Enantioselective friedel-crafts reactions of aromatic amines with ethyl glyoxylate in pyridinium-based ionic liquids

Article abstract of DOI:10.1071/CH06221

The Friedel?Crafts reactions of aromatic amines with ethyl glyoxylate in pyridinium-based ionic liquids (ILs) are investigated using 2,2?-dihydroxy-1,1?- binaphthyl (BINOL) metal complexes as chiral catalysts. Reaction parameters such as the catalyst?IL c

Full text of DOI:10.1071/CH06221

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2006, 59, 468–472
www.publish.csiro.au/journals/ajc
Enantioselective Friedel–Crafts Reactions of Aromatic Amines
with Ethyl Glyoxylate in Pyridinium-Based Ionic Liquids
Sanjay V. MalhotraA,B andYing XiaoA
A
Department of Chemistry and Environmental Science, New Jersey Institute of Technology,
University Heights, Newark, NJ 07102, USA.
Corresponding author. Email: malhotra@njit.edu
B
The Friedel–Crafts reactions of aromatic amines with ethyl glyoxylate in pyridinium-based ionic liquids (ILs) are
ꢀ
ꢀ
investigated using 2,2 -dihydroxy-1,1 -binaphthyl (BINOL) metal complexes as chiral catalysts. Reaction parame-
ters such as the catalyst–IL compositions, reactant compositions, reaction time, and reaction temperature are studied.
High yields and excellent enantioselectivities are achieved under the relatively mild conditions. The ILs can also
be recycled and reused as opposed to traditional solvent systems.
Manuscript received: 21 June 2006.
Final version: 28 July 2006.
Introduction
reaction. Therefore, based on our earlier studies,[18] we
embarked on the investigation of the utility of pyridinium-
based ILs as solvents for the titled reaction. Herein we report
the first study using pyridinium-based ILs as solvents for the
asymmetric Friedel–Crafts acylation of aromatic amines with
ethyl glyoxylate.
Recently, ionic liquids (ILs) have gained much attention as
viable substitutes for volatile organic solvents in organic
[1]
[2]
synthesis and other processes. It is mainly a result of
their unique characteristics,[ such as chemical and thermal
stability, no measurable vapour pressure, non-flammability,
strongly solvating but non-coordinating character, high
polarity, and ease of recycling. They have been investi-
gated in various organic and organometallic syntheses such
3]
Results and Discussion
+
Two ILs, 1-ethyl-pyridinium trifluoroacetate ([EtPy] -
[
4]
[5]
[6]
−
3
[CF COO] ) and 1-ethyl-pyridinium tetrafluoroborate
as hydrogenation, oxidation, Diels–Alder reaction,
Heck reactions,
[
7]
[8]
+
−
Friedel–Crafts reactions,
Beckman
([EtPy] [BF ] ) were employed for this study (Fig. 1).
4
[9]
[10]
polymerization,[11]
rearrangements, hydroformylation,
To find a suitable catalyst, chiral ligand, and sol-
vent system, the reaction of ethyl glyoxylate with N,N-
and others.[ However, most of these studies have mainly
focussed on non-asymmetric reactions. Enantioselective
reactions in ILs are still at a preliminary stage. The polar
and non-coordinating nature of ILs suggests their potential
for asymmetic induction and it may have a profound effect
on their reactivity and selectivity in a reaction. Asymmetric
transformations in ILs can be achieved by (a) using chi-
ral reagents, chiral substrates, or chiral catalysts and IL as
12]
dimethylanilines was initally studied (Fig. 2). Lewis acids
i
Ti(OPr ) and Cu(OTf) were tested in the presence of the
4
2
chiral ligands BINOL and Br-BINOL. The catalyst–ligand
complexes were prepared as per the literature method.[
The molar ratio of N,N-dimethylaniline (1a), ethyl glyoxy-
late (2), and the catalyst complex was maintained at 1:2:0.2.
The results are summarized in Table 1.
17]
[
13]
solvent, or (b) using chiral ILs.
As the data shows, the yield and enantiomeric excess (e.e.)
of the reaction depend on the catalyst and solvent system
chosen. The chiral ligands (R)-BINOL and (R)-BINOL-Br
give comparable yields. However, (R)-BINOL-Br results
The asymmetric Friedel–Crafts reaction of aromatic
compounds with α-dicarbonyl compounds is an important
reaction in organic synthesis.[ It provides a simple proce-
dure for the preparation of optically active aminomandelic
acid derivatives, which are important building blocks for
many biologically significant compounds.[ Earlier stud-
ies on the Friedel–Crafts reaction of glyoxylate with N,N-
dimethylanilines resulted in high yields and an enantiomeric
14]
15]
CF COOϪ
BF4Ϫ
3
Nϩ
Nϩ
[
16]
excess when a tert-butyl bisoxazoline copper(ii) complex
ꢀ
ꢀ
ϩ
Ϫ
or a 2,2 -dihydroxy-1,1 -binaphthyl (BINOL) titanium(iv)
ϩ
Ϫ
[EtPy] [BF ]
4
[
EtPy] [CF COO]
3
[
17]
complex
was used in organic solvents. To our knowledge
there has been no report on the application of ILs for this
Fig. 1. Pyridinium-based ILs.
10.1071/CH06221 0004-9425/06/070468
©
CSIRO 2006

Enantioselective Friedel–Crafts Reactions in Pyridinium-Based Ionic Liquids
469
in a higher e.e., which is similar to an earlier report,[17]
with the Lewis acid catalyst, which results in a higher yield
and enantioselectivity. However, a more systematic study is
required to fully understand the system.
and suggests that an electron-withdrawing group (–Br) at
ꢀ
the 6,6 -positions of BINOL is favorable to improve the
enantioselectivity of the reaction. In addition, a higher enan-
tioselectivity is seen with Ti(OPr )4 as catalyst compared to
Table 2 shows the effect of reaction time for the Friedel–
Crafts reaction of 1a with 2 catalyzed by a BINOL–Ti(OPr )4
i
i
Cu(OTf)2. Interestingly, when these reactions are carried out
in ethylpyridinium tetrafluroborate, lower yields and selec-
tivity are observed in all cases.This suggests that the anion of
the IL can influence the rate, yield, and selectivity of such an
electrophilic substitution reaction. Here, the trifluroacetate
anion is more electron withdrawing compared to triflurobo-
rate, therefore, it has a stronger and more selective interaction
complex at room temperature.There is no change in the selec-
tivity after the initial 4 h of the reation time.This suggests that
the interaction of the catalyst–ligand system with the carbo-
cation formed from the aromatic amine is fast and creates
100
9
8
7
0
0
0
O
HO
OEt
60
50
40
R
O
*
Chiral ligand with Ti(OPri )
Pyridinium IL
R
OEt
4
ϩ
H
ϩ
Ϫ
(R)-BINOL–[EtPy] [CF COO]
3
O
3
0
ϩ
Ϫ
(
(
(
R)-BINOL-Br–[EtPy] [CF COO]
NMe2
1
3
NMe2
3
2
20
ϩ
Ϫ
R)-BINOL–[EtPy] [BF ]
4
Br
10
ϩ
Ϫ
R)-BINOL-Br–[EtPy] [BF ]
4
a: R ϭ H
b: R ϭ Cl
c: R ϭ Br
OH
OH
OH
OH
0
0
4
8
12
16
20
24
28
d: R ϭ Me
e: R ϭ OMe
Br
Time (h)
(R)-BINOL
(R)-BINOL-Br
Fig. 3. Friedel–Crafts reactions of N,N-dimethylaniline with ethyl
glyoxylate in various catalyst–solvent systems at room temperature.
Fig. 2. Asymmetric Friedel–Crafts reaction.
Table 1. Friedel–Crafts acylation of N,N-dimethylaniline 1a with ethyl glyoxylate 2
Room temperature, 24 h
Entry
Ligand
Catalyst
Solvent
Isolated yield [%]
e.e.A [%]
B
i
1
(R)-BINOL
(R)-BINOL
(R)-BINOL
(R)-BINOL
Ti(OPr )4
Toluene
CH2Cl2
THF
99
99
99
84
73
99
86
74
86
68
87
70
77
71
48
77
48
91
88
59
74
38
87
53
B
2
B
3
+
−
4
5
6
[EtPy] [CF3COO]
+
−
(R)-BINOL
[EtPy] [BF4]
B
(R)-BINOL-Br
(R)-BINOL-Br
(R)-BINOL-Br
(R)-BINOL
(R)-BINOL
(R)-BINOL-Br
(R)-BINOL-Br
Toluene
+
−
−
−
7
8
9
[EtPy] [CF3COO]
+
−
[EtPy] [BF4]
+
Cu(OTf)2
[EtPy] [CF3COO]
+
−
1
1
1
0
1
2
[EtPy] [BF4]
+
[EtPy] [CF3COO]
+
−
[EtPy] [BF4]
A
B
Determined by HPLC on a chiralcel OD-H column.
Ref. [17].
Table2. EffectofreactiontimeontheacylationofN,N-dimethylaniline1awithethylglyoxylate
2
in various ligand–solvent systems at room temperature
Time [h]
Isolated yield [%]/e.e.A [%]
(
R)-BINOL–
(R)-BINOL-Br–
[EtPy] [CF3COO]
(R)-BINOL–
[EtPy] [BF4]
(R)-BINOL-Br–
[EtPy] [BF4]
+
−
+
−
+
−
+
−
[
EtPy] [CF3COO]
4
8
1
2
2
3
36/81
55/81
77/78
81/77
84/77
85/77
39/89
57/88
81/88
85/88
86/88
86/87
35/51
50/49
68/49
71/49
73/48
74/48
37/61
54/61
69/59
73/59
74/59
74/58
6
0
4
0
A
Determined by HPLC on a chiralcel OD-H column.

4
70
S. V. Malhotra and Y. Xiao
a chiral pocket, followed by the reaction with glyoxylate to
give consistent selectivity.
The data show that electron-withdrawing groups, namely
Cl and Br , deactivate the benzene ring, and therefore,
−
−
The yields versus reaction time for different catalyst–
solvent systems are plotted in Fig. 3. In all cases the
lower the yield. However, the enantioselectivity is increased,
which may be attributed to a steric factor, as has previously
+
−
[17]
results with [EtPy] [CF3COO] are better than those
been reported.
On the other hand, an electron-donating
+
−
with [EtPy] [BF4] . For the reaction investigated here,
group, namely CH3, appears to activate the benzene ring
and promotes the reaction. Therefore, the yields (entries 4
and 9) are increased for the reaction in comparison to the
those with N,N-dimethylaniline (entries 1 and 6), while the
e.e. values decrease. Entries 5 and 10 show that the yield
and e.e. values of the reaction with the aromatic amine that
contains a CH3O– group have decreased dramatically, even
though CH3O– is an electron-donating group.This is because
the product contains a hydroxy and a methoxy group, which
can form a six-membered ring with the catalyst, thus inhibit
a combination of the (R)-BINOL-Br–Ti complex with
+ −
[
EtPy] [CF3COO] seems to be ideally suited and has the
greatest effect in increasing the reaction rate.
Once the optimum conditions for the title reaction in IL
were discovered, a host of aromatic amines were investigated
for Friedel–Crafts acylation with 2. Table 3 shows the results
i
of this study which employed the (R)-BINOL-Br–Ti(OPr )4
+
−
complex in [EtPy] [CF3COO] at room temperature
for 24 h.
Table 3. Friedel–Crafts reaction of various aromatic amines
Amine Ligand Isolated yield [%]
1a (–H) (R)-BINOL 84
Entry
e.e.A [%]
1
2
3
4
5
6
7
8
9
77
81
73
68
63
88
91
85
81
74
1b (–Cl)
(R)-BINOL
79
72
86
54
86
80
74
89
55
1c (–Br)
(R)-BINOL
1d(–CH3)
1e (–OCH3)
1a (–H)
1b (–Cl)
1c (–Br)
(R)-BINOL
(R)-BINOL
(R)-BINOL-Br
(R)-BINOL-Br
(R)-BINOL-Br
(R)-BINOL-Br
(R)-BINOL-Br
1d(–CH3)
1e (–OCH3)
1
0
A
Determined by HPLC on a chiralcel OD-H column.
Table 4. Friedel–Crafts reaction at different temperatures
◦
Isolated yield [%]/e.e.A [%]
Entry
Amine
Temp. [ C]
Time [h]
Solvent
(
R)-BINOL
(R)-BINOL-Br
+
−
1
2
3
4
5
6
7
8
9
1a (–H)
1a (–H)
1c (–Cl)
1d (–Br)
1b (–CH3)
1e (–OCH3)
1a (–H)
1b (–Cl)
1c (–Br)
0
0
0
0
0
36
36
36
36
36
36
48
48
48
48
48
[EtPy] [BF4]
47/10
81/85
74/83
67/80
83/79
49/70
77/89
70/87
60/83
78/84
44/73
50/16
82/91
76/94
68/87
85/85
50/77
77/92
71/96
62/89
80/87
46/79
+
−
[EtPy] [CF3COO]
0
−20
−20
−20
−20
−20
1
1
0
1
1d (–CH3)
1e (–OCH3)
A
Determined by HPLC on a chiralcel OD-H column.
Table 5. Recycling and reuse of ILs
+
−
+
−
Recycle
number
[EtPy] [BF4]
[EtPy] [CF3COO]
Recovered
Isolated yield [%]/
Recovered
[wt.-%]
Isolated yield [%]/
[
wt.-%]
e.e. [%]A
e.e. [%]A
0
1
2
3
4
5
—
92
94
91
93
92
74/59
72/58
73/58
72/57
71/56
69/55
—
93
93
92
94
92
86/88
84/87
83/87
82/87
83/86
82/85
A
Determined by HPLC on a chiralcel OD-H column.

Enantioselective Friedel–Crafts Reactions in Pyridinium-Based Ionic Liquids
471
the reaction. A decreased reaction temperature should slow
down the reaction but help to improve selectivity. To test this
was obtained on top. Because N-ethyl-pyridinium bromide is hygro-
scopic, the silver(i) trifluoroacetate used was in excess. Therefore, a
1 M solution of N-ethyl-pyridinium bromide was added drop by drop.
This step was repeated until no more AgBr was seen to form. To ensure
no excess N-ethyl-pyridinium bromide in the mixture, the solution was
subsequently tested with 1 M silver(i) trifloroacetate solution. Finally,
the mixture was stirred for 10 min, and the precipitate of AgBr was fil-
tered off. Any colour and impurities present in the synthesized ILs were
removed by passing the ILs through a charcoal column with distilled
◦
hypothesis we also carried out the reactions at 0 and −20 C
and the results are shown in Table 4.
It must be noted that decreasing the reaction temperature
◦
+
−
to near 0 C leads to freezing of [EtPy] [BF4] . Therefore,
very low yields and selectivity are seen (entry 1). Therefore,
+
−
the reactions were only investigated in [EtPy] [CF3COO] .
In all cases, a decrease in temperature gave lower over-
all yields as expected, but the enantioselectivities increased
significantly.
[
26]
water.
Finally, the water was removed by rotary evaporation under
◦
vacuum at ∼65 C. The resulting ILs were dried in an oven overnight at
◦
6
5 C to remove any residual moisture.
Friedel–Crafts Reactions using the ILs
WealsoinvestigatedthereusabilityandefficiencyoftheIL
for the reaction of 1a with 2 catalyzed by the (R)-BINOL-Br–
titanium complex. The recycling process involved washing
the used ILs with diethyl ether. Any organic residue left in
the IL layer could be separated by the ether wash.The IL layer
The reactions were carried out under N2 atmosphere in oven-dried
◦
glassware. ILs were dried overnight in an oven at 70 C. Chiral ligands
(
0.05 mmol, 10 mol-%) and catalyst (0.05 mmol, 10 mol-%) were added
to 1 mL of pyridinium-based IL. The mixture was stirred until the solid
was completely dissolved. During this step, gentle heating was needed
was then separated and evaporated under reduced pressure at
◦
(
∼45–60 C). The system was then cooled to the specific temperature.
◦
6
[
5 C. Successive runs were performed with the recovered IL
In the meantime, ethyl glyoxylate was prepared by the distillation of a
commercially available ethyl glyoxylate–toluene solution. It should be
noted that toluene distilled first at around 110 C, while ethyl glyoxy-
+
−
+
−
EtPy] [BF4] or [EtPy] [CF3COO] for the acylation at
◦
room temperature for 24 h.
late (∼98% GC) was left as residue. Freshly distilled ethyl glyoxylate
As results in Table 5 show, both ILs could be recovered
efficiently and almost without loss of activity and selectivity,
as the yields and e.e. are not affected even after the fifth run
with the recovered IL. However, yields of the IL recovery
from the used catalyst–IL system were relatively low.
(
1 mmol) and amine (0.5 mmol) were added to the reaction system.After
a specific reaction time, 3 mL of water and 3 mL of diethyl ether were
added to quench the reaction. It should be noted that after adding water,
i
Ti(OPr )4 became a white suspension, which could be filtered off easily.
However, this phenomenon did not occur for Cu(OTf)2. The water layer
was evaporated under reduced pressure and the ILs could be recycled
and purified. They were then reused. The organic layer was concen-
trated under reduced pressure and the product was purified by flash
chromatography on silica gel using hexanes/ethyl acetate (70/30) as an
eluent.
Conclusions
Pyridinium-based ILs are desirable media for the asymmet-
ric Friedel–Crafts reaction of aromatic compounds with ethyl
glyoxylate. The reaction proceeds at a relatively low tem-
perature and a high product yield and e.e. are achieved.
(
R)-2-[4-(Dimethylamino)phenyl]-2-hydroxyacetic
Acid Ethyl Ester 3a
◦
−1
+ −
White solid, m.p. 104 C. HPLC (1.0 mL min ) tr/min 11.7, 12.6
major). δH (500 MHz, CDCl3) 1.22 (t, 3H, CH2CH3), 3.00 (s, 6H,
[
EtPy] [CF3COO] is found to be the best solvent to effi-
(
ciently substitute for the traditional organic solvents. Appli-
cations of the pyridinium-based ILs as solvents for a number
of other reactions are under investigation in our laboratory.
N(CH3)2), 3.47 (d, 1H, OH), 4.20 (q, 2H, CH2CH3), 5.10 (d, 1H, CH),
6.71 (m, 2H, Ar), 7.22–7.26 (m, 2H, Ar).
(
R)-2-[2-Chloro-4-(dimethylamino)phenyl]-2-hydroxyacetic
Acid Ethyl Ester 3b
Colourless oil. HPLC (1.0 mL min 1) tr/min 13.2 (minor), 16.8
major). δH (500 MHz, CDCl3) 1.22 (t, 3H, CH2CH3), 2.99 (s, 6H,
N(CH3)2), 3.60 (d, 1H, OH), 4.24 (q, 2H, CH2CH3), 5.43 (d, 1H, CH),
.67 (dd, 1H, Ar), 6.91 (d, 1H, Ar), 7.22 (d, 1H, Ar).
Experimental
−
All reactions were carried out under a nitrogen atmosphere. The reac-
tion products were analyzed using a Hewlett Packard CP-3800 HPLC
equipped with a chiralcel OD-H column (hexanes/propan-2-ol, 90/10).
The enantiomeric excesses were determined by the area ratios of each
chromatographic peak. H NMR spectroscopy was employed to confirm
the products (500 MHz, in CDCl3).
(
6
1
(
R)-2-[2-Bromo-4-(dimethylamino)phenyl]-2-hydroxyacetic
Acid Ethyl Ester 3c
Colourless oil. HPLC (1.0 mL min 1) tr/min 11.3, 16.5 (major). δH
−
Synthesis of 1-Ethyl-pyridinium Trifluoroacetate
and 1-Ethyl-pyridinium Tetrafluoroborate
(
(
6
500 MHz, CDCl3) 1.20 (t, 3H, CH2CH3), 2.92 (s, 6H, N(CH3)2), 3.58
d, 1H, OH), 4.22 (q, 2H, CH2CH3), 5.40 (d, 1H, CH), 6.66 (dd, 1H,Ar),
+
−
Two ILs ,1-ethyl-pyridinium trifluoroacetate ([EtPy] [CF3COO] ) and
.94 (d, 1H, Ar), 7.18 (d, 1H, Ar).
+ −
-ethyl-pyridinium tetrafluoroborate ([EtPy] [BF4] ), were prepared
following literature methods.
1
[
19]
(
R)-2-[2-Methyl-4-(dimethylamino)phenyl]-2-hydroxyacetic
Trifluoroacetic acid or tetrafluoroboric acid (0.2 mol) were slowly
added to a stirred slurry of silver(i) oxide (0.1 mol) and distilled water
Acid Ethyl Ester 3d
Light yellow oil. HPLC (1.0 mL min 1) tr/min 11.1, 16.0 (major).
δH (500 MHz, CDCl3) 1.24 (t, 3H, CH2CH3), 2.43 (s, 3H,ArCH3), 2.96
s, 6H, N(CH3)2), 3.43 (d, 1H, OH), 4.21 (q, 2H, CH2CH3), 5.29 (d, 1H,
CH), 6.55–6.61 (m, 2H, Ar), 7.13 (d, 1H, Ar).
−
(
50 mL). To avoid photodegradation of Ag2O, the reaction mixture was
fully covered with aluminum foil. The mixture was stirred continuously
until the reaction was complete, which was indicated by the formation
of a solution. A solution of N-ethyl-pyridinium bromide (0.2 mol) was
added to the reaction mixture. As the reaction took place and the ILs
formed, a yellow precipitate of silver(i) bromide was observed. The
mixture was stirred at room temperature for a certain time until no
more precipitate formed. The stirring was stopped and the precipitate
allowed to settle to the bottom of the beaker until a clear aqueous layer
(
(
R)-2-[2-Methoxy-4-(dimethylamino)phenyl]-2-hydroxyacetic
Acid Ethyl Ester 3e
Colourless oil. HPLC (1.0 mL min 1) tr/min 17.4, 20.5 (major). δH
−
(500 MHz, CDCl3) 1.21 (t, 3H, CH2CH3), 2.96 (s, 6H, N(CH3)2), 3.61

4
72
S. V. Malhotra and Y. Xiao
(
d, 1H, OH), 3.84 (s, 3H, OCH3), 4.21 (q, 2H, CH2CH3), 5.19 (d, 1H,
[8] J. Ross, J. Xiao, GreenChem. 2002, 4, 129. doi:10.1039/B109847K
[9] S. Guo, Y. Deng, Catal. Commun. 2005, 6, 225. doi:10.1016/
J.CATCOM.2005.01.003
CH), 6.26 (dd, 1H, Ar), 6.31 (d, 1H, Ar), 7.10 (d, 1H, Ar).
[
[
[
10] P. Wasserscheid, H. Waffenschmidt, J. Mol. Catal. Chem. 2000,
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(
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(
[
[
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