Chiral Cobalt(III) Complexes as Bifunctional Bronsted Acid-Lewis Base Catalysts for the Preparation of Cyclic Organic Carbonates

Article abstract of DOI:10.1002/cssc.201501365

Stereochemically inert cationic cobalt(III) complexes were shown to be one-component catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide at 50 C and 5 MPa carbon dioxide pressure. The optimal catalyst possessed an iodide coun

Full text of DOI:10.1002/cssc.201501365

DOI: 10.1002/cssc.201501365
Full Papers
Chiral Cobalt(III) Complexes as Bifunctional Brønsted Acid–
Lewis Base Catalysts for the Preparation of Cyclic Organic
Carbonates
Yuri A. Rulev,[a] Vladimir A. Larionov,[a] Anastasia V. Lokutova,[a] Margarita A. Moskalenko,[a]
Ol’ga L. Lependina,[a] Victor I. Maleev,[a] Michael North,*[b] and Yuri N. Belokon*[a]
Stereochemically inert cationic cobalt(III) complexes were
shown to be one-component catalysts for the synthesis of
cyclic carbonates from epoxides and carbon dioxide at 508C
and 5 MPa carbon dioxide pressure. The optimal catalyst pos-
sessed an iodide counter anion and could be recycled. A cata-
lytic cycle is proposed in which the ligand of the cobalt com-
plexes acts as a hydrogen-bond donor, activating the epoxide
towards ring opening by the halide anion and activating the
carbon dioxide for subsequent reaction with the halo-alkoxide.
No kinetic resolution was observed when terminal epoxides
were used as substrates, but chalcone oxide underwent kinetic
resolution.
Introduction
Recently, we introduced a new class of stereochemically inert
coordinatively saturated chiral complexes of cobalt(III) as pro-
moters of asymmetric carbon–carbon bond forming reac-
tions.[1–6] The use of chiral octahedral complexes, where the
metal ion is not involved in the catalytic act and only serves to
position and activate organic groups of a ligand, is a fast
evolving new field of research.[7–15] An efficient group of such
catalysts developed by our group are cationic complexes of
cobalt(III) and Schiff bases obtained from substituted salicylic
aldehydes and (R,R)- or (S,S)-cyclohexane diamine (Figure 1).[5,6]
The NÀH bonds of the coordinated ligands were activated by
the Lewis acidity of the central metal ion, becoming better
Brønsted acids (so called Lewis-acid activated Brønsted cataly-
sis[16]). The formation of strong hydrogen bonds with the coun-
ter anions was established by X-ray single crystal analysis and
1H NMR spectroscopy.[5] It was this hydrogen bonding function
of the NH groups that activated the substrate. Thus, the com-
plexes became “Brønsted acids in disguise”. Additionally, the
counter anions of the complexes were involved in the activa-
tion of another substrate in bimolecular reactions, such as cya-
nosilylation of aldehydes.[17]
We believed that other important reactions, involving simul-
taneous catalysis by both nucleophilic and electrophilic groups
of the catalysts, could also be efficiently promoted with our
cationic complexes. Therefore, we decided to investigate the
atom efficient coupling of carbon dioxide and oxiranes leading
to cyclic carbonates. Cyclic carbonates have applications as sol-
vents in organic synthesis, electrolytes for lithium-ion batteries,
and as chemical intermediates for biodegradable polymer syn-
thesis.[18–22] Catalysis of this reaction is generally believed to
proceed as shown in Scheme 1, with both the nucleophilic
opening of the oxirane (usually by a halide anion) and electro-
philic activation of both epoxide and carbon dioxide (by
strong Lewis acids or Brønsted acids) involved in the transition
states.[23,24]
There are a number of catalytic systems used for producing
cyclic carbonates, involving mostly Lewis acids with added am-
monium halides, providing the necessary nucleophilic compo-
nents.[25,26] The simplest catalysts include alkali metal salts[27–29]
and quaternary ammonium or phosphonium salts.[30–33] The ad-
dition of fluorinated alcohols[34] or phenols[35] to the ammoni-
um salts supplied an additional Brønsted acid component and
improved the catalytic performance.[24,34,35]
Figure 1. D(S,S) stereochemically inert chiral CoIII complexes 1–7.
[a] Y. A. Rulev, Dr. V. A. Larionov, A. V. Lokutova, Dr. M. A. Moskalenko,
Dr. O. L. Lependina, Dr. V. I. Maleev, Prof. Y. N. Belokon
Nesmeyanov Institute of Organoelement Compounds
Moscow 19991 (Russia)
Cationic catalysts 1–7 seemed to offer a convenient new bi-
functional mono-component catalytic system, possessing the
necessary nucleophilic and electrophilic components inside the
same ionic pair. The complexes are robust and their counter
anions can be easily interchanged.[5] In addition, the chirality of
the complexes may provide the opportunity for the enantio-
E-mail: yubel@ineos.ac.ru
[b] Prof. M. North
Green Chemistry Centre of Excellence
Department of Chemistry
University of York
Heslington, York, YO10 5DD (UK)
E-mail: Michael.North@york.ac.uk
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bis-silver salt with silver oxide followed by the reaction with
complex 1 in methanol, with the precipitated silver chloride
being removed by filtration (route b).
Styrene oxide 8a was used as a model substrate to test the
catalytic efficiency of complexes 1–7 in the reaction with
carbon dioxide. Preliminary results showed that to obtain
good conversions using 2 mol% catalyst, reactions had to be
carried out at a minimum of 508C and 5 MPa pressure of
carbon dioxide (Scheme 3). No side reaction was observed
Scheme 1. Proposed mechanism of cyclic carbonate synthesis.
meric recognition of racemic oxiranes in the condensation
with carbon dioxide. In this paper we report the use of the cat-
ionic stereochemically inert cobalt(III) complexes 1–7 as cata-
lysts for the synthesis of cyclic carbonates.
Scheme 3. Synthesis of cyclic carbonates using catalysts 1–7.
under these conditions with the conversion of styrene oxide
8a corresponding to the formation of styrene carbonate 9a.
The results are shown in Table 1. The observed activity of the
counter anions was in the order of their nucleophilicities (Cl<
Br<I, Table 1, entries 1–3). On the other hand, the complexes
with basic sulfonate and carboxylate counter anions had very
disappointing activities (Table 1, entries 4–6). Surprisingly, com-
plex 7 with a bis-sulfonate counter anion was twice as active
as the corresponding monosulfonate complex 5 (Table 1, en-
tries 5 and 7).
Results and Discussion
Complex 1 was prepared as described earlier.[5] The counter-
anion exchange to form complexes 2–7 was conducted by two
different routes (Scheme 2). Route a relied on a simple two
phase ion-exchange experiment, as previously reported.[5] In
this way a twentyfold excess of the corresponding alkali metal
salt in a water solution was stirred with a dichloromethane so-
lution of complex 1, giving complete exchange of the chloride
counter anion for bromide, iodide, benzoate, and p-toluenesul-
fonate in the organic solution of the complex. However, this
approach failed in the case of doubly charged counter anions,
such as terephthalate and 2,5-naphthalene disulfonate. In
these cases, the appropriate di-acid was converted into the
Table 1. Coupling of CO2 and styrene oxide promoted by complexes 1–
7.[a]
Entry
Catalyst
Conversion
TOF[b] [hÀ1
]
1
2
3
4
5
6
7
1
2
3
4
5
6
7
15
34
95
2
7
3
0.31
0.71
1.98
0.04
0.15
0.06
0.31
15
[a] Reaction conditions: 2 mol% catalyst (calculated relative to CoIII), no
solvent, 508C, 5 MPa CO2, 24 h. [b] TOF=turnover frequency=(moles of
product)/(moles of catalyst)”time.
The effect of tetrabutylammonium bromide (TBAB) addition
to the catalytic system was next studied using 3-phenoxypro-
pylene oxide 8b as substrate; the results are presented in
Table 2. Notably, TBAB is a better catalyst than complex 1
(Table 2, entries 1 and 2) and the catalytic effects of both pro-
moters were almost additive (Table 2, entries 1–3). However,
the introduction of more nucleophilic counter anions as in
complexes 2 and 3 made TBAB inclusion in the reaction much
less important (Table 2, entries 4–7). Increasing in the amount
Scheme 2. Counter-anion exchange of complex 1.
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Table 2. Coupling of CO2 and 3-phenoxypropylene oxide promoted by
complexes 1–3.[a]
Entry
Catalyst
TBAB
Yield[b] [%]
TOF[c] [hÀ1
]
1
2
3
4
5
6
7
8[d]
–
1
1
2
2
3
3
3
Yes
No
Yes
No
Yes
No
Yes
No
48
24
50
50
77
76
87
>90
2.0
1.0
2.1
2.1
3.2
3.2
3.6
>1.9
[a] Reaction conditions: 1 mol% of the catalyst, 1 mol% of TBAB co-cata-
lyst if added, neat 3-phenoxypropylene oxide, 508C, 5 MPa pressure of
CO2, 24 h. [b] Determined by weight of the isolated carbonate after purifi-
cation on SiO2. [c] TOF=turnover frequency (moles of product)/(moles of
catalyst)”time. [d] 2 mol% of complex 3 used.
of complex 3 to 2 mol% gave complete conversion of epoxide
8b to cyclic carbonate 9b without the need for any TBAB
(Table 2, entry 8). The data summarized in Tables 1 and 2 testify
to the importance of the nucleophilic component in the cata-
lysts and also proved the efficiency of the hydrogen-bond do-
nating catalysis by the cobalt(III) moiety.
Scheme 4. A plausible mechanism of CO2 reaction with epoxides promoted
by cobalt(III) complexes 1–7 with XÀ corresponding to the counter anion of
the complex.
The activity pattern of the bifunctional catalysts 1–6 can be
rationalized on the basis of the array of hydrogen bonds in the
structure of 1 (Figure 2)[5] and the mechanism assumed to be
The rate-limiting step of the reaction is, most likely, the ring
opening of the epoxide by the counter anion,[34] which ex-
plains the importance of the nucleophilicity of the counter
anion. The basicity of the anion, on other hand, should inhibit
the reaction by competing with the substrate for the catalyst’s
hydrogen bonds. This explains the greater catalytic activity of
3 relative to 1.
The catalytic cycle shown in Scheme 4 was supported by
1H NMR data of complexes 1 and 3 in CDCl3 before (Figure 3a
and 4a) and after (Figure 3b and 4b) addition of ten equiva-
lents of propylene oxide. The salient feature of the initial spec-
tra was the very significant difference in the chemical shifts of
the two diastereotopic hydrogens comprising the NH2 groups
of the ligands. One reason for this was a significant magnetic
anisotropic screening of the pro-R hydrogen atom by the C=N
bond of the neighboring ligand (Figure 2), moving its reso-
nance to higher fields (around 2–3 ppm). In contrast, the pro-S
proton is involved in the formation of hydrogen bonds with
the halide anion, according to X-ray data (Figure 2).[5] The
stronger the hydrogen bond, the greater the chemical shift of
the proton situated in the low field region of the spectrum.[5,6]
For example, for complex 1, the signal for this NH group is sit-
uated at 6.53 ppm, whereas for complex 3 it is found at
5.55 ppm. The addition of propylene oxide had almost no
effect on the spectra of complex 1 (Figure 3b), but significantly
moved the signal of the pro-S NH proton to lower fields (from
5.55 to 5.75 ppm) for complex 3. Almost no changes in chemi-
cal shifts were observed for the other protons of this complex.
Evidently, for complex 3, propylene oxide was able to hydro-
gen bond to the pro-S NH proton more effectively than the
iodide counter anion. For complex 1, propylene oxide could
Figure 2. A schematic presentation of the array of hydrogen bonds in the
ion pair of D(S,S)-1–3 with a halide anion, as revealed by X-ray analysis.[5]
operating in a binary hydrogen bond donor (HBD)/tetralkyl
ammonium halide catalytic system,[24,34] as outlined in
Scheme 4. The first step of the catalytic cycle is the reversible
formation of the hydrogen-bond supported adduct of the ep-
oxide and the cobalt complex. Both the counter anion and the
epoxide are combined in the single supramolecular structure
with the epoxide activated towards attack by the counter
anion. The pre-orientation of the two reagents is expected to
compensate for the entropic penalty associated with the bimo-
lecular reaction of the epoxide ring opening. The halohydrin
anion formed by epoxide ring opening is stabilized by the
same array of hydrogen bonds. The next step involves the co-
ordination of a carbon dioxide molecule to the intermediate to
form a hydrogen-bond stabilized adduct. The next and final
steps of the cycle involve the intra-complex attack of the acti-
vated carbon dioxide molecule on the halohydrin anion, fol-
lowed by carbon–halogen bond cleavage, most likely promot-
ed by hydrogen bonding of the catalyst to the leaving group,
and the departure of the cyclic carbonate from the complex.
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Table 3. Coupling of CO2 and various epoxides promoted by complex
3.[a]
Entry
t [h]
Epoxide
Yield [%]
Selectivity[b] [%]
1
2
3[d]
4
5
6
7
3
6
8c
8c
8c
8d
8b
8a
8e
8 f
8g
78[c]
69
>99
>99
>99
>99
>99
>99
>99
>99
80
24
24
24
24
24
24
24
75[c]
89
76[e]
85
74
8
9[f]
16[c]
60
[a] Reaction conditions: neat epoxide, 2 mol% of complex 3, 508C, 5 MPa
CO2, the carbonates were purified chromatographically and the yields are
the isolated ones unless indicated otherwise. [b] The selectivity criteria re-
flect the absence of any other reaction products, including polymers and
1
diols. [c] The yield was determined by H NMR spectroscopy with the cat-
alyst tert-butyl groups serving as the internal standard. [d] Under 1 MPa
pressure of CO2 and at room temperature. [e] Reaction conditions: neat
epoxide, 1 mol% of complex 3, 508C, 5 MPa CO2. [f] In 0.1 mL toluene
using 10 mol% of the catalyst.[36]
Figure 3. 1H NMR spectra of complex 1 a) before and b) after addition of
propylene oxide.
bonate 9c (Table 3, entry 3). Cyclohexene oxide 8 f, as expect-
ed, was relatively unreactive, giving only 16% yield of cyclic
carbonate 9 f (with cis-configuration) under the experimental
conditions. No kinetic resolution of terminal epoxides, assessed
by analyzing the enantiomeric purity of the remaining initial
epoxides at the later stages of the conversion, was detected.
However, in the case of the internal epoxide chalcone oxide
8g, kinetic resolution did occur and the enantiomeric purity of
the remaining epoxide 8g was 55% (determined by chiral
HPLC) at 60% conversion.[36] It may be that the positioning of
the epoxide by hydrogen bonding to the chiral catalyst was
more rigid than for simple epoxides owing to the presence of
an additional carbonyl group, also capable of forming hydro-
gen bonds with the catalyst.
The stability complex 3 was investigated by catalyst recy-
cling experiments using propylene oxide 8c at 508C and
5 MPa carbon dioxide pressure with a reaction time of 24 h, as
this was required for some other epoxides. After the first 24 h
period, the yield of cyclic carbonate 9c was determined by
1H NMR spectroscopy, and then another portion of propylene
oxide was added to the reaction mixture. This procedure was
repeated four times and the catalyst survived for 4 days with
only a small loss of its catalytic activity in the final run (Table 4,
entries 1–4). After the fourth cycle the propylene carbonate
was distilled under reduced pressure and the remaining cata-
lyst again reused with no loss of its catalytic activity (compare
Figure 4. 1H NMR spectra of complex 3 a) before and b) after addition of
propylene oxide.
1
not compete with the strongly hydrogen-bonded chloride
counter anion and hence was not activated towards nucleo-
philic ring opening.
Table 3 entry 1 and Table 4 entry 5). The H NMR spectrum of
complex 3 recovered after this fifth cycle was identical to that
of the initial complex. It was also possible to recover the cata-
lyst chromatographically on SiO2 and reuse it. To further illus-
trate the stability of the catalyst, two reactions were carried
out at the higher temperature of 1008C (Table 4, entries 6 and
7). Even at this temperature the catalyst could be recovered
unchanged (as determined by 1H NMR analysis) and reused
without loss of activity.
The most efficient catalyst (complex 3) exhibited a broad
substrate scope. The results are summarized in Table 3. As ex-
pected, the best substrates were terminal epoxides, for which
the yields of cyclic carbonate were close to 100% under the
standard conditions (Table 3, entries 1–7). Propylene oxide 8c
underwent reaction even at ambient temperature and 1 MPa
carbon dioxide pressure, giving a 75% yield of propylene car-
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Complex 3: A solution of 1 (100 mg, 0.133 mmol) in CH2Cl2 (5 mL)
was added to a solution of KI (440 mg, 2.66 mmol) in water (5 mL).
The mixture was stirred for 4 h, then the organic layer was separat-
ed, dried (MgSO4), and evaporated under reduced pressure to
leave complex 3 (97 mg, 90%) as a brown powder. 1H NMR
(400 MHz, CDCl3): d=8.07 (s, 2H), 7.18 (d, J=2.0 Hz, 2H), 7.03 (d,
J=1.9 Hz, 2H), 5.55 (t, J=10.2 Hz, 2H), 4.12 (t, J=9.5 Hz, 2H), 2.98
(s, 4H), 2.53 (d, J=12.3 Hz, 2H), 2.06–1.49 (m, 10H), 1.25 (s, 22H),
0.94 ppm (s, 18H); 13C NMR (101 MHz, CDCl3): d=162.59, 161.74,
142.14, 136.32, 129.83, 128.60, 117.98, 70.35, 58.39, 35.06, 33.75,
31.40, 31.33, 30.80, 29.51, 25.44, 23.64 ppm; X-ray fluorescence
data: Co/I=1/1, no Cl was detected; [a]D =1940 (c=0.0063,
MeOH);
Table 4. Recyclability of catalyst 3.[a]
Entry
Cycle
Catalyst concentration [m]
T [oC]
Yield [%]
1
2
3
4
5
6
7
1
2
3
4
5[b]
1
2[c]
0.284
0.142
0.094
0.071
0.28
50
50
50
50
50
100
100
100
85
75
100
100
0.142
0.142
100
100
[a] Reaction conditions: neat 8c (50 mg, 0.06 mL, 0.86 mmol), 5 MPa CO2,
24 h, 2 mol% of complex 3 for reactions carried out at 508C and 1 mol%
of catalyst 3 for reactions carried out at 1008C. The second and subse-
quent cycles included the addition of a fresh 50 mg portion of 8c to the
reaction mixture after each 24 h reaction period. [b] Reaction conditions:
neat 8c (50 mg, 0.06 mL, 0.86 mmol), complex 3 recovered from the ex-
periment of entry 4 following distillation of 9c, 5 MPa CO2, 3 h. [c] Reac-
tion conditions: neat 8c (50 mg, 0.06 mL, 0.86 mmol), complex 3 recov-
ered from the experiment of entry 6 following distillation of 9c, 5 MPa
CO2, 24 h.
Complex 4: A solution of 1 (100 mg, 0.133 mmol) in CH2Cl2 (5 mL)
was added to a solution of benzoic acid (324 mg, 2.66 mmol) and
Na2CO3 (140 mg, 1.33 mmol). The mixture was stirred for 4 h, then
the organic layer was separated, dried (MgSO4), and evaporated
under reduced pressure to leave complex 4 (75 mg, 67%) as
1
a brown powder. H NMR (400 MHz, CDCl3): d=8.01 (s, 4H), 7.40 (t,
J=18.8 Hz, 5H), 7.15 (d, J=2.3 Hz, 2H), 6.98 (d, J=2.3 Hz, 2H),
4.10 (t, J=10.0 Hz, 2H), 2.87 (d, J=9.6 Hz, 2H), 2.80–2.62 (m, 2H),
1.78 (m, 9H), 1.46–1.38 (m, 3H), 1.23 (s, 22H), 0.93 ppm (s, 18H);
13C NMR (101 MHz, CDCl3): d=157.54, 157.22, 137.28, 130.99,
125.33, 124.55, 124.65, 123.75, 122.98, 113.50, 65.61, 53.54, 30.34,
28.98, 26.84, 26.61, 25.74, 24.72, 20.61, 19.01 ppm; X-ray fluores-
cence data: no Cl was detected; [a]D =1992 (c=0.00059, MeOH).
Conclusions
The stereochemically inert cationic complexes 1–7 were found
to be catalytically active in cyclic carbonate synthesis starting
from carbon dioxide and epoxides. The activity of the one
component bifunctional system originated from the hydrogen-
bond donating ability of the coordinated ligand and nucleo-
philic participation of the counter anion. The complexes are
robust, simple to prepare, and easy to recycle.
Complex 5: A solution of 1 (100 mg, 0.133 mmol) in CH2Cl2 (5 mL)
was added to a solution of p-toluene sulfonic acid (456 mg,
2.66 mmol) and Na2CO3 (140 mg, 1.33 mmol). The mixture was
stirred for 4 h, then the organic layer was separated, dried
(MgSO4), and evaporated under reduced pressure to leave complex
1
5 (92 mg, 78%) as a brown powder. H NMR (400 MHz, CDCl3): d=
8.04 (s, 2H), 7.79 (d, J=7.8 Hz, 2H), 7.20 (d, J=7.5 Hz, 4H), 7.01 (s,
2H), 5.53 (d, J=9.7 Hz, 2H), 3.91 (t, J=9.4 Hz, 2H), 2.93 (d, J=
9.7 Hz, 2H), 2.78 (d, J=10.6 Hz, 2H), 2.38 (s, 3H), 2.14–1.60 (m,
10H), 1.51 (d, J=12.9 Hz, 2H), 1.27 (d, J=10.0 Hz, 22H), 0.95 ppm
(s, 18H); X-ray fluorescence data: Co/S=1/1, no Cl was detected;
[a]D =1674 (c=0.00037, MeOH).
Experimental Section
Commercial reagents were used as received unless stated other-
wise. Column chromatography was performed using Silica Gel Kie-
1
selgel 60 (Merck). H NMR and 13C NMR spectra were recorded on
Bruker Avance 300 and Bruker Avance III-400 (operating at 300 and
400 MHz for protons, respectively) spectrometers. Optical rotations
were measured on a PerkinElmer 341 polarimeter in a 5 cm cell. El-
emental analysis was performed by the Laboratory of Elemental
Analysis INEOS RAS. X-ray fluorescence data was measured on
a VRA-30 spectrometer.
Complex 6: A solution of 1 (100 mg, 0.133 mmol) in MeOH (5 mL)
was added to a solution of terephthalic acid (11 mg, 0.065 mmol)
and silver(I) oxide (15 mg, 0.065 mmol) in MeOH (5 mL). The solu-
tion was stirred for 4 h, then precipitate was filtered and the filtrate
was evaporated under reduced pressure to leave complex 6
(62 mg, 58%) as a brown powder. 1H NMR (400 MHz, CDCl3): d=
8.04 (s, 4H), 7.18 (s, 4H), 7.01 (s, 2H), 4.08 (s, 2H), 2.83 (m, 2H),
2.73 (s, 1H), 2.04–1.56 (m, 14H), 1.47 (d, J=11.8 Hz, 2H), 1.25 (s,
20H), 0.96 ppm (s, 18H); 13C NMR (101 MHz, CDCl3): d=162.20,
162.04, 142.05, 135.84, 129.47, 128.50, 118.19, 70.35, 58.25, 35.08,
33.72, 31.62, 31.35, 30.49, 29.47, 25.34, 23.73 ppm; X-ray fluores-
cence data: no Cl was detected; [a]D =2034 (c=0.000625, MeOH);
Calculated for C92H136Co2N8O8.3H2O: C, 66.81; H. 8.65; N, 6.77%;
Found: C, 66.88; H, 8.88; N, 6.60.
Complex 1: Prepared as reported in the literature.[5] 1H NMR
(400 MHz, CDCl3): d=8.03 (s, 2H), 7.17 (d, J=2.4 Hz, 2H), 7.00 (d,
J=2.5 Hz, 2H), 6.49 (s, 2H), 3.94 (t, J=9.8 Hz, 2H), 2.91 (d, J=
11.1 Hz, 2H), 2.78 (s, 2H), 2.28 (d, J=9.8 Hz, 2H), 1.93 (d, J=
11.3 Hz, 4H), 1.84 (d, J=13.1 Hz, 2H), 1.77–1.64 (m, 4H), 1.63–1.46
(m, 2H), 1.24 (s, 20H), 0.93 ppm (s, 18H); [a]D =2304 (c=0.00067,
MeOH).
Complex 2: A solution of 1 (100 mg, 0.133 mmol) in CH2Cl2 (5 mL)
was added to a solution of KBr (316 mg, 2.66 mmol) in water
(5 mL). The mixture was stirred for 4 h, then the organic layer was
separated, dried (MgSO4), and evaporated under reduced pressure
to leave complex 2 (92 mg, 87%) as a brown powder. 1H NMR
(400 MHz, CDCl3): d=8.02 (s, 2H), 7.15 (s, 2H), 6.98 (s, 2H), 6.13 (s,
2H), 3.99 (s, 2H), 2.90 (d, J=10.6 Hz, 4H), 2.41 (d, J=12.0 Hz, 2H),
2.01–1.51 (m, 12H), 1.21 (s, 22H), 0.91 ppm (s, 18H); 13C NMR
(101 MHz, CDCl3): d=162.37, 161.89, 142.20, 136.32, 129.84, 128.55,
117.94, 70.29, 58.27, 35.07, 33.74, 31.45, 31.31, 30.67, 29.48, 25.36,
23.65 ppm; X-ray fluorescence data: Co/Br=1/1, no Cl was detect-
ed; [a]D =1630 (c=0.00063, MeOH).
Complex 7: A solution of 1 (100 mg, 0.133 mmol) in MeOH (5 mL)
was added to a solution of 2,5-naphthalenedisulfonic acid (19 mg,
0.065 mmol) and silver(I) oxide (15 mg, 0.065 mmol) in methanol
(5 mL). The solution was stirred for 4 h, then precipitate was fil-
tered and the filtrate was evaporated under reduced pressure to
leave complex 7 (73 mg, 64%) as a brown powder. 1H NMR
(400 MHz, CDCl3): d=8.43 (s, 1H), 8.04 (s, 2H), 8.00 (d, J=8.5 Hz,
1H), 7.91 (d, J=8.3 Hz, 1H), 7.18 (s, 2H), 7.01 (s, 2H), 5.37 (s, 2H),
3.87 (s, 2H), 2.95–2.69 (m, 4H), 2.04–1.33 (m, 16H), 1.25 (s, 18H),
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0.94 ppm (s, 18H); 13C NMR (101 MHz, CDCl3): d=162.40, 161.90,
142.18, 136.27, 132.99, 129.79, 129.04, 128.53, 125.34, 124.12,
118.00, 118.00, 70.34, 58.15, 35.06, 33.74, 31.49, 31.32, 30.61, 29.48,
25.27, 23.58 ppm; X-ray fluorescence data: Co/S=1/1, no Cl was
detected; [a]D =1567 (c=0.00059, MeOH); Calculated for
C94H138Co2N8O10S2.5H2O: C, 62.30; H. 8.23; N, 6.18%; Found: C,
62.34; H, 7.69; N, 6.21.
Acknowledgements
The authors gratefully thank the financial support from a joint
RFBR and Royal Society research grant No 13-03-92601-KO-a and
RFBR research grant No 14-03-31262 young_a.
Keywords: carbon dioxide · catalyst · cyclic carbonate ·
epoxide · kinetic resolution
Synthesis of cyclic carbonates: All reactions were carried out in
autoclaves using the conditions specified in Tables 1–4. After com-
pletion of the experiment, the reaction mixture was analyzed by
1H NMR spectroscopy and passed through a pad of silica to sepa-
rate the catalyst. Column chromatography (SiO2, first hexane, then
EtOAc/hexane, 1:4) was used to separate the starting material and
product. For substrate screening, the yield was determined by
1H NMR spectroscopy with the catalyst tert-butyl groups (d=
0.94 ppm) serving as the internal standard. For the catalyst recycla-
bility study, each new portion of propylene oxide was added to
the reaction mixture after finishing the reaction (without any addi-
tional purification). For the catalyst recyclability study with styrene
oxide column chromatography (SiO2, first hexane, then EtOAc/
hexane 1:4, then methanol) was used to separate the catalyst from
cyclic carbonate.
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ChemSusChem 2016, 9, 216 – 222
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