Fast equilibrium of zwitterionic adduct formation in reversible fixation-release system of CO2 by amidines under dry conditions

Article abstract of DOI:10.1016/j.tet.2013.04.110

We investigated the fixation of CO₂ by several amidines in solution and found that simple monocyclic amidines fixed CO₂ under dry conditions to quantitatively afford the corresponding bicarbonates through hydrolysis of the zwitterion

Full text of DOI:10.1016/j.tet.2013.04.110

Tetrahedron 69 (2013) 5476e5480
Contents lists available at SciVerse ScienceDirect
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journal homepage: www.elsevier.com/locate/tet
Fast equilibrium of zwitterionic adduct formation in reversible
fixationerelease system of CO by amidines under dry conditions
2
a
,
b
a
c
a,b,
*
Naoto Aoyagi , Yoshio Furusho , Yoshihisa Sei , Takeshi Endo
a
Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan
Research Project of Advanced Materials, Yamagata Promotional Organization for Industrial Technology, 2-2-1 Matsuei, Yamagata 990-2473, Japan
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
We investigated the fixation of CO2 by several amidines in solution and found that simple monocyclic
amidines fixed CO under dry conditions to quantitatively afford the corresponding bicarbonates through
2
hydrolysis of the zwitterionic adducts by adventitious water. In contrast, linear amidines did not form the
bicarbonates under the same conditions, while 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) could not ef-
Received 2 February 2013
Received in revised form 18 April 2013
Accepted 21 April 2013
Available online 30 April 2013
fectively trap CO
bubbling at a temperature as low as 60 C. The CO
2
. The CO
2
fixed by the monocyclic amidines was quantitatively released by argon
ꢀ
2
fixationerelease process could be repeated several
Keywords:
Amidines
Carbon dioxide fixation
Zwitterions
times without loss of the high efficiency.
Ó 2013 Elsevier Ltd. All rights reserved.
Bicarbonate
NMR spectroscopy
1. Introduction
amidines and guanidines have been utilized as efficient catalysts for
CO -insertion reactions. The CO -binding reactions seem to occur
2 2
7
The increasing concentration of carbon dioxide (CO
mosphere has resulted in the rising global temperature and serious
environmental problems, while CO is also regarded as a cheap,
green C1 resource. Development of efficient methods for CO ad-
sorption has thus become an important research area in recent
years. One of the most well-researched processes for CO capture is
based on chemically reversible CO fixation by primary or sec-
ondary amines at ambient temperature to give ammonium carba-
2
) in the at-
simultaneously with the capture of water molecules to yield bi-
carbonates and have been recently applied to switchable ionic
4
bee
2
liquids and surfactant.
unable to efficiently trap CO
dry conditions and their CO
amount of water (e.g., 700 ppm for DBU) for the formation of the
On the other hands, the amidines are
even at high pressure of CO under
-binding process requires a certain
1
2
2
2
2
2
5
b
2
bicarbonate salts. The CO
to involve a zwitterionic adduct between the amidine and CO
existence of which was also reported to occur between CO and the
2
-binding reactions have been suggested
2
, the
mates, and CO
2
release from the ammonium carbamates upon
2
heating.2 Styrene- or siloxane-based polymers bearing amino
free amino groups of proteins. Recently, Villiers et al. reported the
isolation and full characterization including single crystal X-ray
8
3
groups have been also studied for application to CO
2
fixation.
However, these processes require 2 equiv of amines for CO
2
fixa-
analysis of the zwitterionic adduct between 1,5,7-triazabicyclo
2
,3
5d
tion and high temperature for CO
The use of amidines and guanidines, such as 1,8-diazabicyclo
5.4.0]undec-7-ene (DBU) for reversible CO fixationerelease pro-
cesses is considered more attractive than the use of primary or
secondary amines, since the CO fixation requires 1 equiv of the
amidines and the CO release proceeds readily at low tempera-
ture. We have also reported reversible CO fixation by polymers
bearing cyclic amidines under wet conditions. In addition,
2
release.
[4.4.0]dec-5-ene (TBD) and CO
2
.
However, all attempts to iso-
late and characterize the zwitterionic adducts between amidines,
such as DBU and CO especially in solution have not yet been
[
2
2
13
successful, although several solid-state C NMR studies revealed
that zwitterionic carbamate species are contained in the solid
2
2
mixtures obtained by evaporation of reaction mixture of amidines
4
,5
5a,e
2
and CO
velopment of CO
requires comprehensive information of the mechanism for CO
2
in dry organic solvents, such as CH
3
CN.
The de-
6
2
recycling and catalytic reactions with amidines
2
capture and release in various reaction media. Moreover, the effect
of the chemical structures of amidines on their binding and release
of CO would be quite useful knowledge for further design and
2
*
Corresponding author. Tel.: þ81 948 22 9210; fax: þ81 948 21 9032; e-mail
addresses: tendo@moleng.fuk.kindai.ac.jp, tendo@me-jsr.fuk.kindai.ac.jp (T. Endo).
0
040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.04.110

N. Aoyagi et al. / Tetrahedron 69 (2013) 5476e5480
5477
4
synthesis of amidine-based CO
lysts. Accordingly, we have investigated the reversible fix-
ationerelease behavior of CO by several amidines including DBU
2
-binding materials as well as cata-
a sufficient amount of water. These results clearly showed that the
reactions of linear amidines with CO were greatly influenced by
2
2
the amounts of water in the reaction mixtures.
under dry conditions, highlighting the mechanism of their CO
binding in solution (Scheme 1).
2
In contrast with these linear amidines, the bicyclic amidines, 1e
(DBN) and 1f (DBU), reacted with the dry CO
ature under the dry conditions, and small amounts of white pre-
cipitates started to form in anhydrous CH CN within 1 h. After
2
at ambient temper-
3
addition of anhydrous ether, the bicarbonate salts, 2e and 2f, were
precipitated as white solids in low yields (Table 1, entries 5 and 6).
The bicarbonates 2 may form through hydrolysis of the zwitterionic
2
intermediates between the amidines 1 and CO by adventitious
water during the reaction and/or the isolation procedure owing to
5
their hygroscopic nature. The compositional formula of the salt 2f
was confirmed by elemental analysis, showing that the salt consists
of the amidinium and bicarbonate ions (Found: C, 55.99; H, 8.44; N,
1
3.07. Calc. for C10
H
18
N
2
O
3
: C, 56.06; H, 8.47; N, 13.07%). The higher
than those of 1aed can be
reactivities of 1e and 1f toward CO
2
attributed to its higher nucleophilicity, owing to their cyclic
structures. However, the fixation efficiencies using 1e and 1f (yields
of 2e and 2f) were 27% and 39% under the above conditions, which
was less satisfactory in economical and practical usages. In contrast,
the use of an equimolar amount of water as hydrolytic agent
resulted in remarkably increased yields of 92% and 91%, re-
spectively (entries 5 and 6). On the other hand, no precipitation was
2
Scheme 1. CO fixation by amidines.
observed when much drier CO
2
(H
CN. This result suggested that the amount of
water contained in the reaction system is an important factor for
the CO fixation by the cyclic amidine.
2
O content<0.5 ppm) was
5
b
employed in dry CH
3
2
. Results and discussion
First, we examined the CO
2
2
fixation by four linear amidines, N,N-
Next, we investigated the CO2 fixation behavior of two mono-
0
9
0
13
dimethyl-N -octylacetamidine
hexylacetamidine 1b, N,N-dimethyl-N -octylformamidine 1c,
1a,
N,N-dimethyl-N -cyclo-
cyclic amidines, 1-methyl-1,4,5,6-tetrahydropyrimidine 1g and
10
0
11
14
1,4,5,6-tetrahydropyrimidine 1h. Upon bubbling the dry CO into
2
0
12
and N,N-dimethyl-N -cyclohexylformamidine 1d, with dry bub-
bling CO (200 mL/min, H O content <5 ppm) for 1 h in anhydrous
acetonitrile (CH CN) under the dry conditions (Table 1, entries
e4). In all cases, the reaction mixtures remained homogeneous
an anhydrous CH CN solution of 1g (0.5 M), white solids started to
3
2
2
exothermically precipitate in anhydrous CH CN within 10 min. After
3
3
1 h, the bicarbonate salt 2g was isolated in an almost quantitative
5
1
yield, which indicates that the adsorbability of 1g under the dry
and colorless with no indication of precipitate formation under the
dry conditions. Although trace amounts (<1%) of the bicarbonates
were obtained from the acetamidines, 1a and 1b, under the wet
conditions, the formamidines, 1c and 1d, did not form the bi-
carbonates at all in the presence of equimolar amount of water. In
most cases, the amidines were completely recovered as colorless
CO2 atmosphere was comparable to that in sufficiently wet CO
atmosphere (Table 1, entry 7). The compositional formula of 2g was
2
6
also confirmed by elemental analysis and it was further character-
1
13
ized by H and C NMR. Furthermore, the structure of 2g was un-
15
ambiguously confirmed by X-ray crystallographic analysis (Fig. 1).
However, the highly deliquescent character of the bicarbonate salt
ꢀ
liquids by evaporation at 25 C, whereas the linear amidines were
2
reported to react smoothly and quantitatively with CO to give the
bicarbonate salts as white precipitates in wet CH
3
CN containing
Table 1
CO
2
fixation by the amidines under dry and wet conditionsa
Entry
Amidine
Yield/%b
Dry conditionc
Wet conditiond
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
0
0
0
<1
<1
0
0
0
27
39
95
>99
92
91
94
>99
1g
1h
a
Reaction conditions: 5 mmol of 1aeh, 10 mL of anhydrous CH
3
CN, CO
2
bubbling
ꢀ
(
200 mL/min, H
2
O content <5 ppm) at 25 C for 1 h.
b
Isolated.
c
Water only came from the CO
2
gas and/or adventitiously.
d
After completion of CO
a syringe.
2
bubbling, water (5 mmol, 1 equiv) was added via
Fig. 1. Single crystal X-ray structure of 2g with thermal ellipsoids of 50% probability.

5478
N. Aoyagi et al. / Tetrahedron 69 (2013) 5476e5480
2
g hampered gaining its precise thermogravimetric analysis (TGA)
data and melting point. The monocyclic amidine 1h was also shown
to react exothermically with the dry CO in anhydrous CH CN and
2
3
the bicarbonate salt 2h was isolated quantitatively in 1 h (Table 1,
entry 8). The 2h thus obtained did not deliquesce in the air for, at
least, a couple of days. The compositional formula of 2h was also
confirmed by elemental analysis and it was further characterized by
1
13
2
H and C NMR. The higher ability of 1g and 1h to trap CO could be
attributed to the less steric hindrance around the nucleophilic ni-
trogen atoms when compared to those in 1e and 1f.
The thermal stability of 2h was studied by TGA. The analytical
ꢀ
value of the first weight loss at 106 C corresponded to that of CO
2
and H
2
O (anal. 40.1%, calcd 41.8%), and the second weight loss at
ꢀ
159 C corresponded to that of the amidine (anal. 56.2%, calcd.
ꢀ
2
within 1 h at 60 C under nitrogen
5
8.2%). 2h also released CO
bubbling (200 mL/min) in anhydrous dimethyl sulfoxide (DMSO).
The high efficiency of the CO fixationerelease cycle using 1h
was demonstrated in anhydrous DMSO by bubbling the dry CO at
2
2
ꢀ
ꢀ
2
5 C and bubbling Ar at 60 C in an alternate manner. The mixture
ꢀ
was preheated at 60 C under bubbling Ar for 30 min before the
2 2
initial CO fixation step. The CO fixation efficiency of 1h was es-
timated from the weight change of the reaction mixture by using
a blank DMSO solution. As shown in Fig. 2, both the fixation and
release proceeded quantitatively within 1 h by bubbling CO
2
and Ar,
Fig. 3. Partial 13C NMR spectra (100 MHz, 25 ꢀC) of a solution of 1g in CD
with bubbling CO gas followed by addition of an excess water.
3
CN (0.1 M)
respectively. Furthermore, the CO fixing efficiency did not change
2
2
between the first and third fixation steps. The bicarbonate salt 2h
was isolated in an almost quantitative yield (98%) from the reaction
mixture after the third fixation step and it was also subjected to
elemental analysis, which showed that the salt was highly pure as
2
the zwitterionic carbamate 1g-CO appeared at 131.3 ppm, which
16
was different from that of dissolved CO
2
3
in CD CN (125.6 ppm).
ꢁ
The CO2 signal shifted to a high magnetic field with increasing
the salt obtained after the first CO
.13; N, 19.21. Calc. for C
2
O
fixation step (Found: C, 41.30; H,
: C, 41.09; H, 6.90; N, 19.17%).
bubbling time, while the methine signal did not change very much.
This chemical shift change indicates that 1g-CO exchanged its CO2
2
ꢁ
7
5
H
10
N
2
3
These results proved that 1h was a highly efficient recyclable CO
adsorbent in the presence of an extremely low amount of water.
2
moiety with the dissolved CO
cordingly, the CO2 moiety of 1g-CO
2
faster than the NMR time scale. Ac-
and the dissolved CO gave
ꢁ
2
2
the one signal with the averaged chemical shift, which shifted to
a higher magnetic field with bubbling time, as the amount of the
dissolved CO
2
increased. After 2 min of CO
2
bubbling, an excess
water was added to the CD
HCO3 appeared at 160.6 ppm along with that of the dissolved
3
CN solution, a new signal attributable to
ꢁ
17
CO
hydrolysis of 1g-CO
mixture was consistent with the C NMR result. We have similarly
confirmed the zwitterionic adduct formation between 1f and CO in
anhydrous CD CN at the initial stage of the CO -binding reaction as
the intermediate for the bicarbonate 2f (Fig. S3).
Contrary to our expectation, we found by 13C NMR that the
2
at 125.6 ppm, showing the formation of the bicarbonate 2g by
1
2
. The H NMR spectral change of the reaction
13
2
3
2
linear acetamidine 1a also formed the zwitterionic adduct 1a-CO
with CO in anhydrous CD CN (Fig. S1). Upon bubbling the dry CO
into an anhydrous CD
2
2
3
2
3
CN solution of 1a, the amidine signal (NeC]
N) shifted to a low magnetic field by ca. 2 ppm and a new signal
ꢁ
derived from the carbamic CO2 group at 125.8 ppm, which was
2
shown to exist in a fast equilibrium with the dissolved CO mole-
cules. Upon addition of an excess of water, a new signal assignable
ꢁ
to HCO3 ion appeared at 160.5 ppm, accompanied by a low
magnetic field shift of the amidine carbon by ca. 2 ppm. The fact
that the reaction of 1a with CO
recovery of 1a (Table 1, entry 1) indicated that the zwitterionic
adduct 1a-CO (or the bicarbonate 2a formed by hydrolysis of 1a-
CO ) was highly soluble in CH CN owing to the octyl substituent
and the captured CO was released during evaporation of the
2 3
in CH CN resulted in the complete
Fig. 2. Reversible CO
2
fixation and release by 1h. (a) Flow rate of CO
2
and Ar: 200 mL/
2
min; (b) the CO
2
fixation efficiency was calculated based on the formation of the bi-
2
3
carbonate 2h.
2
13
3 3
CH CN. In contrast, the C NMR spectrum of 1c in CD CN did not
We further investigated the CO
2
-binding reaction of 1g in dry
CN by 13C and H NMR spectroscopy, focusing on the initial stage
of the reaction (Figs. 3 and S4). Upon bubbling the dry CO into
a solution of 1g in CD CN at ambient temperature for 1 min, the
show any significant change after bubbling CO gas, indicating that
the carbamic zwitterion 1c-CO2 was not present in a detectable
amount, or the equilibrium state exists very close to the side of the
2
1
CD
3
2
3
free 1c and CO (Fig. S2). The difference in the CO -binding behavior
2
2
signal of the methine carbon (NeC]N) shifted to a lower magnetic
field of 153.1 ppm and a new signal assignable to the CO2 moiety of
could be attributed to the methyl group on the amidine methine
moiety of 1a, which enhances the nucleophilicity of the amidine
ꢁ

N. Aoyagi et al. / Tetrahedron 69 (2013) 5476e5480
5479
nitrogen atom by its electron-donating nature. However, 13C NMR
study revealed that upon addition of an excess water, 1c was
completely converted to the bicarbonate 2c, probably because the
mixture of N-methyl-1,3-diaminopropane (4.41 g, 50 mmol) in
anhydrous toluene (50 mL) at room temperature, and the mixture
ꢀ
was stirred at 80 C for 24 h. The mixture was evaporated in vacuo,
excess water shifted the equilibrium of 1c with CO
2
to the side of 2c.
and the residue was distilled under reduced pressure to give 1g
(4.63 g, 94%) as a colorless liquid; Bp 57e59 C/1.0 kPa. H NMR
Thus, the 13C NMR results revealed that the carbamic zwitterion
formation was highly affected by the chemical structures of ami-
dines under anhydrous conditions, whereas the amidines gave the
bicarbonates in the presence of a sufficient amount of water irre-
spective of the structures.
ꢀ
1
ꢀ
(400 MHz, CDCl
3
, 25 C)
d
(ppm): 1.84 (quin, J¼6.0 Hz, 2H,
NCH
NCH
2
CH
CH
2
CH
CH
2
N), 2.82 (s, 3H, NCH
3
), 3.11 (t, J¼6.0 Hz, 2H,
2
2
2
NCH
3
), 3.27 (t, J¼6.0 Hz, 2H, NCH
2
CH
, 25 C)
2
CH
d
2
NCH
(ppm): 20.6
NCH ), 45.1
), 149.8 (NCHN). IR (neat, cm ): 2928, 2848
NCH ).
3
), 6.92
1
3
ꢀ
(s, 1H, NCHN). C NMR (100 MHz, CDCl
3
(NCH CH
2 2
CH
2
N), 39.3 (NCH
NCH
3
2
), 42.2 (NCH CH
2
CH
2
3
ꢁ
1
3
. Conclusion
2
(NCH CH
2
CH
2
3
(
CH), 1634 (C]N), 1313 (CH
2
3
In conclusion, we found that the simple monocyclic amidines
fixed CO
2
quantitatively under dry conditions to afford the corre-
4.2.2. 1,4,5,6-Tetrahydropyrimidine (1h).14 N,N-Dimethylformamide
dimethyl acetal (6.26 g, 52.5 mmol) was added to the mixture of 1,3-
diaminopropane (3.71 g, 50 mmol) in anhydrous toluene (50 mL) at
sponding bicarbonates through hydrolysis of the zwitterionic ad-
ducts by adventitious water. In contrast, use of the linear amidines
did not form the bicarbonates under the same dry conditions, while
ꢀ
room temperature, and the mixture was stirred at 80 C for 24 h. The
DBU and DBN cannot effectively trap CO
2
. The CO
2
fixed by the
mixture was evaporated in vacuo, and the residue was distilled
monocyclic amidines was quantitatively released after heating at
under reduced pressure to give 1h (3.71 g, 88%) as a colorless liquid;
ꢀ
ꢀ
1
ꢀ
3
, 25 C) d (ppm): 1.83
6
0
C with Ar bubbling. The CO
2
fixationerelease efficiency
Bp 80e82 C/0.1 kPa. H NMR (400 MHz, CDCl
CH CH
remained unchanged after the second cycles under the dry condi-
(quin, J¼6.0 Hz, 2H, NCH
2
2
2
N), 3.30 (t, J¼6.0 Hz, 4H,
13
1
13
tions. We also disclosed by the C and H NMR studies that the
zwitterionic adduct formation between the amidines and CO in
NCH
(100 MHz, CDCl
(NCH CH CH
2
CH
2
CH
2
NH), 4.28 (br s, 1H, NH), 7.11 (s, 1H, NCHN). C NMR
ꢀ
2
3
, 25 C)
d
(ppm): 21.2 (NCH
2 2 2
CH CH N), 41.2
ꢁ1
solution is reversible and in equilibrium faster than the NMR time
scale. This can account for why characterization of zwitterionic
carbamates in solution was not successful despite tremendous ef-
forts. In addition, the effect of the chemical structures of amidines
2
2
2
NH), 147.3 (NCHN). IR (neat, cm ): 3309 (NH), 2932,
2851 (CH), 1635 (C]N).
4.3. Typical procedure for CO
2
fixation
2 2
on the CO capture is remarkable; the acetamidine can bind CO to
form a zwitterionic adduct in solution as effectively as the cyclic
amidines, while the formamidine formed the zwitterionic adduct
Dry CO gas (>99.999%, H O <5 ppm) was bubbled into a solu-
2 2
tion (0.2 L/min) of the amidine (5 mmol) in anhydrous CH
3
CN
2
O
ꢀ
with CO
2
in a detectable amount under the same conditions. This
(10 mL) at 25 C. After 1 h under bubbling CO
2
, anhydrous Et
CN. The
resulting white precipitate was filtered off, washed with anhydrous
Et to give amidinium
O (10 mLꢂ2) and dried in a stream of CO
bicarbonate as a white powder.
can be explained in terms of the nucleophilicity of the amidine ni-
trogen atoms as well as the steric hindrance, and could be quite
important information for further design and synthesis of amidine-
(20 mL) was added to a suspension of the salt in CH
3
2
2
and guanidine-based materials for CO
insertion. Although the zwitterionic adducts between amidines and
CO are less stable than their guanidine counterparts, we believe
2 2
fixation and catalysts for CO
2
4.3.1. 1-Methyl-1,4,5,6-tetrahydropyrimidinium bicarbonate (2g). Yield
1
ꢀ
2
O, 25 C) d (ppm): 2.03 (quin,
that incorporation of bulky substituents would kinetically increase
their stability, and such attempts to isolate and characterize zwit-
763 mg, 95%. H NMR (400 MHz, D
J¼6.0 Hz, 2H, NCH CH CH N), 3.16 (s, 3H, NCH
NCH CH CH NCH
), 3.40 (t, J¼6.0 Hz, 2H, NCH
H, NCHN). C NMR (100 MHz, D
(ppm): 18.8 (NCH CH CH N), 37.3 (NCH
6.1 (NCH CH CH NCH
2
2
2
3
), 3.32 (t, J¼6.0 Hz, 2H,
terionic amidines-CO
2
adducts are ongoing in our laboratory.
2
2
2
3
2
CH
2
CH
2
NCH
3
), 7.82 (s,
1
3
ꢀ
1
d
2
O, 0.4% dioxane (67.4 ppm), 25 C)
4
4
4
. Experimental section
2
2
2
3 2 2 2 3
), 42.1 (NCH CH CH NCH ),
ꢁ
4
2
2
2
3
), 152.7 (NCHN), 161.2 ðHCO Þ. IR (KBr,
3
ꢁ
1
ꢁ
.1. General
cm ): 3412 (OH), 2950 (CH), 1696 (C]N), 1407 ðOCO Þ. Anal. Calcd
2
for C
6 12 2 3
H N O : C, 44.99; H, 7.55; N, 17.49. Found: C, 45.14; H, 7.81; N,
.1.1. Materials and instruments. All starting materials and dehy-
17.47.
drated solvents were purchased from Aldrich, Wako Pure Chemical
Industries (Osaka, Japan), and Tokyo Kasei Kogyo (Tokyo, Japan),
and were distilled prior to use. The melting points were measured
using a Yanaco MP-S3 melting point apparatus (Kyoto, Japan) and
were uncorrected. The NMR spectra were obtained using a JEOL
4.3.2. 1,4,5,6-Tetrahydropyrimidinium
bicarbonate
728 mg, >99%. Mp 90e92 C. H NMR (400 MHz, D O, 25 C)
CH CH
(2h). Yield
d (ppm):
ꢀ
1
ꢀ
2
2.00 (quin, J¼6.0 Hz, 2H, NCH
2
2
2
N), 3.40 (t, J¼6.0 Hz, 4H,
1
3
NCH
2
CH CH
2 2
N), 7.87 (s, 1H, NCHN). C NMR (100 MHz, D
2
O, 0.4%
1
ꢀ
ECS-400 spectrometer operating at 400 MHz for H and 100 MHz
dioxane (67.4 ppm), 25 C)
d
(ppm): 18.5 (NCH
2
CH
2
CH
2
N), 38.6
13
ꢁ
ꢁ1
for C. The IR spectra were recorded as KBr discs or pellets on
a JASCO FT/IR-460 Plus spectrometer. The elemental analyses were
performed by the Perkin Elmer 2400II CHNS/O Analyzer. Ther-
mogravimetric analysis (TGA) was performed at heating and cool-
(NCH
2
CH
2
CH
2
N), 152.1 (NCHN), 161.2 ðHCO Þ. IR (KBr, cm ): 3433
3
ꢁ
(OH, NH), 2980 (CH), 1685 (C]N), 1402 ðOCO Þ. Anal. Calcd for
2
5 10 2 3
C H N O : C, 41.09; H, 6.90; N,19.17. Found: C, 41.13; H, 7.13; N,19.25.
ꢀ
ꢀ
ing ratio of 5 C/min over the temperature range of 25e200 C on
a Seiko EXSTER6000 system with TGA-6200 instruments under
nitrogen atmosphere (flow rate: 200 mL/min). The single crystal X-
4.4. X-ray crystallographic data of 2g
X-ray diffraction data for 2g were collected on a Bruker Smart
ray data were collected on a Bruker Smart Apex CCD-based X-ray
APEX-II CCD-based X-ray diffractometer with Mo-Ka radiation
ꢀ
ꢀ
diffractometer with Mo-K
a
radiation (l¼0.71073 A).
(l
¼0.71073 A) at 90 K.
Single crystals of 2g [C
6
H
12
N
2
O
3
, MW¼160.18] suitable for X-ray
4
.2. Preparation of the cyclic amidines
analysis were grown from a solution in anhydrous acetone, and
3
a single colorless crystal with dimensions 0.20ꢂ0.20ꢂ0.15 mm was
4
.2.1. 1-Methyl-1,4,5,6-tetrahydropyrimidine (1g).13 N,N-Dimethylf-
selected for intensity measurements. The unit cell was monoclinic
ormamide dimethyl acetal (6.26 g, 52.5 mmol) was added to the
1
with the space group P2 /n. Lattice constants with Z¼4,

5
480
N. Aoyagi et al. / Tetrahedron 69 (2013) 5476e5480
ꢁ1
cm 3
ꢁ
,
m
(MoK
a
)¼0.115 mm
,
F(000)¼340,
r
2
b
calcd¼1.436
g
2501e2509; (f) Guo, Z.; Feng, Y.; Wang, Y.; Wang, J.; Wu, Y.; Zhang, Y. Chem.
Commun. 2011, 9348e9350; (g) Yan, Q.; Zhou, R.; Fu, C.; Zhang, H.; Yin, Y.; Yuan,
J. Angew. Chem., Int. Ed. 2011, 50, 4923e4927.
ꢀ
ꢀ
ꢀ
q
max¼57.30 were a¼6.7400(7) A, b¼10.7652(11), c¼10.4216(11) A,
ꢀ
ꢀ
3
¼103.1400(10) , and V¼736.37(13) A . A total of 3396 reflections
2
5. For studies on mechanism of CO binding by amidines and guanidines, see: (a)
were collected, of, which 1295 reflections were independent
int¼0.0097). The structure was refined to final R ¼0.0268 for 1293
data [I>2 (I)] with 144 parameters and wR
¼0.0737 for all data,
GOF¼1.044, and residual electron density max/min¼0.164/
Perez, E. R.; Santos, R. H. A.; Gambardella, M. T. P.; De, M. L. G. M.; Rodrigues-
Filho, U. P.; Launay, J.-C.; Franco, D. W. J. Org. Chem. 2004, 69, 8005e8011; (b)
Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.; Liotta, C. L. J. Org.
Chem. 2005, 70, 5335e5338; (c) Pereira, F. S.; deAzevedo, E. R.; da Silva, E. F.;
Bonagamba, T. J.; da Silva Agostíni, D. L.; Magalh a~ es, A.; Job, A. E.; P eꢁ rez
Gonz ꢁa lez, E. R. Tetrahedron 2008, 64, 10097e10106; (d) Villiers, C.; Dognon, J.-
P.; Pollet, R.; Thu eꢁ ry, P.; Ephritikhine, M. Angew. Chem., Int. Ed. 2010, 49,
(R
1
s
2
ꢀ
ꢁ3
ꢁ
0.217 e A
.
3
465e3468; (e) Pereira, F. S.; Lincon da Silva Agostini, D.; do Espirito Santo, R.
D.; deAzevedo, E. R.; Bonagamba, T. J.; Job, A. E.; Gonzalez, E. R. P. Green Chem.
011, 13, 2146e2153.
6. (a) Endo, T.; Nagai, D.; Monma, T.; Yamaguchi, H.; Ochiai, B. Macromolecules
004, 37, 2007e2009; (b) Ochiai, B.; Yokota, K.; Fujii, A.; Nagai, D.; Endo, T.
Supplementary data
2
1H and 13C spectra of isolated compounds and experimental
2
data of linear amidines (1aed) and amidinium bicarbonates (2e
and 2f) are available. Supplementary data related to this article can
be found at http://dx.doi.org/10.1016/j.tet.2013.04.110.
Macromolecules 2008, 41, 1229e1236; (c) Nagai, D.; Endo, T. J. Polym. Sci., Part A:
Polym. Chem. 2009, 47, 653e657; (d) Barkakaty, B.; Morino, K.; Sudo, A.; Endo, T.
Green Chem. 2010, 12, 42e44; (e) Barkakaty, B.; Morino, K.; Sudo, A.; Endo, T. J.
Polym. Sci., Part A: Polym. Chem. 2011, 49, 545e549; (f) Aoyagi, N.; Furusho, Y.;
Endo, T. Chem. Lett. 2012, 41, 240e241; (g) Aoyagi, N.; Furusho, Y.; Endo, T. J.
Polym. Sci., Part A: Polym. Chem. 2013, 51, 1230e1242.
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