Asymmetric Transfer Hydrogenation of Imines in Water by Varying the Ratio of Formic Acid to Triethylamine

Article abstract of DOI:10.1021/acs.orglett.5b00889

Asymmetric transfer hydrogenation (ATH) of imines has been performed with variation in formic acid (F) and triethylamine (T) molar ratios in water. The F/T ratio is shown to affect both the reduction rate and enantioselectivity, with the optimum ratio being 1.1 in the ATH of imines with the Rh-(1S,2S)-TsDPEN catalyst. Use of methanol as a cosolvent enhanced reduction activity. A variety of imine substrates have been reduced, affording high yields (94-98%) and good to excellent enantioselectivities (89-98%). In comparison with the common azeotropic F-T system, the reduction with 1.1/1 F/T is faster.

Full text of DOI:10.1021/acs.orglett.5b00889

Letter
pubs.acs.org/OrgLett
Asymmetric Transfer Hydrogenation of Imines in Water by Varying
the Ratio of Formic Acid to Triethylamine
Vaishali S. Shende, Sudhindra H. Deshpande, Savita K. Shingote, Anu Joseph, and Ashutosh A. Kelkar*
Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Pune, India
S
* Supporting Information
ABSTRACT: Asymmetric transfer hydrogenation (ATH) of imines has been
performed with variation in formic acid (F) and triethylamine (T) molar ratios
in water. The F/T ratio is shown to affect both the reduction rate and
enantioselectivity, with the optimum ratio being 1.1 in the ATH of imines with
the Rh-(1S,2S)-TsDPEN catalyst. Use of methanol as a cosolvent enhanced
reduction activity. A variety of imine substrates have been reduced, affording high yields (94−98%) and good to excellent
enantioselectivities (89−98%). In comparison with the common azeotropic F−T system, the reduction with 1.1/1 F/T is faster.
nantiomerically enriched active amines are significant
synthetic precursors for biologically active molecules in
was found that ATH of imine is pH dependent. Significant
improvement in imine conversion with excellent enantiose-
lectivity was observed for a wide range of imine derivatives with
Rh-(1S,2S)-TsDPEN catalyst under an optimum F/T ratio in
water with a short reaction time.
To initiate this study, we examined the ATH of imine 1a as a
model substrate to amine 2a as the product (Scheme 1) using
E
medical, pharmaceutical, agricultural sciences, flavor, and
fragrance industries.1 Different methods have been utilized for
the synthesis of enantiomerically pure amines in the past few
years.2 Asymmetric transfer hydrogenation (ATH) of imines is
one of the most popular methods due to its operational
simplicity and avoidance of the use of hazardous hydrogen gas
and pressure vessels.3 Various chiral catalysts have been
investigated for ATH of imines, but the most outstanding to
date are the Ru and Rh complexes with the optically active N-
toluenesulfonyl-1,2-diphenylethylenediamine (TsDPEN) li-
gand4 in organic solvents with a formic acid−triethylamine
azeotrope as a H donor.2−4 The choice of reaction medium and
H donor is important in achieving an ATH reaction with high
efficiency. The metal catalyzed ATH of imines is mostly
performed in an azeotropic mixture of formic acid (HCOOH)
and triethylamine (NEt3) (F−T), with the F/T molar ratio
being 5:2 or with HCOONa as the hydrogen source and water
as the solvent.3−5 Recently we have reported a significant
enhancement in ATH of imines in water with the use of
methanol as a cosolvent.6 The activity as well as enantiose-
lectivity is very good with an azeotropic F/T and Rh or Ru
complex/TsDPEN catalyst system.2−6 However, it is observed
in the literature that the main drawback of ATH in an
azeotropic F/T is that some catalytic complexes exhibit sluggish
activity and require a longer induction period under acidic
conditions.7 ATH of ketones has been investigated in water
using F/T as the hydrogen donor, and the activity as well as
enantioselectivity was found to be dependent on the F/T ratio
(initial pH of the reaction mixture).7b,8 To the best of our
knowledge there are no reports on the effect of the F/T ratio
(initial pH of the reaction mixture) on the activity and
enantioselectivity of ATH of imines with water as solvent, and
there are very few reports on pH dependent ATH of imines.10
Herein we wish to report our results on the ATH of imines in
water with F/T as the H donor. Variation of the F/T ratio in
ATH of imines with water as a solvent was investigated, and it
Scheme 1. ATH of Imine 1a by Varying FA−TEA Ratio
CH2Cl2 and H2O as solvents (Table 1). The precatalyst was
generated by treating (1S,2S)-Ts-DPEN (0.0075 mmol) with
[RhCl2(Cp*)2] (0.0025 mmol) in water (1 mL) at 40 °C for 1
h,11 and the reduction was started by introducing the
HCOOH−NEt3 (F/T) azeotrope (1.0 mL ; molar ratio F/T
= 2.5/1) and 1a with a substrate/catalyst (S/C) ratio of 100:1.
Table 1. Comparison of ATH of 1a with 2.5/1 F/T in
a
CH2Cl2 and in H2O
f/t
ratio
time
(min)
conv
b
ee
(%)
c
entry
catalyst
solvent
(%)
1
2
3
4
5
Rh-TsDPEN
Rh-TsDPEN
Rh-TsDPEN
Rh-TsDPEN
Rh-TsDPEN
2.5/1
2.5/1
2.5/1
5.0/1
5.0/1
CH2Cl2
H2O
10
99
9
89
d
60
nd
2
H2O
1440
180
300
99
0
d
H2O
nd
nd
d
H2O
2
a
Reaction conditions: 1a, 0.5 mmol; [RhCp*Cl2]2, 0.0025 mmol;
(1S,2S)-TsDPEN, 0.0075 mmol; F/T, 1 mL; H2O, 1 mL; temp, 40 °C.
b
c
Determined by GC equipped with HP-1 column. Determined by
d
HPLC equipped with chiral column. Not determined.
Received: March 26, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00889
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
As reported in the literature,2a the reaction proceeded smoothly
with CH2Cl2 as solvent with F/T (2.5/1) as the H donor and
99% of imine 1a was converted into (R)-2a in 10 min with 89%
ee (Table 1, entry 1).
To our surprise, with H2O as solvent, a much slower reaction
rate was observed (Table 1, entry 2). Thus, only 9% conversion
was observed for the reduction of 1a at 40 °C in 1 h; the
conversion rose to 99% after a prolonged time of 1440 min
with a decrease in ee to 2% (Table 1, entry 2). The major
difference was that the pH value of the azeotrope−water system
was 3 at the beginning of the reaction, while that of the F/T
azeotrope (2.5/1)−CH2Cl2 (organic solvent) was 5. We have
investigated the effect of the initial pH of F/T by systematically
varying the molar ratio of F/T in water, and the results are
presented below.
In order to understand the role of pH in detail, ATH of 1a
was investigated by systematically varying the F/T ratio in a
range of 1.5 to 0.5, and the results obtained for ATH of 1a
(conversion) after 60 min of reaction time with intermediate
sampling are presented in Figure 2. As shown in Figure 2, the
The ATH of imine 1a (0.5 mmol) was performed in water (1
mL) at various initial solution pH values by adjusting the F/T
molar ratios in a 0.5 to 5.0 range; the total solution volume was
kept constant at 2.0 mL. Figure 1a shows the graph of turnover
Figure 2. Conversion versus reaction time for ATH of imine 1a using
Rh-TsDPEN in F/T mixtures at different initial F/T ratios at 40 °C.
1a, 0.5 mmol; F/T, 1 mL; H2O, 1 mL; [RhCp*Cl2]2, 0.0025 mmol;
(1S,2S)-TsDPEN, 0.0075 mmol.
reduction of imine 1a proceeded slowly at F/T ratio 1.5 to 1.4
(pH range 4.1 to 4.5) and a 1a conversion of 30−50% was
observed in 60 min. The reaction accelerated with the decrease
in F/T ratio between 1.3 to 1.1 (pH range 4.6−5.1), and
complete conversion of 1a was obtained in just 10 min at an F/
T ratio of 1.1. A remarkable acceleration in rate was observed
for ATH of 1a at F/T ratio 1.1/1 in water compared to those
with azeotropic F/T (2.5/1) in water, and the results are
presented in Figure 3. With a further decrease in F/T ratio to
Figure 1. (a) TOF against initial solution pH values; (b) ee against
initial solution pH for the reduction of imine 1a (0.5 mmol) by
HCOOH−NEt3 in water (2 mL total volume) with [RhCp*Cl2]2
(0.0025 mmol), (1S,2S)-TsDPEN (0.0075 mmol) at 40 °C. The initial
pH values were determined by varying the HCOOH/NEt3 molar
ratios from 5.0/1.0 to 0.5/1.0.
frequency (TOF) as a function of initial pH of the F/T−water
mixture (see Table S1 in Supporting Information (SI) for
details), while Figure 1b shows the ee versus initial pH of the
F/T−water mixture. The reaction barely took place at low pH
values (pH range 2.4−3.1) and slowly accelerated at pH 3.1. At
high F/T ratios (F/T ratio 5.0−2.0, pH range 2.4−3.8), the
reaction medium was strongly acidic and only 19% conversion
was achieved in 24 h. As seen in Table 1 with the ATH of imine
1a at F/T ratio 5.0 (initial pH 2.4), the reaction did not
proceed even after 180 min of reaction time and only 2%
conversion was attained at 300 min (Table 1, entries 4−5). The
results observed indicate sluggish activity in terms of conversion
and ee and a long induction period under acidic conditions.
Zhou et al. have observed similar results for ATH of ketones.
Activity increased significantly with the increase in initial pH of
the F/T solution in the range 4.8−5.6, and the highest TOF
value of 594 h−1 was observed at pH 5.1. Activity decreased
with the further increase in pH of the solution and decreased
significantly with the increase in pH beyond 9.4. TOF values
varied significantly with the change in initial pH values of F/T
mixtures. The observed results confirm the pH dependent ATH
of imines in water. The higher rates at pH values greater than 4
could be due to the increased concentration of HCOO−. At pH
>4, HCOOH (pKa = 3.6) exists predominately as HCOO−,
which is essential for the formation of the rhodium formato
complex as per the mechanism of ATH reaction.
Figure 3. Comparison of ATH of 1a in 1.1/1 F/T in water and 2.5/1
F/T in water: 1a, 0.5 mmol; total volume, 2 mL; at 40 °C.
1.0 and 0.9, the pH of the reaction mixture became basic (pH
8.7−9.4) and the reduction slowed and reached completion in
30 and 60 min, respectively. The rate of reduction decreased
significantly under strongly basic conditions (F/T, 0.8 to 0.5;
pH range 9.7−10.3). Thus, the results of the F/T ratio variation
show that ATH of imines proceeded effectively under slightly
acidic conditions (pH 5.1).
Enantioselectivities were determined at the end of the
reaction, and the results are presented in Figure 1b. The results
indicate that at acidic pH (2.4−4.1) ee values were very low
(3−56%). As the pH increased, the ee values increased. Thus,
for reactions with pH values in a 5.1−10.3 range, ee values were
high (82−91%) and remained constant up to pH 10.3. As seen
from Figure 1a and b, both the rate of reduction and ee were
strongly dependent on the initial pH value and best results were
observed at an initial pH of 5.1 (F/T ratio: 1.1/1). Variation in
ee with changes in F/T ratio from 1.5:1 to 0.5:1 with time
B
DOI: 10.1021/acs.orglett.5b00889
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
sampling is presented in Figure S1 in the SI. The ee was in a
range of 54−58% at F/T ratio 1.5 and increased to ∼80% for
F/T ratio 1.4:1 and 1.3:1 indicating strong dependence on the
initial pH of the reaction mixture. The ee values were in an 84−
89% range for experiments with the F/T ratio in a 1.2:1 to 0.5:1
range indicating the ee was not significantly affected by the
higher basicity of the reaction mixture. These results with
decreased ee values at acidic pH resemble those of aqueous
ATH8,11,12 and F/T variation of ketones in water.7,8
The results obtained clearly show that the activity and ee of
ATH of imines are pH regulated, similar to ATH of
ketones.10−13Based on the literature reports14 and results
obtained in the present work, we propose two catalytic cycles
likely to operate under acidic and basic conditions, depending
on the F/T molar ratios (Scheme S1 and discussion, SI).Under
strong acidic conditions the Rh-TsDPEN chelate is broken
resulting in lower activity and ee. At pH 5.1 and above, the
chelate is intact giving good activity and ee.The best results for
ATH of 1a were obtained at F/T ratio 1.1/1 and pH 5.1, which
is a slightly acidic pH. Thus, probably iminium ion formation
occurred very efficiently at this pH value. The iminium ion
formed might be entering into the catalytic cycle and accepts
hydride from Rh-hydride species, forming amine 2a as the
product.
conversion was achieved in 6 min with 89% ee with methanol
as cosolvent (Table 2, entry 2). The rate of reduction of 1a was
slow for all other catalysts tested as compared to the Rh based
catalyst (Table S2, SI) however; activity increased with the use
of methanol as a cosolvent for both Ru and Ir catalysts. The
enhanced reaction rate with methanol as a cosolvent could be
attributed to the fact that cosolvents have H-bond donor and/
or acceptor groups for aqueous solubility and a small
hydrocarbon region that serves to disrupt the strong H-bond
network of pure water, thereby increasing the solubility of
reactants/products in the reaction mixture.
Optimum results in terms of both activity and enantiose-
lectivity for ATH of 1a in the F/T−H2O/MeOH system were
obtained with the Rh-TsDPEN catalyst, and hence a screening
of various imine derivatives (Scheme 2) was performed with S/
Scheme 2. ATH of Imines with Rh-TsDPEN in 1.1/1 F/T
The efficiency of various catalysts was tested with (1S,2S)-
TsDPEN ligands for ATH of imine 1a with F/T (molar ratio of
1.1/1) in water, and the best results were obtained with the Rh-
TsDPEN catalyst (Table S2, SI). In our recently published
paper we observed significant enhancement in activity for ATH
of imines in water with HCOONa as the hydrogen source with
methanol as a cosolvent.6 Based on these results we tested
methanol as a cosolvent for catalyst screening with F/T (molar
ratio of 1.1/1) in water (Table 2, entry 2). From the results, the
Rh catalyst was found to be the most active catalyst and ATH
reaction of 1a achieved the highest conversion of 99% with 89%
ee in a short reaction time of 10 min using the Rh-TsDPEN
catalyst and water as solvent (Table 2, entry 1) and 99%
Table 2. ATH of Imines with Rh-TsDPEN in F/T (1.1/1)−
a
H2O/MeOH
C ratio 200 with the Rh catalyst; the results are presented in
Table 2. Imines with alkyl groups on the imino carbon
displayed 99% conversions in 6 min (95−98% yields) with ee’s
of 83−99% (Table 2, entries 3−9). The chain length of the
alkyl group had little effect on the enantioselectivity (Table 2,
entry 6). The imine 1h having a bulky phenyl group on the
imino carbon required 60 min to achieve 87% conversion with
an ee of 5%, and in this case a second aromatic system
apparently interferes with selective catalyst binding, resulting in
a low ee value (Table 2, entry 10).2a
With an aromatic group with more substituents, the activity
was very sluggish and only 29% conversion was attained for
imine 1i in 60 min with a 2% ee value (Table 2, entry 11). The
investigations were extended to include the reduction of cyclic
sulfonamide and β-carboline derivatives (Table 2, entries 12−
14). Cyclic sulfonamides have been widely used in asymmetric
reactions.16 β-Carboline derivative 3a was obtained in 95%
yield with 91% enantioselectivity (Table 2, entry 12), and 3b
was obtained in 92% yield with 97% enantioselectivity (Table 2,
entry 13). Cyclic sulfonamide 5 was reduced in 6 min with a
96% yield and 32% ee (Table 2, entry 14). When ATH of imine
7 was performed at F/T ratio 1.1/1, a 96% yield obtained in 15
min with 81% enantioselectivity (Table 2, entry 15). We also
performed ATH of 1a at an S/C ratio of 500 under similar
d
entry
imine
s/c
time (min)
yield (%)
ee (%)
b
c
1
1a
1a
1a
1b
1c
1d
1e
1f
100
100
200
200
200
200
200
200
200
200
200
200
200
200
200
500
10
6
99
89
89
89
85
88
88
83
97
99
5
c
2
99
3
6
98
95
95
96
98
97
98
4
6
5
6
6
6
7
6
8
6
9
1g
1h
1i
6
c
10
11
12
13
14
15
16
60
60
10
10
6
87
c
29
2
3a
3b
5
95
92
96
96
92
91
97
32
81
88
7
15
10
1a
a
Reaction conditions: Imine, 1 mmol; [RhCp*Cl2]2, 0.0025 mmol;
(1S,2S)-TsDPEN, 0.0075 mmol; F/T (1.1/1) 1 mL; H2O, 0.5 mL;
b
MeOH, 0.5 mL; at 40 °C. F/T (1.1/1) 1 mL; H2O, 1 mL.
c
d
Conversion determined by GC using HP1 column. ee determined
by Chiral HPLC.
C
DOI: 10.1021/acs.orglett.5b00889
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, NMR spectra, and HPLC traces of
products. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b00889.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: aa.kelkar@ncl.res.in.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.5b00889
Org. Lett. XXXX, XXX, XXX−XXX