A powerful aluminum catalyst for the synthesis of highly functional organic carbonates

Article abstract of DOI:10.1021/ja311053h

An aluminum complex based on an amino triphenolate ligand scaffold shows unprecedented high activity (initial TOFs up to 36 000 h^-1), broad substrate scope, and functional group tolerance in the formation of highly functional organic carbonates prepared from epoxides and CO₂. The developed catalytic protocol is further characterized by low catalyst loadings and relative mild reaction conditions using a cheap, abundant, and nontoxic metal.

Full text of DOI:10.1021/ja311053h

Communication
pubs.acs.org/JACS
A Powerful Aluminum Catalyst for the Synthesis of Highly Functional
Organic Carbonates
Christopher J. Whiteoak,† Nicola Kielland,† Victor Laserna,† Eduardo C. Escudero-Adan,† Eddy Martin,†
́
and Arjan W. Kleij*,†,‡
†Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
‡Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
S
* Supporting Information
bifunctional catalyst systems for this reaction have been
ABSTRACT: An aluminum complex based on an amino
triphenolate ligand scaffold shows unprecedented high
activity (initial TOFs up to 36 000 h−1), broad substrate
scope, and functional group tolerance in the formation of
highly functional organic carbonates prepared from
epoxides and CO2. The developed catalytic protocol is
further characterized by low catalyst loadings and relative
mild reaction conditions using a cheap, abundant, and
nontoxic metal.
reported combining a Lewis acid and a nucleophile which is
required for the ring opening of the epoxide.4
To date, most of the reported catalyst systems require
relatively high catalyst loadings, and as a result (initial) turnover
frequencies (TOFs) are rarely >200 h−1 even at elevated
temperatures (>100 °C), thus limiting their potential for
industrial application. In this context, recent work reported by
Sakai has shown important progress in the field communicating
a bifunctional Mg(porphyrin) catalyst having TOFs as high as
12 000 h−1.5 Though this is indeed a significant improvement,
the catalyst system was only reported to be effective for the
conversion of terminal epoxides (Scheme 1; R′ = H).
Previously, Minakata reported on a facile route toward more
functional iodinated (unsaturated) cyclic carbonates elegantly
making use of trapping a carbonic acid derivative by a
hypoiodite source under mild conditions.6 Unfortunately, this
procedure does not allow for the introduction of other
functional groups and cannot address the synthesis of sterically
encumbered disubstituted carbonates (Scheme 1, R and R′)
and oxetanes. Thus, the development of a catalyst system able
to combine unusual reactivity (high TOF) with a broad
substrate scope and high functional group tolerance to date still
represents a challenging task.
arbon dioxide (CO2) can be considered as an abundant,
C
low cost, and renewable carbon source, but its use as such
is limited as a result of its high stability and therefore low
reactivity. Synthetic procedures which utilize CO2 as a carbon
feedstock are currently generating great interest within the
scientific community,1 as these may offer viable alternative
routes toward various organic structures that are currently
derived from fossil fuel based resources.2 The catalysis of the
atom-efficient (cyclo)addition of CO2 to epoxides which yields
useful cyclic or polycarbonate products (Scheme 1)3 is gaining
momentum, as a variety of industrial targets incorporating these
carbonates have been developed.1a−d Many different binary/
The Lewis acidity of aluminum has already been successfully
exploited to provide catalyst systems for the cycloaddition of
CO2 to epoxides.7 Of these reports, the most developed
aluminum based catalysts are the bimetallic Al(salen)
complexes reported by North and co-workers which provide
a system active under ambient conditions, although due to the
catalyst loadings required high TOFs were not achieved and
only terminal epoxides proved to be suitable substrates.8 More
recently, Darensbourg also studied a bifunctional Al(salen)
framework in the context of organic carbonate formation.9
Motivated by these previous works and the fact that Al
represents a nontoxic, earth-abundant metal, we set out to
develop a catalyst system that shows the required features
needed for the efficient conversion of a wide range of highly
functional/substituted epoxides and oxetanes.
Scheme 1. Synthesis of Organic Carbonates from Epoxides
and CO2
a
We reasoned that the use of an amino triphenolate ligand
showing strong inductive effects (see Scheme 1) would favor
the high oxophilic nature of the complexed metal center and
a
Bottom: synthesis of Al catalyst 1 from a hexachlorinated amino
trisphenol and its X-ray structure (Al center in yellow and one axial
THF ligand is also present).
Received: November 9, 2012
Published: January 10, 2013
© 2013 American Chemical Society
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a
create a new powerful catalyst with high potential for the
activation of epoxides and oxetanes. The envisioned Al
complex10 was easily synthesized in high yield (80%) and
purity by treatment of the known ligand, H3L,11 and AlMe3 in
THF. The resultant complex was initially investigated as a
catalyst for the cycloaddition of CO2 to 1,2-epoxyhexane in
combination with n-tetrabutylammonium iodide (TBAI) as a
cocatalyst.
Scheme 2. Scope in Terminal Epoxides
At a catalyst loading of 0.05 mol % and a TBAI loading of
0.25 mol % we were able to achieve quantitative conversion of
this substrate at 90 °C within 2 h (Table 1, entry 2, TOF = 960
Table 1. Synthesis of Cyclic Carbonate 2a from CO2 and 1,2-
a
Epoxyhexane
bc
,
d
entry 1 (mol %) cocat. (mol %) t (h) conv. (%) TOF (h−1
)
1
2
3
4
5
6
7
8
9
−
0.05
0.25
0.25
0.05
0.25
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
2
17
96
13
46
38
33
6
−
960
2
−
0.05
2
−
460
e
2
0.0025
0.0010
0.0005
0.0005
0.0005
0.0005
0.0005
0.0005
0.0005
2
7600
16 500
24 000
26 000
24 000
24 000
15 000
29 000
36 000
2
0.5
1
13
18
24
15
29
36
1.5
2
f
10
11
12
13
e
2
g
h
2
2
a
Conditions: 1,2-epoxyhexane (1.00 g, 10.0 mmol), Al complex 1
(quantity indicated), TBAI (quantity indicated), 90 °C, 10 bar initial
a
Cyclic (bis)carbonates 2b−2m obtained from their corresponding
epoxides using 1/TBAI as catalyst system. General conditions: neat,
1.00 g of substrate, molar ratio 1/TBAI = 1:5, 70 °C, 18 h, 10 bar
initial CO2 pressure, 30 mL autoclave. The observed selectivity for the
b
1
CO2 pressure, 30 mL autoclave. Determined by H NMR of the
c
crude reaction mixture. Selectivity for the cyclic carbonate product
d
e
>99%. Initial TOF/h/1 during the first 2 h. Using an Al(salen)2O
f
catalyst at the same [Al] loading. TON = 112 000 after 18 h; average
of three runs. Using PPN-I. Using PPN-Br.
1
cyclic carbonate product was >99% in all examples determined by H
g
h
NMR in CDCl3. Note that in the synthesis of products 2f−2k 0.5 mL
of MEK (co)solvent was used.13
h−1). We then investigated the use of reduced amounts of Al
complex 1 (entries 5−10) and were delighted to find that
considerable conversions could still be achieved resulting in
very high initial TOFs of up to 24 000 h−1 per aluminum metal.
To assess whether this activity was maintained throughout the
reaction, the activities were also measured after 0.5, 1, and 1.5 h
(entries 7−9) showing that the TOF reported after 2 h remains
virtually unchanged in the initial stage of the reaction.
Importantly, this indicates that the catalyst is stable under
these conditions; under these same conditions the TON
reached 112 000 after 18 h.12 When compared to the TBAI
cocatalyst alone (Table 1, entries 1 and 3) significantly higher
conversions were observed in the presence of Al complex 1,
with TOFs as high as 36 000 h−1 when TBAI was replaced by
PPN-I or PPN-Br as the cocatalyst (PPN = bis-
(triphenylphosphine)iminium, entries 12 and 13). This high
reactivity of the binary catalyst 1/TBAI was also observed with
other substrates (Scheme 2) giving initial TOFs of 27 000 h−1
(2h), 24 000 h−1 (2d), and 17 000 h−1 (2f).
newly developed Al-complex 1 shows higher initial TOFs
(entries 2 versus 4, and 10 versus 11).
Having established that Al complex 1 is a highly active
catalyst for organic carbonate formation (Table 1), we then
focused on further application of its promising reactivity
potential and first investigated the scope of the cycloaddition of
CO2 to a variety of highly functionalized terminal epoxides
providing organic carbonates 2b−2m (Scheme 2). In some
cases it was necessary to use methylethylketone (MEK) as a
(co)solvent during the conversion to prevent problems
associated with either high viscosity and/or the solid nature
of the reaction mixture. In general, low amounts of catalyst 1
(0.05−0.10 mol %) and cocatalyst (0.25−0.50 mol %) were
already effective under comparatively mild conditions (70 °C,
pCO2 = 10 bar).14 All of the substrates studied were
0
conveniently converted into the corresponding (bis)carbonates
2b−2m with high selectivity (>99%), and the products were
isolated in good to excellent yields, indicating the high
versatility of this catalyst system. Interestingly, the catalyst
system 1/TBAI not only tolerates functionalities such as alkene,
alkyne, alkyl halide, alcohol, ether, and substituted aryl groups
When compared with a μ-oxo-Al(salen) dinuclear complex
which may be regarded as one of the benchmark systems,8 the
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(2b−2g) but also allows for conversion of even more
challenging substrates that contain fragments having potentially
competing O- or N-donor atoms that may be able to
coordinate to the Al metal center and compromise catalytic
conversion. Carbonates 2h−2k incorporating morpholine,
ester, amide, and sulfonate ester groups could also be obtained
in reasonable to good isolated yields (40−84%); in some cases
effects arising from isolation through column purification
resulted in lower isolated yields. Bis-epoxide substrates were
also cleanly converted into their bis-carbonate products 2l and
2m in good yields (73−94%). These combined results clearly
demonstrate the catalytic potential of Al-complex 1 showing
the amplified scope for the conversion of highly functional
terminal epoxides.
diffraction studies further supported the proposed configu-
rations (see Supporting Information (SI)). It should be noted
that there are very few reports of the synthesis of 2q16 through
catalytic cycloaddition of CO2 to stilbene oxide, marking the
current protocol as a rare example of a catalyst able to mediate
this conversion affording a high yield of 2q.
Oxetanes are the closest homologues to epoxides but have
historically received far less attention. Their lower ring strain
results in lower conversion rates, making these compounds
significantly more challenging substrates, as ring opening is
usually considered to be the rate-limiting step. There have been
limited reports of vanadium, aluminum, chromium, and iron
catalysts capable of catalyzing the cycloaddition of CO2 to
oxetanes.17 Conversion under various reaction conditions of
two different oxetanes leading to the six-membered carbonates
1s and 1t are shown in Table S3 and Scheme S1 (SI) where
again MEK was used as a (co)solvent due to the solid nature of
the products, thus enabling optimization of these conversions
(best results in Scheme 3).
To further exemplify and challenge the catalytic potential of
Al-complex 1, internal epoxides with increasing steric bulk and
oxetanes were then chosen as reaction partners (Scheme 3,
a
Scheme 3. Scope for Internal Epoxides/Oxetanes
It was possible to obtain almost quantitative and selective
conversion to carbonate 2s (95% yield) after only 4 h at 70 °C
(Table S3, entry 3). Longer reaction times lead to a lower
selectivity for 2s, as the formation of increasing amounts of
polycarbonates were observed in the product mixture (Table
S3, entry 4). At a lower reaction temperature (50 °C),
prolonged reaction times however did not lead to any
detectable polymer formation (Table S3) pointing to a
temperature onset for polymer formation starting from 2s.18
It is known that cyclic carbonates derived from oxetanes are the
kinetic products and are susceptible to polycarbonate formation
unlike their epoxide-derived homologues. Carbonate 2t is
significantly less accessible as a result of the increased steric
effects posed by the 3,3′-positioned substituents, and therefore
increased reaction times were required. Though this led to a
higher yield of 2t (Scheme 3; 26%), much lower selectivities
(54%) were observed due to polycarbonate/ether formation
(see Table S3) as a result of ring-opening polymerization and
partial decarboxylation.19 Also in this case, by lowering the
reaction temperature to 50 °C virtually complete selectivity for
2t was noted albeit at the expense of the substrate conversion
(19%) and isolated yield (12%).
a
Cyclic carbonates 2n−2t obtained using 1/TBAB as catalyst system.
In conclusion, we have presented an easily accessible and
new amino triphenolate complex based on a cheap, earth-
abundant, and nontoxic metal (i.e., Al) that has been
demonstrated to be a highly active and versatile catalyst for
organic carbonate formation. Remarkably initial TOFs as high
as 36 000 h−1 during catalysis of the cycloaddition of CO2 to
epoxides were obtained with TONs exceeding 100 000. To the
best of our knowledge, these TOFs are among the highest
reported for a homogeneous catalyst system in the context of
cyclic carbonate formation. The catalyst system also displays a
wide substrate scope and functionality tolerance. It thus allows
for the preparation of highly functional cyclic carbonate
products derived from terminal, double, and internal epoxides,
and oxetanes. We are currently investigating the catalytic
potential of Al-complex 1 in the formation of organic products
derived from CO2 and other suitable substrates.
General conditions: 1.00 g of substrate, 1/TBAB = 1:5, 70 °C, 18 h, 10
bar initial CO2 pressure, 30 mL autoclave. Selectivity for the cyclic
carbonate product >99% in all examples (unless stated otherwise)
determined by 1H NMR in CDCl3. Further details: (i) for 2o−2q the
reaction time was 42 h and temperature was 90 °C using 0.5 mL of
MEK13 as (co)solvent; (ii) for 2r the ratio 1/TBAB was 1:10; (iii) for
2s and 2t TBAI was used. For 2s the reaction time was 4 h, and for 2t
it was 66 h; (iv) the selectivity toward the trans product in the case of
1
2o−2q was >99% as determined by H NMR in CDCl3.
synthesis of 2n−2t). These internal epoxides (except for 2r)
and oxetanes are widely considered to be very difficult
substrates, and as a result much less success has been achieved
with these substrates.15 The use of 1/TBAB (TBAB = n-
tetrabutylammonium bromide) as a catalyst system turned out
to be most beneficial for the conversion of these internal
epoxides into their cyclic carbonates using low loadings of 1
(0.1−0.5 mol %), whereas the use of TBAI gave poorer results.
To obtain reasonable yields of products 2p and 2q, a reaction
temperature of 90 °C was required. The trans disposition of
both substituents in carbonates 2o−2q was easily determined
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and full analytical data, and copies of
relevant NMR and IR spectra for known and new compounds.
1
by H NMR spectroscopy; for 2o and 2q single-crystal X-ray
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(12) The average TOF was further investigated up to a reaction time
of 6 h showing a slight decrease to 12 700 h−1; see the SI (Table S1).
Additionally, the influence of air and water was also studied in the
conversion of substrates 2b, 2d, and 2f′; only a small effect of added
water was noted on the product yield observed for the conversion of
substrate 2d; see the SI (Table S2). The combined data show that the
used catalyst system is highly robust and, furthermore, can be handled
in air.
(13) MEK was used when solid substrates were employed; this
solvent is known to be among the best solvents for CO2 dissolution
and thus for an optimal conversion; see for instance ref 4b.
(14) For the substrate scope we used NBu4X (X = Br, I) as
cocatalysts, as these are cheaper, more accessible, and effective enough
compared with the PPN salts.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
akleij@iciq.es
■
Author Contributions
C.J.W. and N.K. contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
(15) Successful synthetic approaches towards such carbonates
include: (a) Sanders, D. P.; Fukushima, K.; Coady, D. J.; Nelson, A.;
Fujiwara, M.; Yasumoto, M.; Hedrick, J. L. J. Am. Chem. Soc. 2010,
132, 14724. (b) Li, C.-Y.; Wu, C.-R.; Liu, Y.-C.; Ko, B.-T. Chem.
Commun. 2012, 48, 9628.
We thank ICIQ, ICREA, and the Spanish Ministerio de
́
Economıa y Competitividad (MINECO) through Project
́
́
CTQ-2011-27385 for support and Dr. Noemı Cabello, Sofıa
́
Arnal, and Vanessa Martınez for the MS analyses.
(16) Recently Gabriele and co-workers reported a Pd-catalyzed route
towards multiply substituted cyclic carbonates by an oxidative
carbonylation of 1,2- and 1,3-diols. The yield for carbonate 2q in
this case was only 39%. See: Gabriele, B.; Mancuso, R.; Salerno, G.;
Veltri, L.; Costa, M.; Dibenedetto, A. ChemSusChem 2011, 4, 1778.
(17) (a) Darensbourg, D. J.; Horn, A., Jr; Moncada, A. I. Green Chem.
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Belmonte, M.; Benet-Buchholz, J.; Kleij, A. W. Adv. Synth. Catal. 2012,
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(19) Kricheldorf, H. R.; Jenssen, J. J. Macromol. Sci., Chem. 1989, A26,
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