Synthesis of a bipyridine-derived achiral thiourea trifluoromethanesulfonic acid salt and its application as an additive in organocatalytic asymmetric reactions

Article abstract of DOI:10.1016/j.tetlet.2013.08.004

The host-guest complex of a proline-thiourea bipyridine trifluoromethanesulfonic acid salt can catalyze organocatalytic asymmetric reactions such as aldol, Michael, and Mannich in polar protic medium with high stereoselectivities. The privileged bipyridin

Full text of DOI:10.1016/j.tetlet.2013.08.004

Tetrahedron Letters 54 (2013) 5677–5681
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Tetrahedron Letters
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Synthesis of a bipyridine-derived achiral thiourea
trifluoromethanesulfonic acid salt and its application
as an additive in organocatalytic asymmetric reactions
Ayhan Sıtkı Demir a, , Sinan Basceken a,b,
⇑
a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
b Department of Chemistry, Hitit University, 19030 Corum, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 February 2013
Revised 18 July 2013
Accepted 1 August 2013
Available online 11 August 2013
The host–guest complex of a proline–thiourea bipyridine trifluoromethanesulfonic acid salt can catalyze
organocatalytic asymmetric reactions such as aldol, Michael, and Mannich in polar protic medium with
high stereoselectivities. The privileged bipyridine backbone and the thiourea motif are essential to the
activity and enantioselectivity through hydrogen bonding interactions.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Host–guest complex
Proline–thiourea interaction
Asymmetric aldol reaction
Enantioselective Michael reaction
Asymmetric Mannich reaction
Organic reactions in water and aqueous medium have attracted
a great deal of attention and have become highly desirable.
Although organocatalytic asymmetric reactions are normally car-
ried out in organic solvents, it has been demonstrated that the
addition of water has a positive effect on the process,1,2 but the
use of an excess amount decreases the reactivity and stereocontrol
of the reaction.3 The search for simple chiral molecules capable of
promoting enantioselective reactions in water and in aqueous
medium has seen significant activity.4
NCS
H
N
H
N
CF3
NH2
N
S
N
CF3
F3C
CF3
CF3
S
N
N
acetone, 48 h, rt
F3C
N
H
N
H
H2N
II
I
CF3SO3H
CHCl3, rt
Barbas and co-workers developed a catalytic direct asymmetric
aldol reaction that was performed in water without the addition of
organic solvents.5 The use of proline furnished low selectivities and
the bifunctional catalyst system demonstrated excellent reactivity,
diastereoselectivity, and enantioselectivity in water. Kobayashi
et al. reported the catalytic diastereo- and enantioselective Man-
nich-type reaction of a hydrazono ester with silyl enol ethers in
aqueous medium with zinc fluoride and a chiral diamine ligand.6
Prolinamide and its derivatives appear to be the most efficient
organocatalysts for Barbas–List aldol reactions.7 Prolinamides de-
rived from amino alcohols or amino phenols,8 amines, diamines,9
and prolyl sulfonamides10 have been explored as bifunctional cat-
alysts to promote stereoselective aldol reactions in water.1 Accord-
ingly, we decided to explore bipyridine-derived achiral thiourea
H
N
H
N
CF3
CF3SO3
H
N
S
CF3
CF3
S
N H
CF3SO3
F3C
N
H
N
H
III
Scheme 1. Synthesis of bipyridine trifluoromethanesulfonic acid salt III.
II11 in water soluble form, that is, water soluble salt III, and to test
its utility in Barbas–List aldol reactions in water.
The bipyridine derived achiral thiourea II was converted into its
trifluoromethanesulfonic acid salt III as shown in Scheme 1.
Aldol reaction. Initially, the catalytic activity of the salt III in
water was tested. Various reaction conditions were examined as
described for Barbas–List aldol reactions (Table 1). The aldol prod-
uct was obtained in ee: 99% and dr: 98/2 and 85% yield.
⇑
Corresponding author. Tel.: +90 (0)364 227 7000; fax: +90 (0)364 227 7005.
E-mail address: sinanbasceken@hitit.edu.tr (S. Basceken).
Deceased on June 24th 2012.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.004

5678
A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681
Table 1
Thus, special design is required for performing asymmetric
reactions in water. Here we report the efficient enamine-based
organocatalytic direct asymmetric aldol reactions between cyclic
ketones and aldehydes using bipyridinium trifluoromethanesulfo-
nate salt III as the additive in water without any organic solvent.
Proline-catalyzed direct aldol reactions have been shown,
experimentally and computationally, to proceed through enamine
intermediates. Based on this initial proposal, we propose that the
interaction of the thiourea–bipyridine moiety with proline in the
transition state (TS) is as shown in Figure 1. The aldehyde is acti-
vated by hydrogen bonding with the carboxyl group of the proline
in a manner such that carbon–carbon (C–C) bond formation takes
place from its re face. The presence of the bipyridine moiety re-
stricts the conformation and makes the carboxyl group a better
hydrogen-bond donor. The main advantage of this model is that
the transition state is stabilized through hydrogen bonding. There-
fore, a small change in the pKa value of an organic compound
would affect its catalytic activity and selectivity in the aldol
reaction.11,12
The enantioselective direct aldol reaction of various aldehydes 1 with cyclohexanone
2 in watera
OH
O
O
III
O
(S)-proline
Ar
anti-3a-f
dr (anti/syn)b eec (%)
+
Ar
1a-e
H2O, rt
2
Entry
Aldehyde (Ar)
Yield (%)
85
80
60
65
70
78
(Conf.)d
1
2
3
4-NO2C6H4
3-NO2C6H4
4-BrC6H4
4-CF3C6H4
2-MeOC6H4
4-NO2C6H4
98/2
98/2
91/9
91/9
99/1
44/56
99
99
98
98
96
98
(2R,10S)
(2R,10S)
(2R,10S)
(2R,10S)
(2R,10S)
(2R,10S)
4
5
6e
a
The reactions were conducted with (S)-proline (20 mol %), bipyridinium tri-
fluoromethanesulfonate salt III (10 mol %), cyclohexanone or cyclopentanone
(16 equiv), aldehyde (0.125 mmol), and H2O (20 L) for 12 h.
l
b
Determined by chiral HPLC analysis of the mixture of anti/syn product.
Determined by chiral HPLC.
Determined by comparison of their specific rotations with those reported for
c
d
As proposed for the Hajos–Eder–Sauer–Wiechert reaction,11 we
assume that the key intermediate of the direct intermolecular
asymmetric aldol reactions described is an enamine formed
between proline and the corresponding ketone donor substrate.
In analogy to the mechanism of aldolase antibody 38C2,11 this
enamine with a bipyridine thiourea moiety in the backbone attacks
the carbonyl group of the aldehyde acceptor with high enantiofa-
cial selectivity, which is imposed by a highly organized tricyclic
hydrogen bonded framework resembling a metal-free Zimmer-
man–Traxler type transition state (Scheme 2).11 This readily
anti-3a–f.
e
Cyclopentanone was used.
The isolated products showed the same stereochemical out-
come as those obtained via Barbas–List reactions. To the best of
our knowledge, this is the first organocatalytic asymmetric aldol
reaction in water with unmodified proline (host) and a bipyri-
dine-derived achiral thiourea trifluoromethanesulfonic acid salt
(guest), which gives excellent diastereo- and enantioselectivities.
Various examples were studied and the aldol products were ob-
tained in comparably good yields (60–85%) and with very high
selectivity (up to 99% ee) (Table 1). The reactions with aldehydes
bearing electron-withdrawing groups proceeded smoothly to af-
ford the aldol adducts in 12 h (Table 1, entries 1–4), with a good
level of diastereoselectivity. For the electron-rich aromatic alde-
hyde (Table 1, entry 5), the reaction required a longer time
(24 h). In entry 6, after the addition of cyclopentanone, p-nitro-
benzaldehyde was added, and stirring was continued for 12 h.
The isolated aldol product showed excellent enantioselectivity
(98%), but much lower diastereoselectivity.
Host
CF3
O
N
H
H
O
N
H
O
CF3
N
Ar
S
H
Guest
N
O
S
H
O
To test the ability and flexibility of the catalytic system, (S)-tert-
leucine (host) was used instead of proline under similar conditions.
The aldol reactions furnished anti products with excellent ee
(>98%) and good yield (80%). The reactions were conducted with
(S)-tert-leucine (20 mol %), bipyridine trifluoromethanesulfonic
acid salt III (10 mol %), cyclohexanone (16 equiv), 4-nitrobenzalde-
F3C
O
Figure 1. Proposed TS for the enantioselective aldol reaction.
host
hyde (0.125 mmol), and water (20 lL) for 12 h.
O
-H2O
N
HN
+
‘To make a good asymmetric catalyst perfect’, the role of
suitable additives can be crucial in enhancing the reactivity and
stereoselectivity of the catalytic system.11 Therefore, we decided
to use, instead of bipyridinium trifluoromethanesulfonate salt III,
pyridine, 2,6-dimethylpyridine, and bipyridine as additives in the
proline-catalyzed organocatalytic asymmetric reactions. The re-
sults should provide information on the discriminating hydrogen
bonding effect of thiourea or the basic effects of the pyridine moi-
ety. However, when pyridine and pyridine derived additives were
used in place of the thiourea, no reaction took place and we did
not obtain any product. This fact demonstrates clearly that the
hydrogen bonding effect of proline–thiourea plays the most impor-
tant role, and not the basicity of the pyridine moiety. In addition,
under similar conditions, the aldol reaction without bipyridinium
trifluoromethanesulfonate salt III did not lead to any product.
Therefore, the use of water as a reaction solvent is not always
practical for asymmetric catalytic reactions because water often
inhibits the catalyst activity or alters the enantioselectivity by
interrupting ionic interactions and hydrogen bonds critical for
stabilizing the transition states of the reaction intermediates.5
O
O
H
H
O
R
O
L*
L*
guest
O
OH
R
N
O
+H2O
R
O
N
H
OH
O
H
H
O
O
*
L
L*
re-facial attack
CF3
si-enamine + re-aldehyde
TS
OH
O
HN
HN
=
O
L*
CF3
R
+
H
HN
S
O
L*
anti
N
CF3SO3
H
Scheme 2. Proposed mechanism of the aldol reaction.

A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681
5679
explains the observed re facial enantioselectivity. Typically, anti-
diastereoselectivity is observed in the proline–thiourea catalyzed
aldol reaction.11
of the proline forms an assembly with the thiourea, in turn enhanc-
ing the reactivity and selectivity of the catalyst (Fig. 2).
Based on the observed stereoselectivities, we have proposed a
plausible catalytic mode for this asymmetric conjugate addition
(Scheme 3). Presumably, the in situ formed enamine intermediate
between aldehydes 4 and the proline–thiourea catalyst adopts the
E-conformation. Subsequently, a new C–C bond is formed by the
Michael reaction. To expand further the scope of amino acid
based catalysis, we examined other important C–C bond-forming
reactions (Table 2). In this study, we tested the activity of this cat-
alyst for Michael addition reactions in water. In contrast to the al-
dol reaction, Michael addition with this water soluble catalyst did
not form any product. Next, we carried out this reaction in a mix-
ture of water/organic solvent (toluene) and the reaction was mon-
itored by thin layer chromatography. After 24 h, the expected
Michael product was formed in 98% ee and 18/82 dr as shown in
Table 2, entry 4. The Michael reaction with (S)-tert-leucine fur-
nished the corresponding product with 36/64 dr, but with reversed
enantioselectivity (80% ee) (Table 2, entry 5). We also used (S)-
tryptophan as the catalyst, which interestingly furnished the same
diastereoselectivity (36/64) but with opposite enantioselectivity
(78% ee) (Table 2, entry 6).
Host
O
O
N
N
O
O
H
CF3
H
N
H
N
Ar
S
CF3
N
O
O
H
Guest
S
F3C
O
Figure 2. Proposed TS for the enantioselective Michael reaction.
We continued to evaluate the scope of the reaction by testing
the Michael addition of butyraldehyde 4b to nitroalkene 5b, which
led to a good isolated yield (65%) and high diastereoselectivity (16/
84) with high enantioselectivity (85%) (Table 2, entry 7). Addition-
ally, the reaction between isovaleraldehyde 4c and trans-4-bromo-
b-nitrostyrene (5b) gave the Michael adduct in good yield (60%)
and high stereoselectivity (Table 2, entry 8). Accordingly, we car-
ried out the Michael reaction in the absence of bipyridinium tri-
fluoromethanesulfonate salt III in toluene/water under the same
conditions, but unfortunately, no product was formed. This result
indicates that when the catalyst includes an acid group,5 the reac-
tion does not proceed in a toluene/water system. This may be be-
cause the proline is soluble, whereas the reactants are less miscible
in water. In a biphasic system, the interactions required for the
reaction do not occur. In the case of the Michael reaction using
the proline–thiourea host–guest complex, the catalyst additive
assembles with the reactants through hydrophobic interactions,
excluding water molecules from the organic phase. In this concen-
trated organic phase the reaction occurs efficiently to afford Mi-
chael adducts with high enantioselectivities, probably facilitated
host
O
-H2O
N
HN
O
O
+
H
H
O
O
L*
L*
4a
Ar
guest
NO2
O2N
Ar
Ar
N
+H2O
N
O
O
H
H
H
O
O
N
OH
O
L*
O
L*
5a-d
TS
Ar
O
HN
O2N
6a-f
O
H
+
O
L*
syn
by hydrogen bonds between the thiourea salt and proline in the TS.
We assume that the Michael reaction would proceed according
Scheme 3. Proposed mechanism of the Michael reaction.
to a modified Seebach’s model,13 in which the carboxylate moiety
Table 2
Michael reaction of aldehydes and nitroalkenes in toluene/watera
R2
III
O
O
(S)-proline
NO2
(R)
NO2
+
(S)
toluene/H2O, rt
R1
R2
R1
4a-c
5a-d
6a-f
Entry
Aldehyde
R1
Nitrostyrene
R2
Yield (%)
dr (anti/syn)b
eec (%)
(Conf.)d
1
2
3
CH3
CH3
CH3
CH3
CH3
CH3
CH2CH3
CH(CH3)2
H
67
72
70
71
66
62
65
60
18/82
34/66
20/80
18/82
36/64
36/64
16/84
5/95
93
97
82
98
80
78
85
90
(2R,3S)
(2R,3S)
(2R,3S)
(2R,3S)
(2S,3R)
(2S,3R)
(2R,3S)
(2R,3S)
4-Br
4-OMe
2-Br
4-Br
4-Br
4-Br
4-Br
4
5e
6f
7
8
a
The reactions were conducted with (S)-proline (20 mol %), bipyridinium trifluoromethanesulfonate salt III (10 mol %), aldehyde (5 equiv, 20
(0.125 mmol), toluene (1.0 mL), and H2O (20 L) for 24 h.
lL), nitrostyrene
l
b
Determined by chiral HPLC analysis of the mixture of anti/syn product.
Determined by chiral HPLC.
Determined by comparison of their specific rotations with those reported.
(S)-tert-Leucine was used.
c
d
e
f
(S)-Tryptophan was used.

5680
A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681
addition to the unsaturated electrophile 5. Here, the proline–thio-
urea moiety supports the substrate orientation and provides the
basis for the enantioselective coupling. Next, the re face of the en-
amine intermediate attacks the re face of trans-b-nitrostyrene to
give the corresponding preferred syn Michael adducts6a–f
(Scheme 3). Nevertheless, the precise catalytic mechanism needs
further investigation.13
reaction conditions were modified using toluene/water as the reac-
tion solvent, the desired product 11 was formed. The reaction with
(S)-tert-leucine resulted in the formation of the Mannich product,
however, with low enantioselectivity (50%) and in low yield (36%).
In order to explain the observed stereoselectivity, it was pro-
posed that the reaction follows an enamine mechanism and in-
volves a transition state as shown in Figure 3. This is similar to
the earlier proposed transition state of the corresponding aldol
reaction (see Fig. 1).
The proposed mechanism of the proline–thiourea catalyzed
Mannich reaction is depicted in Scheme 5. On the basis of the
previous mechanism for proline-catalyzed Mannich reactions, we
reasoned that this asymmetric Mannich reaction could also
proceed via an enamine pathway because the nucleophilic addition
of the in situ generated enamine would be faster to an imine than
to an aldehyde.14 The (E)-configurations are assumed for both the
enamine and the imine. Typically, Mannich products are formed
Mannich reaction. Another representative application was the
Mannich reaction in water. The catalytic, diastereo-, and enantio-
selective Mannich-type reaction of a hydrazono ester with silyl
enol ethers in aqueous media has been achieved with zinc fluoride
and a chiral diamine ligand. The use of water and a small amount
of TfOH was essential for the reactions to proceed in high yields.6
In our initial experiment, we found that the host–guest proline–
thiourea system could catalyze the three-component Mannich
reaction of aliphatic aldehydes 4c–e, p-anisidine 8, and acetone 7
in toluene/water to give the expected b-amino-carbonyl com-
pounds 9a–c with high enantioselectivities (up to 85% ee) in good
yields, as shown in Table 3. Accordingly, under similar conditions,
the Mannich reaction in the absence of bipyridinium trifluoro-
methanesulfonate salt III did not lead to any product. This result
is the same as those previously described in this study on enam-
ine-based organocatalytic asymmetric reactions, such as aldol
and Michael. The effectiveness of the proline–thiourea catalysts
compared to proline can be attributed to the solubility of the cat-
alysts. Although proline dissolves in water, the thiourea salt is only
partially soluble in water and forms an organic phase with the
aldimine and ketone in which the Mannich reaction proceeds
efficiently.
III
(S)-proline
87% ee and 60% yield
MeO
OMe
O
O
HN
N
+
toluene/H2O, rt
7
H
NO2
11
NO2
10
50% ee and 36% yield
More effective was the two-component Mannich reaction of
aldimine 10, derived from the aromatic aldehyde, p-nitrobenzalde-
hyde (1a), and p-anisidine (8), with acetone 7 in toluene/water in
the presence of 10 mol % of the catalyst. The Mannich product
was isolated in 87% ee and 60% yield as shown in Scheme 4. The
reactions were conducted with (S)-proline (20 mol %), bipyridini-
(S)-tert -leucine
III
Scheme 4. Two-component Mannich reaction.
Host
um trifluoromethanesulfonate salt III (10 mol %), acetone (50
lL),
aldimine (0.0625 mmol), toluene (2 mL), and water (10 L) for
l
O
N
PMP
H
CF3
CF3
O
H
H
N
O
48 h. Accordingly, the same conditions were applied to the asym-
metric Mannich reaction of acetone 7 with aldimine 10 in water.
Unfortunately, no product was formed. However, when the
H
N
N
R
R'
S
N
H
Guest
O
Table 3
S
Three-component Mannich reactions with various aliphatic aldehydes (PMP =
O
F3C
p-methoxyphenyl)a
OMe
Figure 3. Proposed TS for the enantioselective Mannich reaction.
III
9a-c
O
O
(S)-proline
H2N
+
O
HN
O
+
-H2O
R
toluene/H2O, rt
N
HN
O
O
OMe
4c-e
7
H
R
+
H
8
O
O
L*
L*
Entry
1
Aldehyde
Product
Yield (%)
eeb (%)
85
(Conf.)c
NAr
RCHO
-H2O
PMP
+
O
R
H
O
HN
ArNH2
60
(S)
Ar
NH
Ar
N
O
R
H
O
O
PMP
PMP
+H2O
N
N
H
H
O
O
HN
HN
O
R
2
3
55
58
80
80
(R)
(R)
O
OH
O
L*
L*
si-facial attack
si-enamine + si-imine
TS
Ar
R
a
The reactions were conducted with (S)-proline (20 mol %), bipyridinium tri-
fluoromethanesulfonate salt III (10 mol %), acetone (50 L), p-anisidine
L) at room
NH
O
HN
+
l
O
(0.0625 mmol), aldehyde (0.0625 mmol), toluene (2 mL), and H2O (10
l
H
O
temperature for 48 h.
L*
b
Determined by chiral HPLC with an OD-H column.
Determined by comparison of their specific rotations with those reported.
c
Scheme 5. Proposed mechanism for the Mannich reaction.

A. S. Demir, S. Basceken / Tetrahedron Letters 54 (2013) 5677–5681
5681
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selectively by the enamine to allow for protonation of its lone pair
and compensation of negative charge formation. This mechanism
is similar to that proposed for the proline-catalyzed aldol reaction
(Scheme 2).14
In summary, we have found that the host–guest proline–thio-
urea bipyridinium trifluoromethanesulfonate salt III complex can
catalyze directly enantioselective aldol reactions in water with
unmodified proline and with excellent stereoselectivities (up to
98:2 dr, >99% ee). In contrast to aldolreactions, the Michael and
Mannich reactions were not successful in water. We found that
these reactions in water/toluene gave the desired products with
high stereoselectivities. More detailed studies on the mechanisms
and synthetic applications of this supramolecular catalyst in enan-
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Acknowledgments
We gratefully acknowledge the Scientific and Technological Re-
search Council of Turkey (TÜBITAK), the Turkish Academy of Sci-
ences (TÜBA), and Middle East Technical University (METU).
Thanks are also due to Professor Metin Balci for editorial
assistance.
Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/
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