Michael additions of methylene active compounds to chalcone in ionic liquids without any catalyst: The peculiar properties of ionic liquids

Article abstract of DOI:10.1002/chem.200600870

Michael additions of malonodinitrile as well as several other reagents to chalcone have been found to proceed well in pure ionic liquids, without the addition of any catalyst. The catalytic effect of the residual acidity caused by hydrolysis of ionic liqu

Full text of DOI:10.1002/chem.200600870

DOI: 10.1002/chem.200600870
Michael Additions of Methylene Active Compounds to Chalcone in Ionic
Liquids without any Catalyst: The Peculiar Properties of Ionic Liquids
[a]
ˇ
Mµria Mecˇiarovµ and Stefan Toma*
Abstract: Michael additions of malonodinitrile as well as several other reagents to
chalcone have been found to proceed well in pure ionic liquids, without the addi-
tion of any catalyst. The catalytic effect of the residual acidity caused by hydrolysis
of ionic liquids anions was excluded because HCl in dichloromethane did not cata-
lyse the Michael addition of malonodinitrile. Piperidine was tested as the catalyst
and was found to be a much better catalyst in ionic liquids than in dichlorome-
thane. Therefore, the following question arose: what is the effect of ionic liquids
À
Keywords: C H
ionic liquids
compounds
organic synthesis
activation
methylene active
Michael addition
·
·
·
·
À
on the dissociation constants of C H acids?
Introduction
tion time is a rather long two to three days and the best en-
antiomeric excess (ee) and yields were achieved after the ad-
dition of an equivalent amount of base (piperidine or piper-
azine).
In recent years, ionic liquids have emerged as frequently
used “green” solvents for many organic reactions, including
transition-metal and biocatalysed reactions.[21–28] Dell’Anna
found that [bmim]BF4 is a good solvent for the addition of
À
The Michael addition is one of the most frequently used C
C bond-forming reactions in organic synthesis.[1–5] The cata-
lysts used include different bases, such as the rubidium salt
of l-proline.[6–8] Organocatalysts have also been used for Mi-
chael additions of aldehydes and ketones to unsaturated car-
bonyl derivatives or nitrostyrenes.[9–16] Reactions are usually
carried out either in DMSO or by using an excess of one re-
agent as the solvent. High amounts of the catalyst (10–
20 mol%) should be used and reaction times are very long
20–180 h). l-Proline alone was found to be a rather ineffec-
tive catalyst, but its catalytic efficiency raised considerably
after the addition of up to 80 mol% of trans-2,5-dimethylpi-
perazine or piperazine as described by Hanessian.[12] McMil-
lanꢀs catalyst also gave just medium yields upon the Michael
addition of different aldehydes to methyl vinyl ketone, but
yields were raised after the addition of 4-ethoxycarbonylca-
techol.[17] Ley et al. described the best catalyst for the Mi-
chael addition of ketones to nitrostyrene[18] and similarly of
nitroalkanes to cyclohex-2-ene-1-one and benzylideneace-
tone[19] as well as dimethyl malonate[20] to 5-(pyrrolidin-2-
yl)-1H-tetrazole. They screened a range of solvents and
found that dichloromethane was the best; however, the reac-
acetylacetone to methyl vinyl ketone when NiCAHTRE(UNG acac)2 is used
as the catalyst.[29] Yadav described [bmim]BF4 as an excel-
lent solvent for the Michael additions of b-ketoesters to
methylvinyl ketone, cyclohexenone and cyclopentenone
when copper(ii) triflate was used as the catalyst.[30] We have
found that l-proline in ionic liquids is a very good catalyst
for the Michael addition of aliphatic aldehydes and ketones
to b-nitrostyrenes.[31] Rasalkar very recently described the l-
proline-catalysed Michael addition of ketones to nitrostyr-
ene.[32] He tested several ionic liquids and 1-methoxyethyl-3-
methylimidazolium methanesulfonate ([MOEMIM]OMs)
was found to be the best. To achieve good yields, it was nec-
essary to prolong the reaction time up to 60 h and catalyst
loading had to be increased up to 40 mol% to achieve
75% ee. Very recently, Hagiwara described[33] the organoca-
talysed addition of aliphatic aldehydes to methyl vinyl
ketone in ionic liquid [bmim]PF6. 2-(S)-(1-Morpholinome-
thyl)piperidine was found to be the best organocatalyst, but
the yields of the product were only medium with 11–
51% ee.
ˇ
ˇ
[a] Dr. M. Meciarovµ, Prof. Dr. S. Toma
Department of Organic Chemistry
Faculty of Natural Sciences, Comenius University
Mlynskµ dolina CH2, 842 15 Bratislava (Slovakia)
Fax : (+421)2-6029-6690
In our recent work, we described the successful Michael
additions of thiols to unsaturated ketones in ionic liquids
without the addition of any base.[34] The main aim of this
E-mail: toma@fns.uniba.sk
1268
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Chem. Eur. J. 2007, 13, 1268 – 1272

FULL PAPER
work was therefore to examine if the Michael addition of
carbon nucleophiles can proceed in ionic liquids without any
basic catalyst.
Results and Discussion
To start our research, we decided to study solvent effects on
the reaction of malonodinitrile (1a) to chalcone (2).
(Scheme 1). Reactions were performed in several ionic
Scheme 1. Michael addition of methylene active compounds to chalcone.
IL=ionic liquid.
Figure 1. Structures of ionic liquids. IL1=1-Butyl-3-methyl imidazolium
hexafluorophosphate ([bmim]PF6); IL2=1-butyl-1-methylpyrrolidinium
tris(pentafluoroethyl)trifluorophosphate; IL3=1-butyl-3-methylimidazo-
lium ethylsulfate ([emim]SO4Et, ECOENGTM212); IL4=1-hexyl-3-
methylimidazolium bis(trifluoromethylsulfonyl)imide; IL5=1-butyl-3-
methylimidazolium trifluoromethanesulfonate; IL6=1-hexyl-3-methyl-
imidazolium tris(pentafluoroethyl)trifluorophosphate; IL7=tributyl-
liquids. All the ionic liquids that we used were very pure
(no UV absorbance from 200–400 nm) and neutral with the
exception of IL1 (pH 6.2; its hydrolysis was described in ref-
erence [35]), IL5 (pH 4.2) and IL7 (pH 2.3), which are
acidic, and IL8, which is basic. The results are summarised
in Table 1. Structures of ionic liquids are given in Figure 1.
AHCTRE(GNU ethyl)phosphonium diethylphosphate; IL8=1-butyl-3-methylimidazoli-
um dicyanamide.
nucleophiles could be an acid-catalysed reaction, as we dis-
covered in the additions of thiophenol to chalcone.[34] At-
tempts on the HCl-catalysed reaction in dichloromethane
(Table 1, Entry 9) proved that the reaction cannot be cata-
lysed by acids. From the results given in Table 1 it can be
concluded that ionic liquids have a special effect on the Mi-
chael addition of malonodinitrile to chalcone.
Table 1. Study of the solvent effect on the addition of malonodinitrile
(1a) to chalcone (2) (22 h, room temperature) without any catalyst.[a]
Entry
Solvent
Yield of 3a [%]
1
2
3
4
5
6IL
7
8
9
IL1
IL2
IL3
IL4
IL5
6
IL7
IL8
CH2Cl2
76
53
95
15a
0[a]
10[a]
88[a]
95
We can speculate that malononitrile is acidic enough to
be partially dissociated and ionic liquids are able to enhance
the nucleophility of its anion. It is of interest to note that
the higher nucleophilicity of halide anions in IL than in con-
ventional solvents was described by Chi et al.,[38–40] and for
water by Kim.[41] The higher nucleophilicity of halide anions
in ionic liquids was made questionable in papers by
Welton[42,43] and Landini.[44] Landini[44] proved that the nucle-
ophilicities of halide ions and the azide ion in [hexmim]PF6
are comparable with their nucleophility in methanol and is
slightly lower than in DMSO. On the other hand, Welton[43]
found that amines are more nucleophilic in 1-butyl-1-meth-
ylimidazolium trifluoromethylsulphonate and 1-butyl-1-
methylpyrrolidinium trifluoromethylsulphonate than in di-
chloromethane and acetonitrile. Lancaster[45] also proved
the higher nucleophilicity of amines in ionic liquids, but
found that the nucleophilicity of halide anions is lower in
ionic liquids than in dichloromethane and that the nucleo-
philicity order of different halides strongly depends on the
structure of ionic liquids.
0 (0[b]
)
[a] The same results were obtained when 5 mol% of l-proline as the cat-
alysts was used. [b] 3–4 drops of HCl was added to the reaction mixture.
Reactions went smoothly in IL1, IL2 and IL3, which are
more or less neutral, but very good results were also ach-
ieved in acidic ionic liquid IL7 and in basic ionic liquid IL8.
The observation of the catalyst-free Michael addition in
ionic liquids is very surprising, even though Michael addi-
tion in pure [bmim]BF4 was briefly mentioned in a paper by
Fan,[36] but without explanation. The observation of Ranu
that Michael additions of carbon nucleophiles proceed well
in [bmim]OH is not surprising, because this is in fact a basic
solvent.[37] On the other hand, the product 3a was isolated
in very low yield (15 and 10%) in IL4 and IL6 and no addi-
tion reaction was observed in IL5. We do not have any ex-
planation for this fact. Due to the fact that the Michael ad-
dition of malonodinitrile was also successful in acidic ionic
liquids, we decided to test if the Michael addition of carbon
It was of interest to know if this peculiar property of ionic
liquids would be apparent upon the Michael addition of
other methylene active compounds and we decided, there-
fore, to extend the range of nucleophiles..
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1269

S. Toma and M. Me›iarovµ
53% yield in IL2 and 95% yield in IL3 and in IL8. Methyl
cyanoacetate (1e) gave 34% of 3e without any catalyst in
IL1 after 90 h at room temperature, while only traces of 3e
were detected (TLC) after 22 h. However, the product 3e
was isolated in 54% yield in IL3 (22 h at room tempera-
ture). Similarly, nitromethane (1 f) was unreactive in IL2,
while the product 3 f was isolated in 71% yield when the re-
action was performed in IL3 under the same conditions. On
the other hand, we do not have any explanation as to why
acetylacetone (1b) did not undergo reaction, while diben-
zoylmethane (1i) gave a reasonable yield (28%, 8 h, 808C in
IL3) of the product 3i. It is of interest to note that Gryko[46]
also described Michael additions with dibenzoylmethane
and its analogues, but not with acetylacetone. 2-Phenylpro-
panal did not undergo Michael addition without the pres-
ence of any catalyst or in IL3 (acidic IL) or IL8 (basic IL).
Prolonging the reaction time to 100 h did not affect the re-
action course (Table 2, entries 5 and 6). The same results
were obtained when the Michael addition of cyclohexanone
(1h) on chalcone was performed (Table 2, entries 15
and 16).
The results of the additions without any catalyst are given
in the Table 2. We checked the possibility of performing the
addition of nine different methylene active compounds 1a–i
Table 2. Michael additions of selected active methylene compounds to
chalcone (2) in ionic liquids without any catalyst.
From the results given in Table 2, it follows that ionic liq-
uids are good media for catalyst-free Michael additions with
a range of nucleophiles and we can speculate how they in-
fluence the reaction course. The explanation, which suggests
the higher nucleophilicity of carbon nucleophiles in ionic
liquids, analogously as was described for water,[41]
amines[43,45] and thiols[34] is not sufficient because we have
used methylene active compounds and not a real nucleo-
phile, which would be formed from such a neutral reagent
by addition of base. It is necessary to stress that we did not
add any base to the reaction mixture by which a carbon
nucleophile (anion) can be formed. It seems, therefore, rea-
Entry Nucleophile Solvent Reaction conditions Product Yield [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1b
1b
1d
1d
1e
1e
1 f
1 f
1h
1h
1i
IL1
IL2
IL3
IL8
IL3
IL8
IL3
IL5
IL1
IL3
IL2
IL3
IL3
IL8
IL3
IL3
22 h/RT
22 h/RT
22 h/RT
22 h/RT
22 h/RT
100 h/RT
100 h/RT
100 h/RT
90 h/RT
22 h/RT
16h/80 8C
8 h/808C
22 h/RT
100 h/RT
22 h/RT
8 h/808C
3a
3a
3a
3a
–
76
53
95
95
0[a]
0[a]
39
–
3d
3d
3e
3e
–
3 f
–
–
41
34
54
9
10
11
12
13
14
15
16
0[a]
71
0[a]
0[a]
0[a]
À
sonable to believe that dissociation constants of C H acids
–
3i
are very different in ionic solvents than in conventional sol-
vents, or that ionic liquids are shifting oxo–enol tautomeria
of methylene active compounds towards the their enol
forms. The other possible explanation for the observed
behaviour is described by anion stabilization by ionic
liquids.[47,48]
1i
28
[a] Only starting material was detected in the reaction mixture.
to chalcone (2) in different ionic liquids. The most reactive
nucleophile 1a gave the product 3a in 76% yield in IL1,
The fact that dimethyl malonate (1c) and 2-nitropropane
(1g) did not undergo reaction with l-proline catalysis in di-
chloromethane was observed by Ley et al.[18–20] They rea-
soned that this is caused by their lower acidity and a stron-
ger base should be added to the reaction mixture. Their rea-
soning was right, as after the addition of one equivalent of
the stronger base (piperidine or meso 2,5-dimethylpipera-
zine) to the reaction mixture, they achieved high yields of
the addition products, even though the reaction time was
rather long (up to three days).
We decided, therefore, to examine the effect of l-proline
or piperidine on the reaction course. Results are given in
Table 3.
Michael addition of malonodinitrile (1a) was going well
with both l-proline and piperidine as the catalysts. Piperi-
dine was a much better catalyst, because just 2 h was
enough to reach 91% yield of the product 3a, whereas 22 h
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Chem. Eur. J. 2007, 13, 1268 – 1272

The Peculiar Properties of Ionic Liquids
FULL PAPER
Table 3. Results of the Michael addition of carbon nucleophiles to chal-
cone (2) under catalysis in different solvents at room temperature.
Experimental Section
Entry
Donor
Solvent
Catalyst[a]
t [h]
Product
Yield [%]
General: NMR spectra were measured on a Varian Gemini 2000 spec-
trometer operating at 300 MHz (1H NMR) and 75 MHz (13C NMR); tet-
ramethylsilane was used as an internal standard. MS spectra were mea-
sured at Micromass ZMD ESI (80 eV) system and substances were dis-
solved in acetonitrile/water (80:20). Elemental analysis were performed
on a Carlo–Erba instrument. Enantioselectivity for the purified products
was determined by HPLC (Krüss P3002RS instrument) on a chiral
column Chiralcel OD-H by using n-hexane/2-propanol (90:10 v/v) as an
eluent and cellulose tris(3,5-dimethylphenylcarbamate) coated on 5 mm
silica-gel as a packing composition. MS data were obtained on a Hew-
lett–Packard, Agilent 1100 Series MSD HPLC-MS instrument. Organo-
catalysts and starting materials were purchased in reagent grade (Aldrich,
Acros, Fluka, Merck) and used without further purification. Ionic liquids
were purchased from Solvent Innovation and from Merck.
1
2
3
4
5
6
7
8
1a
1a
1c
1c
1c
1c
1g
1g
1g
1g
1g
1g
IL3
IL3
IL3
IL3
CH2Cl2
CH2Cl2
IL1
IL1
IL3
l-proline
piperidine
l-proline
piperidine
l-proline
piperidine
l-proline
piperidine
l-proline
piperidine
l-proline
piperidine
22
2
22
1
22
22
12[b]
3
22
3
3a
3a
3a
3c
3c
3c
3g
3g
3g
3g
3g
3g
95
91
0[c]
59
0[c]
0[c]
31
90
25
95
0[c]
0[c]
9
10
11
12
IL3
CH2Cl2
CH2Cl2
22
22
[a] 5 mol% of the catalysts were used. [b] Reaction at 808C. [c] Only
General experimental procedure: The ionic liquid (1 mL) was degassed
by stirring under reduced pressure (oil pump), then catalyst (5 mol%)
and chalcone 2 (0.208 g, 1.0 mmol) were added and the mixture was stir-
red for 15 min at room temperature. Nucleophile 1 (1.5 mmol) was then
added and the resulting reaction mixture was stirred intensively for the
specified time and temperature (see Tables 1, 2, and 3). The product was
extracted by using several portions of diethyl ether and the combined ex-
tracts were evaporated in vacuo and purified by column chromatography
on SiO2 (hexane/ethyl acetate 4:1 or hexane/dichloromethane 2:1). Prod-
ucts were isolated as pure materials and their structure was proven by
1H NMR spectra and new compounds were completely characterised.
The spectroscopic characteristics of already known products, 3a,[49] 3c,[50]
3d,[51] 3e,[52] 3 f,[53] 3g[54] and 3i[55] were in agreement with published data.
starting material only was detected in the reaction mixture.
were necessary to achieve 95% of 3a when using l-proline
as a catalyst.
It is not surprising that dimethyl malonate (1c) and 2-ni-
tropropane (1g) did not react without any catalyst because
they are less acidic than other reagents. The fact that di-
methyl malonate (1c) and 2-nitropropane (1g) did not un-
dergo reaction with l-proline catalysis in dichloromethane
was observed by Ley et al.[18–20] They reasoned that it is due
to their lower acidity and a stronger base should be added
to the reaction mixture. Addition of dimethyl malonate (1c)
also failed under l-proline catalysis both in IL3 and in di-
chloromethane (Table 3, entries 3 and 5). Starting material
was only detected in the reaction mixture after 22 h at room
temperature. The reaction catalysed with 5 mol% of piperi-
dine in IL3 gave, after 1 h, 59% of 3c, while after 22 h in di-
chloromethane only traces of 3c were detected by TLC
(Table 3, entries 4 and 6).
Acknowledgements
This work was supported by the Slovak Grant Agency VEGA, grant No.
1/0072/03. We are grateful to CYTEC Canada for donation of ionic
liquid IL7. NMR measurements were enabled by the Slovak State Pro-
gram project No. 2003SP200280203.
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temperature when the reaction was performed in IL1 and
95% of 3g was obtained in IL3 (Table 3, entries 8 and 10).
l-Proline-catalysed addition of 1g on chalcone in IL1 after
22 h gave 31% of the adduct 3g and 25% in IL3 (Table 3,
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À
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Received: June 21, 2006
Published online: November 8, 2006
1272
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Chem. Eur. J. 2007, 13, 1268 – 1272