Role of pre-organization around the azaphosphatrane catalyst's active site in the conversion of CO2 to cyclic carbonates

Article abstract of DOI:10.1016/j.catcom.2014.04.004

Various N-substituted azaphosphatranes have been prepared and successfully applied to the synthesis of styrene carbonates from CO₂ and epoxides. Enhancement of the catalytic properties of the azaphosphatrane was achieved upon pre-organization o

Full text of DOI:10.1016/j.catcom.2014.04.004

Catalysis Communications 52 (2014) 26–30
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Role of pre-organization around the azaphosphatrane catalyst's active
site in the conversion of CO2 to cyclic carbonates
Bastien Chatelet a, Erwann Jeanneau b, Jean-Pierre Dutasta a, Vincent Robert c,
Alexandre Martinez a, , Véronique Dufaud
⁎
d,⁎⁎
a
Laboratoire de Chimie, École Normale Supérieure de Lyon, CNRS, UCBL, 46 allée d'Italie, 69364 Lyon, France
b
c
Centre de Diffractométrie Henri Longchambon, Université Claude Bernard Lyon1, Site CLEA-Bâtiment ISA, 5 rue de La Doua, F-69100 Villeurbanne Cedex, France
Laboratoire de Chimie Quantique, Institut de Chimie, UMR CNRS 7177, Université de Strasbourg, 4 rue Blaise Pascal, F-67070 Strasbourg, France
Laboratoire de Chimie, Catalyse, Polymères, Procédés (UMR 5265), CNRS, Université Claude Bernard Lyon1, CPE Lyon, 43 Bd du 11 novembre 1918, 69616 Villeurbanne cedex, France
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Various N-substituted azaphosphatranes have been prepared and successfully applied to the synthesis of styrene
carbonates from CO2 and epoxides. Enhancement of the catalytic properties of the azaphosphatrane was
achieved upon pre-organization of the active site through hydrogen-bonding.
© 2014 Elsevier B.V. All rights reserved.
Received 24 January 2014
Received in revised form 19 March 2014
Accepted 5 April 2014
Available online 13 April 2014
Keywords:
Azaphosphatranes
Cyclic carbonates
CO2 chemical valorization
Homogeneous catalysis
Epoxides
1. Introduction
conditions [17–19]. Recent advances include the use of binary catalysts
[7,20] based on the association of a Lewis acid (e.g. Al, Zn, Mn, Cr, Co
Conversion of CO2 to industrially relevant compounds has recently
attracted much interest for the development of sustainable and greener
alternatives to fossil fuel based resources [1–3]. CO2 has the advantage
of being non-toxic, abundant and is particularly useful as a substitute
of phosgene. In this regard, the cycloaddition of CO2 to epoxides to pro-
duce cyclic carbonates is one of the most promising and eco-friendly
methodologies to exploit carbon dioxide as a renewable raw material
[4–8]. Organic carbonates are important building blocks that have
found numerous applications as green solvents, additives to gasoline,
thickeners for cosmetics, electrolytes for lithium batteries and as useful
intermediates for the production of plastics and fine chemicals [9–12].
Although commercially manufactured for about 50 years [13] using
mainly quaternary ammonium or phosphonium salts [14–16] as cata-
lysts, the industrial synthesis of ethylene and propylene carbonates
from CO2 and corresponding epoxides still requires fairly drastic
reaction conditions (high temperatures and pressures) and the use of
relatively pure CO2. In the past decades, numerous catalytic systems
have been developed to achieve higher performance under milder
among others) and a nucleophile (e.g. the counteranion of an onium
salt) or the development of one-component systems which contain
both functions within a single catalytic site [5,21]. In these systems,
the Lewis acid and the nucleophile act cooperatively at the epoxide
through oxygen atom coordination and nucleophilic ring-opening,
respectively.
Although significant progress has been made, several disadvantages
need to be circumvented to produce greener catalysts and the process-
es: low catalyst stability and reactivity, the need for a cocatalyst and/or
cosolvent, the limited scope of substrates and, in most cases, the pres-
ence of expensive and toxic metals. In this respect, organocatalysis
may represent a powerful tool capable to respond to increasingly strin-
gent regulatory issues concerning metal contamination of pharmaceuti-
cal products or consumer chemical feedstocks. In addition to quaternary
ammonium and phosphonium salts [22], ionic liquids [23,24], betaine-
based structures [25], organic bases among other organocatalysts have
been reported to efficiently enable CO2 fixation into cyclic carbonates.
Phenol associated with organic bases such as 4-dimethylaminopyridine
(DMAP) was also successfully applied to this transformation [26]. Howev-
er, unlike metal-mediated catalyzed approaches, elevated temperatures
and pressures (90–140 °C, 10–140 bar) were required in most cases to
achieve sufficient conversion. More recently, Kleij et al. reported that
the combination of phenolic-based compounds such as catechol and/or
⁎
Corresponding author. Tel.: +33 472728863.
⁎⁎ Corresponding author. Tel.: +33 472431792.
E-mail addresses: Alexandre.Martinez@ens-lyon.fr (A. Martinez),
veronique.dufaud@univ-lyon1.fr (V. Dufaud).
http://dx.doi.org/10.1016/j.catcom.2014.04.004
1566-7367/© 2014 Elsevier B.V. All rights reserved.

B. Chatelet et al. / Catalysis Communications 52 (2014) 26–30
27
pyrogallol with nBu4NI co-catalyst led to a very efficient catalytic system
able to operate under solvent-free and relatively mild reaction conditions
(25–45 °C, PCO2 = 10 bar, 2–5 mol% catalyst), thus fulfilling some of the
criteria towards sustainability [4]. The authors attributed this improve-
ment to both synergistic and stabilizing effects provided by the polyphe-
nolic structures through multiple hydrogen-bonding.
In our effort to contribute to the overall trend for clean and effective
CO2 conversion, we have recently described the use of azaphosphatranes
(AZAPs), which are the acidic conjugates of the Verkade's superbases
[27–29], as efficient single-component, metal-free catalysts for the syn-
thesis of cyclic carbonates from CO2 and epoxides (Scheme 1) [30]. In
that work, we showed that these organocatalysts were able to operate
under low CO2 pressures and relatively mild temperatures (1 bar, 80–
100 °C) even at catalyst loadings of 0.1 mol%. We also demonstrated
that the presence of bulky substituents on the equatorial nitrogen atoms
strongly correlate with higher catalyst activity and stability over several
days of reaction. Based on kinetic studies and previous literature prece-
dents [31,32] we proposed a mechanism which involves the insertion of
CO2 into the P\N bond of the AZAP unit as the rate determining step.1
In the present contribution, we wish to further explore the different
aspects of the catalyst structural dependence through careful ad-
justment of the substitution patterns around the phosphorus site. As
we shall see, the pre-organization of the active site through hydrogen-
bonding will lead to the creation of highly reactive pockets with im-
proved performance in the cycloaddition of CO2 to epoxides.
Scheme 1. Coupling of styrene oxide with CO2 catalyzed by azaphosphatranes (R = alkyl
or aromatic moieties).
1H NMR (CDCl3, 298 K, 500.1 MHz): δ 6.41 (d, 3JH–F = 9.18 Hz, 6H,
ArH); 3.76 (s, 9H, ArOCH3); 3.75 (s, 6H, ArCH2N); 2.58 (t, 3J =
6.01 Hz, 6H, N(CH2)2N); 2.50 (t, 3J = 5.80 Hz, 6H, N(CH2)2N).
13C NMR (CDCl3, 298 K, 125.7 MHz) δ 163.2 (d, J = 12.6 Hz, CAr);
161.2 (d, J = 12.6 Hz, CAr); 160.0 (t, J = 14.6 Hz,CAr); 97.7 (d, J =
30.4 Hz, CArH); 55.7 (OCH3); 54.4 (ArCH2N); 46.5 (NCH2CH2N); 40.2
(NCH2CH2N).
19F NMR (CDCl3, 298 K, 282.2 MHz): δ −115 ppm.
2.1.2. Compound 1b
In an ice-bath cooled round bottom flask, tris(dimethylamino)phos-
phine (0.410 mL, 2.25 mmol) was dissolved in acetonitrile (15 mL).
Phosphorus trichloride (98 μL, 1.12 mmol) was then added drop-wise.
The reaction mixture was vigorously stirred at 0 °C for 0.5 h, and a solu-
tion of tris(2,6-difluoro-4-methoxybenzyl)tren (1.88 g, 3.06 mmol) in
acetonitrile (10 mL) was added drop-wise. The reaction mixture was
then stirred overnight at room temperature. The solvent was removed
under reduced pressure and the crude compound was chroma-
tographed on silica gel eluting with CHCl3/MeOH 15/1 to give pure 1b
as an off-white powder (1.01 g, 49% yield).
2. Experimental
Commercial reagents were used without further purifications. 1H,
13C, 31P and 19F NMR spectra were recorded on Bruker spectrometers
at 500.1, 125.7, 202.4 and 282.2 MHz respectively. Chemical shifts (δ)
are referenced with respect to TMS (1H, 13C), H3PO4 85% (31P) or TFA
ESI-HRMS m/z calcd for C30H34F6N4O3P (M+) 643.2267, found
643.2236.
(
19F). Compounds 1a and 1c were synthesized according to literature
1H NMR (CDCl3, 298 K, 500.1 MHz): δ 6.48 (d, 3JH–F = 9.57 Hz, 6H,
ArH), 6.00 (d, 1JP–H = 508 Hz, 1H, P–H); 4.09 (d, 3JP–H = 14.60 Hz, 6H,
ArCH2N); 3.79 (s, 9H, ArOCH3); 3.49–3.46 (m, 6H, N(CH2)2N); 2.97–
2.92 (m, 6H, N(CH2)2N).
procedures [33,34]. Yields were estimated during catalytic runs by 1H
NMR with a Bruker Avance 300 spectrometer at 300.1 MHz. Mass spec-
tra were performed by the Service Central d'Analyses, CNRS, France. CO2
of a purity of 99.99% was commercially available and used without fur-
ther purification.
13C NMR (CDCl3, 298 K, 125.7 MHz): δ 163.2 (d, J = 11.7 Hz, CAr);
161.2 (d, J = 11.7 Hz, CAr); 105.2 (m, CAr); 98.2 (d, J = 29.06 Hz,
CArH); 55.9 (OCH3); 46.6 (d, J = 7.09 Hz, N(CH2)2N); 38.5 (d, J =
6.71 Hz, N(CH2)2N); 38.1 (d, J = 20.4 Hz, ArCH2N).
2.1. Syntheses of new compounds
31P NMR (CDCl3, 298 K, 202.4 MHz) δ: −14.4 (d, 1JP–H = 508 Hz).
19F NMR (CDCl3, 298 K, 282.2 MHz): δ −114.6 ppm.
2.1.1. Tris(2,6-difluoro-4-methoxybenzyl)tren
In a round bottom flask, tris(2-aminoethyl)amine (tren) (546 mg,
3.74 mmol) was dissolved in methanol (10 mL) and ice-bath cooled.
Then, a solution of 2,6-difluoro-4-methoxybenzaldehyde (2.00 g,
11.60 mmol, 3.1 eq) in a 1:1 (v/v) mixture of chloroform/methanol
(10 mL) was added drop-wise. The reaction was slowly warmed up to
room temperature and stirred overnight. Methanol (10 mL) was then
added. Subsequent portions of NaBH4 (878 mg, 23.20 mmol) were
added to the ice-cooled mixture over a thirty-minute period and the
mixture was stirred for another hour. Solvents were evaporated. A solu-
tion of 10% NaOH in water was added (25 mL), and the resulting mix-
ture was extracted with toluene (3 × 30 mL). The combined organic
phases were extracted with HCl 1 M (3 × 50 mL) and the combined
aqueous phases were then basified with an aqueous 10% NaOH solution.
The solution was then extracted with toluene (3 × 100 mL) and the
organic layers were dried over Na2SO4, filtered, and the solvent was re-
moved under reduced pressure to give a pale yellow oil (1.88 g, 82%).
ESI-HRMS m/z calcd for C30H37F6N4O3 (MH+) 615.2764, found
615.2745.
2.2. Catalytic procedures
Reactions were carried out in a 5 mL Schlenk tube equipped with
a rubber septum. In a typical run, the reactor was charged with
50 mmol of styrene oxide (SO), 0.05 mmol of catalyst (1a–c) and 2,4-
dibromo-mesitylene used as an internal standard. Carbon dioxide was
first bubbled into the solution for 5 min to saturate the liquid phase. A
balloon filled with carbon dioxide was then connected to the Schlenk
through a needle to maintain a constant atmosphere of carbon dioxide
during the course of the reaction. The mixture was placed in a thermo-
static oil bath at the required temperature and stirred at a stirring speed
of 1000 rounds min−1. After each 24 hour period, the reaction mixture
was cooled to room temperature and an aliquot was analyzed by 1H
NMR. Based on the SO integrated intensity in the NMR spectrum, addi-
tional substrate was added to the mixture to recover the initial substrate
to catalyst ratio. Carbon dioxide was bubbled through the solution for
15 min at room temperature, the CO2 reservoir was connected to the
reaction vessel, which was returned to the thermostatic bath. For the
catalytic tests run over a course of 24 h, yields were determined by 1H
NMR following the same procedure but starting with 5 mmol of styrene
oxide and 1 mol% of the catalysts (1a–c).
1
Note that the participation of the deprotonated AZAP species in the catalytic cycle has
been ruled out considering the high pKa value of the AZAP acid–base couple (pKa ~ 33)
and the absence of a strong base in the reaction medium.

28
B. Chatelet et al. / Catalysis Communications 52 (2014) 26–30
a
b
c
Fig. 1. Targeted azaphosphatranes 1a–c to probe the structure–activity effects.
2.3. Computational method
with CO2 to produce styrene carbonate (SC) as a model reaction. In co-
herence with the requirements of highly sustainable green methodolo-
gies, we chose to work close to atmospheric pressure (1 bar CO2) at
relatively low temperatures (100 °C) and catalyst loadings (0.1 and
1 mol%). Catalytic tests were first performed over a 24 hour period
with a catalyst loading of 1 mol%. All catalysts 1a–c exhibited good cat-
alytic activities with respectively 73% (1a), 86% (1b) and 92% (1c) yields
in styrene carbonate. In order to investigate more accurately the role of
these structural changes on catalyst reactivity and stability, a second se-
ries of reactions was carried out where the catalyst loading was further
decreased to 0.1 mol% and the reaction time extended to 96 h. The prog-
ress of the reaction was monitored by 1H NMR sampling every 24 h
(Fig. 2).
At these higher conversions, we can see that the reference
organocatalyst 1a has a limited stability as shown by its steadily de-
creasing activity over the four-day period. On the contrary, 1b and 1c
showed nearly constant activity during the course of the experiment.
Initial catalytic activities were found to be respectively twice and
three times higher with 1b and 1c than with 1a highlighting the key
role played by the stereoelectronic properties of the surrounding
amino groups on the rate of the reaction. Introduction of specific groups
on the benzene ring in 1b and 1c also strongly increases the stability of
the catalysts: 1b and 1c practically maintain their initial activity over the
four-day test resulting in much higher TONs (respectively 500 and 800)
than for the reference catalyst 1a (200). This stability enhancement
could likely be related to the ability of the fluoro and methoxy Lewis
base substituents to create H-bonding with the acidic P–H+ site. Indeed,
Verkade et al. have already reported the occurrence of this type of inter-
actions between the OMe unit and the P–H+ site in AZAP 1c [33] and
similar interactions are probably effective in 1b. In our case, we suggest
that hydrogen bonding can favor the regeneration of the AZAP core.
Moreover, we would like to stress that the Van der Waals radius of hy-
drogen and fluorine are very close (1.20 and 1.47 Å respectively). Thus,
the stability enhancement observed for 1b with respect to 1a is likely
due to H-bonding between the benzyl substituents and the P–H+ site.
We thus became interested in investigating further the relative abil-
ity of catalysts 1b and 1c to achieve intramolecular hydrogen bonding
with the P–H+ site, and addressed this question by means of density
functional theory (DFT) calculations. In order to validate our approach,
we first decided to compare the X-ray structure of 1a in the solid state
to that obtained in the gas phase by the mean of DFT calculation
(Figs. S8 and S9 in the Supporting information) [35]. The optimized
bond lengths and valence angles are in good agreement with the
Ab initio evaluations were performed using the Gaussian 03 package
within a restricted DFT framework. A combination of unrestricted BP86
functional and triple-zeta 6-311G all electron basis sets has proven to be
very satisfactory for geometry optimizations [35]. We checked that our
results did not suffer from the arbitrariness of the exchange correlation
functional using the hybrid B3LYP functional.
3. Results and discussion
Azaphosphatranes possess several remarkable features that make
them particularly interesting candidates for the activation of both CO2
and epoxides. First, these protonated aminophosphines, built from the
tris(2-aminoethyl)-amine (tren) scaffold, present a well-defined tricy-
clic structure around the phosphorus atom which allows for greater sta-
bility. The soft organic cation, in which the charge is delocalized around
the HPN core, does not strongly associate with hard halide anion. Thus,
the anion is more prone to react as nucleophile to initiate the epoxide
ring-opening step. Finally, the nature of the lipophilic pocket formed
by a ring of amino groups around the central PH unit provides a unique
catalytic space to probe the structure/activity effects.
Three N-substituted azaphosphatranes were considered during the
course of this study (Fig. 1): AZAP 1a containing a p-methoxybenzyl
substituent was chosen as a benchmark structure since the aromatic
moiety can provide a versatile platform for fine-tuning of the substitu-
tion pattern. AZAPs bearing benzyl rings ortho-substituted by fluorine
atoms (1b) or a methoxy group (1c) were also synthesized. We antici-
pated that these substituents could favor intramolecular hydrogen
bonding with the P–H+ moiety, providing a cavity above the reactive
center which may protect it from degradation.
Taking advantage of the high versatility of the AZAP synthesis, cata-
lysts 1a–c were prepared following a two-step reaction sequence
(Scheme 2). The key substituent was introduced in the form of an alde-
hyde in the triple alkylation of tris(2-aminoethyl)amine (tren) and treat-
ment with NaBH4. Subsequent reaction with PCl(NMe2)2 led to the
desired substituted azaphosphatranes in respectable yields (ca. 50%).
31P NMR spectra of catalysts 1a–c exhibited a single resonance at −12.4
(1a), −14.4 (1b) and −10.9 ppm (1c) as expected for AZAP derivatives.
Full characterization of 1b has been displayed in Experimental section and
Supporting information.
The effect of the catalyst structure on the catalytic performance has
been investigated in detail using the coupling of styrene oxide (SO)
Scheme 2. Synthetic sequence to azaphosphatrane catalysts.

B. Chatelet et al. / Catalysis Communications 52 (2014) 26–30
29
900
800
700
600
500
400
300
200
100
0
catalytic activity and stability. We have shown that the introduction of
specific substituents at appropriate positions on the aromatic rings led
not only to an increased activity but also to more robust catalysts. This
improved efficiency has been attributed to the ability of F and OMe
Lewis base groups to form H-bonding with the acidic P–H+ inducing
larger stabilizing effects. This study demonstrates that by fine design
of reactive site structure remarkable reactivity gain can be achieved
for organocatalysts.
1a
1b
1c
Acknowledgments
The authors would like to acknowledge the funding from the French
Ministry of Research (Project ANR-2010JCJC 7111 [(Sup)3Base]).
0
20
40
60
80
100
Time (h)
Appendix A. Supplementary data
Fig. 2. Influence of the azaphosphatrane substitution on catalyst reactivity. Conditions:
styrene oxide (50.0 mmol), catalyst 1a–c (0.05 mmol), CO2 (1 bar), 100 °C. TONs were
determined by 1H NMR using 2,4-dibromomesitylene (2.0 mmol) as an internal reference.
NMR data, X-ray and DFT optimized molecular structures of 1a,
X-ray structure of 1c. Supplementary data to this article can be found
online at http://dx.doi.org/10.1016/j.catcom.2014.04.004.
X-ray data, thus, the structures of the two other catalysts 1b and 1c were
optimized with respect to all geometrical parameters, using a BP86/DFT
approach leading to global minima (Fig. 3). In their optimized geometry,
catalysts 1a–c displayed some similar features, in particular the distance
between the phosphorus and the nitrogen in both equatorial and apical
position are characteristic of an azatrane unit (around 2.2 A and 2.3 Å,
respectively). Interestingly, intramolecular hydrogen bonding between
the P–H+ site and the fluorine or the methoxy groups are likely to occur
with both catalysts 1b and 1c (the averages of the F…H–P and O…H–P
distances are 2.8 Å and 3 Å respectively), whereas such hydrogen bond-
ing is not possible with 1a. This is consistent with our aforementioned
hypothesis concerning the enhanced stability of AZAPs 1b and 1c
when compared to 1a during the CO2 conversion: the presence of intra-
molecular hydrogen bonds probably protects the active site from degra-
dation. Indeed, the loss of catalytic activity observed for catalyst 1a may
be related to the ring-opening of the AZAP tricyclic structure after CO2
insertion into the P\N bond [31,32]. Such an intermediate has already
been reported to be highly reactive to nucleophilic species, in particular
water [31,32]. Thus, intramolecular interactions may reinforce the sta-
bility of the catalyst by favoring the regeneration of the polycyclic struc-
ture of the azaphosphatrane unit at the end of the catalytic cycle, which
is consistent with our proposed mechanism [30]. X-ray structure of 1c
confirms the occurrence of hydrogen bonding between P–H+ and the
benzyl substituent (P–H+…O–CH3 distance of 3.2 Å is observed. See
Supporting information).
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In summary, differently N-substituted azaphosphatranes have been
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ate from CO2 and epoxides. The effects of changes in the local substitu-
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Fig. 3. DFT optimized structures of catalysts 1a, 1b and 1c (typical H-bond lengths (Å) in 1b and 1c are displayed).

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