Synthesis of Cyclic Carbonates Catalysed by Aluminium Heteroscorpionate Complexes

Article abstract of DOI:10.1002/chem.201500790

New aluminium scorpionate based complexes have been prepared and used for the synthesis of cyclic carbonates from epoxides and carbon dioxide. Bimetallic aluminium(heteroscorpionate) complexes 9-14 were synthesised in very high yields. The single-crystal X-ray structures of 12 and 13 confirm an asymmetric κ²-NO-μ-O arrangement in a dinuclear molecular disposition. These bimetallic aluminium complexes were investigated as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide in the presence of ammonium salts. Under the optimal reaction conditions, complex 9 in combination with tetrabutylammonium bromide acts as a very efficient catalyst system for the conversion of both monosubstituted and internal epoxides into the corresponding cyclic carbonates showing broad substrate scope. Complex 9 and tetrabutylammonium bromide is the second most efficient aluminium-based catalyst system for the reaction of internal epoxides with carbon dioxide. A kinetic study has been carried out and showed that the reactions were first order in complex 9 and tetrabutylammonium bromide concentrations. Based on the kinetic study, a catalytic cycle is proposed.

Full text of DOI:10.1002/chem.201500790

DOI: 10.1002/chem.201500790
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&
Cyclic Carbonates |Hot Paper|
Synthesis of Cyclic Carbonates Catalysed by Aluminium
Heteroscorpionate Complexes
Javier Martínez,[a, b] JosØ A. Castro-Osma,[a] Amy Earlam,[a] Carlos Alonso-Moreno,[b]
Antonio Otero,*[b] Agustín Lara-Sµnchez,*[b] Michael North,*[a] and Antonio Rodríguez-
DiØguez[c]
Abstract: New aluminium scorpionate based complexes
have been prepared and used for the synthesis of cyclic car-
bonates from epoxides and carbon dioxide. Bimetallic alumi-
nium(heteroscorpionate) complexes 9–14 were synthesised
in very high yields. The single-crystal X-ray structures of 12
and 13 confirm an asymmetric k2-NO-m-O arrangement in
a dinuclear molecular disposition. These bimetallic alumini-
um complexes were investigated as catalysts for the synthe-
sis of cyclic carbonates from epoxides and carbon dioxide in
the presence of ammonium salts. Under the optimal reaction
conditions, complex 9 in combination with tetrabutylammo-
nium bromide acts as a very efficient catalyst system for the
conversion of both monosubstituted and internal epoxides
into the corresponding cyclic carbonates showing broad
substrate scope. Complex 9 and tetrabutylammonium bro-
mide is the second most efficient aluminium-based catalyst
system for the reaction of internal epoxides with carbon di-
oxide. A kinetic study has been carried out and showed that
the reactions were first order in complex 9 and tetrabutyl-
ammonium bromide concentrations. Based on the kinetic
study, a catalytic cycle is proposed.
materials[3,5] and as a phosgene replacement.[6] The main chal-
lenge is to overcome the reaction thermodynamics due to the
rather low heat of formation (DHf0 =À394 kJmolÀ1) and stan-
dard Gibbs energy of formation (DGf0 =À395 kJmolÀ1) of
carbon dioxide.[7]
Introduction
Meeting the world’s growing energy demand and shifting
from a fossil fuel-dependent industry to a sustainable and bio-
based one are the major challenges for the first half of the
21st century. On one hand, 90% of commercially available or-
ganic chemicals produced by the chemicals industry are pro-
duced from crude oil.[1] On the other hand, due to the utilisa-
tion of fossil fuels to produce energy, the concentration of
carbon dioxide in the atmosphere is increasing every year.[2]
Therefore, carbon dioxide is increasingly being seen as a sus-
tainable chemical feedstock for the chemicals industry as it is
abundant, economic and non-toxic.[3] It can be used as starting
material for the synthesis of organic molecules,[3,4] polymeric
One of the most studied reactions of carbon dioxide is its re-
action with epoxides (1) to afford either cyclic- (2) or poly-
carbonates (3) (Scheme 1) which are 100% atom-economical
reactions and amongst the most important commercial pro-
cesses using carbon dioxide as starting material.[8] Even though
this reaction is highly exothermic due to the release of the ep-
oxide strain energy, it requires a suitable catalyst to lower the
high activation barrier.[9]
[a] J. Martínez, Dr. J. A. Castro-Osma, A. Earlam, Prof. M. North
Green Chemistry Centre of Excellence, Department of Chemistry
The University of York, York, YO10 5DD (UK)
E-mail: michael.north@york.ac.uk
[b] J. Martínez, Dr. C. Alonso-Moreno, Prof. A. Otero, Dr. A. Lara-Sµnchez
Departamento de Química Inorgµnica, Orgµnica y Bioquímica, and Centro
de Innovación en Química Avanzada (ORFEO-CINQA)
Facultad de Ciencias y Tecnologías Químicas
Universidad de Castilla-La Mancha
Scheme 1. Synthesis of cyclic- and polycarbonates.
Both cyclic- and polycarbonates have many industrial appli-
cations.[10] Aliphatic polycarbonates are potential replacements
for aromatic polycarbonates, derived from phosgene and bi-
sphenol-A, which are polymers with outstanding properties for
the electronics, optical media, automotive and healthcare in-
dustries.[5] Moreover, aliphatic polycarbonates can be used for
the production of polyethercarbonate polyols, a key building
block for the sustainable synthesis of polyurethanes, a polymer
Campus Universitario, 13071 Ciudad Real (Spain)
E-mail: Antonio.Otero@uclm.es
Agustin.Lara@uclm.es
[c] Dr. A. Rodríguez-DiØguez
Departamento de Química Inorgµnica, Facultad de Ciencias
Universidad de Granada. Avda de Fuentenueva s/n, 18071 Granada (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500790.
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with many applications.[11] Much research is being devoted to
commercialise this reaction and promising catalysts have al-
ready been developed.[12] Cyclic carbonates have numerous
commercial applications,[13] including as electrolytes for lithi-
um-ion batteries,[14] sustainable solvents,[15] degreasing
agents,[16] and chemical intermediates.[17] The synthesis of cyclic
carbonates from epoxides and carbon dioxide has been a com-
mercial process for more than sixty years, but current commer-
cial processes use relatively inefficient onium salts as catalysts
which require the use of high temperatures and high carbon
dioxide pressures.[18]
esters.[27] Recently, we have reported the use of a range of thio-
acetamidate (e.g., 7) or acetamidate (e.g., 8) heteroscorpionate
aluminium complexes in combination with quaternary ammo-
nium salts as a highly active catalyst system for the synthesis
of cyclic carbonates at room temperature and one bar pressure
of carbon dioxide (Figure 2).[28]
The synthesis of cyclic carbonates has been extensively in-
vestigated in recent years and many highly active catalyst sys-
tems which combine a metal complex and an onium salt as co-
catalyst have been developed.[19] The nature of the metal
centre in metal complexes plays an important role in their cat-
alytic performance and aluminium complexes are one of the
most efficient catalyst systems for this reaction.[20] The choice
of ligand is also a key parameter that determines the catalytic
activity of these complexes, with Schiff base ligands being the
most widely studied.[21] The bimetallic aluminium(salen) com-
plex 4 (Figure 1) in combination with tetrabutylammonium
bromide (TBAB) is the most active catalyst system for the syn-
thesis of cyclic carbonates from terminal epoxides and carbon
dioxide at room temperature and one bar pressure.[22] The in-
corporation of the cocatalyst into the salen ligand to afford
one-component and immobilised aluminium(salen) catalysts 5
and 6 (Figure 1) was also achieved and these complexes were
also found to be very efficient catalysts.[23] Complex 4 and
silica supported catalyst 6 were shown to be compatible with
waste carbon dioxide present in either simulated or real power
station flue gas from combustion of gas or coal.[24] Recently we
have reported that complex 4 can act as an efficient catalyst
for the synthesis of cyclic carbonates from terminal epoxides
and carbon dioxide without the need for a cocatalyst under
optimal reaction conditions, which can simplify the purification
step, decrease the cost of the catalyst system and avoid corro-
sion issues associated with the halide anions.[25]
Figure 2. Aluminium(scorpionate) complexes for the synthesis of cyclic car-
bonates from terminal epoxides and carbon dioxide.
Inspired by the high catalytic activity displayed by the alumi-
nium oxo-bridge complex 4, the work described herein, is the
result of a rational development of a new family of bimetallic
aluminium(scorpionate) complexes containing an oxo-bridge
between the two aluminium centres and their application as
catalysts for the conversion of epoxides into the corresponding
cyclic carbonates. Notably, a combination of complex 9 and
TBAB is a very efficient catalyst system for the conversion of
a broad range of internal epoxides to cyclic carbonates with,
unusually, higher catalytic activity for the synthesis of cyclo-
pentene carbonate than cyclohexene carbonate.
Results and Discussion
Synthesis and structural characterisation: The bimetallic
alkyl-aluminium complexes [{AlR2(k2-bpzbe)}(m-O){AlR3}] (R=Me
9, Et 10), [{AlR2(k2-bpzte)}(m-O){AlR3}] (R=Me 11, Et 12) and
[{AlR2(k2-(R,R)-bpzmm)}(m-O){AlR3}] (R=Me 13, Et 14) were syn-
thesised by an alkane elimination route involving reaction of
the previously reported alcohol containing heteroscorpionate
precursors, bpzbeH[29] [bpzbe=1,1-bis(3,5-dimethylpyrazol-1-
yl)-3,3-dimethyl-2-butoxide], bpzteH[29] [bpzte=2,2-bis(3,5-di-
methylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH[29]
[(R,R)-bpzmm=(1R)-1-{(1R)-6,6-dimethyl-bicyclo[3.1.1]-2-
hepten-2-yl}-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide],
with
two equivalents of the corresponding trialkylaluminium
(Scheme 2). The reactions were carried out in toluene and,
after the appropriate workup, the complexes were isolated in
good yields (~85%) as colourless solids for complexes 9–12
and as yellow solids for complexes 13 and 14. The second alu-
minium centre is coordinated through a dative bond to the
oxygen atom from the alkoxide moiety (Scheme 2). Complexes
9–12 were isolated as racemates, whilst compounds 13 and 14
were prepared as single enantiomers. The complexes are
stable in air for several days in the solid state at room temper-
ature.
Figure 1. Aluminium(salen) complexes for the synthesis of cyclic carbonates
from terminal epoxides and carbon dioxide.
The use of metal complexes bearing heteroscorpionate li-
gands as catalysts for commercially important processes has
been recently reviewed.[26] In recent years, several heteroscorpi-
onate aluminium complexes have been reported and found to
exhibit catalytic activity in the polymerisation of cyclic
The solution-state structures of complexes 9–14 were char-
1
acterised by spectroscopic methods. The H and 13C{1H} NMR
spectra of 9–14 exhibit two distinct sets of pyrazole resonan-
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Scheme 2. Synthesis of complexes 9–14.
ces, indicating the existence of two types of pyrazole rings.
1
The H NMR spectra of these complexes show two singlets for
each of the H4, Me3 and Me5 pyrazole protons, one singlet for
each of the methine groups (CH bridge of pyrazole rings and
CHa), and the signals corresponding to the R’ moieties of the
heteroscorpionate ligands as well as the alkyl ligands. It is
worth noting that some pyrazole proton signals appear as
broad resonances (Figure 3), a finding that indicates fluxional
behaviour, which is thought to be due to an exchange process
between a coordinated and a non-coordinated pyrazole ring
(see discussion below). This type of behaviour has previously
been observed in tetrahedral aluminium complexes supported
with acetamidate or thioacetamidate heteroscorpionate li-
gands.[27] NOESY-1D NMR experiments allowed the unequivocal
assignment of most H NMR resonances and H–13C heteronu-
clear correlation (g-HSQC) experiments were carried out to
assign the resonances corresponding to C4, Me3 and Me5 of
the pyrazole rings and alkyl groups (see Experimental Section).
The data obtained support a tetrahedral disposition for each
aluminium atom, with the heteroscorpionate ligand coordinat-
ed in a k2-NO-m-O coordination mode, bridging the two alumi-
nium centres as depicted in Scheme 2.
Scheme 3. Structures for the four stereoisomers of compounds 9–12.
become resolved into a number of separate peaks for each of
the two diastereoisomers (Scheme 3) (relative intensities 5:1)
at À408C (Figure 4).
1
1
1
Figure 4. Variable temperature H NMR spectra in the region from 8.2 to
&
*
4.5 ppm for compound 12 in [D8]toluene. ( ) major isomer; ( ) minor
isomer.
Thus, an exchange process between coordinated and non-
coordinated pyrazole rings is taking place and this involves in-
terconversion from one diastereoisomer to the other. It is
worth noting that, due to the coordination mode of the li-
gands, the methine carbon bridging the two pyrazole rings is
a stereocentre. For complexes 9–12, which contain a racemic
heteroscorpionate ligand there are four stereoisomers
(Scheme 3), whilst for complexes 13 and 14, which contain an
enantiopure heteroscorpionate ligand, there are only two dia-
stereoisomers (Scheme 4).
Figure 3. 1H NMR spectrum of complex [{AlEt2(k2-bpzte)}(m-O){AlEt3}] (12).
To investigate fluxionality within complexes 9–14, a variable
temperature NMR study was carried out and the spectra for
compound 12 (Figure 4) provide a good example of the spec-
tral changes observed. The VT NMR analysis showed that the
resonances of the alkyl group directly bonded to the alumini-
um centre, as well as those of the pyrazole rings, broaden and
The solid-state structures of complexes 12 and 13 were de-
termined by single crystal X-ray diffraction. The corresponding
MERCURY drawings are depicted in Figure 5.[30] The crystallo-
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viation from ideal values [range 94.08(5)8 to 117.86(6)8 for 12,
95.33(13)8 to 116.9(3)8 for 13] and in all compounds the most
acute angle of 94.08(5)8 for 12 and 95.33(13)8 for 13 is ob-
served for N(1)-Al(1)-O(1), which is constrained by the bite of
the heteroscorpionate ligand. The distances between N(3) and
Al(1) of 3.451(5) Š for 12 and 3.362(6) and 3.603(6) Š for 13 are
too long to be considered as bonding or an interaction be-
tween N(3) and the Al(1) atom, probably due to the steric hin-
drance caused by the alkyl groups. The bond distances
Al(2)ÀO(1) of 1.9409(11) Š for 12 and 1.917(3) and 1.918(3) Š
for 13 are longer than the Al(1)ÀO(1) distances of 1.8351(11),
1.831(3) and 1.824(3) Š for 12 and 13, respectively, and hence
corroborate that the second aluminium is coordinated through
a dative bond to the oxygen from the alkoxide moiety
(Figure 5). The AlÀC distances [ranges 1.9709(15)–2.0000(16)
and 1.956(4)–1.992(5) Š for 12 and 13, respectively] are in
good agreement with literature data.[27,31] The Al(1)ÀN(1) bond
distances of 1.9768(13) Š for 12 and 1.956(4) and 1.960(4) Š for
13 are similar to those in other aluminium pyrazolyl com-
plexes.[27,32]
Scheme 4. Structures for the two diastereoisomers of compounds 13 and
14.
graphic data and selected bond interatomic distances and
angles are given in Tables S1–S3 of the supporting information.
In both complexes, the heteroscorpionate ligands are k2-NO-m-
O coordinated to the aluminium centres. In addition, the two
aluminium centres within each complex are coordinated to
two and three alkyl ligands, respectively. Thus, these com-
plexes consist of a dinuclear structure in which each alumini-
um atom adopts a distorted tetrahedral geometry. The crystal-
line structures are consistent with those proposed in Scheme 2
on the basis of solution-state NMR and other analytical data.
Given the coordination mode of the heteroscorpionate ligand,
complexes 12 and 13 are chiral. In the solid state, complex 12
crystallises as a racemic mixture of a single diastereoisomer
(SR/RS) and the structure of the RS enantiomer is depicted in
Figure 5. Complex 13 has an enantiopure heteroscorpionate
ligand, a situation that could generate two diastereoisomers
(Scheme 4). The compound crystallises as an enantiopure com-
plex in the chiral space group P1 and the absolute configura-
tion of the enantiomer in the unit cell is SR.
Synthesis of cyclic carbonates: Complexes 9–14 were initially
screened for the conversion of styrene oxide 1a into styrene
carbonate 2a at 258C and one bar carbon dioxide pressure
under solvent free conditions for 24 h using 5 mol% of both
complex 9–14 and TBAB (Scheme 5). Six reactions involving
the same epoxide were carried out in parallel in magnetically
stirred glass vials within a large conical flask containing cardice
pellets with a rubber stopper pierced by a deflated balloon.
1
Each reaction was then analysed by either H NMR spectrosco-
py or gas chromatography without any purification to deter-
mine the conversion of epoxide 1a into cyclic carbonate 2a.
The results are shown in Table 1. No polycarbonate formation
was detected under the aforementioned reactions conditions.
Complexes 9–14 were found to be moderately
active for the synthesis of styrene carbonate under
these conditions. As can be seen from entries 1–6, bi-
metallic aluminium complexes 9 and 14 were the
most active catalysts, probably due to the higher sol-
ubility of those complexes in the epoxide under the
reaction conditions rather than to any steric effect. In
order to further increase the conversion, the effect of
increasing the reaction temperature to 508C was in-
vestigated whilst keeping the pressure of carbon di-
oxide at one bar (Table 1). As expected this is benefi-
cial and quantitative conversion of styrene oxide 1a
into the corresponding cyclic carbonate 2a can be
achieved within 24 h using a combination of bimetal-
Figure 5. MERCURY perspective drawings of compounds 12 (left) and 13 (right). Thermal
ellipsoids are set at 50% probability and hydrogen atoms are omitted for clarity. Mercury
is the crystal structure visualisation software of the Cambridge structural database.
lic complexes 9–14 and TBAB. Control experiments
showed that neither TBAB nor complex 9 alone dis-
played significant catalytic activity in the absence of
the other catalyst component under these reaction
conditions (Table 1, entries 7 and 8).
Compounds 12 and 13 exhibit similar structural features.
The dihedral angle between the N(1)-Al(1)-O(1) and C(1)-Al(1)-
C(2) planes (87.288 for 12, and 89.208 and 89.608 for 13, re-
spectively) are consistent with a tetrahedral geometry. Further-
more, the angles around the Al(1) atom show considerable de-
As complex 9 was one of the most active catalysts it was se-
lected for further optimisation and the effect of the cocatalyst
was investigated. It was found that the order of catalytic activi-
ty was Br > I > Cl > F (Table 2, entries 1–4) for tetrabutylam-
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Table 2. Influence of cocatalyst on the catalytic activity of complex 9.[a]
[c]
Entry
Cocatalyst
T [8C]
Conversion [%][b]
TOF [hÀ1
]
1
2
3
4
5
6
7
8
Bu4NF
Bu4NCl
Bu4NBr
Bu4NI
DMAP
NMI
PPNCl
PPNBr
Bu4NBr
PPNCl
PPNBr
50
50
50
50
50
50
50
50
25
25
25
0
0.00
0.49
0.80
0.73
0.00
0.00
0.81
0.73
0.38
0.10
0.38
59
96
87
0
Scheme 5. Synthesis of the cyclic carbonates 2a–k using complexes 9–14.
Table 1. Synthesis of carbonate 2a catalysed by complexes 9–14.[a]
0
97
90
45
12
46
9
10
11
[c]
[c]
Entry Catalyst Cocatalyst 258C [%][b] TOF [hÀ1
]
508C [%][b] TOF [hÀ1
]
[a] Reactions carried out at 1 bar CO2 pressure for 24 h using 5 mol% of
complex 9 and 5 mol% of cocatalyst. [b] Conversion determined by
1H NMR spectroscopy or gas chromatography of the crude reaction mix-
ture after 24 h. [c] TOF=moles of product/(moles of catalyst · time).[19p,21a]
1
2
3
4
5
6
7
8
9
Bu4NBr
Bu4NBr
Bu4NBr
Bu4NBr
Bu4NBr
Bu4NBr
Bu4NBr
–
75
35
41
45
35
74
1
0.63
0.29
0.34
0.38
0.29
0.62
0.01
0.00
96
95
88
96
97
97
5
0.80
0.79
0.73
0.80
0.81
0.81
0.04
0.00
10
11
12
13
14
–
9
0
0
Table 3. Optimisation of the synthesis of carbonates 2a and 2b using
catalyst 9 and Bu4NBr.[a]
[a] Reactions carried out at 1 bar CO2 pressure for 24 h using 5 mol% of
complex and 5 mol% of Bu4NBr cocatalyst. [b] Conversion determined by
1H NMR spectroscopy or gas chromatography of the crude reaction mix-
ture after 24 h. [c] TOF=moles of product/(moles of catalyst · time).[19p,21a]
[c]
Entry
9 [mol%]
Bu4NBr [mol%]
Conversion [%][b]
TOF [hÀ1
]
1
2
3
4
5
6
7
8
9
10
0.5
1
2
2.5
5
0.5
1
2
2.5
5
0.5
1
2
2.5
5
0.5
1
2
2.5
5
81
83
87
87
96
20
36
65
68
94
0.68
0.69
0.73
0.73
0.73
0.80
0.17
0.30
0.54
0.57
monium salts at 508C and one bar carbon dioxide pressure.
These results are in agreement with previous work[22,28] and
suggest that a balance between nucleophilicity and leaving
group ability is required. 4-Dimethylaminopyridine (DMAP) and
N-methylimidazole (NMI) were not good cocatalysts as no
cyclic carbonate was formed from reactions where they were
added to complex 9 (Table 2, entries 5–6). Bis(triphenylphos-
phoranylidene)ammonium chloride (PPNCl) and bis(triphenyl-
phosphoranylidene)ammonium bromide (PPNBr) (Table 2, en-
tries 7–8) showed excellent catalytic activity at 508C and one
bar carbon dioxide pressure. To determine the optimal cocata-
lyst, TBAB, PPNCl and PPNBr were selected for a second screen-
ing in which the temperature was reduced from 508C to 258C
(Table 2, entries 9–11). TBAB and PPNBr were found to be more
active than PPNCl under these conditions. Since TBAB is avail-
able at much lower cost than PPNBr, it was selected as the op-
timal cocatalyst.
[a] Reactions carried out at for 24 h using catalyst 9 and Bu4NBr cocatalyst
at 508C and 1 bar carbon dioxide. [b] Conversion determined by 1H NMR
spectroscopy or gas chromatography of the crude reaction mixture after
24 h. [c] TOF=moles of product/(moles of catalyst · time).[19p,21a]
Since complex 9 contains five methyl groups which could
be hydrolysed, a study to determine the effect of small
amounts of water on its catalytic activity was carried out at
room temperature. As shown in Table 4 and Figure 6, the addi-
tion of 0.5 mol% of water (enough to hydrolyse just 1 in every
25 aluminium bound methyl groups in complex 9) increased
the conversion of styrene oxide 1a to styrene carbonate 2a
from 36 to 55% (Table 4, entry 3). A further increase in the
amount of water added was however detrimental and the con-
version decreased steadily to just 17% when 20 mol% water
(enough to hydrolyse all the aluminium methyl bonds) was
added (Table 4, entry 11).
The effect of reducing the catalyst loading was investigated
whilst keeping a constant catalyst to cocatalyst ratio of 1:1
(Table 3). Styrene oxide 1a and decylene oxide 1b were select-
ed as aromatic and aliphatic substrates to ensure that the re-
sults were representative of a range of substrates. For styrene
oxide (Table 3, entries 1–5), conversions higher than 80% were
obtained even when using 0.5 mol% of catalyst and cocatalyst.
However, when using decylene oxide as substrate 5 mol% of
catalyst and cocatalyst was needed in order to get over 70%
conversions at 508C and one bar carbon dioxide pressure
(Table 3, entries 6–10). It was therefore concluded that 5 mol%
of catalyst and cocatalyst should be used when screening ep-
oxides, though lesser amounts may be required for particularly
reactive substrates.
The reaction of eleven terminal epoxides 1a–k with carbon
dioxide to form the corresponding cyclic carbonates 2a–k was
investigated using 5 mol% of complex 9 and TBAB under sol-
vent free conditions at 508C and 1 bar carbon dioxide pres-
sure. Reactions were carried out for 24 h, then analysed by
1H NMR spectroscopy to give a conversion and the cyclic car-
bonate was isolated and purified to give the chemical yield.
The results are shown in Table 5. In general, good to excellent
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Table 4. Influence of water content on the catalytic activity of complex
9.[a]
Table 5. Conversion of epoxides into cyclic carbonates using catalyst 9
and Bu4NBr.[a]
[c]
[d]
Entry
H2O [mol%]
Conversion [%][b]
TOF [hÀ1
]
Entry
Substrate
Conversion [%][b]
Yield [%][c]
TOF [hÀ1
]
1
2
3
4
5
6
7
8
0
36
45
55
53
50
40
37
31
30
28
17
0.30
0.38
0.46
0.44
0.42
0.33
0.31
0.26
0.25
0.23
0.14
1
2
3
4
5
6
7
8
1a
1b
1d
1e
1 f
1g
1h
1k
96
89
86
90
75
92
88
100
74
32
12
22
4218
70
65
0.62
0.27
0.10
0.18
0.34
0.58
0.54
0.74
0.25
0.5
0.75
1
1.5
2
2.5
5
10
20
88
9
10
11
[a] Reactions carried out at 508C and 1 bar CO2 pressure for 24 h using
5 mol% of catalyst 9 and 5 mol% of Bu4NBr cocatalyst. [b] Conversion de-
termined by 1H NMR spectroscopy of the crude reaction mixture after
24 h. [c] Yield of pure isolated cyclic carbonate. [d] TOF=moles of prod-
uct/(moles of catalyst · time).[19p,21a]
[a] Reactions carried out in duplicate at 25oC and 1 bar CO2 pressure for
24 h using 5 mol% of catalyst 9 and 5 mol% of Bu4NBr cocatalyst.
[b] Conversion determined by 1H NMR spectroscopy or gas chromatogra-
phy of the crude reaction mixture after 24 h. [c] TOF=moles of product/
(moles of catalyst · time).[19p,21a]
Table 6. Conversion of epoxides 1a–k into cyclic carbonates 2a–k using
catalyst 9 and Bu4NBr.[a]
[d]
Entry
Substrate
Conversion [%][b]
100
100
[e]
Yield [%][c]
TOF [hÀ1
]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
85
86
79
86
62
86
77
67
61
82
94
0.71
0.72
0.66
0.72
0.52
0.57
0.64
0.56
0.51
0.68
0.78
100
100
100
100
100
100
100
100
9
10
11
1j
1k
[a] Reactions carried out at 508C and 10 bar CO2 pressure for 24 h using
5 mol% of catalyst 9 and 5 mol% of Bu4NBr cocatalyst. [b] Conversion de-
termined by 1H NMR spectroscopy of the crude reaction mixture after
24 h. [c] Yield of pure isolated cyclic carbonate. [d] TOF=moles of prod-
uct/(moles of catalyst · time).[19p,21a] [e] The volatile nature of epoxide 1c
meant that conversion could not be determined.
Figure 6. Influence of water on the conversion of styrene oxide 1a into sty-
rene carbonate 2a catalysed by complex 9 and Bu4NBr (5 mol% each) at
room temperature and 1 bar CO2. Data shown are the average of two ex-
periments with the individual experiments being used to indicate the error
bars.
conversions to the cyclic carbonates were achieved, but disap-
pointingly poor yields were obtained under these reaction
conditions when some aliphatic epoxides were used as sub-
strates.
To overcome this issue, reactions were carried out in
a sealed reactor at 508C and 10 bar carbon dioxide pressure
and the results obtained are shown in Table 6. In general, all of
the terminal epoxides were converted into the corresponding
cyclic carbonates in good to excellent yields with selectivity to-
wards the cyclic carbonate product higher than 99%. Notably,
the combination of complex 9 and TBAB can quantitatively
convert alkyl and aryl epoxides as well as those functionalised
with halides, alcohols and ethers into the corresponding cyclic
carbonates under these reaction conditions showing that the
catalyst system is tolerant of various functional groups.
In order to expand the substrate scope, the complex 9/TBAB
catalyst system was investigated using internal epoxides
Scheme 6. Synthesis of cyclic carbonates 16a–g using complex 9 and TBAB
as catalyst system.
(Scheme 6). These are more challenging substrates for cyclic
carbonate synthesis, though significant achievements have
been recently reported.[33]
The formation of seven disubstituted cyclic carbonates 16a–
g from their corresponding internal epoxides 15a–g and
carbon dioxide was firstly trialled using 5 mol% of both com-
Chem. Eur. J. 2015, 21, 9850 – 9862
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plex 9 and TBAB under solvent free conditions at 508C and
10 bar carbon dioxide pressure in a sealed reactor for 24 h. The
results are shown in Table 7. Under these conditions, cyclic car-
bonates 16a–g can be obtained in 6–99% yield (Table 7, en-
tries 1, 3–6, 8, 10). For some cyclic carbonates, the yield was
rather low and the effect of increasing the reaction tempera-
ture to 1008C and the reaction pressure to 50 bar was investi-
gated (Table 7, entries 2, 7, 9, 11). Using higher temperature
and pressure was beneficial to the catalytic activity, giving
good yields except for 1,1-disubstituted epoxide 15g for
which the yield of cyclic carbonate 16g is still very low. No
polycarbonate was observed under the reaction conditions
used showing that the selectivity towards cyclic carbonate
product is higher than 99%. The combination of complex 9
and TBAB is the second most active aluminium catalyst system
for this transformation and displayed higher catalytic activity
than bimetallic salen and acen aluminium catalysts,[34] being
outperformed only by the aminotriphenolate aluminium com-
plexes reported by Kleij and co-workers.[33] We were delighted
to find that the catalyst system displayed an uncommon
higher catalytic activity towards cyclopentene oxide than cy-
clohexene oxide under the reaction conditions probably due
to the lower ring strain calculated for 16b compared to
16a.[35]
synthesis of cyclic carbonates 16a–f catalysed by complex 9
and TBAB also occurs with retention of the epoxide stereo-
chemistry. Apart from cis-1,2-dimethylethylene carbonate (15c)
which was obtained as a 94:6 mixture of cis- and trans-isomers,
all other cyclic carbonates were obtained with 100% retention
of epoxide stereochemistry.
To verify that the formation of cyclic carbonates from termi-
nal epoxides catalysed by complex 9 and TBAB also occurs
with retention of the epoxide stereochemistry, the reaction of
cis- and trans-deuterated epoxide 1b and carbon dioxide was
investigated. The results obtained at 508C and 10 bar carbon
dioxide pressure indicated that the reaction proceeds with re-
tention of stereochemistry, giving a 93:7 mixture of cis- and
trans-isomers of deuterated decylene carbonate (2b) from cis-
deuterated decylene oxide (1b) and a 93:7 mixture of trans-
and cis-isomers of deuterated decylene carbonate (2b) from
trans-deuterated decylene oxide (1b). These results are consis-
tent with the generally proposed mechanism for cyclic carbon-
ate synthesis catalysed by a Lewis acid and nucleophile combi-
nation, in which the epoxide undergoes two substitution reac-
tions at the less hindered carbon atom (Scheme 7).
Table 7. Conversion of epoxides 15a–g into cyclic carbonates 16a–g
using catalyst 9 and Bu4NBr.[a]
[d]
Entry Substrate T [8C] P [bar] Conversion [%][b] Yield [%][c] TOF [hÀ1
]
1
2
3
4
5
6
7
8
15a
15a
15b
15c
15d
15e
15e
15 f
15 f
15g
15g
50
100 50
10
43
32
47
99
79
63
30
65
21
71
6
0.27
0.39
0.83
0.66
0.53
0.25
0.54
0.18
0.59
0.05
0.12
100
100
96
100
97
83
25
77
[e]
50
50
50
50
100 50
50 10
100 50
50 10
100 50
10
10
10
10
Scheme 7. Synthesis of cis-deuterated decylene carbonate 2b from cis-deu-
terated decylene oxide 1b using complex 9 and TBAB as catalyst system.
9
10
11
[e]
14
A kinetic study was undertaken to investigate the mecha-
nism of cyclic carbonate formation catalysed by complex 9
and TBAB under solvent free conditions. As previously report-
ed,[25] under these conditions in the early stages of the reac-
tion, the reaction follows zero order kinetics as the epoxide
acts as substrate and reaction solvent, but as the reaction pro-
gresses the cyclic carbonate becomes the main species in the
reaction mixture and the reaction follows first order kinetics.
Reactions were carried out using styrene oxide 1a as substrate
with an excess of carbon dioxide at 508C and 1 bar carbon di-
oxide pressure. Samples were taken from the reaction every
20 min and analysed by gas chromatography to determine the
conversion of styrene oxide 1a into styrene carbonate 2a.
From previous work,[28] the general rate equation may be
written as shown in Equation (1). As the concentration of
carbon dioxide, complex 9 and TBAB do not change during
the reaction, they can be included in the observed reaction
constant, giving Equation (2). Since the concentration of epox-
ide 1a does not change during the early stage of the reaction
[a] Reactions carried out at 50–1008C and 10–50 bar CO2 pressure for
24 h using 5 mol% of catalyst 9 and 5 mol% of Bu4NBr cocatalyst.
[b] Conversion determined by H NMR spectroscopy of the crude reaction
mixture after 24 h. [c] Yield of pure isolated cyclic carbonate. [d] TOF=
moles of product/(moles of catalyst · time).[19p,21a] [e] The volatile nature
of epoxide 15g meant that conversion could not be determined.
1
The use of internal epoxides 15a–g as substrates gave an
opportunity to obtain an insight into the reaction mechanism
and study the stereochemistry of cyclic carbonate formation
catalysed by complex 9 and TBAB. The catalytic cycle previous-
ly proposed for the synthesis of cyclic carbonates using scorpi-
onate aluminium catalysts,[28] involves a double inversion at
the less hindered carbon atom of the epoxide, therefore epox-
ide stereochemistry should be retained during the reaction
and this was found to be the case for reactions catalysed by
complex 8 and TBAB.[28]
1
Analysis of the H and 13C NMR spectra of products 16a–f
and comparison with literature data[33,36] confirmed that the
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and the kinetics can be fitted to zero order kinetics, the rate
equation for the early stages of the reaction can be written as
Equation (3). However, as the reaction progresses and the con-
centration of epoxide 1a decreases, the rate equation may be
expressed as Equation (4), showing that the reaction is first
order in epoxide 1a concentration.
a
b
c
d
ð1Þ
ð2Þ
ð3Þ
ð4Þ
Rate ¼ k½1 aŠ ½CO2Š ½9Š ½Bu4NBrŠ
Rate ¼ kobs½1 aŠa, where kobs ¼ k½CO2Š ½9Š ½Bu4NBrŠ
b
c
d
Figure 8. Plot of the observed first order rate constant versus [Bu4NBr]. Reac-
tions were carried out at 508C under solvent free conditions with
[9]=332 mm and [Bu4NBr]=83–332 mm. Data shown are the average of
two experiments with the individual experiments being used to indicate the
error bars.
a
b
c
d
Rate ¼ k0obs, where k0obs ¼ k0½1 aŠ ½CO2Š ½9Š ½Bu4NBrŠ
b
c
d
Rate ¼ k1obs½1 aŠ, where k1obs ¼ k1½CO2Š ½9Š ½Bu4NBrŠ
To study the order with respect to the concentration of com-
plex 9 and TBAB two set of reactions were carried out in dupli-
cate at four different concentrations of complex 9 or TBAB,
keeping the concentration of the other catalyst component
constant.[30] The log[9]/logk0obs and log[9]/logk1obs plots showed
slopes of 0.90 and 0.98, respectively, indicating that the reac-
tion is first order with respect to the concentration of 9 (c=
1).[30] This was confirmed by a plot of either k0obs or k1obs versus
[9] which also showed a good fit to a straight line (Figure 7).
Rate ¼ k0obs, where k0obs ¼ k0½1 aŠ½CO2Š½9Š½Bu4NBrŠ
Rate ¼ k1obs½1 aŠ, where k1obs ¼ k1½CO2Š½9Š½Bu4NBrŠ
ð5Þ
ð6Þ
Equations (5) and (6) have the same form as those previously
determined for the synthesis of cyclic carbonates catalysed by
acetamidate and thioacetamidate scorpionate aluminium com-
plexes.[28] Therefore, since the stereochemistry and reaction ki-
netics are the same, the mechanism previously proposed for
complexes 7 and 8 (Scheme 7) appears to also apply to com-
plex 9. Firstly, the epoxide is activated by coordination to one
of the aluminium centres. Then, bromide provided by the
TBAB attacks the less hindered carbon to form a halo-alkoxide.
Carbon dioxide is inserted into the aluminium-oxygen bond to
afford a metal carbonate which can ring-close to give the
cyclic carbonate and regenerate the TBAB and aluminium com-
plex 9.
Conclusion
Figure 7. Plot of the observed first order rate constant versus [9]. Reactions
were carried out at 508C under solvent free conditions with
[Bu4NBr]=332 mm and [9]=83–332 mm. Data shown are the average of
two experiments with the individual experiments being used to indicate the
error bars.
In conclusion, the rational design and synthesis of a new
family of bimetallic oxo-bridge heteroscorpionate aluminium
complexes bearing an alkoxide as the pendant donor arm is re-
ported. NMR and single-crystal X-ray studies allowed us to es-
tablish a k2-NO-m-O coordination mode for the complexes.
Complexes 9–14 in combination with a cocatalyst act as cat-
alysts for the synthesis of cyclic carbonates from epoxides and
carbon dioxide. This study has led to the discovery of bimetal-
lic aluminium complex 9, which in combination with TBAB
forms an efficient catalyst system for the synthesis of cyclic car-
bonates from carbon dioxide and epoxides. The catalyst
system displays a broad substrate scope catalysing the synthe-
sis of cyclic carbonates from functionalised terminal and inter-
nal epoxides in good to excellent yields. Cyclic carbonates
were synthesised in a very selective manner and no poly-
carbonate was observed. Cyclopentene carbonate was ob-
tained in 99% yield with 100% cis selectivity when complex 9
and TBAB were used as catalyst system, showing that this
system displays higher reactivity towards cyclopentene oxide
than cyclohexene oxide.
The first-order dependence of the reaction rate on the con-
centration of complex 9 indicates, as previously reported for
processes catalysed by complexes 7 and 8,[28] that there is no
cooperative catalysis between aluminium ions. The
log[Bu4NBr]/logk0obs and log[Bu4NBr]/logk1obs plots had slopes
of 0.80 and 0.84, respectively, again suggesting that the pro-
cess is first order with respect to the concentration of TBAB
(d=1).[30] This was also confirmed by a plot of either k0obs or
k1obs versus [Bu4NBr] which also showed a good fit to a straight
line (Figure 8).
Therefore, the rate equation for the synthesis of styrene car-
bonate 2a from styrene oxide 1a catalysed by complex 9 and
tetrabutylammonium bromide may be expressed as Equa-
tion (5) during the early stages of the reaction or by Equa-
tion (6) during the latter stages of the reaction.
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C6D6, 298 K): d=5.93 (s, 1H, CH), 5.70 (s, 1H, H4’), 5.22 (s, 1H, H4),
4.59 (s, 1H, CHa), 2.18 (s, 3H, Me3), 2.05 (s, 3H, Me3’), 1.93 (s, 3H,
Me5’), 1.32 (s, 3H, Me5), 0.91 (s, 9H, -C(CH3)3], 1.65–1.49 (m, 15H, Al-
CH2CH3), 0.52–0.10 ppm (m, 10H, Al-CH2CH3); 13C{1H}-NMR
(125 MHz, C6D6, 298 K): d=150.1, 148,9, 143.3, 135.9 (C3,3’, C5,5’),
107.9 (CH4’), 106.9 (CH4), 83.6 (CHa), 65.7 (CH), 36.0 [C(CH3)3], 27.2
[C(CH3)3], 13.3 (Me3), 12.5 (Me3’), 10.1 (Me5’), 9.4 (Me5), 10.7, 10.3,
(Al-CH2CH3), 2.2 ppm (Al-CH2CH3); elemental analysis calcd (%) for
C26H50Al2N4O: C 63.90, H 10.31, N 11.47; found: C 64.12, H 10.52, N
11.31.
Whilst many catalyst systems have been developed for the
reaction of terminal epoxides with carbon dioxide, only a few
catalyst systems act as efficient catalyst systems for the synthe-
sis of cyclic carbonates from internal epoxides. Seven cyclic
carbonates were synthesised from internal epoxides in excel-
lent conversions, although higher reaction temperature and
pressure were used to achieve good yields. Nevertheless, the
catalyst system reported herein is the second most active alu-
minium-based catalyst for this transformation, outperformed
only by the recently reported aluminium aminotriphenolate
complexes.[33]
Synthesis of [{AlMe2(k2-bpzte)}(m-O){AlMe3}] (11): The synthesis of
11 was carried out in an identical manner to 9, using bpzteH (1 g,
3.0 mmol) and AlMe3 (2m in toluene, 3.0 mL 6.0 mmol). Compound
1
11 was isolated as a white solid (1.28 g, 94%). H NMR (500 MHz,
C6D6, 298 K): d=7.34 (d, 3JH-H =7.4 Hz, 2H, Ho-MePh), 6.93 (d,
3JHÀH =7.4 Hz, 2H, Hm-MePh), 6.12 (s, 1H, CH), 6.08 (s, 1H, CHa),
5.72 (s, 1H, H4’), 5.03 (s, 1H, H4), 2.20 (s, 3H, Me3’), 1.98 (s, 3H,
MePh), 1.91 (s, 3H, Me5’), 1.85 (s, 3H, Me5), 0.16, À0.07 [s, 6H,
Al(CH3)2], À0.32 ppm [s, 9H, Al(CH3)3]; 13C{1H} NMR (125 MHz, C6D6,
Experimental Section
All manipulations were performed under nitrogen, using standard
Schlenk techniques. Solvents were predried over sodium wire (tolu-
ene, n-hexane) and distilled under nitrogen from sodium (toluene)
or sodium-potassium alloy (n-hexane). Deuterated solvents were
stored over activated 4 Š molecular sieves and degassed by several
freeze-thaw cycles. Microanalyses were carried out with a Perkin–
298 K): d=150.8, 148.9, 144.2, 138.2, 136.6, 134.8 (C3,3’, C5,5’, Cipso,p
-
MePh), 128.2 (Cm-MePh), 126.3 (Co-MePh), 107.8 (CH4’), 107.3 (CH4),
77.8 (CHa), 70.1 (CH), 20.5 (MePh), 13.3 (Me3’), 12.4, (Me3), 10.1
(Me5’), 9.3 (Me5), À4.2, À5.6, [Al(CH3)2], À7.1 ppm [Al(CH3)3]; ele-
mental analysis calcd (%) for C24H38Al2N4O: C 63.70, H 8.46, N 12.38;
found: C 63.98, H 8.67, N 12.14.
1
Elmer 2400 CHN analyser. H and 13C NMR spectra were recorded
on a Varian Inova FT-500 spectrometer and referenced to the resid-
ual deuterated solvent. NOESY-1D spectra were recorded with the
following acquisition parameters: irradiation time 2 s and number
of scans 256, using standard VARIANT-FT software. Two-dimension-
al NMR spectra were acquired using standard VARIANT-FT software
and processed using an IPC-Sun computer. Commercially available
chemicals (Alfa, Aldrich, Fluka) were used as received. Ligands
bpzbeH, bpzteH and (R,R)-bpzmmH were prepared according to lit-
erature procedures.[29] Gas chromatography was performed on an
Agilent 7820A instrument fitted with a TCD detector using a HP-IN-
NOWAX 19091N-133 capillary column (30 m”0.25 mm) with argon
as the carrier gas. The conditions used were: initial temperature
608C, hold at initial temperature for 3 min then ramp rate
Synthesis of [{AlEt2(k2-bpzte)}(m-O){AlEt3}] (12): The synthesis of
12 was carried out in an identical manner to 9, using bpzteH (1 g,
3.0 mmol) and AlEt3 (1m in hexane, 6.8 mL 6.8 mmol). Compound
1
12 was isolated as a white solid (1.51 g, 96%). H NMR (500 MHz,
3
3
C6D6, 298 K): d=7.26 (d, JH-H =7.4 Hz, 2H, Ho-MePh), 6.93 (d, JH-H
=
7.4 Hz, 2H, Hm-MePh), 5.97 (s, 1H, CHa), 5.92 (s, 1H, CH), 5.72 (s,
1H, H4’), 5.07 (s, 1H, H4), 2.22 (s, 3H, Me3’), 2.07 (s, 3H, Me3), 1.98 (s,
3H, MePh), 1.83 (s, 3H, Me5’), 1.06 (s, 3H, Me5), 1.60–1.40 (m, 15H,
Al-CH2CH3), 0.80–0.40 [m, 4H, Al(CH2CH3)2], 0.25 ppm [m, 6H,
Al(CH2CH3)3]; 13C{1H} NMR (125 MHz, C6D6, 298 K): d=151.2, 149.1,
144.4, 138.3, 136.7, 135.2 (C3,3’, C5,5’, Cipso,p-MePh), 129.2 (Cm-MePh),
128.4 (Co-MePh), 107.7 (C4’), 107.3 (C4), 78.4 (CHa), 70.5 (CH), 20.5
(MePh), 13.0 (Me3’), 12.6 (Me3), 9.8 (Me5’), 9.4 (Me5), 10.4
[Al(CH2CH3)3], 10.3, 10.1 [Al(CH2CH3)2], 4.7, 2.8, [Al(CH2CH3)2],
0.8 ppm [Al(CH2CH3)3]; elemental analysis calcd (%) for
C29H48Al2N4O: C 66.64, H 9.26, N 10.72; found: C 66.82, H 9.42, N
10.53.
158CminÀ1 to 2208C; hold at final temperature for 6 min; tR
=
8.11 min (styrene oxide), tR = 15.48 min (styrene carbonate). Specif-
ic rotations [a]2D were measured at 228C on a Perkin–Elmer 241 Po-
larimeter equipped with a Na lamp operating at 589 nm with
a path length of 10 cm.
Synthesis of [{AlMe2(k2-bpzbe)}(m-O){AlMe3}] (9): In 250 cm3
Schlenk tube, bpzbeH (1.0 g, 3.4 mmol) was dissolved in dry tolu-
ene (70 mL) and heated to 508C. A solution of AlMe3 (2m in tolu-
ene, 3.4 mL, 6.8 mmol) was added and the reaction mixture was
stirred at this temperature for 2 h. Removal of the volatiles under
reduced pressure yielded 9 as a white solid. The product was
washed with n-hexane (25 mL) and recrystallized from toluene at
Synthesis of [{AlMe2(k2-bpzmm)}(m-O){AlMe3}] (13): The synthesis
of 13 was carried out in a manner identical to that for 9, using
(R,R)-bpzmmH (1 g, 2.8 mmol) and AlMe3 (2m in toluene, 2.8 mL,
5.6 mmol). Compound 13 was isolated as a yellow solid (1.24 g,
92%). [a]2D5 = +21.9 (c=0.1, toluene); 1H NMR (500 MHz, C6D6,
298 K): d=6.19 (s, 1H, Hd), 6.03 (s, 1H, CH), 5.71 (s, 1H, H4’), 5.22 (s,
1H, CHa), 5.19 (s, 1H, H4), 2.36 (m, 2H, Hh), 2.16 (s, 3H, Me3’), 2.14
(m, 3H, He,b), 1.95 (s, 3H, Me3), 1.86 (s, 3H, Me5’),1.85(m, 1H, Hf),
1.37 (s, 3H, Me5), 1.14, 0.37 (s, 6H, Meg), 0.19, À0.01 [s, 6H,
Al(CH3)2], À0.35 ppm [s, 9H, Al(CH3)3]; 13C{1H} NMR (125 MHz, C6D6,
298 K): d=150.6, 148.8, 143.8, 142.0, (C3,3’, C5,5’), 136.0 (Cc), 123.3
(Cd), 108.0 (C4’), 107.0 (C4), 78.1 (CHa), 66.2 (CH), 43.4 (Cb), 40.4 (Cf),
37.7 (Cg), 31.9 (Ch), 30.8 (Ce), 26.0, 20.1 (Meg), 13.3 (Me3’), 12.3 (Me3),
10.2 (Me5’), 9.7 (Me5), À4.2, À5.2 [Al(CH3)2], À7.1 ppm [Al(CH3)3]; el-
emental analysis calcd (%) for C26H44Al2N4O: C 64.71, H 9.19, N
11.61; found: C 64.85, H 9.42, N 11.42.
1
À268C to give compound 9 as a white solid (1.32 g, 93%). H NMR
(500 MHz, CDCl3, 298 K): d=6.19 (s, 1H, CH),6.05 (s, 1H, H4’),5.76 (s,
1H, H4), 4.56 (s, 1H, CHa), 2.36 (s, 3H, Me3’), 2.16 (s, 3H, Me5’), 2.07
(s, 3H, Me5), 1.99 (s, 3H, Me3), 0.92 (s, 9H, -C(CH3)3], À0.53, À0.58
(s, 6H, Al(CH3)2], À1.19 ppm (s, 9H, Al(CH3)3]; 13C{1H} NMR
(125 MHz, CDCl3, 298 K): d=150.1, 148.9, 143.9, 136.1 (C3,3’, C5,5’),
108.7 (CH4’), 107.1 (CH4), 83.6 (CHa), 66.3 (CH), 36.0 [C(CH3)3], 27.6
[C(CH3)3], 13.7 (Me3), 13.1 (Me3’), 10.9 (Me5’), 10.8 (Me5), À5.0, À5.2
[Al(CH3)2], À5.3 ppm [Al(CH3)3]; elemental analysis calcd (%) for
C21H40Al2N4O: C 60.26, H 9.63, N 13.39; found: C 60.52, H 9.85, N
13.12.
Synthesis of [{AlEt2(k2-bpzmm)}(m-O){AlEt3}] (14): The synthesis of
14 was carried out in a manner identical to that for 9, using (R,R)-
bpzmm (1 g, 2.8 mmol) and AlEt3 (1m in hexane, 5.6 mL,
5.6 mmol). Compound 14 was isolated as a yellow solid (1.44 g,
93%). [a]2D5 = +11.6 (c=0.1, toluene); 1H NMR (500 MHz, C6D6,
Synthesis of [{AlEt2(k2-bpzbe)}(m-O){AlEt3}] (10): The synthesis of
10 was carried out in an identical manner to 9, using bpzbeH (1 g,
3.4 mmol) and AlEt3 (1m in hexane, 6.8 mL 6.8 mmol). Compound
1
10 was isolated as a white solid (1.51 g, 91%). H NMR (500 MHz,
Chem. Eur. J. 2015, 21, 9850 – 9862
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298 K): d=6.13 (s, 1H, Hd), 5.93 (s, 1H, CH), 5.71 (s, 1H, H4’), 5.21 (s,
1H, H4), 5.05 (s, 1H, CHa), 2.34 (m, 2H, Hh), 2.20 (bs, 1H, Hb), 2.18 (s,
3H, Me3’), 2.09 (s, 3H, Me3), 2.00 (m, 2H, He), 1.87 (s, 3H, Me5’),1.84
(m, 1H, Hf), 1.34 (s, 3H, Me5), 1.13, 0.28 (s, 6H, Meg), 1.60–1.40 (m,
15H, Al-CH2CH3), 1.10–0.40 [m, 4H, Al(CH2CH3)2], 0.23 ppm [m, 6H,
Al(CH2CH3)3]; 13C{1H} NMR (125 MHz, C6D6, 298 K): d=150.9, 148.9,
144.0, 141.6, (C3,3’, C5,5’), 136.3 (Cc), 123.0 (Cd), 107.9 (C4’), 106.8 (C4),
78.5 (CHa), 65.8 (CH), 43.8 (Cb), 40.4 (Cf), 37.7 (Cg), 31.7 (Ch), 30.7
(Ce), 26.0, 20.0 (Meg), 13.0 (Me3’), 12.5 (Me3), 10.1 (Me5’), 9.8 (Me5),
10.4 [Al(CH2CH3)3], 9.7 [Al(CH2CH3)2], 4.9, 3.1, [Al(CH2CH3)2], 0.4 ppm
[Al(CH2CH3)3]; elemental analysis calcd (%) for C31H54Al2N4O: C
67.36, H 9.85, N 10.14; found: C 67.52, H 10.02, N 9.97.
2902, 1781 cmÀ1; HRMS (ESI+): calcd m/z 125.0209 [M+Na]+;
found: 125.0215.
1,2-Butylene carbonate (2d): Obtained as a colourless liquid
1
(167.0 mg, 86%). H NMR (500 MHz, CDCl3, 298 K): d=4.5–4.7 (1H,
m, OCH), 4.49 (1H, t J 8.1 Hz, OCH2), 4.05 (1H, dd J 6.3, 5.3 Hz,
OCH2), 1.6–1.9 (2H, m, CH2), 1.00 ppm (3H, t J 7.1 Hz, CH3); 13C{1H}
NMR (125 MHz, CDCl3, 298 K): d=155.2, 78.1, 69.1, 27.0, 8.6 ppm. IR
(neat): n˜ =2938, 2917, 1801 cmÀ1; HRMS (ESI+): calcd m/z 139.0366
[M+Na]+; found: 139.0364.
1,2-Hexylene carbonate (2e): Obtained as a colourless liquid
1
(148.0 mg, 62%); H NMR (500 MHz, CDCl3, 298 K): d=4.68 (1H, qd
J 7.5, 5.4 Hz, OCH), 4.52 (1H, t J 8.1 Hz, OCH2), 4.06 (1H, dd J 8.4,
7.2 Hz, OCH2), 1.6–1.9 (2H, m, CH2), 1.2–1.6 (4H, m, 2”CH2),
0.91 ppm (3H, t J 7.1 Hz, CH3); 13C{1H} NMR (125 MHz, CDCl3,
298 K): d=154.8, 77.0, 69.2, 33.4, 26.3, 22.1, 13.5 ppm; IR (neat):
n˜ =2941, 2922, 2899, 1796 cmÀ1; HRMS (ESI+): calcd m/z 167.0679
[M+Na]+; found: 167.0682.
General procedure for catalyst screening at 1 bar pressure
Styrene oxide 1a (1.7 mmol), catalyst (83.0 mmol) and Bu4NBr
(26.7 mg, 83.0 mmol) were placed in a sample vial fitted with a mag-
netic stirrer bar and placed in a large conical flask. Cardice pellets
were added to the conical flask which was fitted with a rubber
stopper pierced by a deflated balloon. The reaction mixture was
stirred at 508C for 24 h, then the conversion of styrene oxide 1a to
styrene carbonate 2a was determined by analysis of a sample by
1H NMR spectroscopy.
1,2-Dodecylene carbonate (2 f): Obtained as a colourless liquid
1
(286.0 mg, 86%); H NMR (500 MHz, CDCl3, 298 K): d=4.6–4.7 (1H,
m, OCH), 4.48 (1H, t J 8.5 Hz, OCH2), 4.03 (1H, dd J 8.5, 7.5 Hz,
OCH2), 1.6–1.7 (2H, m, CH2), 1.2–1.4 (16H, m, 8”CH2), 0.84 ppm
(3H, t J 7.0 Hz, CH3); 13C{1H} NMR (125 MHz, CDCl3, 298 K): d=
155.1, 69.3, 33.8, 31.8, 29.5, 29.4, 29.3, 29.2, 29.1, 24.3, 22.6,
14.0 ppm. IR (neat): n˜ =2931, 2832, 1798 cmÀ1; HRMS (ESI+): calcd
m/z 251.1618 [M+Na]+; found: 251.1621.
General procedure for synthesis of cyclic carbonates at 10
bar pressure
4-Chlorostyrene carbonate (2g): Obtained as
a white solid.
(256.0 mg, 77%); m.p. 66–698C (lit.[22,23,28] 68–698C); 1H NMR
(500 MHz, CDCl3, 298 K): d=7.42 (2H, d J 8.5 Hz, ArH), 7.32 (2H, d J
8.5 Hz, ArH), 5.68 (1H, t J 7.9 Hz, OCH), 4.82 (1H, t J 8.4 Hz, OCH),
4.31 ppm (1H, t J 7.8 Hz, OCH2); 13C{1H} NMR (125 MHz, CDCl3,
298 K): d=154.6, 135.9, 134.4, 129.6, 127.3, 76.8, 71.1 ppm; IR
(neat): n˜ =2973, 2698, 2121, 2017, 1971, 1793 cmÀ1; HRMS (ESI+):
calcd m/z 220.9976 [M+Na]+; found: 220.9977.
An epoxide 1a–1k or 15a–g (1.7 mmol), catalyst 9 (28.7 mg,
83.0 mmol) and Bu4NBr (26.7 mg, 83.0 mmol) were placed in a multi-
point reactor with a magnetic stirrer bar. The reaction mixture was
stirred at 508C for 24 h. The conversion of epoxide to cyclic car-
bonate was then determined by analysis of a sample by 1H NMR
spectroscopy. The remaining sample was filtered through a plug of
silica, eluting with CH2Cl2 to remove the catalyst. The eluent was
evaporated in vacuo to give either the pure cyclic carbonate or
a mixture of cyclic carbonate and unreacted epoxide. In the latter
case, the mixture was purified by flash chromatography using a sol-
vent system of first hexane, then hexane/EtOAc (9:1), then hexane/
EtOAc (3:1), then EtOAc to give the pure cyclic carbonate. Cyclic
carbonates 2a–2k and 16a–g are all known compounds and the
spectroscopic data for samples prepared using catalyst 9 were con-
sistent with those reported in the literature.[22,23,28,33,36]
4-Bromostyrene carbonate (2h): Obtained as
a white solid.
(225.0 mg, 67%); m.p. 72–758C (lit.[22,23,28] 68–698C); 1H NMR
(500 MHz, CDCl3, 298 K): d=7.58 (2H, dd J 8.1, 2.0 Hz, ArH), 7.25
(2H, dd J 8.4, 1.8 Hz, ArH), 5.68 (1H, t J 7.9 Hz, OCH), 4.82 (1H, t J
8.4 Hz, OCH2), 4.31 ppm (1H, t J 7.8 Hz, OCH2); 13C{1H} NMR
(125 MHz, CDCl3, 298 K): d=154.5, 134.8, 132.5, 127.5, 123.9, 76.8,
70.9 ppm; IR (neat): n˜ =2951, 2522, 2161, 2017, 1981, 1801,
1771 cmÀ1; HRMS (ESI+): calcd m/z 264.9471 [M+Na]+; found:
264.9460.
Glycerol carbonate (2i): Obtained as a colourless liquid (50.0 mg,
61%); 1H NMR (500 MHz, [D6]DMSO, 298 K): d=5.22 (1H, t J 5.5,
OH), 4.7–4.8 (1H, m, OCH), 4.45 (1H, t J 8.3 Hz, CH2O), 4.24 (1H, dd
J 8.1, 5.8 Hz, CH2O), 3.62 (1H, ddd J 12.5, 5.5, 2.6 Hz, CH2OH),
3.46 ppm (1H, ddd J 12.6, 5.6, 3.3 Hz, CH2OH); 13C{1H} NMR
(125 MHz, [D6]DMSO, 298 K): d=155.7, 77.5, 66.4, 61.1 ppm; IR
(neat): n˜ =3382, 2901, 1799 cmÀ1; HRMS (ESI+): calcd m/z 141.0158
[M+Na]+; found: 141.0156.
Styrene carbonate (2a): Obtained as a white solid. (232.0 mg,
1
85%). M.p. 49–518C (lit.[22,23,28] 50–518C); H NMR (500 MHz, CDCl3,
298 K): d=7.3–7.5 (5H, m, ArH), 5.70 (1H, t J 8.0 Hz, PhCHO), 4.82
(1H, t J 8.4 Hz, OCH2), 4.36 ppm (1H, t J 8.6 Hz, OCH2); 13C{1H} NMR
(125 MHz, CDCl3, 298 K): d=154.8, 135.8, 129.7, 129.2, 125.8, 76.7,
71.2 ppm; IR (neat): n˜ =3060, 3029, 2961, 2903, 1791, 1599 cmÀ1
HRMS (ESI+): calcd m/z 187.0366 [M+Na]+; found: 187.0369.
;
1,2-Decylene carbonate (2b): Obtained as a colourless liquid
1
3-Chloropropylene carbonate (2j): Obtained as a colourless liquid.
(286.0 mg, 86%); H NMR (500 MHz, CDCl3, 298 K): d=4.8–4.6 (1H,
1
(185.0 mg, 82%); H NMR (500 MHz, CDCl3, 298 K): d=4.98 (1H, m,
m, OCH), 4.50 (1H, dd J 8.4, 7.8 Hz, OCH2), 4.04 (1H, dd J 8.4,
7.2 Hz, OCH2), 1.6–1.9 (2H, m, CH2), 1.1–1.6 (12H, m, 6”CH2),
0.86 ppm (3H, t J 6.8 Hz, CH3); 13C{1H} NMR (125 MHz, CDCl3,
298 K): d=155.2, 77.1, 69.5, 34.0, 31.9, 29.4, 29.2, 29.1, 24.5, 22.7,
14.2 ppm; IR (neat): n˜ =2916, 2851, 1800 cmÀ1; HRMS (ESI+): calcd
m/z 201.1485 [M+H]+; found: 201.1493.
OCH), 4.59 (1H, t J 8.5 Hz, CH2Cl), 4.41 (1H, dd J 9.0, 8.7 Hz, CH2Cl),
3.79 (1H, dd J 12.0, 6.5 Hz, CH2O), 3.73 ppm (1H, dd J 12.5, 4.0 Hz,
CH2O); 13C{1H} NMR (125 MHz, CDCl3, 298 K): d=154.2, 74.3, 67.0,
43.7 ppm; IR (neat): n˜ =3451, 1971, 1803 cmÀ1; HRMS (ESI+): calcd
m/z 158.9819 [M+Na]+; found: 158.9815.
Propylene carbonate (2c): Obtained as
a
colourless liquid
3-Phenoxyproplylene carbonate (2k): Obtained as a white solid.
(305.0 mg, 94%); m.p. 94–978C (lit.[22,23,28] 94–958C); 1H NMR
(500 MHz, CDCl3, 298 K): d=7.2–7.5 (2H, m, 2”ArH), 7.04 (1H, t J
7.5 Hz, ArH), 6.9–7.0 (2H, m, 2”ArH), 5.0–5.1 (1H, m, OCH), 4.5–4.7
(2H, m, OCH2), 4.26 (1H, dd J 10.6, 4.2 Hz, CH2OPh), 4.16 ppm (1H,
1
(135.0 mg, 79%); H NMR (500 MHz, CDCl3, 298 K): d=4.8–4.9 (1H,
m, OCH), 4.57 (1H, t J 8.3 Hz, OCH2), 4.04 (1H, dd J 8.3, 7.4 Hz,
OCH2), 1.50 ppm (3H, d J 6.3 Hz, CH3); 13C{1H} NMR (125 MHz,
CDCl3, 298 K): d=154.7, 73.2, 70.5, 19.2 ppm; IR (neat): n˜ =2961,
Chem. Eur. J. 2015, 21, 9850 – 9862
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
dd J 10.6, 3.6 Hz, CH2OPh); 13C{1H} NMR (125 MHz, CDCl3, 298 K):
d=158.1, 154.5, 129.7, 122.0, 114.6, 74.1, 66.9, 66.2 ppm; IR (neat):
n˜ = 3429, 3061, 2989, 2924, 2328, 1791 cmÀ1; HRMS (ESI+): calcd
m/z 217.0471 [M+Na]+; found: 217.0482.
using known ratios of 1a and 1b and used to determine the rela-
tive concentration of styrene oxide 1a and styrene carbonate 2a.
X-ray crystallographic structure determination
cis-1,2-Cyclohexene carbonate (16a): Obtained as a white solid.
(110 mg, 47%); m.p. 35–378C (lit.[28,33,36] 34–358C); 1H NMR
(400 MHz, CDCl3, 298 K): d=4.6–4.7 (m, 2H, CHO), 1.8–2.0 (4H, m,
2xCH2CHO), 1.5–1.7 (2H, m, CH2), 1.3–1.4 ppm (2H, m, CH2); 13C{1H}
NMR (100 MHz, CDCl3, 298 K): d=155.2, 75.7, 26.7, 19.2 ppm; IR
Colourless and yellow crystals (for 12 and 13, respectively) were
obtained by diffusion of toluene and were mounted on a glass
fibre and used for data collection on a Bruker D8 Venture with
Photon detector equipped with graphite monochromated MoKa ra-
diation (l=0.71073 Š). Lorentz-polarisation and empirical absorp-
tion corrections were applied. The structures were solved by direct
methods and refined with full-matrix least-squares calculations on
F2 using the program SHELXS97.[37] Anisotropic temperature factors
were assigned to all atoms except for hydrogen atoms, which are
riding their parent atoms with an isotropic temperature factor arbi-
trarily chosen as 1.2 times that of the respective parent.[38] Several
crystals of 13 were measured and the structure was solved from
the best data we were able to collect. Final R(F), wR(F2) and good-
ness of fit agreement factors, details on the data collection and
analysis can be found in Table S1. Selected bond lengths and
angles are given in Tables S2 and S3 (see the Supporting Informa-
tion). CCDC 1045942 (12) and 1045943 (13) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(neat): n˜ = 2933, 2861, 1784 cmÀ1
165.0522 [M+Na]+; found: 165.0522.
; HRMS (ESI+): calcd m/z
cis-1,2-Cyclopentene carbonate (16b): Obtained as a white solid.
1
(210 mg, 99%); m.p. 30–338C (lit.[33,36] 29–308C); H NMR (400 MHz,
CDCl3, 298 K): d=5.00–5.20 (m, 2H, CHO), 2.00–2.20 (2H, m, CH2),
1.50–1.90 ppm (4H, m, CH2); 13C{1H} NMR (100 MHz, CDCl3, 298 K):
d=155.6, 81.9, 32.3, 21.6; IR (neat): n˜ =2967, 2871, 1789 cmÀ1
HRMS (ESI+): calcd m/z 151.0366 [M+Na]+; found: 151.0360.
;
cis-2,3-Butene carbonate (16c):[33,36] Obtained colourless liquid in
a 94:6 mixture of cis- and trans-isomers (153 mg, 79%); 1H NMR
(400 MHz, CDCl3, 298 K): d=4.82 (m, 2H, CH), 1.35 ppm (6, d J
6.2 Hz, CH3); 13C{1H} NMR (100 MHz, CDCl3, 298 K): d=154.7, 76.1,
14.5 ppm; IR (neat): n˜ = 2960, 2899, 1787 cmÀ1; HRMS (ESI+): calcd
m/z 139.0366 [M+Na]+; found: 139.0365.
trans-2,3-Butene carbonate (16d): Obtained as a white solid
(121 mg, 63%); m.p. 30–328C (lit).[33,36] 29–308C); 1H NMR
(400 MHz, CDCl3, 298 K): d=4.32 (m, 2H, CH), 1.44 ppm (6, d J
5.9 Hz, CH3); 13C{1H} NMR (100 MHz, CDCl3, 298 K): d=154.6, 80.0,
18.5 ppm; IR (neat): n˜ =2955, 2871, 1776 cmÀ1; HRMS (ESI+): calcd
m/z 139.0366 [M+Na]+; found: 139.0369.
Acknowledgements
The authors gratefully acknowledge financial support from the
European Union Seventh Framework Programme FP7-NMP-
2012 under grant agreement number 309497, the Ministerio
de Economía y Competitividad (MINECO), Spain (Grant No.
CTQ2011-22578) and the Junta de Comunidades de Castilla-La
Mancha, Spain (Grant No. PEII-2014-013A).
trans-1,2-Diphenylethylene carbonate (16e): Obtained as a white
1
solid. (260 mg, 65%); m.p. 109–1108C (lit.[33,36] 110–111 8C); H NMR
(400 MHz, CDCl3, 298 K): d=7.35–7.45 (m, 6H, ArH), 7.25–7.35 (m,
4H, ArH), 5.42 ppm (s, 2H, CH); 13C{1H} NMR (100 MHz, CDCl3,
298 K): d=154.4, 135.1, 129.8, 129.3, 126.1, 85.0, 80.8, 18.4 ppm; IR
(neat): n˜ =3051, 2977, 1812, 1458 cmÀ1; HRMS (ESI+): calcd m/z
263.0679 [M+Na]+; found: 263.0676.
trans-1-Phenyl-2-methylethylene carbonate (16 f): Obtained as
a white solid. (210.0 mg, 71%); m.p. 112–1158C (lit.[33,36] 110–
111 8C); 1H NMR (400 MHz, CDCl3, 298 K): d=7.30–7.50 (5H, m,
ArH), 5.12 (1H, d, J 8.0 Hz, CH), 4.59 (1H, m, CH), 1.55 ppm (3H, d,
J 8.0 Hz, CH3); 13C{1H} NMR (100 MHz, CDCl3, 298 K): d=154.4,
135.1, 129.8, 129.3, 126.1, 85.0, 80.8, 18.4 ppm; IR (neat): n˜ =3010,
Keywords: aluminium complexes · carbon dioxide · catalysis ·
cyclic carbonates · epoxides
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Received: February 26, 2015
Published online on June 1, 2015
Chem. Eur. J. 2015, 21, 9850 – 9862
www.chemeurj.org
9862
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim