Higher enantioselectivities in thiourea-catalyzed Michael additions under solvent-free conditions

Article abstract of DOI:10.1016/j.tet.2013.12.029

Enantioselective Michael additions catalyzed by hydrogen-bonding catalysts produce many important compounds. Solvent-free reaction conditions in a ball mill can provide an improved enantioselectivity over the reaction in solution, due to lack of disruptiv

Full text of DOI:10.1016/j.tet.2013.12.029

Tetrahedron 70 (2014) 901e905
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Higher enantioselectivities in thiourea-catalyzed Michael additions
under solvent-free conditions
ꢁ
*
ꢀ
Martina Hestericova, Radovan Sebesta
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska dolina CH-2, SK-84215 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
Enantioselective Michael additions catalyzed by hydrogen-bonding catalysts produce many important
compounds. Solvent-free reaction conditions in a ball mill can provide an improved enantioselectivity
over the reaction in solution, due to lack of disruptive solvation of reagents. A range of 15 structurally
Received 30 September 2013
Received in revised form 28 November 2013
Accepted 10 December 2013
Available online 18 December 2013
different hydrogen-bonding organocatalysts were tested in two Michael additions to
b-nitrostyrene
under solvent-free conditions and compared with corresponding experiments in solution. With several
thiourea catalysts, these Michael additions proceeded with higher enantioselectivities under solvent-free
conditions than in solution.
Keywords:
Asymmetric organocatalysis
Hydrogen bond
Thiourea
Ó 2013 Elsevier Ltd. All rights reserved.
Michael addition
Mechanochemistry
1. Introduction
of the Sheldon E-factor and the increase of chemical process envi-
ronmental sustainability.11
Catalysis mediated by hydrogen bonding, which is widely
present in the nature, has recently become a prominent non-
covalent organocatalytic approach enabling a broad spectrum of
transformations.1 Many enzymes, such as serine proteases and al-
dolases,2 HIV-1 protease,3 4-chlorobenzoyl CoA-dehalogenase,4 or
chymotrypsine,5 exert their catalytic activities through hydrogen
bonding. Low molecular weight hydrogen-bond organocatalysts
operate by a biomimetic approach that includes orientation of re-
agents and stabilization of the transition state. Bifunctional and
trifunctional organocatalysts often try to mimic enzymatic systems
in simultaneous activation of both electrophile and nucleophile.6,7
Organocatalysis, due to the typically benign nature of its cata-
lysts, is also compatible with many non-traditional methodologies
in organic synthesis, such as mechanical activation, ultrasound ir-
radiation, microwave heating, non-conventional solvent or flow
techniques.8,9 Ball milling is a mechanochemical technique well
established in the chemical industry, which is being increasingly
used also in organic synthetic laboratories.10 The solvent-free re-
actions profit from higher reactivities, because the reactants are not
solvated. Furthermore, bypassing the solvent use or eventual re-
duction of the amount of solvents is a key way to achieve reduction
Potential for the use of ball milling in asymmetric organo-
catalysis has already been recognized.12 Bolm and co-workers were
the first to show that the aldol reaction can be performed under ball
milling conditions using proline as a catalyst.13,14 Other aldol re-
actions, operating via enamine activation, have then been de-
scribed in the ball mill too. Najera and co-workers used BINAM
prolineamides as catalysts.15e17 Juaristi and co-workers showed
that dipeptides based on proline are useful catalysts for mecha-
nochemical aldol reactions.18e20 Enantioselective Michael addition
of aldehydes to nitroalkenes, catalyzed by pyrrolidine derivatives,
also work well under solvent-free conditions.21 Other organo-
catalytic activation modes have been much less explored under
solvent-free conditions. Chimni and co-workers demonstrated that
even simple grinding is an effective tool for Michael additions
catalyzed by several Cinchona-based organocatalysts.22 Asymmet-
ric phase-transfer catalysis benefited from solvent-free conditions,
as Lamaty’s group showed during alkylations of glycine Schiff ba-
ses.23 Organocatalysis mediated by hydrogen-bond donors also
work well using ball milling. Xu and co-workers reported
squaramide-catalyzed Michael addition of 1,3-dicarbonyl com-
pounds to nitroalkenes.24 More recently, Bolm and co-workers
showed that chiral thioureas catalyze 1,4-addition of nitroketones
to nitroalkenes.25 Furthermore, they also noticed higher stereo-
selectivity of the Michael addition under solvent-free conditions
than in solution.
* Corresponding author. E-mail addresses: sebesta@fns.uniba.sk, radovan.se-
ꢁ
besta@gmail.com (R. Sebesta).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.12.029

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M. Hestericova, R. Sebesta / Tetrahedron 70 (2014) 901e905
902
Houk and co-workers showed that the energy differences be-
tween hydrogen bonds are sensitive to solvents. These energy
differences decrease rapidly with increasing solvent polarity.26 It
has been showed that enantioselectivities of hydrogen bond cata-
lyzed reactions are higher in less polar solvents than in polar
ones.27e29 Furthermore, the formation of parasitic hydrogen bonds
between protic solvents and reagents may further decrease yields
and enantioselectivities of such reactions.30 This also implies that
hydrogen bond catalysis may proceed better in the absence of
solvent.
In this context we decided to investigate the effect of solvent-
free reaction conditions on organocatalytic reactions mediated by
hydrogen-bonding catalysts. Specifically, we would like to compare
organocatalytic reactions performed in organic solvent with those
performed under solvent-free conditions. In this respect, chiral
thioureas seem like an excellent choice of catalysts, because of their
widespread use in asymmetric organocatalysis.6,31e33
As benchmark reactions, we selected two Michael additions,
because these reactions are useful for the synthesis of many bi-
ologically relevant compounds. Nitroalkenes are among the most
attractive Michael acceptors, because their nitrogroup is a strong
electron-acceptor and it can be easily transformed into a number of
other functional groups.34 It is also well established that thioureas
activate the nitroalkene through two hydrogen bonds.35 Further-
more, bifunctional catalysts can often interact with a nucleophile
too. As nucleophiles, we used indole and dimethyl malonate, which
both can be directed to the electrophile via secondary hydrogen
bond interaction. Calculations show that activation of reactants can
even be reversed. In either way, bifunctional catalysts activate both
reacting partners and stabilize corresponding transition states via
a network of several hydrogen bonds.36 The first model reaction
leads to optically active indole derivatives, which can be useful for
the synthesis of biologically active compounds, such as tryptamines
and 1,2,3,4-tetrahydro-b
-carbolines,34 inhibitors of exonic splicing
accelerator,37 inhibitors of cyclin-dependent kinase,38 or indole-3-
methanol, which can suppress the cancer development by inhib-
iting cell proliferation.39 The second model reaction is a Michael
addition of dimethyl malonate to trans-b-nitrostyrene, which is
Fig. 1. Organocatalysts used in this study.
widely used to screen the effectiveness of asymmetric organo-
catalysts.40 It produces an optically active derivative of dimethyl
malonate, which can be useful for instance in the synthesis of
pregabalin, a drug for epilepsy and neuropathic pain treatment.41
diamine (C5)29 and a bis(thiourea) C6.50 Furthermore we have
screened also two squaramides C7a,b.51,52
The solvent-free reactions were carried out using a vibrational
ball mill in a container made from stainless steel with one steel ball
with 5 mm diameter. Using two balls often resulted in stacking balls
inside the container. The vibration frequency was 20 Hz. The
loading was approximately 0.3 mmol, with 20 mol % of catalyst in
all cases. The reactions in solution were performed in dichloro-
methane at 0.1 mmol scale at room temperature for 72 h. For the
sake of comparison, no attempts were made to optimize experi-
ments with individual catalysts. Blank reactions without any cata-
lysts, under otherwise identical conditions were also performed.
The reaction in the ball mill afforded 50% of adduct 3 after 6 h. Use
of acidic additives such as benzoic or mandelic acid (20 mol %)
increased the yield of adduct 3 to 80 and 95%, respectively, and
these experiments were used for the preparation of racemic sam-
ples for the determination of enantiomer purity. The blank reaction
in dichloromethane afforded adduct 3 in 35% yield after 72 h. Table
2. Results and discussion
We started our investigation with the Michael addition of indole
(1) to trans-b-nitrostyrene (2) (Scheme 1). The addition afforded
product 3 as a main product. The reactions were carried out in a ball
mill in the absence of solvent and for comparison in solution, ex-
emplified by a typical solvent for this reaction, dichloromethane.
Scheme 1.
1 collects data from the reaction of indole (1) with
(2) using 15 hydrogen-bond donating organocatalysts C1eC7.
The addition of indole (1) to -nitrostyrene (2) proceeded with
generally good yields in solution. On the other hand under solvent-
free conditions there was larger variation in yields ranging from 19
to 95% of product 3. We have also performed two solvent-free ex-
periments with just dry stirring. With catalysts C2b and C4a, con-
versions were 19% and 26%, respectively, after 36 h. It means that
b-nitrostyrene
As the hydrogen-bond catalysts we selected a range of typical
thioureas and squaramides (Fig. 1). We have used Jacobsen-type
thioureas C1aec,42e44 Takemoto-type catalysts C2a,b,28,45 then
four thioureas with aminoalcohols moieties (C3aed),34,46,47 as well
as two thioureas containing Cinchona alkaloid moiety (C4a,b).48,49
In addition we have tested also thiourea having a binaphthyl
b

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M. Hestericova, R. Sebesta / Tetrahedron 70 (2014) 901e905
903
Table 1
Table 2
The Michael addition of indole (1) to
b-nitrostyrene (2)
The Michael addition of dimethyl malonate (4) to
b
-nitrostyrene (2)
Entry
Catalyst
Ball milla
Yieldc (%)
Solutionb
Yieldc (%)
Entry
Catalyst
Ball milla
Yieldc (%)
Solutionb
Yieldc (%)
erd (R/S)
erd (R/S)
erd (R/S)
erd (R/S)
1
2
3
4
5
6
7
8
d
50
40
38
90
63
19
79
65
95
95
24
42
26
33
40
45
50:50
47:53
30:70
49:51
42:58
11:89
29:71
52:48
52:48
47:53
10:90
63:37
49:51
40:60
39:61
45:55
35
50
92
88
82
50
56
90
95
95
60
70
83
95
60
66
50:50
50:50
38:62
44:56
50:50
60:40
42:58
50:50
54:46
45:55
60:40
60:40
42:58
52:48
40:60
49:51
1
2
3
4
5
6
7
8
d
47
65
78
71
85
85
89
78
65
66
60
50:50
40:60
24:76
90:10
24:76
86:13
70:30
75:25
60:40
96:4
70
60
90
74
90
44
36
89
70
11
90
50:50
50:50
6:94
C1a
C1b
C1c
C2a
C2b
C3a
C3b
C3c
C3d
C4a
C4b
C5
C2a
C2b
C3a
C3b
C3c
C3d
C4a
C4b
C5
65:35
40:60
60:40
75:25
79:21
55:45
62:38
51:49
9
9
10
11
10
11
12
13
14
15
16
C6
45:55
a
Reactions were carried out with 2 equiv of dimethyl malonate, 1 equiv of
-nitrostyrene, 20 mol % of K2CO3, and 20 mol % of catalyst for 6 h.
b
b
C6
C7a
C7b
b
Reactions were carried out with 2 equiv of dimethyl malonate, 1 equiv of
-nitrostyrene, 20 mol % of K2CO3, and 20 mol % of catalyst in toluene at ꢀ30 ꢁC for
72 h.
a
c
Reactions were carried out with 1 equiv of nitrostyrene, 1 equiv of indole, and
20 mol % of catalyst for 6 h.
Yield of isolated product 5.
Enantiomeric ratio was determined by HPLC analysis with a chiral OD-H Chir-
d
b
Reactions were carried out with 1 equiv of nitrostyrene, 1 equiv of indole, and
acel column.
20 mol % of catalyst in CH2Cl2 for 72 h.
c
Yield of isolated product 3.
Enantiomeric ratio was determined by HPLC analysis with a chiral OD-H Chir-
d
in the enantiomeric purity of product 5 (Table 2, entries 4, 6, and
10). In one instance, Takemoto catalyst C2b afforded product 5 with
higher enantiomer purity in solution than under solvent-free
conditions in the ball mill (Table 2, entry 3). Interestingly, cata-
lysts C2a, C3c, C3d, and C5 even showed increase in yields in the
ball mill (Table 2, entries 4, 6, 7, and 10).
acel column.
dry stirring is less effective than ball milling, because under ball
milling conditions similar results were obtained in just 6 h. From
experiments in dichloromethane solution, product 3 was obtained
in quite low enantiomer purity. Interestingly, certain catalysts were
capable of significant increase of the enantiomeric ratio under
solvent-free conditions. This can be seen in experiments with cat-
alysts C1b, C2b, C3a, and C4a, which increased the enantiomer ratio
up to 10:90 (Table 1, entries 3, 6, 7 and 11).
The two Michael additions, which we have tested, proceed
similarly under solvent-free conditions and in solution. However, in
several instances, solvent-free conditions led to higher enantiose-
lectivities of the reactions. The reaction in solvent was markedly
better only in one experiment (Table 2, entry 3). The decrease of
enantiomeric ratio in case of reactions carried out in solvent can be
caused by a solvation of reactants, which to a certain extent dis-
rupts hydrogen bonds with catalysts. The extent of the difference is
not easy to evaluate because other parameters, which can influence
enantioselectivity, may be at play. The reaction temperature is
probably one of the most important ones. In solution, the reaction
temperature is easy to monitor and control. On the other hand, in
common laboratory ball mills it is not possible. Furthermore, the
content of the container heats itself during the milling as a result of
friction and impacts of balls with material and walls. With respect
to enantioselectivities in these two particular Michael additions,
there were also quite fast non-catalyzed reactions. These back-
ground reactions decrease the enantiomer purities of products
even when catalysts were used.
Based on the results of the addition of indole (1) we chose
catalysts that performed well and tested them on another Michael
addition. The addition of dimethyl malonate (4) to trans-b-nitro-
styrene (2) affords diester 5 (Scheme 2). Based on the data from the
literature, we decided to carry out the solution reaction in toluene,
as the authors achieved best yields and enantioselectivity in this
solvent.53 Furthermore, there are reports of decreased enantiose-
lectivity in more polar solvents like THF or acetonitrile.28 In this
reaction 2 equiv of dimethyl malonate (4) was used. An attempt to
reduce the amount of dimethyl malonate (4) to 1 equiv did not
bring any improvement.
We noticed a decrease in yields in the case of solvent-free
conditions, especially in the reaction of indole. This was probably
caused by partial destruction of the catalyst during mechano-
chemical activation, and possibly as a result of local high temper-
atures in the reaction vessel.
Scheme 2.
At present, it is difficult to come up with some generalization
with respect to catalyst structure, which can be more suitable for
hydrogen-bonding catalysis under solvent-free conditions. One
can, however, observe some correlation between the pKa of thio-
ureas with their better performance in the ball mill. Linear free
energy relationship between pKa of thiourea catalysts and enan-
tioselectivity was already described by Luo, Cheng and co-workers
for the Michael addition in toluene.54 The catalysts, which per-
formed better under solvent-free conditions fall in the range of pKa
10e14.55 Jacobsen-type thioureas with considerably higher
pKaw19 did not show any significant improvement of enantiose-
lectivity under solvent-free conditions. This would imply that hy-
drogen bonds need to be of higher strength in order to manifest
improvement under solvent-free conditions over the reaction in
Also in the case of addition of dimethyl malonate (4) to
b-
nitrostyrene (2) we ran blank reactions without catalysts. Under
solvent-free conditions, compound 5 was obtained in 47% yield
after 4.5 h of milling. On the other hand, in toluene the yield of
compound 5 was 70% after 72 h at ꢀ30 ꢁC. The results from ex-
periments with dimethyl malonate (4) and nitrostyrene (2) using
10 organocatalysts are collected in Table 2.
The reaction of dimethyl malonate (4) with b-nitrostyrene (2)
afforded product 5 in higher yields under solvent-free conditions
and in solution. One can again see, from data in Table 2, that several
catalysts were able to catalyze the reaction with higher enantio-
selectivity under solvent-free conditions than in solution. In par-
ticular, catalysts C3a, C3c, and C5 show a significant improvement

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M. Hestericova, R. Sebesta / Tetrahedron 70 (2014) 901e905
904
solution. This may seem counterintuitive, but higher enantiose-
lectivity under solvent-free conditions in ball mill can be a result of
a balance between two effects. On one side a positive effect of non-
solvated reagents and thus stronger hydrogen bonds can be coun-
tered with a negative effect of higher temperatures, which in-
trinsically occur under mechanical mixing.
atmosphere at room temperature. The reaction mixture was stirred
for 72 h. The mixture was then concentrated under reduced pres-
sure and purified by flash chromatography (silica gel, hexane/EtOAc
4:1) to afford the product 3. Yields are summarized in Table 1.
4.3.1. (S)-3-(2-Nitro-1-phenylethyl)-1H-indole (3). Brown solid; mp
98 ꢁC; 84% ee; 1H NMR (300 MHz, CDCl3):
d 8.07 (s, 1H, NH), 7.44 (d,
3. Conclusion
J¼7.9 Hz, 1H), 7.38e7.30 (m, 5H), 7.30e7.26 (m, 1H), 7.20 (ddd,
J¼9.2, 5.5, 1.5 Hz, 1H), 7.11e7.01 (m, 2H), 5.23e5.15 (m, 1H), 5.07
(dd, J¼12.4, 7.5 Hz, 1H), 4.94 (dd, J¼12.4, 8.4 Hz, 1H); 13C NMR
We have evaluated 15 structurally different thiourea and
squaramide organocatalysts in two Michael additions under
solvent-free conditions and in solution. The experiments showed
(75 MHz, CDCl3):
d
¼139.2, 139.2, 128.9, 127.8, 127.6, 126.1, 122.7,
121.6, 120.0, 118.9, 114.5, 111.4, 79.5, 41.5; chiral HPLC (Daicel Chir-
that Michael additions of indole and dimethyl malonate to
b
-
alcel OD-H) hexanes/2-propanol¼70:30, flow rate: 0.75 mL/min;
nitrostyrene under solvent-free conditions in the ball mill pro-
ceeded with higher enantioselectivities than in solution with sev-
eral thiourea catalysts. Lack of solvation seems, at least in some
cases, beneficial for enantioselective hydrogen-bonding catalysis.
Further work on elucidation of this question is in progress in our
laboratory.
l
¼254 nm; tR(minor)¼38.0 min, tR(major)¼47 min; FTIR (solid,
cmꢀ1):
n
¼3397 (m) NeH, 2923 (m) CeH, 1532 (m) NeO, 1453 (m)
CeH, 1376 (m) CeH; LCeMS: m/z 267 [MþH]. Spectral data are in
agreement with those in the literature.34,56
4.4. General procedure for the solvent-free enantioselective
Michael addition of dimethyl malonate to nitrostyrene
4. Experimental section
4.1. General
A mixture of trans-b-nitrostyrene (2) (50 mg, 0.335 mmol), di-
methyl malonate (4) (89 mg, 0.67 mmol), K2CO3 (10 mg,
0.07 mmol), and catalyst (0.067 mmol, 20 mol %) was added to
a 5 mL stainless steel container and was milled in a ball mill at
20 Hz with one stainless steel ball with 0.5 cm radius for 4.5 h. The
reaction was monitored by TLC analysis. Crude reaction mixture
was purified by flash chromatography (silica gel, hexane/EtOAc 4:1)
to afford the product 3. Yields are summarized in Table 2.
All commercially available reagents were purchased in Sigma-
eAldrich and used as received. Solvents were dried over CaH2 or
sodium metal and distilled under inert gas. Purification of reaction
products was carried out by flash column chromatography using
silica gel with 60 A pore size and 40e63
m
m particle size. 1H and 13
C
ꢂ
NMR were recorded on a Varian NMR System 300 spectrometer
running at 300 and 75 MHz, respectively, in CDCl3, acetone-d6 or
DMSO as solvent. Chemical shifts were reported in parts per million
(ppm) downfield from tetramethylsilane, which was used as an
internal standard. Infrared spectra were recorded on FT-IR Thermo
Nicolet iS-10 spectrophotometer. Melting points were recorded on
a Kofler melting point apparatus and were uncorrected. Optical
rotations were measured on a Jasco P-2000 polarimeter. The en-
antiomer ratios of the products were determined by enantiose-
lective HPLC using column with chiral stationary phase (Daicel
Chiracel OD-H) and UV detector (254 nm). Racemic samples were
obtained using benzoic acid or maleic acid as catalysts. Catalysts
C1a, C2b, and C4a are commercially available; the rest of the cata-
lysts were prepared according to literature procedures C1b,43 C1c,44
C2a,45 C3a and C3d,34 C3b,46 C3c,47 C4b,49 C5,29 C6,50 C7a,52 C7b.51
Solvent-free Michael additions were performed in an oscillation
mill MM400 from Retsch company with work frequency 20 Hz
using one stainless steel ball with 0.5 cm radius.
4.5. General procedure for the enantioselective Michael ad-
dition of dimethyl malonate to nitrostyrene in solution
To a stirred solution of trans-b-nitrostyrene 2 (0.1 mmol), K2CO3
(0.02 mmol) and catalyst (20 mol %) in dry toluene (2 ml) was
added dimethyl malonate 4 (0.3 mmol) under an inert atmosphere
at ꢀ30 ꢁC. After being stirred for 72 h, the mixture was concen-
trated under reduced pressure and HV and purified by flash chro-
matography (silica gel, hexane/EtOAc 4:1) to afford the product 3.
Yields are summarized in Table 2.
4.5.1. (S)-Dimethyl 2-(2-nitro-1-phenylethyl)malonate (5). Yellow
solid; mp 56 ꢁC; 88% ee; 1H NMR (300 MHz, CDCl3):
d 7.37e7.27 (m,
1H), 7.23 (dd, J¼7.7, 1.8 Hz, 1H), 4.97e4.83 (m, 1H), 4.25 (td, J¼8.7,
5.6 Hz, 1H), 3.87 (d, J¼9.1 Hz, 1H), 3.76 (s, J¼2.2 Hz, 1H), 3.56 (s, 1H);
13C NMR (75 MHz):
54.8, 53.1, 53.0, 43.0, 29.8; chiral HPLC (Daicel Chiralcel OD-H)
d
¼167.9, 167.3, 136.2, 129.1, 128.5, 127.9, 77.5,
hexanes/2-propanol¼90:10, flow rate: 1.00 mL/min;
l
¼254 nm;
tR(minor)¼24.0 min, tR(major)¼28 min; FTIR (solid, cmꢀ1):
n
¼2953
4.2. General procedure for the solvent-free enantioselective
Michael addition of indole to nitrostyrene
(m) CeH, 1731 (s) C]O, 1551 (s) NeO, 1496 (m) CeCAr, 1239 (m)
NeO, 1167 (m) COeO, 1092 (w) OeCeC; LCeMS: m/z 304.1 [MþNa].
Spectral data are in agreement with those in the literature.57
A mixture of trans-b-nitrostyrene (2) (50 mg, 0.335 mmol), in-
dole (1) (45 mg, 0.385 mmol), and catalyst (0.067 mmol, 20 mol %)
was added to a 5 mL stainless steel container and was milled in
a ball mill at 20 Hz with one stainless steel ball with 0.5 cm di-
ameter for 6 h. The reaction was monitored by TLC analysis. The
crude reaction mixture was purified by flash chromatography (sil-
ica gel, hexane/EtOAc 4:1) to afford the product 3. Yields are
summarized in Table 1.
Acknowledgements
This publication is the result of the project implementation:
26240120025 supported by the Research & Development Opera-
tional Programme funded by the ERDF. Financial support from
Slovak Grant Agency VEGA, Grant No. 1/0543/11 is gratefully ac-
knowledged. We thank Matej Zabka and Rastislav Baran for the
synthesis of some of the catalysts.
ꢁ
4.3. General procedure for the enantioselective Michael ad-
dition of indole to nitrostyrene in solution
References and notes
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