Azaphosphatranes as structurally tunable organocatalysts for carbonate synthesis from CO2 and epoxides

Article abstract of DOI:10.1021/ja402053d

Three azaphosphatranes were used as organocatalysts for the synthesis of cyclic carbonates from CO₂ and epoxides. They proved to be efficient single-component, metal-free catalysts for the reaction of simple or activated epoxides (styrene oxide, epichlorohydrin, glycidyl methyl ether) with CO ₂ under mild reaction conditions, displaying high stability and productivity over several days of reaction. Substitution patterns on the catalyst were shown to affect activity and stability. Kinetic analysis allowed investigation of the reaction mechanism.

Full text of DOI:10.1021/ja402053d

Communication
pubs.acs.org/JACS
Azaphosphatranes as Structurally Tunable Organocatalysts for
Carbonate Synthesis from CO2 and Epoxides
Bastien Chatelet,† Lionel Joucla,† Jean-Pierre Dutasta,† Alexandre Martinez,*,† Kai C. Szeto,‡
and Veronique Dufaud*,‡
́
†
́
Ecole Normale Super
‡Universite
Lyon, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne Cedex, France
́
ieure de Lyon, CNRS, Universite
́
de Lyon, Laboratoire de Chimie, 46 allee
́
d’Italie, 69364 Lyon, France
Claude Bernard Lyon1, CPE
́
de Lyon, Laboratoire de Chimie, Catalyse, Polymer
̀
e, Procedes (C2P2), CNRS, Universite
́
́
́
S
* Supporting Information
very attractive because CO2 can be incorporated into epoxides
with no formation of side products.15
ABSTRACT: Three azaphosphatranes were used as
organocatalysts for the synthesis of cyclic carbonates
from CO2 and epoxides. They proved to be efficient single-
component, metal-free catalysts for the reaction of simple
or activated epoxides (styrene oxide, epichlorohydrin,
glycidyl methyl ether) with CO2 under mild reaction
conditions, displaying high stability and productivity over
several days of reaction. Substitution patterns on the
catalyst were shown to affect activity and stability. Kinetic
analysis allowed investigation of the reaction mechanism.
In the past decades, numerous catalysts have been developed
for this purpose. Quaternary ammonium and phosphonium
salts or alkali metal halides (e.g., KI) are typically used as
homogeneous catalysts. Other homogeneous catalytic systems
have also been reported to be effective in the production of
cyclic carbonates.16−18 Recent examples include
ZnBr2(pyridine)2,19 salen complexes such as Cr-salen-Cl/
base,20 ionic liquids such as imidazolium salts,21 and
polyfluoroalkyl phosphonium iodides.22 However, in most of
these cases, additives and/or cocatalysts as well as organic
solvents are often needed. In addition, unsatisfactory activities,
harsh reaction conditions, and the presence of toxic metals are
still drawbacks that need to be overcome. Hence, the design of
novel, metal-free catalysts with enhanced properties still
remains to be developed toward effective CO2 conversion.
The azaphosphatranes, which are the acidic counterparts of
the well-known proazaphosphatrane superbases (also named
Verkade’s superbases)23 possess several interesting features as
potential catalysts for this reaction related to their charge, the
relative ease and modular character of their syntheses, and their
robustness (Chart 1). They contain a phosphorus atom, which
he development of catalytic processes for utilization of
T
carbon dioxide (CO2), a global-warming gas, is of growing
interest for carbon management and sustainable develop-
ment.1,2 Carbon dioxide is a potentially inexpensive and
abundant renewable C1 building block and is recognized to
be environmentally benign (nontoxic, noncorrosive and
nonflammable). In this regard, the efficient transformation of
CO2 under mild conditions into useful chemical compounds is
very attractive from both an industrial and an academic
viewpoint.3−6 CO2 is thermodynamically very stable, and its
activation requires the use of high-energy substrates or
electroreductive processes;7,8 however, the synthesis of low-
energy target molecules such as organic carbonates represents a
promising alternative to overcome the thermodynamics. The
cycloaddition of CO2 to epoxides to produce five-membered
cyclic carbonates (Scheme 1) is one of the few industrial
synthetic processes that efficiently utilizes CO2 as a raw
material.9−11
Chart 1. Structure of the Azaphosphatranes 2a−c
is stabilized through proton-induced chelation by the tertiary
nitrogen, rendering the phosphorus atom five-coordinate.24
The presence of the free anion is helpful in a large number of
catalytic processes,25 but it is the structure of the cation, with
the very stable partially charged proton ensconced in a large
tunable lipophilic pocket along with the presence of several
tertiary nitrogen atoms within the molecule, that makes this
class of catalysts very interesting, particularly for the CO2
activation reaction. A few synthetic applications of these
molecules have been reported by Verkade’s group.26,27
Cyclic carbonates are widely used as electrolyte components
in lithium batteries, polar aprotic solvents, and intermediates in
the production of pharmaceuticals and fine chemicals.12−14 In
terms of “green chemistry” and “atom economy” this process is
Scheme 1. Synthesis of Cyclic Carbonates from Epoxides
and Carbon Dioxide
Received: February 26, 2013
© XXXX American Chemical Society
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Recently, we reported the first use of these protonated
phosphonium moieties as highly active and robust phase
transfer catalysts.28
resonance at −12 and 2.1 ppm, respectively (Figures S5 and S6
in SI), supporting our assumption which connects their stability
to the bulkiness around the N−P−H bonds (vide infra).
A second series of reactions was carried out with 2b and 2c
under solvent-free conditions and a much higher substrate-to-
catalyst ratio (1000:1) for a much longer period. The progress
of each reaction was monitored by 1H NMR analysis of aliquots
taken every 24 h. After each 24-h period, an appropriate
amount of SO was added to bring the substrate-to-catalyst ratio
back up to 1000. A marked difference between 2b and 2c was
clearly evidenced (Figure 2). The p-methoxybenzyl-substituted
In this contribution, we report on their activity as
organocatalysts for cyclic carbonate synthesis from CO2 and
epoxides. We show that they are indeed efficient single-
component, metal-free catalysts for this reaction, which display
high stability and productivity over several days of reaction. We
considered three azaphosphatranes with various electronic and
steric properties, with Cl− as counteranion, to explore the
influence of the structural features on the catalytic behavior.
Methyl (2a), p-methoxybenzyl (2b), and neo-pentyl (2c)
azaphosphatrane derivatives (Chart 1) were prepared in a two-
step synthesis, which involved first a triple reductive alkylation
of the appropriate aldehyde on the tris-(2-aminoethyl)amine
followed by reaction with PCl(NMe2)2 (Scheme S1 in
Supporting Information [SI]).29,30 The effect of the catalyst
structure on the catalytic performance was first investigated in
detail using the coupling of styrene oxide (SO) with CO2 to
produce styrene carbonate (SC) as a model reaction, and we
especially focused our attention on their initial activity. Given
the long-term goal of producing low-pressure CO2 activation
catalysts, we initially chose to work under pressures of CO2
close to atmospheric pressure with a catalyst loading of 1 mol
%. Finally, to achieve significant and differentiable conversions
at this low CO2 pressure in useful time frames, the reaction was
run at 100 °C.
Figure 2. Effect of the azaphosphatrane substitution on catalyst
stability. Conditions: SO (50.0 mmol), 2b or 2c (0.05 mmol), CO2 (1
atm), 100 °C. TON were determined by 1H NMR spectroscopy using
2,4-dibromomesitylene (2.0 mmol) as an internal standard.
The reaction of SO with CO2 catalyzed by the methyl-
substituted azaphosphatrane 2a led to low carbonate yield
(11%), and the conversion stopped after a few hours (Figure 1,
azaphosphatrane 2b was initially active, but this activity
decreased over the four-day test. The neo-pentyl-substituted
catalyst 2c showed an almost constant reactivity through the
four-day experiment. After this period the crude reaction
mixtures were analyzed by 31P NMR spectroscopy. With 2b
several downfield signals, in addition to that of 2b, were
observed, whereas the spectrum with catalyst 2c remained very
clean (Figures S7 and S8 in SI).
Again, higher steric hindrance around the N−P−H core
correlates with catalyst stability, after CO2 insertion into the P−
N bond. Further experiments showed that at lower temperature
(80 °C) 2b can also perform as a stable and productive catalyst
(Figure 3). In order to produce significant turnovers at 80 °C,
more reactive substrates (i.e., epichlorohydrin and glycidyl
Figure 1. Initial kinetic profile of the catalytic coupling reactions of SO
with CO2. Conditions: SO (5.0 mmol), 2a−c (0.05 mmol), CO2 (1
1
atm), toluene (1 mL), 100 °C. Yields were determined by H NMR
using 2,4-dibromomesitylene (1.0 mmol) as an internal standard.
curve 2a). The 31P NMR spectra analysis of the crude mixture
(Figures S1 and S4 in SI) showed, in addition to the catalyst
signal at −11.0 ppm, the presence of two downfield signals at
7.3 and 5.6 ppm, suggesting some catalyst degradation which
may originate from the insertion of CO2 into the P−N bond, as
previously reported for aminophosphane derivatives31a and also
observed with CS2.31b,c
This result prompted us to investigate more sterically
congested azaphosphatranes, hypothesizing that steric protec-
tion of the P−H site would improve catalyst stability under our
reaction conditions. As expected, bulky p-methoxybenzyl (2b)
and neo-pentyl (2c) azaphosphatranes gave satisfactory results
with 50% yields after 7 h (Figure 1, curves b and c). The 31P
NMR spectra analyses of the crude mixtures issued from the
runs with 2b and 2c each showed only the initial catalyst’s P−H
Figure 3. Activity of p-methoxybenzyl azaphosphatrane (2b) at 80 °C.
Conditions: epoxide (50.0 mmol), 2b (0.05 mmol), CO2 (1 atm), 80
°C. TON were determined by 1H NMR spectroscopy using 2,4-
dibromomesitylene (2.0 mmol) as an internal standard.
B
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Journal of the American Chemical Society
Communication
Scheme 2. Proposed Mechanism for Cyclic Carbonates Synthesis Catalyzed by Azaphosphatrane Derivatives
methyl ether) were used. In this manner, 2b produced high
TON in a very steady fashion over several days (Figure 3),
arriving at more than 1000 turnovers for epichlorohydrin after
four days.
The effect of azaphosphatrane substitution on catalytic activity
was particularly investigated, and it was found that the bulkier
neo-pentyl azaphosphatrane derivative 2c was the most stable
and active species. Detailed kinetic studies allowed us to
propose a mechanism which implies both epoxide, catalyst, and
CO2 in the rate-determining step. The strong structure/activity
correlation opens the route for the design of advanced catalysts.
We plan to fully explore different aspects of this dependence
including the introduction of chiral auxiliary and the
construction of molecular cages around the P−H sites.30
Insight into the reaction mechanism was obtained from a
series of kinetic studies. In order to determine the order with
respect to catalyst, the reaction was carried out without solvent,
under 1 bar of CO2 at 100 °C, and with four different catalyst
loadings between 0.25 to 1.50 mol %. Normal first-order
dependence on epoxide was observed over 5 h of reaction
(Figure S9 in SI), and comparison of the observed rate
constants for the four experiments indicated a first-order
reaction in catalyst (Figure S10 in SI). Rates were also
determined for a series of runs under varying concentrations of
CO2, produced by saturating the solution with a mixture of
CO2 and N2 (10, 20, 50 and 100%) (Table S1 in SI). A first-
order dependence on CO2 was observed (Table S2 in SI). This
is consistent with a rate-determining step involving both
catalyst, epoxide, and CO2.
ASSOCIATED CONTENT
* Supporting Information
■
S
Catalytic procedures, protocol for the determination of isolated
yield, 31P NMR spectra of catalysts (2a−2c) before and after
cycloaddition of CO2 to styrene oxide after 7 h (1 mol %) and
96 h (0.1 mol %) of reaction, and kinetics analysis showing
first-order dependence on epoxide, catalyst, and CO2
concentrations. This material is available free of charge via
the Internet at http://pubs.acs.org.
Given these observations and literature precedents,32,33 we
propose the mechanism illustrated in Scheme 2. SO first forms
an adduct with the catalyst through oxygen via a hydrogen
bond to the phosphonium cation. A tricyclic phosphoryl-
carbamate structure is then obtained by insertion of CO2 into
the P−N bond. This species is highly reactive and very sensitive
to hydrolysis and can lead to degradation as mainly observed
with 2a.31a The bulkier substituents in 2b and 2c act as
protecting groups avoiding degradation. For this scheme, the
observed first-order rate dependences on catalyst, CO2, and SO
seem to indicate that the formation of the adduct is in rapid
equilibrium with free azaphosphatrane and SO and that the
CO2 activation step is rate determining. The reaction proceeds
to product by the nucleophilic attack of the chloride at the
secondary carbon of the epoxide and subsequent attack of the
resultant alkoxide moiety on the activated carbon dioxide. The
proximity of the two activated moieties might also suggest that
these steps are concerted. Subsequent ring-closure would form
the cyclic carbonate and regenerate the azaphosphatrane
catalyst.
AUTHOR INFORMATION
Corresponding Author
Alexandre.Martinez@ens-lyon.fr (A.M.); Veronique.Dufaud@
univ-lyon1.fr (V.D.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
B.C. and L.J. thank the French Ministry of Research (Project
ANR-2010JCJC 7111 [(Sup)3Base]) for financial support.
■
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