Enantioselective Aldol Reactions in Water by a Proline-Derived Cryptand and Fixation of CO2 by Its Exocyclic Co(II) Complex

Article abstract of DOI:10.1021/acs.inorgchem.7b02007

The secondary amine donors present in the bridges of a laterally nonsymmetric oxa-aza cryptand have been derivatized with l-proline to obtain the chiral cryptand L. The cryptand L efficiently catalyzed aldol reactions in water with up to 75% ee. On reacting with Co(II) perchlorate in the presence of KSCN, L readily formed the trinuclear complex {[Co₃(L)₂(NCS)₆]·(15CH₃CN)(5acetone)(6H₂O)} (1). The complex 1 in combination with the cocatalyst tetrabutylammonium bromide (TBAB) formed an efficient catalytic system in the synthesis of cyclic carbonates from CO₂ and epoxides at room temperature and atmospheric pressure.

Full text of DOI:10.1021/acs.inorgchem.7b02007

Article
pubs.acs.org/IC
Enantioselective Aldol Reactions in Water by a Proline-Derived
Cryptand and Fixation of CO by Its Exocyclic Co(II) Complex
2
Dinesh De, Aditya Bhattacharyya, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
*
S Supporting Information
ABSTRACT: The secondary amine donors present in the bridges of a laterally
nonsymmetric oxa-aza cryptand have been derivatized with L-proline to obtain
the chiral cryptand L. The cryptand L efficiently catalyzed aldol reactions in
water with up to 75% ee. On reacting with Co(II) perchlorate in the presence
of KSCN, L readily formed the trinuclear complex {[Co (L) (NCS) ]·
3
2
6
(
15CH CN)(5acetone)(6H O)} (1). The complex 1 in combination with the
3
2
cocatalyst tetrabutylammonium bromide (TBAB) formed an efficient catalytic
system in the synthesis of cyclic carbonates from CO and epoxides at room
2
temperature and atmospheric pressure.
INTRODUCTION
{[Co (L) (NCS) ]·(15CH CN)(5acetone)(6H O)} (1;
3 2 6 3 2
Scheme 1).
Complex 1 has been used to catalyze the reaction between
■
Macrobicyclic cryptands have received steady attention from
researchers during the past few decades due to their ability of
selective and efficient ion binding. Additionally, cryptands have
1
epoxides and CO gas at atmospheric pressure and room
2
temperature in solvent-free condition, to generate cyclic
such structural features that render them important compo-
nents in supramolecular chemistry with a wide range o2f
applications in chemical, biological, and material sciences.
Laterally nonsymmetric oxa-aza cryptands incorporating
secondary amines in the bridges constitute an important class
carbonates. This process not only removes CO from the
2
atmosphere but also produces cyclic carbonates that are key
intermediates in the synthesis of a variety of compounds of
pharmaceutical relevance. Besides, they are useful as polar
aprotic solvents, electrolytes for lithium ion batteries, and so
3
of cryptands. The secondary amines present in the bridges can
13,14
be easily derivatized with different groups forming new
compounds endowed with various properties. The employ-
on.
Cyclic carbonates are also used as constituents of oils
4
and paints and as monomers for the synthesis of polycar-
15
16
ment of a chiral functional group in the cryptand to transform it
to a chiral cryptand is quite fascinating and expands its scope
for several new applications such as asymmetric organic
transformations, chiral recognition, and so on. Herein, we
present the synthesis of the tris-L-proline derivative (L; Scheme
bonates and polyurethanes since they can undergo ring-
opening polymerization. Our approach benefits from eliminat-
ing the commonly used highly toxic carbonylation reagent
phosgene and is 100% atom economical, making it a highly
desirable transformation. To date, there are several catalysts
1
) of an oxa-aza cryptand. L-Proline is a naturally occurring
reported for the coupling of CO with epoxides, comprising
2
inexpensive chiral amino acid and can catalyze many organic
simple alkali metal salts, phosphines, main group metal
5
reactions. The direct intermolecular aldol reaction is an
complexes, transition metal complexes, porous metal oxides,
important carbon−carbon bond forming reaction with highly6
17−29
and MOFs.
However, most of them suffer from low
efficient atom economy. Following the lead provided by List
catalyst stability/reactivity, air sensitivity, the need for
cosolvent, or the requirement of high pressure and/or high
temperature. However, several salen-Co/Al complexes have
been identified as homogeneous catalysts in the formation of
7
and Barbas, L-proline and its derivatives have been used by
8
−11
several groups
for enantioselective intermolecular aldol
12
reactions. A number of L-prolinamides have also been used
for the purpose. However, it should be emphasized that low
loading of the catalyst and a wide range of substrates for
enantioselective aldol reactions in aqueous medium present an
important synthetic challenge. In this study, we demonstrate
the scope of the tris-proline derivative L in asymmetric
intermolecular aldol reactions in water.
cyclic carbonate from CO and epoxides at relatively low
2
30−33
temperatures and pressures.
If the utilization of cyclic
carbonates is to be substantially increased, then new
commercially viable catalysts and processes which operate at
Furthermore, cryptand L reacts with Co(II) in the presence
Received: August 5, 2017
of KSCN at room temperature to form a trinuclear complex,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02007
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Inorganic Chemistry
Article
Scheme 1. Synthesis of Complex 1 from the Tris-Proline Derivative L
close to atmospheric pressure and near room temperature are
required.
nm consistent with octahedral Co(II) complex with the
37,38
CoN O chromophore.
The CD spectrum (Figure S2) of
4
2
L in CH CN showed negative Cotton effect at 280 nm and
3
RESULTS AND DISCUSSION
positive Cotton effect at 275 nm while the complex 1 showed
negative Cotton effect at 295 nm and positive Cotton effect at
■
Structural Characterization. The compound L was
synthesized via the reaction of N-(tert-butoxycarbonyl)-L-
proline with the cryptand. Also, the complex 1 could be
synthesized readily by reacting Co(II) perchlorate and L in the
presence of KSCN. Single-crystal X-ray structure determination
revealed that 1 crystallized in the trigonal chiral space group
2
72 nm and also a monosignate CD signal at 494 nm.
Aldol Reactions Catalyzed by L. The enantioselective
aldol reaction is an important C−C bond forming reaction.
Most of the prior studies on this reaction were done with large
amounts of (10.0 mol %) catalyst loading. Thus, there is a
demand for efficient catalysts, which can function at a lower
loading without affecting much of the enantiopurity and the
duration of reaction. On the other hand, due to increasing
environmental awareness, water should be the ideal alternative
medium for this reaction. It was found that L acted as a very
efficient organocatalyst for enantioselective aldol reactions in an
aqueous medium. Initially, the aldol reaction of cyclohexanone
(0.3 mmol) with 4-nitrobenzaldehyde (0.2 mmol) was studied
by using different loading of the catalyst L (5.0−0.2 mol %) in
1 mL of water (Table 1). It was observed that, with 5.0 mol %
P3 21 (Table S1). The asymmetric unit consisted of one L, one
2
Co(II) with full occupancy, and another Co(II) with half
occupancy, besides three thiocyanate anions and lattice solvent
molecules. The lattice solvent molecules were highly
disordered, and their content was established from thermog-
ravimetric weight-loss analysis (TGA), IR spectra, and
elemental analysis. The values are in good agreement with
34
the PLATON-calculated solvent-accessible void volume,
which is found to be 39% of the unit-cell volume. The
structure showed two L and three Co(II) ions (Figure 1) where
a
Table 1. Optimization of Reaction Conditions
catalyst loading
(mol %)
temp
(°C)
time
(h)
anti/
c
b
d
entry
% yield
syn
% ee
1
2
3
4
5
6
7
5.0
2.0
1.0
0.5
0.2
0.2
0.0
rt
rt
rt
rt
rt
0
2
2
94
62:38
59:41
66:33
57:43
52:48
68:32
75
72
72
69
68
63
89
2
87
4
86
5
84
8
78
rt
24
trace
a
Reaction conditions: aldehyde (0.2 mmol), cyclohexanone (0.3
b
mmol) in 1 mL of H O. Yields of isolated products after silica gel
2
c
column chromatography. Diastereoselectivities were determined by
Figure 1. Coordination environment around (a) each terminal Co(II)
and (b) the middle Co(II) ion. (c) Overall structure of the complex 1.
1
d
H NMR analysis of the crude rection mixture. The ee’s were
determined by HPLC using chiral columns.
each Co(II) ion exhibited distorted octahedral geometry. The
two terminal metal ions had ligation from two proline groups
attached to the same cryptand moiety (Figure 1a) and
of the catalyst in water at room temperature, the reaction was
almost complete in 2 h and the product was obtained in 94%
yield with 75% ee (Table 1, entry 1). Upon gradually reducing
the catalyst loading up to 0.2 mol %, the reaction proceeded
smoothly but required a little longer time for completion (5
h,Table 1, entry 5). In this particular case, we could get the
corresponding product with only up to 68% ee. Further, on
lowering the temperature to 0 °C it was observed that the
enantioselectivity was slighty reduced to 63% (Table 1, entry
6). Finally, the use of a higher loading of catalyst in the reaction
−
additionally two N atoms of two SCN ions.
The middle Co(II) ion was bonded to two such cryptand
units through the third proline group besides two N-bonded
−
SCN anions (Figure 1b). The Co−N and Co−O bond
distances (Table S2) were found to be within normal
3
5,36
ranges.
1
The UV−vis spectrum (Figure S1) of the complex
in MeCN shows three ligand field bands at 620, 535, and 495
B
DOI: 10.1021/acs.inorgchem.7b02007
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Inorganic Chemistry
Article
Table 2. Aldol Reaction of Various Substrates Using Organocatalyst L
e
f
g
Catalyst 1.0 mol %, water medium. Isolated yields after silica gel chromatography. The ee’s were determined by HPLC using chiral columns.
h
1
i
Diastereoselectivities were determined by H NMR analysis of thecrude reaction mixture. Enantiomeric excess could not be determined by chiral
HPLC analysis.
Scheme 2. Proposed Enamine Mechanism of the L Catalyzed Asymmetric Aldol Reaction
with 4-nitrobenzaldehyde with a large excess of cyclohexanone
3 equiv) led to the formation of the final product
quantitatively.
Using the optimized reaction conditions, it was extended to
other substrates using 1.0 mol % of the catalyst, L. A variety of
aromatic aldehydes were tested for the aldol reactions (Table
We assumed that the asymmetric aldol reactions occurred via
an enamine mechanism (Scheme 2). This catalyst might
6
(
facilitate each individual step, including the nucleophilic attack
of the amino group (a), the dehydroxylation of the carbinol
amine intermediate (b), the deprotonation of the iminium
species (c), the carbon−carbon bond forming step (d), and
both steps of the hydrolysis of the iminium−aldol intermediate
2). For the electon-deficient aromatic aldehydes, the reaction
(
e and f).
proceeded smoothly (Table 2, entries 1−4). For the neutral
aromatic aldehyde, the reaction needed a longer time (12 h),
and lower isolated yield was obtained (Table 2, entry 5).
Reactions were not successful with electon-rich aromatic
aldehydes. For most of the substrates, the aldol products
were formed as the major product with excellent yields. Other
ketones were also used under the same conditions, and the
aldol products were obtained in good yields (Table 2, entries
Low catalyst loading, less reaction time, and water as reaction
medium are the key aspects of the catalyst, L. Though the L-
39
proline derived chiral catalysts reported by Wang et al., Wu et
8
40
al., and Zhao et al. showed high catalytic activity, they suffer
from the use of higher loading of the catalyst (>10 mol %) with
longer reaction time (>24 h). Several other groups also showed
the same reaction using a catalyst loading of as high as 20 or 30
mol % to achieve a good isolated yield of the aldol
6−8). In the case of acetaldehyde (Table 2, entry 8),the yield
6,41−45
product.
was lowest (35%) because the enamine intermediate might
attack the carbonyl group of acetaldehyde in addition to that of
Synthesis of Cyclic Carbonates from Epoxides and
CO . There is considerable current interest to develop novel
2
4
-nitrobenzaldehyde. In this particular case, the aldol-
technologies that allow the synthesis of cyclic carbonates from
dehydratation product was major. The desired aldol products
CO and reactive epoxides under more economic conditions,
2
1
13
were confirmed by the H and C NMR, IR, and ESI-MS
analyses (Figures S3−S42).
thus reducing both the cost and greenhouse gas emissions. In
this context, complex 1 was tested as a catalyst for the
C
DOI: 10.1021/acs.inorgchem.7b02007
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
j
Table 3. Coupling of Various Epoxides with CO at Atmospheric Pressure Using 1
2
j
Reactions were carried out in a 10 mL two-necked round-bottom flask at room temperature with CO bubbling. Epoxide (1.0 mL, 1 equiv), complex
2
k
1
1
(0.0005 equiv), TBAB (0.05 equiv). No solvent was added. Selectivity for cyclic carbonate in all systems is 100% based on H NMR spectroscopic
l
m
analysis. Yield of the isolated product obtained after column chromatography. To avoid the loss of volatile propylene oxide caused by CO flow,
2
the reaction was carried out at 0 °C with sluggish CO flow.
2
cycloaddition of various epoxides and CO2 under mild
conditions to yield cyclic carbonates. A combination of 1 and
TBAB showed the superior catalytic activity for the cyclo-
Scheme 3. Proposed Mechanism for the Complex 1
Catalyzed Carbon Dioxide Fixation into Epoxide in the
Presence of TBAB
addition reaction of CO to epichlorohydrin to yield the
2
corresponding cyclic carbonate in good yields (48%) under 1
atm of CO at room temperature, within 24 h (Table 3, entry
2
1
). Furthermore, in the absence of 1, the same reaction
catalyzed by the cocatalyst TBAB alone gave 11% yield (Table
, entry 6). Yet, when the catalyst 1 and cocatalyst TBAB were
combined together, a facile reaction occurred (Table 3, entry
). This result is notably better than the results already
3
1
46
47
described by Lu et al. (41% yield), Jing et al. (16.9%
48
conversion), and Brandenburg et al. (up to 40% yield using
propylene oxide as substrate) under similar experimental
conditions using multichiral Co(III) complexes.
Using the same reaction conditions, a series of epoxides
could be converted to the corresponding cyclic carbonates with
good yields (Table 3). In case of phenyl glycidyl ether (Table 3,
entry 4), 1.0 mL of ethyl acetate was added to prevent the
solidification of the cyclic carbonate. The desired carbonates
1
13
were confirmed by the H and C NMR, IR, and ESI-MS
analyses (Figures S43−S62). Notably, the products were nearly
racemic (Figures S55, S56, S61, and S62), indicating the
influence of the chirality of complex 1 to be minimal.
The plausible mechanism (Scheme 3) for this chemical
fixation reaction was similar to those by the salen Co(III)X
complexes as catalysts and quaternary ammonium salts as
4
9
cocatalysts. The complex 1 promoted the ring-opening of the
D
DOI: 10.1021/acs.inorgchem.7b02007
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Inorganic Chemistry
Article
11H), 2.09−1.66 (m, 9H), 1.41 (s, 27H) ppm (Figure S63). 13C NMR
epoxide with the help of Bu NBr to afford a cobalt-bound
4
(
100 MHz, CDCl , 25 °C, Si(CH ) ): δ = 23.5, 23.7, 24.4, 28.5, 28.6,
alkoxide in an S 2-type reaction. The subsequent addition of
3 3 4
N
2
7
1
8.7, 29.8, 30.3, 30.9, 31.5, 46.4, 46.9, 47.1, 56.2, 56.6, 59.1, 59.4, 68.1,
9.4, 79.5, 79.8, 80.1, 81.4, 113.9, 129.9, 154.4, 154.5, 158.8, 159.8,
72.9, 173.3, 173.5 ppm (Figure S64). ESI-MS (m/z): 1151 (97%) [M
CO to the ring-opened epoxide was preferred by the presence
2
of Bu NBr, which stabilized the polarized intermediate resulting
4
in a metal carbonate capable of cyclization to form cyclic
carbonate with the regeneration of the catalyst.
+
H], 526 (100%) [M/2 + H] (Figure S65). Anal. Calcd for
C H N O : C, 65.72; H, 7.88; N, 9.73%. Found: C, 65.97; H, 8.04;
63
90
8
12
N, 9.79%.
CONCLUSIONS
■
Synthesis of L. L-Boc (1.5 g, 1.30 mmol) was suspended in
We have constructed a chiral aza-oxa cryptand derivatized with
L-proline. The metal free cryptand L acts as a chiral
organocatalyst and efficiently catalyzed the direct aldol reaction
between aldehydes and carbonyl compounds in water in good
yields. The Co(II) complex of the cryptand L in combination
with the cocatalyst Bu NBr acts as a highly efficient catalytic
system for the synthesis of cyclic carbonates from CO and
epoxides under mild reaction conditions. In view of the global
industrial and academic interest in this reaction, it is likely that
further significant developments in catalyst and process design
will occur in the next few years, thus allowing cyclic carbonate
synthesis as one of the technologies which can be used to help
CH Cl (50.0 mL) and cooled in an ice bath with stirring. Exactly 15.0
2
2
mL of TFA was added, and the mixture was stirred for 30 min. After
warming up to room temperature, the stirring continued for 24 h. All
of the solvent was removed in vacuo, the mixture cooled in an ice bath,
and 1 M KOH was added to maintain a pH of 7−8. The product was
extracted with CH Cl and washed with water. The organic layer, after
4
2
2
drying over anhydrous Na
evaporator and then under high vacuum to obtain L as gray solid
SO , was completely dried in a rotary
2
2 4
1
(
°
4
yield: 900 mg, 81%); mp 77−78 °C. H NMR (400 MHz, CDCl , 25
3
C, Si(CH ) ): δ = 7.17−7.10 (m, 3H), 6.75−6.50 (m, 9H), 4.48−
3
4
.24 (m, 3H), 4.03−3.84 (m, 8H), 3.15−3.05 (m, 20H), 2.85−2.77
(m, 6H), 2.41−2.30 (m, 4H), 2.01−1.56 (m, 9H), 1.22 (br s, 4H)
13
ppm (Figure S66). C NMR (100 MHz, CDCl , 25 °C, Si(CH ) ): δ
3
3 4
to limit global CO emissions.
2
=
26.6, 26.7, 29.8, 31.3, 31.4, 44.9, 45.4, 47.6, 47.7, 51.4, 51.7, 52.3,
5
1
2.4, 56.9, 58.1, 58.4, 68.5, 68.7, 68.9, 113.1, 113.3, 113.9, 114.1, 114.6,
14.9, 118.7, 119.3, 129.9, 130.0, 137.8, 157.1, 159.6, 159.7, 174.1 ppm
EXPERIMENTAL SECTION
The details of materials, instruments, and single-crystal X-ray study are
given in the Supporting Information.
■
(
Figure S67). ESI-MS (m/z): 851 (32%) [M + H], 426 (68%) [M/2 +
−1
H] (Figure S68). FT-IR (KBr pellets, cm ): 3425 (broad), 2924 (s),
2853 (s), 1639 (s), 1602 (s), 1446 (s), 1263 (s), 1156 (s), 1043 (s),
Synthesis of L. The meta-N O cryptand was synthesized
5
3
50
following a well-established procedure in our laboratory. Synthesis
of the tris-proline derivative (L) was carried out following the
procedure shown in Scheme 4.
7
82 (s) (Figure S69). Anal. Calcd for C H N O : C, 67.74; H, 7.82;
48 66 8 6
N, 13.17%. Found: C, 67.92; H, 7.98; N, 13.24%.
Synthesis of {[Co (L) (NCS) ]·(15CH CN)(5acetone)(6H O)}
3
2
6
3
2
(1). To a solution of L (850.0 mg, 0.1 mmol) in 5.0 mL of MeOH
Scheme 4. Synthetic Scheme for L
was added Co(II) perchlorate (370.0 mg, 0.1 mmol) in 5.0 mL of
MeOH. A light pink solid precipitated immediately and was collected
by filtration, washed with MeOH, and air-dried. It was taken in 4.0 mL
of MeCN and 1.0 mL of acetone and treated with KSCN (100.0 mg,
0
.1 mmol). Upon addition of KSCN, the color of the solution changed
to dark pink. This solution was filtered, and the filtrate was allowed to
evaporate slowly at room temperature. After 2 days, pink crystals
−1
appeared. Yield: 68%. FT-IR (KBr pellets, cm ): 3453 (broad), 2928
(
(
s), 2872 (s), 2067 (s), 1602 (s), 1489 (s), 1445 (s), 1267 (s), 1157
s), 1042 (s), 749 (s) (Figure S69). Anal. Calcd for
C147H219Co N O S : C, 54.46; H, 6.81; N, 15.99%. Found: C,
3
37 23 6
5
4.85; H, 6.97; N, 16.13%. A similar result was obtained when Co(II)
picrate was used in place of the perchlorate salt.
Caution! The perchlorate/picrate salts must be handled with care as
they are potential explosives in the presence of organic compounds.
General Procedure for the Aldol Condensation Reaction. An
aldehyde (0.2 mmol) was added to a mixture of ketone (0.3 mmol)
and the organocatalyst L (1.0 mol %) in water (1.0 mL) at room
temperature and stirred. Progress of the reaction was monitored by
TLC. After the reaction was over, the reaction mixture was purified by
silica gel column chromatography. The enantiomeric excess (ee) of the
aldol products was determined by chiral HPLC analysis. Relative and
absolute configurations of the products were determined by
1
comparison with the known H NMR, chiral HPLC analysis, and
Synthesis of L-Boc. N-(tert-Butoxycarbonyl)-L-proline (1.5 g, 6.98
mmol), 1,3-diisopropylcarbodiimide (DIC; 1.4 mL, 8.93 mmol), and
-dimethylaminopyridine (DMAP; 100.0 mg) were dissolved in
1
13
optical rotation values. The HPLC data and H and C NMR, IR, and
ESI-MS analyses of all the compounds are given in the Supporting
Information (Figures S3−S42).
General Procedure for the Coupling of Epoxides with CO2.
Reactions were carried out in a 10 mL two-necked round-bottom flask
4
CH Cl (50.0 mL) and then added dropwise to a stirred solution of
2
2
the cryptand (1.0 g, 1.78 mmol) dissolved in CH Cl (50.0 mL) at 5
2
2
°
C. After complete addition, the reaction mixture was stirred at 45 °C
for 48 h. The reaction mixture was then evaporated to dryness and
purified by column chromatography (elution with 5% methanol in
at room temperature under CO atmosphere at 1 bar. A mixture of
2
complex 1 (0.0005 equiv), cocatalyst TBAB (0.05 equiv), and an
epoxide (1.0 mL, 1 equiv) was stirred. The reaction was allowed to go
for a fixed time decided empirically. All cyclic carbonates were isolated
CHCl ) using silica gel (200 mesh) to obtain L-Boc as a white solid
3
1
(
°
3
yield: 1.7 g, 83%); mp 84−85 °C. H NMR (400 MHz, CDCl , 25
3
1
13
C, Si(CH ) ): δ = 7.17−6.49 (m, 12H), 4.63−4.40 (m, 8H), 4.01−
by column chromatography and analyzed through H and C NMR,
3
4
.93 (m, 6H), 3.54−3.42 (m, 9H), 3.15−3.05 (m, 7H), 2.80−2.33 (m,
IR, and ESI-MS spectroscopy (Figures S43−S62).
E
DOI: 10.1021/acs.inorgchem.7b02007
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We gratefully acknowledge the financial support from the
Department of Science and Technology, New Delhi, India, to
P.K.B. and SRF from the Council of Scientific and Industrial
Research, New Delhi, India, to D.D. A.B. thanks CSIR, New
Delhi, India, for a Research Associate fellowship.
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(
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