Synthesis and properties of reversible ionic liquids using CO2, mono- to multiple functionalization

Article abstract of DOI:10.1016/j.tet.2012.06.082

New carbamate based ionic liquids were prepared using different primary amines such as aliphatic, aryl-, di-, and triamines in the presence of organic superbase and CO₂ atmosphere. Many of the prepared compounds can be considered as ionic liqui

Full text of DOI:10.1016/j.tet.2012.06.082

Tetrahedron 68 (2012) 7408e7413
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and properties of reversible ionic liquids using CO2, mono- to multiple
functionalization
*
Gonc¸ alo V.S.M. Carrera, Manuel Nunes da Ponte, Luis C. Branco
ˇ
REQUIMTE, Departamento de Química, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 April 2012
Received in revised form 16 June 2012
Accepted 19 June 2012
Available online 28 June 2012
New carbamate based ionic liquids were prepared using different primary amines such as aliphatic, aryl-,
di-, and triamines in the presence of organic superbase and CO2 atmosphere. Many of the prepared com-
pounds can be considered as ionic liquids with low melting points. The reaction conditions based on sol-
vent, type of base, pressure, reaction time, and temperature were optimized and discussed.
Tetramethylguanidine (TMG) can be envisaged as an alternative base in order to obtain high yields and
purities of carbamate salts specially when primary amines and polyamines are used.
The reversibility studies using n-octylamine/DBU and n-octylamine/tetramethylguanidine systems were
performed. These studies showed that tetramethylguanidine can lead to more reversible systems while
DBU can lead to more stable salts.
Keywords:
CO2
Carbamate
Ionic liquid
Reversible
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Multi-functionalization
1. Introduction
with primary alkyl amines,17 aminoesters,18 and amino-alcohols.19
These reversible systems can be used as solvents with specific
Ionic liquids1 are salts with a melting point below 100 ꢀC (some
are liquids at room temperature), where at least one of the ele-
ments of the salt is organic. As salts, these type of compounds
possess negligible volatility combined with other peculiar proper-
ties that can be tuned by choosing the right combination of cation
and anion. This possibility allowed the use of ionic liquids in a wide
range of applications2 such as organic reaction media,3 extractive
media,4 functional materials,5 and as API’s.6
Carbon dioxide capture and utilization became a hot topic in
recent years related with climate changes by means of anthropo-
genic CO2 production and accumulation. The capture of this com-
pound is actually being performed using aqueous solutions of
amines; however this technology presents some drawbacks7 such
as degradation in the presence of oxygen, corrosion, volatilization,
and energy demand. As alternative the use of ionic liquid/amine
system was proposed. It shows advantages related with their low
volatility and the reduced viscosity when the amine reacts with
CO2.8 Other alternatives consist in the use of task-specific ionic
liquids,9e12 task-specific ionic liquids plus a superbase,13,14 and
polymers.15 In the great majority of these systems, a carbamate or
a carbonate is obtained when CO2 is captured. Using the same
principle of reactivity, Jessop et al.16 created the concept of re-
versible ionic liquid, using an alcohol and a superbase (DBU). Later,
using the same concept, other research groups reported examples
and different properties, such as polarity, before and after reaction
with CO2. Other applications have been explored in order to facil-
itate separation processes between the product and the remaining
elements of the reaction, in particular for ClaiseneSchmidt con-
densation and Heck CeC coupling reactions. Some limitations have
been found in the case of cyanosilylation of cyclohexanone, where
the superbase of the reversible system can reacts with one of the
reactants.20 These reversible systems are good candidates to per-
form CO2 capture. The concept of reversible ionic liquids was also
extended to other Lewis acid gases such as SO2, COS, and CS2.21
Herein, we present the reversible ionic liquid concept based on
the use of CO2 as a reversibility element when combined with alkyl
amines, aryl amines, diamines, and tri-functionalized amines in the
presence of DBU and tetramethylguanidine (TMG) as superbases.
2. Results and discussion
In this study several reactions using amines and CO2 were per-
formed in order to obtain carbamates as anions, combined with
protonated organic superbases as cations (Scheme 1).
Scheme 1. Reaction of carbamate formation using CO2 and a superbase SB such as DBU
* Corresponding author. E-mail address: l.branco@fct.unl.pt (L.C. Branco).
and TMG.
0040-4020/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.06.082

G.V.S.M. Carrera et al. / Tetrahedron 68 (2012) 7408e7413
7409
The reaction conditions have been optimized according to dif-
ferent parameters, such as reaction time, pressure of CO2, and type
of solvent.
with the carbamate group of the anion, provided by TMG based
cation (one NH2 group ) when compared with DBU (one NH group).
In conclusion, TMG can be used as efficient alternative to DBU17
in order to obtain carbamate-based salts in high purity.
When the reaction leads to the formation of the desired
carbamate-based organic salt, the compounds were characterized
by FTIR, 1H and 13C NMR. Compounds 1a and 1b present two peaks
in the region of 163e166 ppm, one concerning the quaternary
amidinium cation, and the other concerning the carbon of carba-
mate functionality. In the infrared spectra, 1a presents a band
centred at 3324 cmꢁ1 indicating the presence of amidinium and
Primaryalkylamines(butylamineandoctylamine), arylamines(1-
naphthylamine, benzylamine, aniline), diamine (1,6-diaminohexane),
and tris(2-ethylamino)amine were tested in order to obtain the re-
spective carbamates as anions. As described before, these reactions
have been done in the presence of the selected superbases (DBU and
tetramethylguanidine) (Fig.1). One equivalent of superbase was used
in the case of mono-amines, while 2 and 3 equiv were required for the
diamine and the tri-functionalized amine, respectively.
Fig. 1. Structures of the cations and anions of the prepared ionic liquids.
Tables 1 and 2 summarize the different reaction conditions and
the correspondent yields of each organic salt. The superbase DBU
was efficient with most of the amines tested. Similar yields (above
70%) were obtained for alkyl amines and aryl amine, however
higher pressures and longer reaction times in the case of 1e was
required. DBU could be efficient as superbase in the case of 1,6-
diaminohexane but no reaction could be achieved with tris(2-
ethylamino)amine. Tetramethylguanidine as an alternative or-
ganic superbase allowed complete reactions (yields of 100%
detected by the analysis of crude NMR) in the case of butylamine 2a
and octylamine 2b. Higher yields and purities were achieved for
primary alkyl amines using the superbase tetramethylguanidine
(TMG) comparing with DBU. Similar behaviour was observed for
aryl amines when the amine functionality is not bonded directly to
the aromatic ring (e.g., benzylamine). Contrarily, no reaction was
observed with 1-naphthylamine and aniline using TMG as super-
base. In general, DBU works better than tetramethylguanidine
when the amine functionality is directly bonded to the aromatic
ring (compounds 1d and 1e versus compounds 2d and 2e). How-
ever, TMG showed a better performance than DBU for multi-func-
tionalized amines.
Table 1
Optimization of reaction conditions in order to obtain carbamate-based salts using
DBU as superbase
Compound Time of reaction (h) Pressure CO2 (bar) Solvent Yieldd (%)
1a
1b
1c
1d
1e
3
3
5
20
Neata
75d
Neata
84d
3
3
5
20
Neata
Neata
85d
83d
8
3
5
20
DCMb
Neata
77d
No reactiond
3
8
20
20
DCMb
DCMb
91d
95d
3
8
8
8
8
8
20
20
40
40
60
60
Neata
Neata
ACNc
ACNc
ACNc
Neata
No reactiond
25d
No reactiond
31e
39e
75e
1f
8
8
20
20
DCMb
DCMb
84d
Two possible mechanistic explanations can be considered: (a)
the differences in reactivity between TMG and DBU superbases for
multi-functionalized amines could be related with the higher pKa of
TMG when compared with DBU. This difference can provide a more
efficient deprotonation of the multi-functionalized amine. (b) The
difference of charge stabilization provided by cations based on TMG
and DBU, related with the more effective hydrogen bond network
1g
No reactiond
a
Without solvent.
Dichloromethane.
Acetonitrile.
b
c
d
e
Yield of reaction at rt calculated from 1H NMR.
Yield of reaction at 40 ꢀC calculated from 1H NMR.

7410
G.V.S.M. Carrera et al. / Tetrahedron 68 (2012) 7408e7413
Table 2
The FTIR spectra of 2a and 2b present carbamate stretching bands
in similar position comparative to 1a and 1b, and it possesses
a band in the region of 1600e1610 cmꢁ1 indicative of guanidine
stretching modes.
Optimization of reaction conditions in order to obtain carbamate-based salts using
tetramethylguanidine as superbase
Compound Time of reaction (h) Pressure CO2 (bar) Solvent Yielda (%)
2a
2b
2c
2d
4
3
20
5
Neata
100d
When aryl amines are used instead of primary alkyl amines,
two different scenarios are observed with tetramethylguanidine
as superbase. When the amine functionality is not bonded di-
rectly to the aromatic ring (benzylamine), the reaction proceeds
with 100% yield (2c), better performance than with DBU (1c).
Differently, when the amine functionality is bonded directly to
the aromatic ring (1-naphthylamine and aniline) no reaction
occurs at the conditions tested. In this case DBU presents a better
performance. The obtained compound 2c, presents characteristic
bands at 1594 and 1561 cmꢁ1 of guanidinium and carbamate
stretches. In addition, two peaks in the region of 163e166 ppm
characteristic of carbamate and guanidinium quaternary carbons
were obtained.
Neata
100d
3
3
20
5
Neata
Neata
100d
100d
8
8
20
20
DCMb
Neata
100d
100d
3
3
8
8
3
8
20
20
20
20
20
40
Neata
DCMb
Neatb
ACNc
ACNc
DCMb
No reactiond
No reactiond
No reactiond
No reactiond
No reactione
No reactiond
With tetramethylguanidine as base, it was possible to obtain
multiple functionalizations with CO2 in high yield as described in
the case of 2f and 2g. This result showed that tetramethylguanidine
could be an alternative, and in same cases better, organic superbase
in order to obtain multi-functionalized salts.
Compound 2f presents characteristic FTIR bands at 1648 and
1608 and 1589 cmꢁ1 of guanidinium and carbamate stretches,
presenting also characteristic peaks in the region of 161e166 ppm
in the 13C spectrum, one peak at 161.78 ppm characteristic guani-
dinium quaternary carbon and a double peak at 165.27 and
165.58 ppm characteristic of carbamate quaternary carbon. The
double peaks are common when chloroform is used as NMR
solvent.
Additionally, compound 2g presents two peaks at 161.77 and
165.08 ppm indicative of the presence of guanidinium quaternary
carbon and carbamate functionality. The FTIR spectrum presents
a band centred at 3333 cmꢁ1 indicative of guanidinium and car-
bamate NH stretching modes. The same spectrum presents bands
at 1624 and 1599 cmꢁ1 indicative of guanidinium and carbamate
stretching modes, respectively.
The syntheses of compounds 1f, 2f, and 2g, based on difunc-
tionalized and tri-functionalized carbamates open good perspec-
tives on a new path to obtain poly-functionalized carbamates. It
should be emphasized that these functionalizations can be very
attractive in order to obtain effective CO2 capture systems, and they
open the possibility of tuning physico-chemical properties simply
by addition of CO2.
2e
2f
8
8
20
60
Neata
ACNc
No reactiond
No reactione
8
20
20
DCMb
Neatb
100d
98d
2g
8
a
Without solvent.
Dichloromethane.
Acetonitrile.
b
c
d
e
Yield of reaction at rt calculated from 1H NMR.
Yield of reaction at 40 ꢀC calculated from 1H NMR.
carbamate NH stretching modes, 1b possesses an equivalent band
centred at 3391 cmꢁ1. Both compounds possess bands at 1648 cmꢁ1
indicative of amidinium stretching modes and a band in the region
of 1570e1600 cmꢁ1 indicative of carbamate stretching.
Considering the aromatic amines, when the base is DBU, the de-
sired products 1c, 1d, and 1e were obtained. Compound 1c presents
two peaks in the region of 164e166 ppm related with the amidinium
quaternary carbon and carbamate carbon. FTIR spectra present one
band centred at 3391 cmꢁ1 indicative of amidinium and carbamate
NH stretching modes, additionally a carbamate stretching at
1574 cmꢁ1 is obtained. Similarly compound 1d presents two peaks in
the regionof160e164 ppm andFTIRbandscentredat3350,1646, and
1577 cmꢁ1. Higher pressures (60 bar) and longer reaction time were
required in order to obtain 75% of product 1e (Table 1). In this case the
reaction performs much better without solvent. Compound 1e
presentsa band at3341 cmꢁ1 indicative ofamidinium and carbamate
NH stretching modes. Bands at 1644 and 1602 cmꢁ1 and two peaks in
the region of 160e166 ppm in the 13C spectra are indicative of car-
bamate carbon and amidininium quaternary carbon.
Differently, the difunctionalized carbamate 1f was obtained with
84% yield using lower pressures of CO2 (20 bar). This is an example of
the possibility to obtain multi-functionalization using CO2. Com-
pound 1f presents a band centred at 3390 cmꢁ1, indication of ami-
dinium and carbamate NH stretching modes. In the same way, the
compound presented one band centred at 1648 cmꢁ1 and another
centred at 1567 cmꢁ1, which can be indication of amidine and car-
bamate stretching, respectively. The compound also presented two
peaks in the region of 164e165 ppm in the 13C spectrum indicative of
amidine and carbamate functionality.
The great majority of the prepared compounds showed melting
points lower than 100 ꢀC, as indicated in Table 3. Compound 2f
presents a melting point of 135 ꢀC, which is in fact a decomposition
temperature; compounds 1f and 2g also started to liberate CO2
during heating, indicative of decomposition before melting. All
remaining compounds presented low melting point and return to
solid state after melt. These low melting points are due to the de-
22
localization of the negative charge
of the carbamate, and
Table 3
Physical state and melting point of the prepared salts
Compound
Physical state
Melting point (ꢀC)
1a
1b
1c
1d
1e
1f
2a
2b
2c
2f
White paste
Yellow paste
Yellow oil
50
58
d
It was not possible to obtain compound 1g after 8 h of reaction
at 20 bar, room temperature, using dichloromethane as solvent.
When the base is changed to tetramethylguanidine, the derived
products 2a and 2b were obtained with 100% yield, a considerable
higher yield than was obtained for compounds 1a and 1b. Com-
pounds 2a and 2b present two bands in the region of 163e166 ppm
in the 13C NMR spectra, one indicative of tetramethylguanidine
quaternary carbon, the other indicative of carbamate functionality.
Purple paste
Yellow paste
Yellow paste
White solid
White solid
White solid
White solid
Orange solid
40
75
55
75
55
60
135
57
2g

G.V.S.M. Carrera et al. / Tetrahedron 68 (2012) 7408e7413
7411
23
Table 4
a possible positive charge delocalization
guanidinium cations.
of amidinium and
Stability/reversibility tests of novel ILs
Concerning the melting point there is no clear trend when pri-
mary alkyl amines are used: Compound 1a presents a melting point
of 50 ꢀC and 1b melts at 58 ꢀC. In this case, when the cation is based
on DBU, there is a decrease of the melting point when the size of the
alkyl chain increases.
Differently, when the superbase is tetramethylguanidine there
is an increment of the melting point going from butyl to octyl
chains; in this case the increment of London interactions leads to an
increase of melting point.
1a
2a
(1) Exposed to air at room temperature, neat
Stablea
Reversiblea
(2) Exposed to air, dissolved in ACN at 60 ꢀ
C
Stableb
Reversibleb,c
Reversiblec
All the stability tests were followed by TLC plates (with methanol/CHCl3 in a volu-
metric 1:1 proportion).
a
Tested for 12 h.
Tested after 90 min.
Tested after 5 h.
b
c
A possible reason for the dissimilar trends observed considering
the different superbases tested can be related with the stronger
interaction between TMG based cation and carbamate group (two
hydrogens per two oxygens in hydrogen bond network), while with
DBU based cation only one hydrogen is available. In both situations
the Londondispersive forces increasewith the incrementof the alkyl
Concerning compounds 1a and 2a the reactions proceed nor-
mally in the presence of CO2, leading to the respective carbamate
salt. The obtained carbamate salts were then exposed either to air
at room temperature (rt) as neat or at 60 ꢀC in acetonitrile as shown
in Scheme 2.
Scheme 2. Reversibility reaction tested using (CO2/air, 60 ꢀC).
chain. The prepared salts based on benzyl and 1-naphthylamine
present lower melting points than the alkyl based salts,1c is liquid at
room temperature and 1d presents a melting point of 40 ꢀC. Both
salts are based on small anions and cations with both the positive
and negative charges delocalized. Probably pi stacking are very
relevant interactions tothe melting point of these salts with 1d, with
an increased pi system leading to higher melting point than 1c.
Compound 1e based on aniline presents a melting of 75 ꢀC. This
melting point is higher than for 1d and 1c. Compound 1e is a small
compound, as the anion is a small, rigid molecule, differently from
compounds 1d and 1c.
Compound 1a is stable to air at room temperature (after a 12 h
exposure), stable to air at 60 ꢀC (after 90 min exposure), and re-
versible after 5 h.
Compound 2a presented a different behaviour. This compound
is reversible when exposed to air at room temperature and at 60 ꢀC.
In this case tetramethylguanidine can lead to more reversible
systems than DBU, of course, this means that DBU leads to more
stable compounds. The preferential use of one of these bases de-
pends on the application.
3. Conclusion
Compounds 1f and 2g started to release CO2 at the temperatures
indicated in Table 1. This might occur because a dianion or a tri-
anion can be relatively unstable when compared with the mono-
functionalized carbamates.
New ionic liquids were prepared using primary alkyl and aryl
amines in combination with DBU and tetramethylguanidine as
superbases. It was possible to obtain dicarbamates and tricarba-
mates, which open new perspectives concerning the possibility to
obtain polycarbamates. These types of multi-functionalized sys-
tems can be relevant in the context of CO2 capture.
Tetramethylguanidine (TMG) can be an alternative base to DBU
in order to obtain high yields of carbamate salts, either when pri-
mary amines or polyamines are used.
Compound 2c possesses a melting point of 60 ꢀC, while com-
pound 1c is liquid at room temperature. With 2c (with tetrame-
thylguanidine) the interaction between cation and anion could be
more effective than the interaction in 1c. Tetramethylguanidine has
a higher number of positions capable of hydrogen bonding than
DBU and this factor might contribute to a higher value of melting
point.
DBU and tetramethylguanidine present different reversibility/
stability profiles, tetramethylguanidine can lead to more reversible
systems, while DBU produces more stable carbamate salts.
The melting point of compound 2g presents an intermediate
value of 57 ꢀC. The conformational degrees of freedom of the anion
and the possibility to delocalize negative charge might compensate
the presence of three negative charges in the same structure of the
anion.
Interestingly compound 2f presents a higher melting point than
the other carbamate salts in Table 3 (135 ꢀC decomposition tem-
perature). In this case factors such as the presence of dianion and
the strong interaction between cation and anion (tetramethylgua-
nidine) might overcome the conformational degrees of freedom of
the hexaalkyl chain.
Compounds 1a and 2a were further tested for reversibility/sta-
bility (Table 4). Both compounds are based on n-octylamine com-
bined either with DBU (1a) or tetramethylguanidine (2a). This
study permitted to check the effect of the superbase on the
stability versus reversibility properties of the carbamate-based
salts.
4. Experimental section
4.1. General
All commercial reagents were used as supplied. n-Octylamine
with a purity of 99%, n-butylamine ꢂ99%, 1-naphthylamine 98%,
benzylamine 99%, aniline 99%, 1,6-diaminohexane 98%, tris-(2-
aminoethyl)amine 96%, and 1,1,3,3 tetramethylguanidine 99%
were supplied by SigmaeAldrich. 1,8-Diazabicyclo[5.4.0]undec-7-
ene (DBU) 99% was supplied by Alfa Aesar.
1H and 13C NMR spectra were recorded on a Bruker AMX400
spectrometer. Chemical shifts are reported downfield in parts per
million from a tetramethylsilane reference.

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G.V.S.M. Carrera et al. / Tetrahedron 68 (2012) 7408e7413
IR spectra were recorded on a Perkin Elmer FTIR Spectrometer,
4.2.4. (1d) [DBUH]þ[1-naphthylNCO2]ꢁ
Spectrum1000.The removalofthevolatilesolventwasaccomplished
under high vacuum. The melting point was measured using the fol-
lowing method: to a capillary tube (open in one side) was introdu-
4.2.4.1. 2,3,4,6,7,8,9,10-Octahydropyrimido[1,2-a]azepin-1-ium
naphthalen-1-ylcarbamate. Prepared following the general pro-
cedure for the synthesis of carbamate salts from 1-naphthylamine
(0.28 g, 1.95 mmol) and DBU (0.3 g, 1.95 mmol) in dichloro-
methane (1 ml) under a 20 bar pressure atmosphere of CO2. The
liquid mixture was stirred for 8 h at room temperature. After that
time, the system was depressurized and a purple paste was
obtained as product after removing the solvent under vacuum
(95%). Mp 40 ꢀC. FTIR nmax (NaCl, CH2Cl2): 3350, 3238, 3045, 2933,
2877, 1646, 1577, 1514, 1460, 1408, 1374, 1323, 1206, 1116, 1089,
ced gaseous CO2, subsequently was introduced
a sample of
carbamate-based salt into the tube, was added more CO2 to the
tube and the tube was sealed with a rubber cap, the melting point
was measured in a Electrothermal melting point apparatus Cat no
IA6304.
4.2. General procedure for the synthesis of carbamate salts
1014, 983, 834, 792, 774 cmꢁ1 1H NMR (400 MHz, CDCl3)
. d:
CO2 was added to a mixture of a primary amine and superbase in
a vial inside a high-pressure reactor until a specific value of pres-
sure was reached (5 or 20 bar). The liquid mixture was stirred at
room temperature during (3e8 h). After the period of reaction, the
system was depressurized and a specific workup was performed.
The compounds were stored at a temperature of 9 ꢀC.
1.54e1.64 (6H, m), 1.83 (2H, m), 2.60 (2H, m), 3.23e3.31 (6H, m),
6.75e6.77 (1H, d, 5.7 Hz), 7.41e7.43 (2H, m), 7.76e7.82 ppm (2H,
m). 13C NMR
d: 20.69, 24.75, 27.45, 29.21, 34.12, 40.51, 48.27, 53.36,
109.40, 118.56, 120.80, 123.47, 124.63, 125.65, 126.22, 128.31,
134.22, 142.08, 160.80, 163.94 ppm.
4.2.5. (1e) [DBUH]þ[phenylNCO2]ꢁ
4.2.1. (1a) [DBUH]þ[octylNCO2]ꢁ
4.2.5.1. Bis(dimethylamino)methaniminium
phenylcarbamate.
4.2.1.1. 2,3,4,6,7,8,9,10-Octahydropyrimido[1,2a]azepin-1-ium oc-
tylcarbamate. Prepared following the general procedure for the syn-
thesis of carbamate salts from n-octylamine (0.255 g, 1.95 mmol) and
DBU (0.3 g, 1.95 mmol) under a 5 bar pressure atmosphere of CO2. The
liquid mixture was stirred for 3 h at room temperature. After that time,
the system was depressurized and white paste was obtained as product
(75%). Mp 50 ꢀC. FTIR nmax (NaCl, CH2Cl2): 3324, 2955, 2921, 2851, 1648,
Prepared following the general procedure for the synthesis of car-
bamate salts from aniline (0.18 g, 1.95 mmol) and DBU (0.3 g,
1.95 mmol) under a 60 bar pressure atmosphere of CO2. The liquid
mixture was stirred for 8 h at 40 ꢀC. After that time the system was
depressurized and a yellow paste was obtained as product (75%). Mp
75 ꢀC. FTIR nmax (NaCl, CH2Cl2): 3341, 3224, 3036, 2931, 2683, 1856,
1644, 1602, 1600, 1567, 1499, 1449, 1421, 1323, 1299, 1242, 1206, 1174,
1607, 1568, 1491, 1473, 1384, 1325, 1207, 1160, 1107, 983, 824, 721 cmꢁ1
1H NMR (400 MHz, CDCl3)
: 0.79 (3H, m), 1.13e1.18 (10H, m), 1.36 (2H,
m), 1.59e1.67 (6H, m), 1.92 (2H, m), 2.79 (2H, m), 3.01 (2H, m),
3.37e3.38 ppm (6H, m). 13C NMR
: 13.89, 19.73, 22.44, 24.12, 26.97,
.
1155, 1108, 1035, 994, 889, 835, 753, 694, 635 cmꢁ1
(400 MHz, D2O) : 1.55e1.65 (6H, m), 1.84e1.88 (2H, m), 2.46e2.49
(2H, m), 3.16e3.19 (2H, t, 5.6 Hz), 3.35e3.43 (4H, m), 6.73e6.78 (3H,
m), 7.11e7.22 ppm (2H, m). 13C NMR
: 18.87, 23.28, 25.83, 28.40,
.
1H NMR
d
d
d
d
29.02, 29.10, 29.27, 30.63, 31.64, 32.12, 38.26, 41.68, 48.43, 53.84, 163.77,
165.44 ppm.
32.75, 37.93, 48.15, 54.09, 116.36, 119.44, 129.47, 146.16, 160.78,
165.87 ppm.
4.2.2. (1b) [DBUH]þ[butylNCO2]ꢁ
4.2.6. (1f) 2[DBUH]þ[hexdiNCO2]ꢁ
4.2.2.1. 2,3,4,6,7,8,9,10-Octahydropyrimido[1,2-a]azepin-1-ium
butylcarbamate. General procedure for the synthesis of carbamate salts
from n-butylamine (0.143 g, 1.95 mmol) and DBU (0.3 g, 1.95 mmol)
under a 5 bar pressure atmosphere of CO2 was followed. The liquid
mixture was stirred for 3 h at room temperature. After that time, the
system was depressurized and yellow paste was obtained as product
(85%). Mp 58 ꢀC. FTIR nmax (NaCl, CH2Cl2): 3391, 2934, 2875, 1648, 1600,
4.2.6.1. 2,3,4,6,7,8,9,10-Octahydropyrimido[1,2-a]azepin-1-ium
hexane-1,6-diyldicarbamate. Prepared following the general pro-
cedure for the synthesis of carbamate salts from 1,6-
diaminohexane (0.11 g, 0.975 mmol) and DBU (0.3 g, 1.95 mmol)
in dichloromethane (2 ml) under a 20 bar pressure atmosphere of
CO2. The liquid mixture was stirred for 8 h at room temperature.
After that time, the system was depressurized and a yellow paste
was obtained as product after removing solvent under vacuum
(84%). Mp 55 ꢀC. FTIR nmax (NaCl, CH2Cl2): 3390, 2933, 2860, 1648,
1447,1360,1324,1207,1197, 984, 834 cmꢁ1.1H NMR (400 MHz, CDCl3)
d:
0.83e0.85 (3H, m), 1.27e1.29 (2H, m), 1.38 (2H, m), 1.62e1.69 (6H, m),
1.93e1.94 (2H, m), 2.80 (2H, m), 3.04e3.06 (2H, m), 3.39 ppm (6H, m).
1567, 1472, 1385, 1324, 1207, 1159, 1107, 984, 833 cmꢁ1
(400 MHz, CDCl3) : 1.27e1.38 (8H, m), 1.58e1.66 (12H, m),
1.86e1.88 (4H, m), 2.60e2.66 (4Hþ2H, m), 3.04 (2H, m),
3.29e3.34 ppm (12H, m). 13C NMR
: 20.66, 24.76, 26.63, 26.81,
.
1H NMR
13C NMR
d: 13.83, 19.97, 24.30, 27.12, 29.10, 32.60, 32.86, 38.74, 39.65,
d
41.47, 48.50, 53.88, 164.06, 165.23 ppm.
d
4.2.3. (1c) [DBUH]þ[BzNCO2]ꢁ
27.53, 29.29, 30.77, 33.78, 40.18, 41.75, 41.94, 48.43, 53.51, 164.09,
164.23 ppm.
4.2.3.1. 2,3,4,6,7,8,9,10-Octahydropyrimido[1,2-a]azepin-1-ium
benzylcarbamate. Prepared following the general procedure for the
synthesis of carbamate salts from benzylamine (0.209 g,
1.95 mmol) and DBU (0.3 g, 1.95 mmol) under a 20 bar pressure
atmosphere of CO2. The liquid mixture was stirred for 3 h at room
temperature. After that time, the system was depressurized and
yellow oil was obtained as product (83%). FTIR nmax (NaCl, CH2Cl2):
3391, 2936, 2864, 2818, 1648, 1574, 1474, 1453, 1382, 1324, 1207,
1159,1107,1028, 984, 820, 740, 701 cmꢁ1. 1H NMR (400 MHz, CDCl3)
4.2.7. (2a) [TMGH]þ[octylNCO2]ꢁ
4.2.7.1. Bis(dimethylamino)methaniminium octylcarbamate. Pre-
pared following the general procedure for the synthesis of carba-
mate salts from n-octylamine (0.34 g, 2.60 mmol) and
tetramethylguanidine (0.3 g, 2.60 mmol) under a 20 bar pressure
atmosphere of CO2. The liquid mixture was stirred for 4 h at room
temperature. After that time, the system was depressurized and
a white solid was obtained as product (100%). Mp 75 ꢀC. FTIR nmax
(NaCl, CH2Cl2): 3333, 2955, 2920, 2850,1636, 1609, 1567,1491, 1472,
d
: 1.58e1.65 (6H, m), 1.87e1.90 (2H, m), 2.74 (2H, m), 3.33e3.34
(6H, m), 4.28 (2H, s), 7.11e7.22 ppm (5H, m). 13C NMR
d
: 19.81, 24.21,
27.05, 29.11, 32.36, 38.43, 45.86, 48.54, 54.00, 126.23, 127.14, 128.11,
141.83, 163.82, 165.57 ppm.
1411, 1332, 1064, 1038, 826 cmꢁ1. 1H NMR (400 MHz, CDCl3)
(3H, m), 1.24 (10H, m), 1.40 (2H, m), 2.64 (1H, m), 2.82 (12H, s),
d: 0.85

G.V.S.M. Carrera et al. / Tetrahedron 68 (2012) 7408e7413
7413
3.04 ppm (1H, m). 13C NMR
d
: 14.03, 22.59, 26.87, 27.05, 29.26,
4.2.11. (2g) [TMGH]33þ[tris(2-ethylNCO2)amine]ꢁ
29.42, 30.81, 31.79, 33.50, 39.37, 41.82, 163.46, 165.45 ppm.
4.2.11.1. Bis(dimethylamino)methaniminium 2,20,200-nitrilotris(-
ethane-2,1-diyl)tricarbamate. Prepared following the general pro-
cedure for the synthesis of carbamate salts from tris(2-aminoethyl)
amine (0.126 g, 0.8595 mmol) and tetramethylguanidine (0.3 g,
2.60 mmol) in dichloromethane (2 ml) under a 20 bar pressure at-
mosphere of CO2. The liquid mixture was stirred for 8 h at room tem-
perature. After that time, the system was depressurized and an orange
solid was obtained as product after removing solvent under vacuum
(98%). Mp 57 ꢀC. FTIR nmax (NaCl, CH2Cl2): 3333, 2941, 2807,1624,1599,
4.2.8. (2b) [TMGH]þ[butylNCO2]ꢁ
4.2.8.1. Bis(dimethylamino)methaniminium
butylcarbamate. -
Prepared following the general procedure for the synthesis of car-
bamate salts from n-butylamine (0.19 g, 2.60 mmol) and
tetramethylguanidine (0.3 g, 2.60 mmol) under a 20 bar pressure
atmosphere of CO2. The liquid mixture was stirred for 3 h at room
temperature. After that time, the system was depressurized and
a white solid was obtained as product (100%). Mp 55 ꢀC. FTIR nmax
(NaCl, CH2Cl2): 3367, 2955, 2784, 1658, 1610, 1566, 1463, 1411, 1063,
1556, 1426, 1406, 1308, 1121, 1041, 1035, 833, 763 cmꢁ1
.
1H NMR
(400 MHz, D2O) : 2.50e2.54 (2.5H, m), 2.64e2.70 (2.5H, m), 2.76 (2H,
d
1038, 864, 836 cmꢁ1
1.30e1.39 (4H, m), 2.64 (1H, m), 2.81 (12H, s), 3.04 ppm (1H, m). 13C
NMR : 13.88, 20.11, 32.93, 39.37, 41.49, 163.49, 165.47 ppm.
.
1H NMR (400 MHz, CDCl3)
d: 0.87 (3H, m),
m), 2.85 (36H, s), 2.88e2.92 (2.5H, m), 2.99e3.02 ppm (2.5H, t,
6.12 Hz), 13C NMR
165.08 ppm.
d: 37.70, 39.23, 51.90, 52.67, 54.31, 161.77,
d
4.2.9. (2c) [tmgh]þ[BzNCO2]ꢁ
Acknowledgements
ˇ
4.2.9.1. Bis(dimethylamino)methaniminium benzylcarbamate. -
Prepared following the general procedure for the synthesis of car-
bamate salts from benzylamine (0.276 g, 2.60 mmol) and
tetramethylguanidine (0.3 g, 2.60 mmol) in dichloromethane
(2 ml) under a 20 bar pressure atmosphere of CO2. The liquid
mixture was stirred for 8 h at room temperature. After that time,
the system was depressurized and a yellow solid was obtained as
product after removing solvent under vacuum (100%). Mp 60 ꢀC.
FTIR nmax (KBr): 3446, 3238, 3180, 3026, 3003, 2929, 2893, 2817,
1677, 1594, 1561, 1494, 1473, 1431, 1419, 1374, 1330, 1302, 1256, 1241,
1200, 1143, 1101, 1065, 1052, 1033, 922, 881, 833, 811, 749, 729, 704,
~
Fundac¸ ao para a Ciencia e a Tecnologia is acknowledged under
PEst-C/EQB/LA0006/2011; PTDC/CTM/103664/2008 and post-
doctoral fellowship GVSMCdSFRH/BPD/72095/2010. The NMR
spectrometers are part of the Natioˇ nal NMR Network (RNRMN) and
~
are funded by Fundac¸ ao para a Ciencia e a Tecnologia (FCT).
References and notes
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New J. Chem. 2007, 31, 1429.
629, 616 cmꢁ1. 1H NMR (400 MHz, CDCl3)
d
: 2.82 (12H, s), 3.84 (1H,
: 39.35, 45.76, 126.15,
s), 4.30 (1H, s), 7.14e7.30 (5H, m). 13C NMR
127.09, 128.02, 141.96, 163.3, 165.06 ppm.
d
4.2.10. (2f) [TMGH]22þ[hexdiNCO2]ꢁ
7. Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Zadigian, D. J.; Price, E. A.;
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4.2.10.1. Bis(dimethylamino)methaniminium hexane-1,6-
diyldicarbamate. Prepared following the general procedure for the
synthesis of carbamate salts from 1,6-diaminohexane (0.15 g,
1.3 mmol) and tetramethylguanidine (0.3 g, 2.60 mmol) in
dichloromethane (2 ml) under a 20 bar pressure atmosphere of
CO2. The liquid mixture was stirred for 8 h at room temperature.
After that time, the system was depressurized and an orange solid
was obtained as product after removing solvent under vacuum
(100%). Mp 135 ꢀC. FTIR nmax (KBr): 3510, 3331, 3281, 3107, 2928,
2856, 1648, 1608, 1589, 1500, 1482, 1465, 1447, 1434, 1424, 1412,
1399, 1364, 1305, 1223, 1197, 1147, 1085, 1078, 1063, 1038, 1009, 992,
872, 848, 832, 815, 791, 726, 698, 682, 647, 618 cmꢁ1
(400 MHz, D2O) : 1.18e1.36 (6H, m), 1.47e1.49 (2H m), 2.65e2.78
(2H, m), 2.83 (24H, s), 2.85e2.88 (2H, m), 13C NMR
: 25.68, 25.76,
.
1H NMR
d
d
25.87, 26.30, 28.14, 28.41, 29.70, 29.95, 39.22, 40.03, 41.47, 41.68,
161.78, 165.27, 165.58.
22. Izgorodina, E. I. Phys. Chem. Chem. Phys. 2011, 13, 4189.
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