Rational investigations in the ring opening of cyclic carbonates by amines

Article abstract of DOI:10.1039/c4gc01032a

Non-isocyanate polyurethanes (NIPUs) constitute a promising alternative for more classical polyurethanes (PUs) as they may display mechanical properties that can match those of PUs and their synthesis does not involve the use of toxic isocyanates. Yet, because of the lower reactivity of carbonates versus isocyanates, the synthesis of NIPUs is not straightforward and generally requires the use of a catalyst. Recently, several groups have reported on the use of different ranges of catalysts for promoting the nucleophilic attack of the amine on the carbonate. However, many of these studies involve the use of highly reactive amine and/or carbonate proscribing a complete panorama of the potentialities of the reaction. Herein, we propose a rational study of the catalyzed aminolysis of four representative cyclic carbonates that reveals that the thiourea organocatalyst 1 outperforms in many aspects, classical inorganic or other organic catalysts. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc01032a

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Rational investigations in the ring opening of
cyclic carbonates by amines†
Cite this: Green Chem., 2014, 16,
4286
M. Blain,a,b L. Jean-Gérard,a R. Auvergne,b D. Benazet,c S. Caillol*b and
B. Andrioletti*a
Non-isocyanate polyurethanes (NIPUs) constitute a promising alternative for more classical polyurethanes
(PUs) as they may display mechanical properties that can match those of PUs and their synthesis does not
involve the use of toxic isocyanates. Yet, because of the lower reactivity of carbonates versus isocyanates,
the synthesis of NIPUs is not straightforward and generally requires the use of a catalyst. Recently, several
groups have reported on the use of different ranges of catalysts for promoting the nucleophilic attack of
the amine on the carbonate. However, many of these studies involve the use of highly reactive amine
and/or carbonate proscribing a complete panorama of the potentialities of the reaction. Herein, we
propose a rational study of the catalyzed aminolysis of four representative cyclic carbonates that
reveals that the thiourea organocatalyst 1 outperforms in many aspects, classical inorganic or other
organic catalysts.
Received 3rd June 2014,
Accepted 16th July 2014
DOI: 10.1039/c4gc01032a
www.rsc.org/greenchem
Introduction
With a global production of 14 Mt in 2006 and 17.5 Mt in
2011, PUs are the 6th most widely used polymer.1 Despite this
undeniable success, increasing concerns related to the use of
(poly)isocyanate monomers as key reagents for the synthesis of
PUs have stimulated the search for alternative synthetic strat-
egies that could afford macromolecules with similar (mechani-
cal) properties without requiring the use of toxic building
blocks. In this context, a recent infatuation in the long-known
aminolysis of carbonates has emerged.2–7 Indeed, the amino-
lysis of carbonates affords carbamate functions structurally
related to the structure obtained when an alcohol reacts with an
isocyanate (Scheme 1). When a cyclic carbonate is reacted with
a polyamine a poly-hydroxyurethane (PHU) is obtained.8–12
Considering the structural similarities between PUs and
PHUs a considerable effort is currently devoted to understand-
ing13,14 and optimizing the ring opening of cyclic carbonates
by amines.15,16
Scheme 1 Carbonate vs. isocyanate strategy.
Due to the well-known lower reactivity of cyclic carbonates
compared with isocyanates,4,5 the use of a catalyst is generally
required for promoting the nucleophilic attack of the amine
on the carbonyl group of the carbonate.17,18 A noticeable
exception involves the use of linear primary amines which
nucleophilicity is generally sufficient for reacting with the car-
bonate without requiring the use of a catalyst, even at room
temperature (vide infra).16 As most PUs are industrially formu-
lated and processed at room temperature, it is highly desirable
to propose a reliable access to PHUs that could proceed in
similar conditions. In addition, synthesizing PHUs at room
temperature would undoubtedly limit the possibility to favor
side reactions, such as the degradation of carbamates to amide
or ureas that could (1) limit the chain growth, or (2) result in
polymers with unwanted functionalities or structure.
aICBMS-UMR 5246 Université Claude Bernard-Lyon 1, Equipe de CAtalyse,
SYnthèse et ENvironnement (CASYEN), Domaine Scientifique de la Doua-Bât.
Curien/CPE, 2° étage aile C, 43 Boulevard du 11 Novembre 1918,
69622 Villeurbanne Cedex, France. E-mail: bruno.andrioletti@univ-lyon1.fr
bEcole Nationale Supérieure de Chimie de Montpellier, 8, Rue Ecole Normale,
34296 Montpellier, France. E-mail: sylvain.caillol@enscm.fr
cJUXTA, 5 rue de la Jalesie, BP 71039, 25401 Audincourt cedex, France
†Electronic supplementary information (ESI) available: Synthetic protocols for
In this context, some of us19–22 and others4,7,12,15,23–25 have
decided to re-investigate the long-known catalysed aminolysis
of carbonates. However, to date, a survey of the corresponding
the preparation of the carbonates (when not commercially available), the cata-
lysts, and complete characterization of the different reaction products. See DOI:
10.1039/c4gc01032a
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literature data reveals the absence of a clear trend regarding ture. At last, the poorly reactive aniline did not react in these
the influence of the amine, the catalyst, and the temperature. conditions. In light of these results, we decided to retain the
Accordingly, we decided to embark a rational study on the cyclohexylamine as nucleophile for the catalyst screening.
aminolysis of carbonates with the aim of providing polymer
Determination of the optimal catalyst
chemists an extensive study on the catalyzed aminolysis of car-
bonates within a context of sustainable chemistry. Different
factors have been investigated. In a first instance, we have
investigated the solvent free reaction of different amines with
the propylene carbonate (which was selected as a typical cyclic
carbonate), in the absence of catalyst. The objective was to
determine a system amine/carbonate that would exhibit a
moderate reactivity in order to evaluate precisely the catalytic
effect. After this preliminary screening, a large range of in-
organic and organic catalysts were tested. The conversion, the
selectivity and the color of the reaction mixture were con-
sidered for this study. Once the optimal reaction time had
been determined, the reactivity of several amines with
different carbonates was investigated providing a rather com-
plete overview of the scope of the reaction.
Both inorganic salts and organocatalysts were investigated.
Inorganic Lewis acids
A large range of little toxic metal salts were first investigated. It
was expected that the Lewis character would enhance the reac-
tivity of the carbonate function22,26 (Table 1).
As mentioned before, the non-catalyzed aminolysis of PC
with the cyclohexylamine proceeds to a very limited extent,
with a conversion of 9% after 1 h, and 45% after 5 h of stirring
at 25 °C. At the exception of Ti(OiPr)4 (entry 7), all the Lewis
acids catalyze the aminolysis reaction with a conversion reach-
ing about 50% after 1 h when Mg, Yb or Bi salts were used.
When the reaction was allowed to proceed for 5 h, in every
case, the conversions increased, reaching a plateau of about
60–75%. Hence, an interesting catalytic effect is observed.
However, for macromolecular applications, chemicals (the
catalyst in our case) that afford color have to be proscribed
because colorless polymers are generally highly desirable.
Similarly, insoluble catalysts should also be banned. Consider-
ing our observations (Table 1) and these aforementioned limit-
ations, organocatalysts were evaluated as well.
Results and discussion
Choice of the amine
Propylene carbonate (PC) being relatively unreactive, and
readily available, it was chosen as the prototypal 5-membered
cyclic carbonate partner for selecting the most appropriate,
non-functionalized amine for the study. In order to provide
data directly usable by polymer chemists, it was decided to
perform the screening neat, as polyurethanes are generally syn-
thesized in the absence of solvent. PC being an excellent
“green” solvent, the screening was performed at 25 °C. A clear
reaction mixture was obtained with the variously substituted
amines investigated (Fig. 1).
As expected, a quantitative GC-MS analysis revealed that the
less hindered, more nucleophilic butyl-, cyclohexylmethyl- and
benzyl-amines displayed the highest reactivities with an
almost complete conversion in about 10 hours, even in the
absence of catalyst. Conversely, the more sterically demanding
cyclohexylamine displayed a limited reactivity with a conver-
sion limited to 66% after 10 hours of stirring at room tempera-
Organocatalysts
A series of organocatalysts encompassing phosphines, bases,
phosphazenes and thioureas was evaluated in the conditions
considered for the screening of the inorganic catalysts
(Scheme 2).
We first evaluated 3 different phosphines stable enough for
allowing an easy handling. Accordingly, 2 triarylphosphines
and a monoalkylbiarylphosphine were investigated (Table 2,
entries 2–4).27
Disappointing results were obtained as the conversions did
not exceed those obtained for the non-catalyzed reactions.
Table 1 Lewis acid-catalyzed aminolysis of PC with cyclohexylaminea
Entry
Catalyst
% Conversionb PC
Observations
1
2
3
4
5
6
7
None
MgBr2
Yb(OTf)3
Fe(OTf)3
FeCl3
Bi(OTf)3
Ti(OiPr)4
9(45)
51(71)
53(77)
40(64)
41(67)
48(70)
12(57)
Colorless
Suspension
Turbid
Brown
Brown
White
Turbid
a CP/cyclohexylamine: 1/1, 25 °C, catalyst loading: 5 mol%, 1 h. b In
parenthesis, conversion after 5 h.
Fig. 1 Conversion of PC with different amines.
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From a mechanistic point of view, even if TBD and a
thiourea display different structures, it is expected that the
catalytic effect could be attributed to similar activation modes
of the carbonate. Indeed, it is commonly admitted that a
thiourea is able to activate a carbonyl function by establishing
two hydrogen bonds with the CO of the carbonyl function in a
6-membered ring fashion. Conversely, as TBD is a strong base,
it is very easily protonated (we observed a reversible protona-
tion of TBD just on standing in air). Based on this assumption,
the resulting guanidinium is able to activate the carbonyl simi-
larly to the thiourea. These analogies may explain why TBD
and the thiourea display similar performances.
In light of the results presented in Tables 1 and 2, it was
decided to complete our study using the TBD and the thiourea
A as catalysts.
Scheme 2 Oganocatalysts used in the study.
Effect of the temperature
Table 2 Organocatalyzed aminolysis of PC with cyclohexylaminea
After selecting the two most promising catalysts, the influence
of the temperature was investigated. To this end, PC and the
cyclohexylamine were reacted at different temperatures in the
presence of TBD and A, respectively (Fig. 2). As expected, an
increase of the temperature up to 100 °C results in a sharp
increase of the kinetics of the reaction. Hence, the conversions
reached 66% and 90% in 12 min when A and TBD were used
as catalysts, respectively. After 1 h, the conversion is complete
in the presence of TBD and attains 90% when A is employed.
Thus, it appears that an increase of the temperature results in
a higher conversion of CP and reveals a higher catalytic activity
of TBD compared with the thiourea. However, it should be
noted that this enhanced reactivity proceeds at the expense of
the selectivity toward the carbamates. Indeed, the presence of
Entry
Catalyst
% Conversion PC
Product ratio 1 : 2
1
2
3
4
5
6
7
8
None
PPh3
MePPh2
Tri(o-toly)phosphine
DABCO
DMAP
DBU
TBD
MTBD
BEMP
P1-tBu
A
9
12
9
35 : 65
50 : 50
55 : 45
63 : 37
54 : 46
55 : 45
42 : 58
56 : 44
50 : 50
55 : 45
44 : 56
45 : 55
48 : 52
7
17
24
38
66
20
24
18
66
61
1
impurities was detected in the H NMR spectra of the carba-
mates when TBD is used (see the ESI†).
9
10
11
12
13
Catalyst loading
B
Next we investigated the catalyst loading. Different catalytic
charges and reaction were screened (Fig. 3).
a CP/cyclohexylamine: 1/1, 25 °C, catalyst loading: 5 mol%, 1 h.
Interestingly, after 10 minutes of reaction, the thiourea A is
a more efficient catalyst than TBD when the catalyst loading is
5 Nitrogen-containing bases were also evaluated28–30 (Table 2,
entries 5–9). All of them catalyzed the aminolysis of CP, the
best catalyst being TBD. This result is in agreement with
former results from the literature that proposed that TBD
could activate both the carbonate and the nucleophile.15,31 In
contrary, phosphazenes that have been proposed as potent
organocatalysts for similar reactions32,33 appeared poorly
efficient in our conditions (entries 10–11). Interestingly, the
thioureas A and B34,35 revealed also a powerful catalytic effect
with a conversion of 66% in the case of the phenylcyclohexyl
thiourea A, matching the conversion obtained with TBD.
Noticeably, in our study, the organocatalysts generally outper-
formed the inorganic catalysts in terms of efficiency but also
because they did not color the reaction mixture. Of note, no
significant selectivity for carbamate 1 or 2 was observed during
our screening.
Fig. 2 Reaction of PC with the cyclohexylamine at different tempera-
tures using TBD and A as catalysts.
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Fig. 3 Effect of the catalyst loading on the aminolysis of PC with
cyclohexylamine.
Fig. 5 Reaction of different carbonates with the cyclohexylamine in the
presence of TBD or A. Carbonate/amine: 1/1, 25 °C, catalyst: 5 mol%,
1 h.
maintained below 1 mol%. However, when increasing the cata-
lytic charge up to 5 mol%, TBD becomes more efficient than
A. These observations are confirmed after 1 h of reaction, but
thiourea A becomes more efficient than TBD with a prolonged
reaction time (10 h). Thus, for large scale application it is
worth noting that the thiourea A is more promising than TBD
because low catalyst loadings are tolerated. This observation
contrasts with what is generally reported in the literature.15
in the absence of catalyst, important conversions of 50% and
63% were measured in the presence of A and TBD, respectively.
In these conditions, the two catalysts displayed a very similar
catalytic activity.
Scope of the reaction: carbonate
To complete the study, we also screened a series of carbonates,
namely the 4-(methoxymethyl)-1,3-dioxolan-2-one, 4-phenyl-
1,3-dioxolan-2-one and 4,6-dimethyl-1,3-dioxan-2-one, a repre-
sentative 6 membered ring carbonate. The cyclohexylamine
was chosen as a nucleophile, and both organocatalysts A and
TBD were investigated.
Interestingly, when screening the different carbonates in
these conditions, at the exception of PC, the thiourea A
revealed a superior or equal activity to TBD (Fig. 5). This obser-
vation is in sharp contrast with what it is generally admitted in
the literature.15 Whereas both 4-(methoxymethyl)-1,3-dioxolan-
2-one and 4-phenyl-1,3-dioxolan-2-one displayed a high reacti-
vity in our conditions with conversions reaching 68 and 63%,
respectively in the presence of catalyst A, the 6-membered ring
4,6-dimethyl-1,3-dioxan-2-one was only proved reactive in the
presence of the thiourea A (57% conversion). This observation
also contrasts with common beliefs that consider that 6-mem-
bered rings are generally more reactive than their 5-membered
ring analogues.13
Scope of the reaction: amine
Having in hands a promising protocol, we decided to investi-
gate the scope of the reaction and tested 4 additional amines
displaying different nucleophilicities: the cyclohexylmethyl-
amine, butylamine, benzylamine and the industrially relevant
IPDA diamine were reacted with 1 (or 2) equivalent of CP, in
the presence of the two selected organocatalysts, at 25 °C for
1 h (Fig. 4).
A careful analysis of the chart (Fig. 4) reveals that the cata-
lytic effect of both TBD and A is very limited when highly
nucleophilic amines are used. In contrast, a dramatic effect is
observed in the case of sterically hindered or deactivated
amines. Thus, when benzylamine reacts to a limited extent
with PC in the absence of catalyst (conv. 28%), in the presence
of A or TBD, the conversion increased to 74% and 94%,
respectively. An even more impressive effect is observed in the
case of IPDA. Whereas IPDA is almost unreactive towards PC
Conclusions
To the best of our knowledge, for the first time, a rational
study about the aminolysis of cyclic carbonates is reported.
Our work demonstrates that in general, organocatalysts display
a higher catalytic activity than inorganic Lewis acids. In
addition, they generally do not bring color or turbidity to the
reaction mixture. This observation is absolutely crucial for
future macromolecular applications. Advantageously, this
study has been carried out in the absence of solvent which is
also relevant for subsequent extension in polycondensation
reactions. Our screening also reveals that TBD and the cyclo-
Fig. 4 Reaction of PC with different amines in the presence of TBD or
A. PC/amine: 1/1, (PC/IPDA: 2/1), 25 °C, catalyst: 5 mol%, 1 h.
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hexylphenyl thiourea A outperform the other catalysts investi-
gated. This observation confirms precedent from the literature.
However, in our conditions, the thiourea A seems more
promising as it cleanly catalyzes the aminolysis of carbonates
even at very low catalyst loading. In addition, it was proven
more efficient when functionalized 5-membered ring or 6-
membered ring carbonates were used.
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Experimental
Chemicals
All the catalysts and IPDI were used as received except the
thioureas synthesized according to the reported literature pro-
cedure.34 Propylene carbonate (PC) was purified by distillation.
Glycerol carbonate (GC) was purified by column chromato-
graphy using ethyl acetate and hexane (60 : 40) as eluent.
4-(Methoxymethyl)-1,3-dioxolan-2-one and 4,6-dimethyl-1,3-
dioxan-2-one were synthesized according to the reported litera-
ture procedure.36
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15 R. H. Lambeth and T. J. Henderson, Polymer, 2013, 54,
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General procedure for the aminolysis of carbonates with
GC/MS quantitative analysis
In a typical procedure, carbonate (1 equiv.), amine (1 equiv.)
and diphenyl ether (0.15 equiv. internal standard) were stirred
together during the time and at the temperature shown in the
table in an open air reaction tube with the quantity of catalyst
used in the table.
16 C. D. Diakoumakos and D. L. Kotzev, Macromol. Symp.,
2004, 216, 37–46.
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General procedure for the aminolysis of carbonates
In a typical procedure, the carbonate (2.6 mmol), amine
(2.6 mmol) and the catalyst (5 mol%) were stirred together for
the appropriate time in an open vessel thermostated at 25 °C.
Acknowledgements
The Association Nationale de la Recherche et de la Technologie
(ANRT) and Juxta are deeply acknowledged for their financial
support so as Mrs Audrey Ledoux for her initial contribution
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