Synthesis of Cyclic Carbonates Catalysed by Chromium and Aluminium Salphen Complexes

Article abstract of DOI:10.1002/chem.201504305

Chromium and aluminium salphen complexes have been found to display remarkable catalytic activity in the synthesis of cyclic carbonates from a range of epoxides and carbon dioxide. The Al(salphen) complex is more reactive towards terminal epoxides at ambient temperature and pressure, whereas the Cr(salphen) complex exhibits higher catalytic activity towards more challenging internal epoxides at elevated temperature and pressure.

Full text of DOI:10.1002/chem.201504305

DOI: 10.1002/chem.201504305
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Homogeneous Catalysis |Hot Paper|
Synthesis of Cyclic Carbonates Catalysed by Chromium and
Aluminium Salphen Complexes
JosØ A. Castro-Osma, Michael North,* and Xiao Wu[a]
Abstract: Chromium and aluminium salphen complexes
have been found to display remarkable catalytic activity in
the synthesis of cyclic carbonates from a range of epoxides
and carbon dioxide. The Al(salphen) complex is more reac-
tive towards terminal epoxides at ambient temperature and
pressure, whereas the Cr(salphen) complex exhibits higher
catalytic activity towards more challenging internal epoxides
at elevated temperature and pressure.
nium.[7] Amongst these, bimetallic aluminium salen complex 4,
reported by North and co-workers (Figure 1),[8] is amongst the
most active catalysts for cyclic carbonate synthesis at room
temperature and pressure in the presence of tetrabutylammo-
nium bromide as a cocatalyst. Moreover, aluminium complex 4
Introduction
Direct chemical fixation of carbon dioxide into organic com-
pounds has received much attention recently, as carbon diox-
ide is a cheap, abundant, non-toxic, and versatile carbon
source[1,2] as well as being one of the most significant green-
house gases.[3] In particular, the transformation from epoxides
1 and carbon dioxide into either cyclic carbonates 2[4] or poly-
carbonates 3[5] is of commercial importance (Scheme 1). There
are a number of applications associated with cyclic carbonates,
such as their use as electrolytes in lithium ion batteries, polar
aprotic solvents, and intermediates in organic synthesis.[6]
Scheme 1. Synthesis of cyclic and polycarbonates.
The biggest challenge in carbon dioxide utilisation is its rela-
tively high kinetic and thermodynamic stability. The thermody-
namic stability can only be overcome by adding energy to the
reaction, either directly or in the form of high energy reactants
such as epoxides or hydrogen whilst the kinetic stability can
be overcome by the use of catalysts. A number of catalytic sys-
tems for the synthesis of cyclic carbonates from epoxides and
carbon dioxide have been developed.[4] In view of the large
scale of production of cyclic carbonates, especially due to the
burgeoning demand for lithium ion batteries, it is highly desir-
able that catalysts for cyclic carbonate synthesis are derived
from inexpensive, Earth-crust abundant metals, such as alumi-
Figure 1. Aluminium-based catalysts 4 and 5.
was shown to have high reusability,[9] as well as being compati-
ble with carbon dioxide generated by the combustion of
methane in a pure oxygen atmosphere.[10] A one-component
aluminium complex, which does not require a separate cocata-
lyst, has been prepared and immobilised analogues have been
developed.[11] The catalytic activity of the immobilised catalysts
were tested in the synthesis of cyclic carbonates from ethylene
and propylene oxide in a gas-phase reactor,[12] and were
shown to be compatible with both simulated[12b] and real flue
gas.[13] More recently, aluminium complex 4 was shown to be
an effective catalyst for the synthesis of cyclic carbonates from
a range of terminal epoxides in the absence of cocatalysts at
elevated temperature and pressure.[14]
[a] Dr. J. A. Castro-Osma, Prof. M. North, Dr. X. Wu
Green Chemistry Centre of Excellence, Department of Chemistry
University of York, York, YO10 5DD (UK)
E-mail: Michael.north@york.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504305.
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Kleij and co-workers have developed a number of amino(tri-
phenolate) based aluminium complexes.[15] Although using
slightly elevated temperature and pressure, aluminium(III) ami-
no(triphenolate) 5 exhibits high catalytic activity with a wide
range of substrates bearing various functional groups. Further-
more, aluminium complex 5 was also shown to be effective for
the conversion of oxetanes into the corresponding six-mem-
bered ring cyclic carbonates.[16]
Although chromium is considered to be a more endangered
element than aluminium,[7] several remarkable catalysts based
around this metal have been reported, particularly for the
preparation of polycarbonates.[5b,d,17] Fewer examples have
been reported for the synthesis of cyclic carbonates. Recently,
Deng and co-workers reported the synthesis of a chromium-
based conjugated microporous polymer and its use as a hetero-
geneous catalyst in the formation of cyclic carbonates from ep-
oxides and carbon dioxide.[18] Therefore, there is still potential
for chromium-based catalysts to be developed as catalysts for
cyclic carbonate synthesis.
In continuation of our efforts to further develop highly
active catalytic systems for the reaction between epoxides and
carbon dioxide, herein, we report the synthesis of chromium
and aluminium based catalysts 6 and 7, both of which contain
a salphen scaffold, as efficient catalysts for the formation of
cyclic carbonates from epoxides and carbon dioxide.
Scheme 2. Synthesis of chromium complex 6 and aluminium complex 7.
Results and Discussion
forded CrIII(salphen) chloride 6 as a brown solid. For the syn-
thesis of aluminium complex 7, salphen ligand 10 was added
to a solution of aluminium triethoxide in refluxing toluene.[7]
After a 16 h reflux, an aqueous work-up afforded Al(salphen)
complex 7 as a bright orange solid.
A new one-component aluminium-based catalyst system 8 has
recently been developed for the reaction between epoxides
and carbon dioxide (Figure 2).[19] However, aluminium complex
The conversion of styrene oxide 1a into styrene carbonate
2a (Scheme 3) was chosen as the reaction to test the catalytic
Figure 2. Aluminium-based catalyst 8.
8, which has the trialkylammonium groups directly attached to
the aromatic ring of the salen ligand, requires a multistep syn-
thesis. Catalysts that exhibit high efficiency as well as being
readily accessible are more attractive. Therefore, the use of
commercially available 4-(diethylamino)salicylaldehyde 9 as
one of the components for the ligand synthesis was investigat-
ed. Given the high catalytic activity previously reported by Kleij
et al. for salphen complexes,[20] it was decided to combine al-
dehyde 9 with 1,2-diaminobenzene to prepare salphen ligand
10 and hence complexes 6 and 7.
The syntheses of chromium complex 6 and aluminium com-
plex 7 are presented in Scheme 2. Condensation of 4-(diethyla-
mino)salicylaldehyde 9 with 1,2-diaminobenzene provided sal-
phen ligand 10 as a yellow solid. Treatment of ligand 10 with
chromium(II) chloride in THF followed by oxidation with air af-
Scheme 3. Synthesis of cyclic carbonates 2a–k using complex 6.
activity of complexes 6 and 7 and the reactions were moni-
tored by 1H NMR spectroscopy. The chromium salphen com-
plex 6 was initially investigated as a catalyst with n-tetrabuty-
lammonium bromide (TBAB) as a cocatalyst, at 258C and 1 bar
carbon dioxide pressure under solvent free conditions for 24 h.
At a catalyst and cocatalyst loading of 2.5 mol%, 63% conver-
sion of styrene oxide to styrene carbonate was achieved after
3 h and this increased to 87% after 6 h and 100% after 24 h
(Table 1, entry 1). Control experiments involving the sole use of
either complex 6 or TBAB gave only trace amounts of cyclic
carbonate product (Table 1, entries 2 and 3), highlighting the
strong synergistic effect observed when using complex 6 and
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halides were slightly more active than the corresponding n-tet-
rabutylammonium salts (Table 2, entries 7 and 8) and bis(tri-
phenylphosphine)iminium bromide (PPNBr) was the most
active cocatalyst, giving 41% conversion to styrene carbonate
3a after 3 h (Table 2, entry 8). However, since TBAB is available
at much lower cost than PPNBr, it was selected as the optimal
cocatalyst.
Table 1. Optimisation of the synthesis of cyclic carbonate 2a using com-
plex 6 and TBAB.[a]
Entry
Catalyst [mol%]
TBAB [mol%]
Conversion [%][b]
1
2
3
4
5
6
7
2.5
2.5
0
0.5
1.0
1.5
2.0
2.5
0
63, 87, 100
<1
2.5
0.5
1.0
1.5
2.0
<1
9, 19, 57
21, 39, 86
33, 55, 96
43, 66, 100
Having determined the optimal reaction conditions to be
1.5 mol% of chromium complex 6 and TBAB at 258C and 1 bar
carbon dioxide pressure, a series of terminal epoxides 1a–k
were then studied as substrates for the formation of the corre-
sponding cyclic carbonates 2a–k. The results are summarised
in Table 3, and in general, good to excellent conversions to the
[a] Reactions were carried out at 258C and 1 bar carbon dioxide pressure
using complex 6 and TBAB in the absence of a solvent. [b] Determined
by 1H NMR spectroscopy after 3, 6, and 24 h.
Table 3. Synthesis of cyclic carbonates 2a–k using complex
6 and
TBAB together. Subsequently, a range of catalyst concentra-
tions were investigated to further reduce the catalyst loading,
whilst keeping the catalyst to cocatalyst molar ratio at 1:1. At
a catalyst loading of 0.5 mol%, low conversions were observed
(Table 1, entry 4), but these increased linearly as the catalyst
concentration was increased to 2 mol% (Table 1, entries 5–7). It
was therefore decided that 1.5 mol% was the optimal loading
for both the catalyst and cocatalyst, affording conversions to
styrene carbonate of 33%, 55% and 96% after 3, 6 and 24 h,
respectively (Table 1, entry 6). All the above reactions gave
cyclic carbonate 2a as the only product and there was no evi-
dence for formation of polymeric or hydrolysis products.
TBAB.[a]
Entry
Substrate
Conversion [%][b]
33, 55, 100
Yield [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
84
59
78
75
78
74
75
61
87
79
73
[d]
–
51, 79, 100
57, 75, 100
4,20, 99
5, 19, 89
7, 29, 80
13,34, 81
9
10
11
46,85, 100[e]
27, 45, 93
25, 58, 100[e]
1j
1k
The influence of the cocatalyst on the reaction between
carbon dioxide and styrene oxide 1a was also investigated
and the results are summarized in Table 2. As expected,[8] it
[a] Reactions were carried out using 1.5 mol% of complex
6 and
1.5 mol% of TBAB at 258C and 1 bar carbon dioxide pressure. [b] Deter-
mined by 1H NMR spectroscopy after 3, 6, and 24 h. [c] Yield of isolated
product after purification by column chromatography. [d] Not determined
owing to the volatility of epoxide 1b. [e] Reactions were carried out at
508C.
Table 2. Influence of cocatalyst on the catalytic activity of complex 6.[a]
[c]
Entry
Cocatalyst
Conversion [%][b]
TOF [hÀ1
]
1
2
3
4
5
6
7
8
TBAF
TBACl
TBAB
TBAI
DMAP
NMI
3, 7, 31
8, 14, 54
33, 55, 100
33, 55, 100
3, 5, 9
1, 2, 6
15, 24, 66
41, 63, 100
0.67, 0.78, 0.86
1.78, 1.56, 1.50
7.33, 6.11, 2.78
7.33, 6.11, 2.78
0.67, 0.56, 0.25
0.22, 0.22, 0.17
3.33, 2.67, 1.83
9.11, 7.00, 2.78
cyclic carbonates were achieved and the products were isolat-
ed in high yields. Epoxides 1i,k are solids at 258C, so, for these
substrates, reactions were carried out at 508C to give a homo-
geneous reaction mixture. Cyclic carbonates 2a–k were again,
the only product formed in these reactions.
PPNCl
PPNBr
A series of internal epoxides 11 a–g were then examined as
substrates to further expand the substrate scope. Internal ep-
oxides are considered to be more challenging substrates for
cyclic carbonate synthesis although an effective method has
recently been reported by Kleij and co-workers.[16] In view of
the known synthetic challenge, reactions with internal epox-
ides were carried out using 1.5 mol% of complex 6 and TBAB,
at 508C and 10 bar carbon dioxide pressure under solvent free
conditions for 24 h (Scheme 4). Under these conditions, cyclic
carbonates 12a–g were isolated in good to excellent yields
except for that derived from epoxide 11 d, illustrating the ver-
satility of catalyst 6 (Table 4). Indeed, this is the first example
of the successful conversion of internal epoxides into the cor-
responding cyclic carbonates using a chromium-based catalyst.
Control experiments using epoxide 11 f as substrate (Table 4,
entries 1–3) showed that under these conditions, no reaction
occurred in the absence of both complex 6 and TBAB and neg-
[a] Reactions were carried out at 258C and 1 bar carbon dioxide pressure
using 1.5 mol% of complex 6 and 1.5 mol% of cocatalyst. [b] Determined
by 1H NMR spectroscopy after 3, 6, and 24 h. [c] TOF=moles of product/
(moles of catalyst ” time).
was found that the nucleophilicity and leaving-group ability of
the cocatalyst determined its activity. Thus, n-tetrabutylammo-
nium fluoride and chloride were poor cocatalysts (Table 2, en-
tries 1 and 2), whilst TBAB and n-tetrabutylammonium iodide
(TBAI) had much higher and similar activity, giving identical
conversions at 258C and 1 bar carbon dioxide pressure
(Table 2, entries 3 and 4). 4-Dimethylaminopyridine (DMAP)
and N-methylimidazole (NMI) were not good cocatalysts as
only 9% and 6% conversions were observed even after 24 h
(Table 2, entries 5 and 6). The bis(triphenylphosphine)iminium
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ards. A MALDI-TOF mass spectrum of the polymer showed a re-
peating unit of 142 Daltons as expected, and showed the pres-
ence of oligomers composed of 3 to 18 monomer units with
alcohols at both chain ends. This indicates that a chain-transfer
process involving moisture within the reactor occurred during
the polymerisation (see the Supporting Information for GPC
and MALDI-TOF traces). Catalysts capable of formation of both
cyclic carbonates and polymeric products have been reported
previously.[5b,d,17,18] This further demonstrates the ability of
chromium-based catalysts to form either cyclic- or poly-
carbonates depending on the monomer structure.
Scheme 4. Synthesis of cyclic carbonates 12a–g.
Stilbene oxide 11 e was also used as a substrate and gave
trans-1,2-diphenylethylene carbonate 12e in 63% isolated
yield (Table 4, entry 8). In this case, the reaction temperature
was raised to 908C as the epoxide is a solid at 508C. There are
very few examples of the synthesis of 12e via catalytic addi-
tion of carbon dioxide to stilbene oxide,[15a,b,22] making com-
plex 6 a rare example of a catalyst able to transform bulky ep-
oxide 11 e into cyclic carbonate 12e in high yield. Complex 6
was also active with other sterically hindered epoxides, 11 f
and 11 g, and provided cyclic carbonates 12 f and 12g in good
yields (Table 4, entries 9 and 10).
Table 4. Synthesis of cyclic carbonates 12a–g using complex 6 and
TBAB.[a]
Entry
Substrate
6
TBAB
[mol%]
Conversion
[%][b]
Yield
[%][c]
[mol%]
1
2
3
4
5
6
7
8
9
10
11 f
11 f
11 f
11 a
11 b
11 c
11 d
11 e
11 f
11 g
0
0
0
1.5
0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0
3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
25
–
–
[d]
68[e]
68[f]
85
[d]
93
–
65[h]
72
[d]
80[g]
63
To test the catalytic activity of aluminium salphen complex 7
in the synthesis of cyclic carbonates from epoxides and carbon
dioxide, styrene oxide was again used as a benchmark sub-
strate. The reaction was carried out using 1.5 mol% of alumini-
um complex 7 and TBAB at 258C and 1 bar carbon dioxide
pressure under solvent free conditions. A 50% conversion to
styrene carbonate 2a was achieved after 3 h (Table 5, entry 1),
68
61
[d]
–
[a] Reactions were carried out at 508C and 10 bar carbon dioxide pres-
sure. [b] Determined by 1H NMR spectroscopy. [c] Yield of isolated prod-
uct after purification by column chromatography. [d] Not determined.
[e] Mixture of 86% cis- and 14% trans-cyclic carbonate product. [f] Yield
of trans-cyclic carbonate product. [g] Yield of polymer. [h] Reaction carried
out at 908C.
Table 5. Synthesis of cyclic carbonates 2a–k using complex
TBAB.[a]
7 and
ligible conversion occurred in the presence of TBAB alone.
Complex 6 alone did have some catalytic activity, but this was
much lower than that observed when complex 6 and TBAB
were both present (compare Table 4 entries 3 and 9).
Entry
Substrate
Conversion [%][b]
50, 75, 100
Yield [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
83
72
79
72
90
88
77
83
77
88
67
[d]
–
In the case of 1,2-dimethyloxirane, both cis- and trans-iso-
mers 11 a,b were used as substrates for the formation cyclic
carbonates 12a,b. It was observed that when the pure trans-
epoxide 11 b was used, the cyclic carbonate product was ob-
tained with retention of configuration and was exclusively
trans-isomer 12b (Table 4, entry 5). The cis epoxide 11 a, how-
ever, gave an 86:14 mixture of cis- and trans-isomers 12a,b of
the cyclic carbonate (Table 4, entry 4). It has been reported
that the synthesis of cyclic carbonates from cis-1,2-dimethylox-
irane 11 a is more challenging than from the trans-isomer
11 b.[21] We were delighted therefore to find that using com-
plex 6 under moderate reaction conditions, both cis- and
trans-epoxides 11 a and 11 b gave cyclic carbonates with the
same isolated yields.
86, 97, 100
97, 100
45, 85, 100
34, 73, 100
60, 75, 100
60, 76, 100
77, 100[e]
9
10
11
1j
1k
41, 60, 100
57, 78, 100[e]
[a] Reactions were carried out using 1.5 mol% of aluminium complex 7
and 1.5 mol% of TBAB at 258C and 1 bar carbon dioxide pressure. [b] De-
termined by H NMR spectroscopy after 3, 6 and 24 h. [c] Yield of isolated
product after purification by column chromatography. [d] Not deter-
mined. [e] Reactions were carried out at 508C.
1
Cyclopentene oxide 11 c was an excellent substrate, giving
cyclic carbonate 12c in 85% yield (Table 4, entry 6). Cyclohex-
ene oxide 11 d was also a substrate for catalyst 6 (Table 4,
entry 7), but gave polycarbonate 3 rather than cyclic carbonate
12d. This was confirmed by comparing H NMR data with pre-
viously reported data.[19] GPC showed that the polycarbonate
a result which was significantly higher than that was obtained
with chromium complex 6 (33%; Table 1, entry 6). Furthermore,
even when compared with dinuclear aluminium salen complex
4 (38% conversion using 1.0 mol% and 62% conversion using
2.5 mol% catalyst after 3 h[8a]), which maybe be regarded as
the state of the art catalyst system, aluminium salphen com-
plex 7 exhibits higher activity. Therefore, a series of terminal
1
had a Mn =1320 and Mw =2639 relative to polystyrene stand-
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epoxides 1a–k were examined as substrates under the same
reaction conditions and gave high conversions to the corre-
sponding cyclic carbonates 2a–k which were also isolated in
high yields (Table 5). All the terminal epoxides tested gave con-
siderably higher conversions in the first few hours of reaction
than were obtained using chromium salphen complex 6 (com-
pare Tables 3 and 5).
cyclic carbonate product 12g was isolated in only 37% yield
(Table 6, entry 9).
For a direct comparison of the rate of conversion of styrene
oxide 1a into styrene carbonate 2a reactions involving cata-
lysts 4, 6 and 7 were carried out with TBAB as cocatalyst at
258C and 1 bar carbon dioxide pressure. The reactions were
carried out for 7 h and samples were removed at intervals for
1H NMR analysis to determine the conversion to cyclic carbon-
ate 2a with time. Figure 3 shows that under the same reaction
conditions, the catalytic activity of aluminium salphen complex
7 is higher than either aluminium salen complex 4 or chromi-
um salphen complex 6.
To investigate the catalytic activity of aluminium complex 7
in the synthesis of disubstituted cyclic carbonates, cyclopen-
tene oxide 11 c was first used as a substrate. Using 1.5 mol%
of complex 7 together with TBAB at 508C and 10 bar carbon
dioxide pressure, a conversion of 95% was observed by
1H NMR and cyclic carbonate 12c was isolated in 73% yield
(Table 6, entry 5). When the same reaction conditions were ap-
Table 6. Synthesis of cyclic carbonates 12a–g using complex 7 and
TBAB.[a]
Entry
Substrate
7
TBAB
[mol%]
Conversion
[%][b]
Yield
[%][c]
[mol%]
1
2
3
4
5
6
7
8
9
11 f
11 f
11 a
11 b
11 c
11 d
11 e
11 f
11 g
0
5
5
5
1.5
1.5
5
5
5
5
0
5
5
1.5
1.5
5
5
5
5
15
49
54
95
76
60[f]
87
37[d]
39[e]
73
60
50
Figure 3. Plot of conversion versus time for styrene carbonate synthesis
~
^
using catalysts 4, 6, and 7. Triangles, Reaction profiles: catalyst 4, cata-
&
lyst 6, catalyst 7.
83
37
The differences in catalytic activity of chromium salphen
complex 6 and aluminium salphen catalyst 7 could be ex-
plained in term of Lewis acidity. Thus, aluminium salphen com-
plex 7 is more Lewis acidic owing to the electron-withdrawing
nature of the bridging oxygen atom, giving it a much faster
rate of conversion for terminal epoxide than complex 6. How-
ever, the bulky nature of bimetallic complex 7 makes it difficult
for it to react with sterically hindered internal epoxides and
monometallic chromium salphen complex 6 is better for these
substrates.
[g]
–
[a] Reactions were carried out at 508C and 10 bar carbon dioxide pres-
sure. [b] Determined by 1H NMR spectroscopy. [c] Yield of isolated prod-
uct after purification by column chromatography. [d] Mixture of 94% cis-
and 6% trans-cyclic carbonate product. [e] Yield of trans-cyclic carbonate
product. [f] Reaction carried out at 908C. [g] Not determined.
plied to cyclohexene oxide 11 d, cis-cyclohexene carbonate
12d was isolated in 60% yield (Table 6, entry 6). When
1.5 mol% of complex 7 and TBAB were used with other disub-
stituted cyclic carbonates 11 a,b,e–g, very low conversions
were observed. By increasing the catalyst concentration to
5 mol%, epoxides 11 a,b,e–g were successfully converted into
cyclic carbonates 12c–g in 37–83% yield. Control experiments
with epoxide 11 f again showed that under these conditions
complex 7 or TBAB alone gave only 5–15% conversion
(Table 6, entries 1 and 2). The same trend was observed for 1,2-
dimethyloxirane as seen when chromium-based complex 6
was employed. The cis- and trans-cyclic carbonates 12a and
12b were isolated in a similar yield (Table 6, entries 3 and 4),
albeit both were much lower than that obtained using com-
plex 6 (cf. entries 4 and 5 of Table 4). Compared to complex 6,
complex 7 catalysed the reaction with much higher retention
of stereochemistry using cis-1,2-dimethyloxirane 11 a, and
a 94:6 ratio of cis- and trans-isomers of cyclic carbonates
12a,b was observed. While the sterically hindered epoxides
11 e and 11 f were successfully converted into the cyclic car-
bonate products 12e and 12 f (Table 6, entries 7 and 8), the re-
action with 1,1-dimethyloxirane 11 g was very sluggish and its
Conclusion
Novel chromium 6 and aluminium 7 salphen complexes de-
rived from 4-(diethylamino)salicylaldehyde 9 and 1,2-diamino-
benzene have been synthesised. A catalytic system using com-
plex 6 or 7 in the presence of TBAB as a cocatalyst has been
developed for the synthesis of cyclic carbonates from epoxides
and carbon dioxide. At 258C and 1 bar carbon dioxide pres-
sure, aluminium salphen complex 7 exhibits higher catalytic
activity for the formation of cyclic carbonates from terminal
epoxides than chromium salphen complex 6. However, chromi-
um salphen complex 6 is more efficient for sterically congested
disubstituted epoxides at elevated temperature and pressure.
Both complex 6 and 7 display higher reactivity towards cyclo-
pentene oxide than cyclohexene oxide.
Although there are a number of catalyst systems that have
been developed for the synthesis of cyclic carbonates from ep-
oxides and carbon dioxide, only a handful of these are active
under mild reaction conditions. Within these, there are even
fewer that can transform internal epoxides to their correspond-
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were added to the conical flask which was then fitted with
a rubber stopper pierced by a deflated balloon. The reaction mix-
ture was stirred at 258C for 24 h. The conversion of epoxide to
cyclic carbonate was determined by analysis of a sample by
1H NMR spectroscopy.
ing cyclic carbonates. We have shown that both chromium
and aluminium salphen complex 6 and 7 are efficient catalysts
for the synthesis of cyclic carbonates from sterically challeng-
ing epoxides.
Experimental Section
General procedure for catalyst screening at 508C and 10 bar
pressure
Synthesis of ligand 10: 4-(Diethylamino)salicylaldehyde (5.0 g,
0.026 mol, 2.0 equiv) was added to a stirred solution of 1,2-diami-
nobenzene (1.4 g, 0.013 mol, 1.0 equiv) in ethanol (250 mL) and
the resulting solution was stirred at room temperature for 16 h. A
bright yellow precipitate was formed and the solution was filtered.
The solid was then washed with cold ethanol and dried to give
ligand 10 as a bright yellow solid (4.8 g, 80%). m.p. 147–1488C;
1H NMR (400 MHz, CDCl3): d=8.42 (s, 2H, 2”NC=H), 7.22–7.12 (m,
4H, Ph), 6.24–6.20 (m, 4H, Ph), 3.38 (q, J=8.0 Hz, 8H, 4”CH2),
1.90 ppm (t, J=8.0 Hz, 12H, 4”CH3); 13C NMR (100 MHz, CDCl3):
d=164.4 (ArC), 160.8 (ArCH), 151.7 (ArC), 142.5 (ArC), 133.7 (ArCH),
126.1 (ArCH), 119.2 (ArCH), 109.5 (ArC), 103.5 (ArCH), 98.1 (ArCH),
44.5 (CH2), 12.7 ppm (CH3); IR (neat): n˜ =3356, 2967, 1611, 1347,
1188, 1013, 961, 784, 751, 698 cmÀ1; HRMS (ESI+): calcd. for
C28H34N4O2 [M+H]+ 459.2755, found 459.2762.
An epoxide (1.66 mmol), catalyst 6 (13 mg, 0.025 mmol) and
Bu4NBr (8 mg, 0.025 mmol) or 7 (82 mg, 0.083 mmol) and Bu4NBr
(27 mg, 0.083 mmol) were placed in a stainless steel autoclave
fitted with a magnetic stirrer bar, and the reactor was heated to
508C before charged with 10 bar pressure of carbon dioxide. The
reaction mixture was stirred for 24 h. The conversion of epoxide to
cyclic carbonate was determined by analysis of a sample by
1H NMR spectroscopy.
Cyclic carbonates 2a–k and 12a–g are all known compounds and
the spectroscopic data of samples prepared using catalysts 6 and
7 were consistent with those previously reported.[8,11,12,15,23,24]
Styrene carbonate (2a): Purification by flash column chromatogra-
phy with n-hexane/EtOAc (6:4) gave a white solid. m.p. 49–518C
(lit.[8,11,12,23] 50–518C); 1H NMR (400 MHz, CDCl3): d=7.41–7.31 (m,
5H, Ph), 5.64 (t, J=8.0 Hz, 1H, OCH), 4.76 (t, J=8.0 Hz, 1H, CH2),
4.28 ppm (t J=8.0 Hz, 1H, CH2);13C NMR (100 MHz, CDCl3): d=
154.7 (C=O), 135.6 (ArC), 129.4 (ArCH), 128.9 (ArCH), 125.7 (ArCH),
77.8 (OCH), 70.9 ppm (OCH2); IR (neat): n˜ =3060, 3029, 2961, 2903,
Synthesis of chromium salphen complex 6: Salphen ligand 10
(0.5 g, 1.09 mmol, 1.0 equiv) was dissolved in dry THF (50 mL)
under argon. Then, anhydrous CrCl2 (0.138 g, 1.09 mmol, 1.0 equiv)
was added and the mixture was stirred for 24 h under argon. Air
was bubbled through the flask for another 24 h. Then, Et2O was
added and the organic layer was washed with a saturated solution
of ammonium chloride (2”25 mL) and brine (2”50 mL). The or-
ganic layer was dried (Na2SO4) and the solvent was evaporated
under reduced pressure to give chromium salphen complex 6 as
a red-brown solid (0.53 g, 91%). m.p. 230–2338C; IR (neat): n˜ =
1791, 1599 cmÀ1
187.0366, found 187.0361.
;
HRMS (ESI+): calcd. for C9H8O3 [M+Na]+
Propylene carbonate (2b): Purification by flash column chroma-
tography with n-hexane/EtOAc (8:2) gave a colourless liquid.
1H NMR (400 MHz, CDCl3): d=4.89–4.80 (m, 1H, OCH), 4.54 (t, J=
8.0 Hz, 1H, CH2), 4.01 (t, J=8.0 Hz, 1H, CH2), 1.47 ppm (d, J=
4.0 Hz, 1H, CH3); 13C NMR (100 MHz, CDCl3): d=155.0 (C=O), 73.5
(OCH), 70.6 (OCH2), 19.3 ppm (CH3); IR (neat ): n˜ =2961, 2902,
1781 cmÀ1; HRMS (ESI+): calcd for C4H6O3 [M+Na]+ 125.0209
found 125.0207.
2972, 1610, 1562, 1347, 1201, 1141, 1014, 825, 788, 754, 650 cmÀ1
;
HRMS (LIFDI): calcd. for C28H32CrN4O2Cl [M]+: 543.16, found 543.14.
Synthesis of aluminium salphen 7: Aluminium triethoxide (2.12 g,
13.08 mmol, 2.0 equiv) in toluene (200 mL) was stirred and heated
to reflux for 1 h. Then, ligand 10 (3.0 g, 6.54 mmol, 1.0 equiv) was
added to the resulting suspension and the reaction refluxed for
16 h. After cooling to room temperature, the solvent was reduced
under reduced pressure and the residue was dissolved in CH2Cl2
(300 mL) and water (300 mL) was added. The resulting mixture was
stirred for 15 min. The layers were separated and the aqueous
layer was extracted with CH2Cl2 (3”100 mL). The combined organic
extracts were washed with brine, dried with MgSO4 and evaporat-
ed under reduced pressure to give the crude product. Purification
by washing the crude product with Et2O gave aluminium salphen
7 as an orange powder (3.84 g, 60%). m.p. 219–2218C; 1H NMR
(400 MHz, CDCl3): d=8.39 (br s, 2H, 2”N=CH), 7.58–7.31 (m, 3H,
Ar), 7.18–6.93 (m, 9H, Ar), 6.93–6.21 (m, 8H, Ar), 3.41 (q, J=4.0 Hz,
16H, 8”NCH2), 1.22 ppm (t J=8.0 Hz, 24H, 8”CH3); 13C NMR
(100 MHz, CDCl3): d=168.1 (ArC), 156.2 (ArCH), 154.4 (ArC), 137.7
(ArC), 136.1 (ArCH), 125.1 (ArCH), 114.4 (ArCH), 110.5 (ArC), 104.0
(ArCH), 101.4 (ArC), 44.5 (CH2), 13.0 ppm (CH3); IR (neat ): n˜ =2968,
1,2-Butylene carbonate (2c): Purification by flash column chroma-
tography with n-hexane/EtOAc (8:2) gave a colourless liquid.
1H NMR (400 MHz, CDCl3): d=4.68–4.53 (m, 1H, OCH), 4.50 (t, J=
8.0 Hz, 1H, CH2), 4.07 (t, J=8.0 Hz, 1H, CH2), 1.87–1.70 (m, 2H,
CH2), 1.02 ppm (t, J=8.0 Hz, CH3); 13C NMR (100 MHz, CDCl3): d=
155.1 (C=O), 78.0 (OCH), 69.0 (OCH2), 26.8 (CH2), 8.4 ppm (CH3); IR
(neat ): n˜ =2938, 2917, 1801 cmÀ1; HRMS (ESI+): calcd for C5H8O3
[M+Na]+ 139.0366, found 139.0364.
1,2-Hexylene carbonate (2d): Purification by flash column chro-
matography with n-hexane/EtOAc (6:4) gave a colourless liquid.
1H NMR (400 MHz, CDCl3): d=4.72–4.70 (m, 1H, OCH), 4.51 (t, J=
8.0 Hz, 1H, CH2), 4.05 (t, J=8.0 Hz, 1H, CH2), 1.84–1.63 (m, 2H,
CH2), 1.48–1.33 (m, 4H, 2”CH2), 0.91 ppm (t, J=8.0 Hz, CH3);
13C NMR (100 MHz, CDCl3): d=155.1 (C=O), 76.8 (OCH), 69.4 (OCH2),
35.6 (CH2), 26.4 (CH2), 22.3 (CH2), 13.8 ppm (CH3); IR (neat): n=
2941, 2922, 2899, 1796 cmÀ1; HRMS (ESI+): calcd for C7H12O3 [M+
Na]+ 167.0679, found 167.0682.
˜
1601, 1568, 1494, 1343, 1209, 1178, 1011, 779, 707, 649, 518 cmÀ1
;
HRMS (ESI+): calcd. for C56H65Al2N8O5 [M+H]+ 983.4727, found
1,2-Dodecylene carbonate (2e): Purification by flash column chro-
matography with n-hexane/EtOAc (8:2) gave a colourless liquid.
1H NMR (400 MHz, CDCl3): d=4.73–4.66 (m, 1H, OCH), 4.52 (t, J=
8.0 Hz, 1H, CH2), 4.06 (t, J=8.0 Hz, 1H, CH2), 1.84–1.70 (m, 2H,
CH2), 1.63–1.26 (m, 16H, 8”CH2), 0.88 ppm (t, J=8.0 Hz, CH3);
13C NMR (100 MHz, CDCl3): d=155.1 (C=O), 77.1 (OCH), 69.4 (OCH2),
33.9 (CH2), 31.8 (CH2), 29.5 (CH2), 29.4 (CH2), 29.3 (CH2), 29.2 (CH2),
29.1 (CH2), 24.3 (CH2), 22.6 (CH2), 14.1 ppm (CH3); IR (neat): n˜ =
983.4703.
General procedure for catalyst screening at 1 bar pressure
An epoxide (1.66 mmol), catalyst 6 or 7 (0.025 mmol) and Bu4NBr
(8 mg, 0.025 mmol) were placed in a sample vial fitted with a mag-
netic stirrer bar and placed in a large conical flask. Cardice pellets
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2931, 2832, 1798 cmÀ1; HRMS (ESI+): calcd for C13H24O3 [M+Na]+
251.1618, found 251.1621.
O), 76.0 (CH), 14.3 ppm (CH3); IR (neat ): n˜ =2960, 2899, 1787 cmÀ1
;
HRMS (ESI+): calcd for C5H8O3 [M+Na]+ 139.0366, found 139.0365.
trans-2,3-Butene carbonate (12b): Purification by flash column
chromatography with n-hexane/EtOAc (7:3) gave a white solid.
m.p. 30–328C (lit).[15,16,24] 29–308C); 1H NMR (400 MHz, CDCl3): d=
4.34–4.32 (m, 2H, 2”CH), 1.45 ppm (d, J=4.0 Hz, 6H, 2”Me);
13C NMR (100 MHz, CDCl3): d=154.3 (C=O), 79.9 (CH), 18.4 ppm
(CH3); IR (neat ): n˜ =2955, 2871, 1776 cmÀ1; HRMS (ESI+): calcd for
C5H8O3 [M+Na]+ 139.0366, found: 139.0364.
1,2-Decylene carbonate (2 f): Purification by flash column chroma-
tography with n-hexane/EtOAc (8:2) gave a colourless liquid.
1H NMR (400 MHz, CDCl3): d=4.73–4.66 (m, 1H, OCH), 4.52 (t, J=
8.0 Hz, 1H, CH2), 4.06 (t, J=8.0 Hz, 1H, CH2), 1.85–1.64 (m, 2H,
CH2), 1.50–1.26 (m, 12H, 6”CH2), 0.87 ppm (t, J=8.0 Hz, CH3);
13C NMR (100 MHz, CDCl3): d=155.1 (C=O), 77.0 (OCH), 69.4 (OCH2),
33.9 (CH2), 31.8 (CH2), 29.3 (CH2), 29.1 (CH2), 29.0 (CH2), 24.3 (CH2),
22.6 (CH2), 14.1 ppm (CH3); HRMS (ESI+): calcd for C11H20O3 [M+
Na]+ 223.1305, found 223.1315.
1,2-Cyclopentene carbonate (12c): Purification by flash column
chromatography with n-hexane/EtOAc (8:2) gave a white solid.
m.p. 30–338C (lit.[15,16,24] 29–308C); 1H NMR (400 MHz, CDCl3): d=
5.11–5.10 (m, 2H, 2”CH), 2.17–2.12 (m, 2H, CH2), 1.83–1.61 ppm
(m, 4H, 2”CH2); 13C NMR (100 MHz, CDCl3): d=155.4 (C=O), 81.8
(CH), 33.1 (CH2), 21.5 ppm (CH2); IR (neat): n˜ =2967, 2871,
1789 cmÀ1; HRMS (ESI+): calcd for C6H8O3 [M+Na]+ 151.0366,
found 151.0368.
3-Chloropropylene carbonate (2g): Purification by flash column
chromatography with n-hexane/EtOAc (6:4) gave a white solid.
m.p. 67–698C (lit.[8,11,12,23] 68–698C);1H NMR (400 MHz, CDCl3): d=
4.98–4.92 (m, 1H, OCH), 4.58 (t, J=8.0 Hz, 1H, OCH2), 4.40 ppm (t,
J=8.0 Hz, 1H, OCH2), 3.78–3.70 ppm (m, 2H, CH2Cl); 13C NMR
(100 MHz, CDCl3): d=154.2 (C=O), 74.3 (OCH), 66.9 (OCH2),
43.7 ppm (CH2Cl); IR (neat ): n˜ =2973, 2698, 2121, 2017, 1971,
1793 cmÀ1; HRMS (ESI+): calcd for C4H5ClO3 [M+Na]+ 158.9819,
found 158.9815.
cis-1,2-Cyclohexene carbonate (12d): Purification by flash column
chromatography with n-hexane/EtOAc (8:2) gave a white solid.
m.p. 35–378C (lit.[15,16,24] 34–358C); 1H NMR (400 MHz, CDCl3): d=
4.70–4.65 (m, 2H, 2”CH), 1.91–1.87 (m, 4H, 2”CH2), 1.68–1.57 (m,
2H, CH2), 1.46–1.32 ppm (m, 2H, CH2); 13C NMR (100 MHz, CDCl3):
d=155.2 (C=O), 75.7 (CH), 26.8 (CH2), 19.2 ppm (CH2); IR (neat ):
n˜ =2933, 2861, 1784 cmÀ1; HRMS (ESI+): calcd for C7H10O3 [M+
Na]+ 165.0522, found 165.0521.
Glycerol carbonate (2h): Purification by flash column chromatog-
1
raphy with n-hexane/EtOAc (6:4) gave a colourless liquid. H NMR
(400 MHz, CDCl3): d=4.83–4.77 (m, 1H, CH), 4.51 (t, J=8.0 Hz, 1H,
OCH2), 4.41 (dd, J=8.0, 4.0 Hz, 1H, OCH2), 3.96 (dd, J=16.0, 4.0 Hz,
1H, CHOH), 3.68 ppm (dd, J=16.0, 4.0 Hz, 1H, CHOH); 13C NMR
(100 MHz, CDCl3): d=155.4 (C=O), 76.6 (OCH), 65.8 (OCH2),
61.6 ppm (CH2OH); IR (neat ): n˜ =3382, 2901, 1799 cmÀ1; HRMS
(ESI+): calcd for C4H6O4 [M+Na]+ 141.0158, found 141.0159.
trans-1,2-Diphenylethylene carbonate (12e): Purification by flash
column chromatography with n-hexane/EtOAc (8:2) gave a white
solid. m.p. 109–1108C (lit.[15,16,24] 110–111 8C); 1H NMR (400 MHz,
CDCl3): d=7.54–7.43 (m, 6H, ArH), 7.33–7.31 (m, 4H, ArH),
5.44 ppm (s, 2H, CH); 13C NMR (100 MHz, CDCl3): d=154.8 (C=O),
134.8 (ArC), 129.8 (ArCH), 129.2 (ArCH), 126.0 (ArCH), 85.4 ppm
(CH); IR (neat ): n˜ =3051, 2977, 1812, 1458 cmÀ1; HRMS (ESI+):
calcd for C15H12O3 [M+H]+ 241.0859, found 241.0863.
3-Phenoxypropylene carbonate (2i): Purification by flash column
chromatography with n-hexane/EtOAc (8:2) gave a white solid.
1
m.p. 94–968C (lit.[8,11,12,23] 94–958C); H NMR (400 MHz, CDCl3): d=
7.30 (t, J=8.0 Hz, 2H, m-Ph), 7.05 (t, J=8.0 Hz, 1H, p-Ph), 6.92 (d,
J=8.0 Hz, 2H, o-Ph), 5.10–5.02 (m, 1H, OCH), 4.71–4.52 (m, 2H,
OCH2), 4.26 (dd, J=11.0, 4.0 Hz, 1H, CH2), 4.16 ppm (dd, J=11.0,
4.0 Hz, 1H, CH2); 13C NMR (100 MHz, CDCl3): d=157.7 (C=O), 154.6
(ArC), 129.7 (ArCH), 122.0 (ArCH), 114.6 (ArCH), 74.0 (OCH), 66.8
(OCH2), 66.2 ppm (CH2); HRMS (ESI+): calcd for C10H10O4 [M+Na]+
217.0471, found 217.0473.
trans-1-Phenyl-2-methylethylene carbonate (12 f): Purification by
flash column chromatography with n-hexane/EtOAc (8:2) gave
a white solid. m.p. 112–1148C (lit.[15,16,24] 110–111 8C); 1H NMR
(400 MHz, CDCl3): d=7.54–7.43 (m, 3H, ArH), 7.37–7.35 (m, 2H,
ArH), 5.13 (d J=8.0 Hz, 1H, PhCH), 4.64–4.57 (m, 1H, CHMe),
1.56 ppm (d, J=8.0 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3): d=
154.3 (C=O), 135.0 (ArC), 129.7 (ArCH), 129.2 (ArCH), 126.0 (ArCH),
84.9 (CH), 80.7 (CH), 18.3 ppm (CH3); IR (neat): n˜ =3010, 2950, 1800,
1459 cmÀ1; HRMS (ESI+): calcd for C10H10O3 [M+H]+ 179.0703,
found 179.0704.
4-Chlorostyrene carbonate (2j): Purification by flash column chro-
matography with n-hexane/EtOAc (6:4) gave a white solid. m.p.
67–698C (lit.[8,11,12,23] 68–698C); 1H NMR (400 MHz, CDCl3): d=7.43
(d, J=8.0 Hz, 2H, ArH), 7.30 (d, J=8.0 Hz, 2H, ArH), 5.66 (t, J=
8.0 Hz, 1H, CH), 4.80 (t, J=8.0 Hz, 1H, CH2), 4.31 ppm (t, J=8.0 Hz,
1H, CH2); 13C NMR (100 MHz, CDCl3): d=154.5 (C=O), 135.8 (ArC),
134.2 (ArC), 129.5 (ArCH), 127.2 (ArCH), 77.2 (OCH), 71.0 ppm
1,1-Dimethylethylene carbonate (12g): Purification by flash
column chromatography with n-hexane/EtOAc (8:2) gave a white
1
solid. m.p. 30–328C (lit.[15,16,24] 29–308C); H NMR (400 MHz, CDCl3):
(OCH2); IR (neat ): n˜ =2973, 2698, 2121, 2017, 1971, 1793 cmÀ1
;
d=4.14 (s, 2H, CH2), 1.52 ppm (s, 6H, 2”Me); 13C NMR (100 MHz,
CDCl3): d=154.6 (C=O), 81.7 (CMe2), 75.4 (CH2), 26.0 ppm (CH3); IR
(neat ): n˜ =2955, 2833, 1780 cmÀ1; HRMS (ESI+): calcd for C5H8O3
[M+Na]+ 139.0366, found 139.0366.
HRMS (ESI+): calcd for C9H7ClO3 [M+H]+ 220.9984, found
220.9976.
4-Bromostyrene carbonate (2k): Purification by flash column chro-
matography with n-hexane/EtOAc (8:2) gave a white solid. m.p.
70–728C (lit.[8,11,12,23] 68–698C); 1H NMR (400 MHz, CDCl3): d=7.58
(d, J=8.0 Hz, 2H, ArH), 7.24 (d, J=8.0 Hz, 2H, ArH), 5.64 (t, J=
8.0 Hz, 1H, CH), 4.80 (t, J=8.0 Hz, 1H, CH2), 4.30 ppm (t, J=8.0 Hz,
1H, CH2); 13C NMR (100 MHz, CDCl3): d=154.5 (C=O), 134.8 (ArC),
132.5 (ArCH), 127.4 (ArCH), 123.9 (ArC), 77.2 (CH), 70.9 ppm (CH2);
IR (neat): n˜ =2951, 2522, 2161, 2017, 1981, 1801, 1771 cmÀ1; HRMS
(ESI+): calcd for C9H7BrO3 [M+Na]+ 264.9471, found 264.9470.
Acknowledgements
We gratefully acknowledge financial support from the Europe-
an Union Seventh Framework Programme FP7-NMP-2012
under grant agreement number 309497.
cis-2,3-Butene carbonate (12a): Purification by flash column chro-
matography with n-hexane/EtOAc (7:3) gave a white solid as a mix-
ture of cis and trans isomers. m.p. 30–318C (lit).[15,16,24] 29–308C);
1H NMR (400 MHz, CDCl3): d=4.84–4.82 (m, 2H, 2”CH), 1.35 ppm
(d, J=8.0 Hz, 6H, 2”Me); 13C NMR (100 MHz, CDCl3): d=154.6 (C=
Keywords: aluminium
epoxides · homogeneous catalysis · salphen ligands
· chromium · cyclic carbonates ·
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Received: October 27, 2015
Revised: November 26, 2015
Published online on January 7, 2016
Chem. Eur. J. 2016, 22, 2100 – 2107
www.chemeurj.org
2107
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