopropylpropan-2-amine by quaternization with bro-
moethane and subsequent metathesis with lithium
bis(trifluoromethanesulfonyl)imide [50]. The structures of
Several authors [43–45] have prepared and character-
ized several DABCO-based ILs [46], which are not only
new and potentially useful solvents for organic synthesis
but, because of the tertiary nitrogen bonded in their
structures, may function as basic TSILs. Guanidine-based
basic ILs have been developed and employed as catalysts
for the aldol [47], Henry [48], and Knoevenagel [49]
reactions without any loss in catalytic activity after 15 runs.
Hardacre [50] published details of a series of Hu¨nig’s base-
tethered ammonium ILs that have been used to catalyze the
Knoevenagel condensation of aldehydes and ketones with
malononitrile and ethyl cyanoacetate.
1
both ILs were proved by H NMR and comparison with
published data [43, 44, 50].
Both ILs were used as reaction media for hydrogen-
transfer reaction of 2-bromoacetophenone (5b) and 4-bro-
moacetophenone (5a) as model substrates (Scheme 1). The
results obtained are given in Table 1.
Hydrogen-transfer reduction of 5b in ionic liquid 2 at
50 °C for 20 h afforded the corresponding alcohol 6b in
63% yield with 72% ee (Table 1, entry 1). Interestingly, the
same reaction carried out at 80 °C (Table 1, entry 2)
resulted in both better yield and higher ee. Good results
(Table 1, entry 3) were observed also at 60 °C and these
conditions were chosen for the reduction of different
ketones. It was also observed that shortening of the reaction
time from 20 to 2 h had a good effect both on yield and ee
(Table 1, entry 4). Reduction of 5b in ionic liquid 1 was
not successful even if it was carried out for 20 h both at 50
and 80 °C (Table 1, entries 5 and 6). Reduction of 5a
proceeded even better, and higher yields and ee of the
product were obtained (Table 1, entries 7 and 8). This is
probably because of the less hindered carbonyl moiety
In our previous paper [51], we examined the possibility
of using ILs as solvents for hydrogen-transfer reduction of
a broad range of aromatic ketones using Noyori’s catalyst.
We found that the best ILs were [emim]EtOSO3 and
ECOENG 500 (Peg-5 cocomonium methyl sulfate) and the
best yields were achieved using sodium formate as reagent.
Reactions were complicated by the fact that this salt is not
soluble in ILs and therefore some water must be added to
the reaction mixture.
No hydrogen-transfer reduction of ketones in basic ILs
has yet been reported. Herein, we report [RuCl2(p-cym-
ene)]2-catalyzed asymmetric transfer hydrogenation of
prochiral ketones in basic Hu¨nig’s base-tethered ammo-
nium ILs.
OH
O
RuCl2(p-cymene)]2 / TsDPEN
Results and discussion
HCOOH / IL / T / t
IL = ionic liquid
Br
Br
Hydrogen-transfer reactions with HCOONa as a source of
H- anions are associated with production of NaCl and
CO2, which are, both industrially and environmentally,
regarded as undesirable waste. As was mentioned above,
T = temperature
6a: 4-Br
6b: 2-Br
5a: 4-Br
5b: 2-Br
t = time
Scheme 1
123

Hydrogen-transfer reduction of aromatic ketones
1191
Table 1 Hydrogen-transfer reaction of 2-bromoacetophenone (5b)
and 4-bromoacetophenone (5a) in basic ILs
Table 2 Hydrogen-transfer reduction of various substrates
Entry
Substrate
Time (h)
Yield (%)
ee (%)
Entry Substrate IL T (°C) Time/h Yield (%) ee (%)
1
5c
5d
5e
5f
2
2
2
2
5
2
5
2
2
2
2
74
84
73
21
65
–
86
78
67
56
76
–
1
2
3
4
5
6
7
8
9
5b
5b
5b
5b
5b
5b
5a
5a
5a
2
2
2
2
1
1
2
2
1
50
80
60
60
50
80
60
60
60
20
20
20
2
63
67
70
87
–
72
91
73
97
–
2
3
4
5
5f
20
20
20
2
6
5g
5g
5h
5i
–
–
7
43
88
83
71
62
68
92
94
85
89
76
91
\5
84
88
–
8
9
20
10
11
5j
5k
O
OH
RuCl2(p-cymene)]2 / TsDPEN
HCOOH / 60°C / t
starting material was isolated from the reaction mixture.
We assume that the basicity of IL 1 is not sufficient for
formation of the active catalytic complex with Ru, which
could explain that no reaction was observed in this IL.
The best conditions that were observed for hydrogen-
transfer reaction of 5a and 5b (entries 4 and 8 in Table 1;
Scheme 2) were applied to other arylketones 5c–5k, whose
structures are given in Fig. 2. Results from the reactions
are summarized in Table 2.
R1
R2
5c-5k
R1
R2
6c-6k
O
N+
N
-
N(SO2CF3)2
Scheme 2
compared to 2-bromoacetophenone. Reduction of 5a in
ionic liquid 1 was not successful either (Table 1, entry 9).
Different behavior of these ILs can be explained by the fact
that IL 2 is more basic (pH of its 10% aqueous solution is
11.24) than IL 1 (pH of its 10% aqueous solution is 9.09).
We measured also the pH of 10% aqueous solutions of the
reaction medium (1 equivalent of basic IL and 2.25
equivalents of formic acid) and found that both were
acidic. Reaction medium (without acetophenone) contain-
ing IL 2 had pH 2.97, whereas that containing IL 1 had pH
2.86. As differences of acidity of these reaction media were
small, we decided to perform an experiment with excess of
IL 1. However, the reaction did not proceed even using
HCOOH–IL 1 = 1:1.2), and after 24 h at 60 °C only
Acetophenone (5c) afforded phenylethanol in good yield
and ee (Table 2, entry 1). Reduction of 4-fluoroacetophe-
none (5d) afforded product with higher ee and higher yield
than reduction of 2-fluoroacetophenone (5e) (entries 2, 3),
similar to the behavior observed for 5b and 5a. Two hours
reaction time was not enough for reduction of 4-methyl-
acetophenone (5f), because the corresponding alcohol was
isolated just in 21% yield (entry 4). Prolonging reaction
time to 5 h resulted in 65% yield of the alcohol 6f having
even higher ee (entry 5). Similar results were obtained in
the reduction of tetralone (5h) and indan-1-one (5i). Both
of the corresponding alcohols were obtained in good yields
and with high ee (entries 8, 9). Satisfactory results were
Fig. 2 Ketones used in
O
O
O
F
O
O
hydrogen-transfer reactions
F
F
5c
5d
5e
5f
5g
O
O
O
O
O
5h
5i
5j
5k
123

ˇ
M. Hut’ka, S. Toma
1192
also obtained with 1-acetylnaphthalene (5j) and 2-acetyl-
furan (5k) (entries 10, 11). 4-Fluorobenzophenone (5g) was
chosen as representative of the diarylketones. Reasonable
yield of the reduction product 6g was achieved only after
prolonging the reaction time (entries 6, 7).
Ts NH
Ph
NH2
Ph
Ts NH
NH2
TsEN
(R,R)-TsDPEN
Fig. 4 The structures of ligands TsDPEN and TsEN
Reduction of 4-hydroxyacetophenone did not proceed,
even after extension of the reaction time to 5 or even 20 h.
The low reactivity of this substrate could be because of the
?M effect of the hydroxyl function (under basic conditions
there is the possibility of formation of phenolate anion with
even stronger ?M effect). 2-Methoxyacetophenone, also,
did not react and combination of the strong ?M effect of
the methoxy group and its steric hindrance are probably
responsible for the negative outcome of the reaction. No
reaction was observed even for 2-acetylpyridine, and we
suppose that this could be because of its strong coordina-
tion to the Ru catalyst and thus its consequential
deactivation.
achiral (IL 2) and chiral (IL 4) basic ILs with addition of
achiral N-tosylated 1,2-ethylendiamine ligand (Fig. 4).
N-Tosylethylene-1,2-diamine was first tested in transfer
reduction of 5a in ionic liquid 2 at 60 °C with HCOONa as
H- source. The reaction went well and the corresponding
racemic alcohol 6a was obtained in 98% yield even after
1.5 h. However, reaction in chiral ionic liquid 4 was a
failure, and no reaction product was observed even after
24 h.
At the suggestion of one referee, we also performed the
reaction in the acidic ionic liquid 1-butyl-2,3-dimethylim-
idazolium diethyleneglycol monosulfate, 10% solution of
which has pH = 1.58. As reduction medium we used the
mixture HCOOH–(Pri)2NEt. We found the reaction went
well at 60 °C, giving, after 5 h, 70% yield of the product
with 93% ee.
Several unsuccessful attempts to recycle the reaction
medium in IL 2 were performed in the reduction of 4-
bromoacetophenone. No reduction of the substrate was
observed even after 24 h reaction time in the second cycle.
We do not have a reasonable explanation for this fact.
It was of interest to investigate if chiral basic ILs can
affect the stereoselectivity of the hydrogen-transfer reac-
tion. Therefore, we synthesized chiral ILs 3 and 4 (Fig. 3).
Stereogenic information in ionic liquid 3 is located in a
cation derived from (R)-2-phenylethylamine; for ionic
liquid 4 it is located in a chiral anion based on L-proline.
Hydrogen-transfer reaction of 5a under the conditions
mentioned above ([RuCl2(p-cymene)]2, HCOOH, 60 °C,
but without chiral TsDPEN ligand) was tested in ionic
liquid 3 to examine whether this chiral medium affected
the stereoselectivity of the reaction. Unfortunately, no
reaction was observed even after 3 days and unconsumed
starting material 5a was isolated. Similarly, there was no
reaction in ionic liquid 4. In this case, formic acid caused
protonation of L-prolinate anion and L-proline precipitated
from the reaction mixture. To avoid this problem we tried
to carry out the reaction in ionic liquid 4 with HCOONa as
hydrogen donor, but without success even after 24 h
reaction time. Because the reaction did not proceed without
ligand we decided therefore to perform the reaction both in
In conclusion, we have shown that hydrogen-transfer
reductions of acetophenones in basic ILs proceed with
comparable enantioselectivity as in neutral or acidic ILs,
but with lower reaction rate. This means the only advan-
tage of basic ILs as the reaction medium in this reaction is
that no external base has to be used, which is beneficial
environmentally.
Experimental
The starting ketones were purchased from Aldrich, Merck,
and Acros and used as received. [RuCl2(p-cymene)]2 cat-
alyst and TsDPEN ligand was purchased from Lancaster.
1H NMR spectra were measured on a 300-MHz Varian
Mercury Plus instrument as solutions in CDCl3 or MeOD
with TMS as the internal standard. enantioselectivity of the
reactions was determined by HPLC using a Chiracel OD-H
column with n-hexane–i-propanol (9:1) as mobile phase at
a flow of 0.75 cm3/min and UV detection at 254 nm. pH
Fig. 3 The structures of basic
ILs 3 and 4
N
N+
O
-OOC
N
N+
N
H
-
N(SO2CF3)2
4
3
123

Hydrogen-transfer reduction of aromatic ketones
1193
and those with basic pH were collected. Into this basic
solution was added, under stirring, a solution of 1.71 g L-
proline (14.9 mmol) in 5 cm3 distilled water. The resulting
mixture was then stirred at r.t. for 20 h. Water was
evaporated using a rotary evaporator and the residue was
dried under high vacuum to obtain 4 as a light brown very
was measured on with INOLAB 720 instrument (WTW
series) using a glass electrode with thermal sensor.
1-Octyl-4-aza-1-azaniabicyclo[2,2,2]octane bis(trifluo-
romethanesulfonyl)imide was prepared according to a
procedure described in literature [43, 44], and 2-[2-
(dimethylamino)ethoxy]ethyl-N,N-diisopropylammonium
bis(trifluoromethanesulfonyl)imide was prepared according
to Ref. [50].
1
viscous liquid (4.6 g, 94%). H NMR (300 MHz, CDCl3):
d = 3.88 (m, 2H, CH2 N?), 3.56 (m, 1H, *CH), 3.47 (m,
6H, ?NCH2CH3, OCH2, CH2O), 3.11 (s, 6H, ?NCH3), 3.17
(m, 1H, NHCH), 3.04 (sept, 2H, iPrCH), 2.81 (m, 1H,
NHCH), 2.66 (t, J = 6.72 Hz, 2H, CH2 N), 2.12 (m, 1H,
*CHCH), 1.85 (m, 1H, *CHCH), 1.73 (m, 2H,
*CHCH2CH2), 1.37 (tt, J = 7.3 Hz, J = 2, 3 Hz, 3H,
CH2CH3), 1.03 (d, J = 6.6 Hz, 12 H, iPrCH3).
(R)-(1-Phenylethyl)[2-(diisopropylamino)ethyl]
dimethylammonium bis(trifluoromethanesulfonyl)imide
(3, C20H33F6N3O4S2)
2-(Diisopropylamino)ethyl chloride (1.56 g, 9.57 mmol)
was added to 1.5 g (R)-N,N-dimethyl-1-phenylethanamine
(10 mmol). The flask was flushed with nitrogen, closed with
a septum, and the mixture was stirred at 60 °C for 4 days. A
light-orange precipitate could be seen in the solution. After
cooling to room temperature 15 cm3 n-hexane was added.
The supernatant solution was decanted and the solid was
washed twice with 15 cm3 n-hexane. The crude light-yellow
crystals were dried at room temperature at high vacuum to
give 3.05 g of the corresponding chloride (73%). This was
dissolved in 45 cm3 distilled water and the solution was
washed with 3 9 15 cm3 CH2Cl2. To the water solution was
then added, under stirring, in small portions, a solution of
15.15 g LiN(SO2CF3)2 (52.7 mmol) in 18 cm3 distilled
water. The mixture was stirred at room temperature for 3 h,
and the upper water layer was separated in a funnel and
washed with 3 9 10 cm3 CH2Cl2. To the bottom (ionic
liquid) layer was added 15 cm3 CH2Cl2, and the solution was
washed with 2 9 10 cm3 of distilled water. CH2Cl2 solu-
tions were combined and dried over MgSO4. After filtration
to remove the MgSO4, CH2Cl2 was evaporated. Product 3
General procedure for hydrogen-transfer reduction
[RuCl2(p-cymene)]2 (3 mg, 0.005 mmol, 5 mol%) and
5 mg TsDPEN (0.012 mmol) were added to 0.6 cm3
degassed ionic liquid. The mixture was stirred under
nitrogen atmosphere at 60 °C for 40 min. The orange
solution was cooled to room temperature and 1 mmol of
the corresponding ketone 5 was then added. The mixture
was stirred at room temperature for 5 min and 1.88 cm3
HCOOH (5 mmol) was then added. The mixture was
heated at 60 °C and stirred for the corresponding time
during which it was monitored by TLC on silica using ethyl
acetate–n-hexane (1:6) as mobile phase. The crude reaction
mixture was purified by column chromatography on silica
(Davisil) with ethyl acetate–n-hexane (1:6) as eluent.
All alcohols 6 as the products of hydrogen-transfer
reactions were characterized by 1H NMR (300 MHz)
spectroscopy. Enantiomeric excess of chiral alcohols was
determined by HPLC using a chiral Chiracel OD-H col-
umn. Results of hydrogen-transfer reductions are given in
Tables 1 and 2.
1
was obtained as a light-yellow oil (9.9 g, 96%). H NMR
(300 MHz, CDCl3): d = 1.02 (d, 12H, 4 9 CH3 from iPr),
1.81 (d, 3H, CH3), 2.94 (m, 2H, CH2 N), 3.04 (m, 2H,
?NCH2), 3.02 (s, 3H, ?NCH3), 3.07 (s, 3H, ?NCH3), 3.28
(m, 2H, 2 9 CH from iPr), 4.75 (q, 1H, *CH), 7.487 (s, 5H,
ArH).
The products obtained were: 1-(4-bromophenyl)ethanol
(6a) [52]: colorless oil (76–91.5%); 1-(2-bromophe-
nyl)ethanol (6b) [53]: colorless oil (62.8–87%);
1-phenylethanol (6c) [54]: colorless oil (73%); 1-(4-fluo-
rophenyl)ethanol (6d) [55]: colorless oil (84.2%), 1-(2-
fluorophenyl)ethanol (6e) [56]: colorless oil (73.2%);
1-(4-methylphenyl)ethanol (6f) [60]: colorless oil (21–
65%); 4-fluorophenylphenylmethanol (6g) [57]: white solid
(42%), m.p.: 46–48 °C; 1,2,3,4-tetrahydronaphthalene-1-ol
(6h) [58]: colorless oil (87.7%); indane-1-ol (6i) [58]:
white solid (82.8%), m.p.: 54-56 °C; 1-(naphth-1-yl)etha-
nol (6j) [59]: colorless oil (71%); 1-(furan-2-yl)ethanol
(6k) [60]: colorless oil (61.5%).
2-(2-(Diisopropylamino)ethoxy)-N-ethyl-N,N-
dimethylethanammonium (S)-pyrrolidine-2-carboxylate
(4, C19H41N3O3)
Reaction was started by preparation of Annex AT in OH
mode. An Annex AT 0806 (27 g, 150 cm3) column was
washed with 750 cm3 distilled water, which is five times
the volume of Annex AT, then with 750 cm3 2 M NaOH
solution [5 9 V(Annex)], and, finally, again with 1.5 dm3
distilled water [10 9 V(Annex)] until the pH of the
washing water was neutral. A saturated solution of 4.4 g
2-[2-(dimethylamino)ethoxy]ethyl-N,N-diisopropylammonium
bromide (13.5 mmol) in 5 cm3 distilled H2O was than
applied to the top of the column, which was then washed
with distilled water. The pH of the fractions was monitored
Acknowledgments Our thanks are due to the Marie Curie Fellow-
ship for a 6 month stay of M.H. at Queen’s University Belfast and to
the VEGA Grant Agency for financial support (grant. no. 1/3569/06).
123

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M. Hut’ka, S. Toma
1194
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