General and Greener Synthesis of Diverse Functional Organic Salts through Schiff Base Chemistry

Article abstract of DOI:10.1055/s-0037-1611783

We report a greener and more-general organic method for the synthesis of functional organic salts containing organic anions through a Schiff base reaction between readily available aldehydes and simple aminoguanidinium salts. This reaction is operationally simple, free of metal salts, and forms water as the sole byproduct. The broad scope and good functional-group compatibility of this method permit its use to provide ready access to a library of more than 70 distinct organic salts, including those of heterocyclic anions, complex pharmaceutical anions, and polyanions, which are difficult to obtain through classical inorganic methods. Moreover, choosing different aldehydes and organic anions provides a convenient method for modulating or improving the functional properties of the designed organic salts, such as their melting points, fluorescence, and energetic properties. We therefore expect that this method will open new opportunities for the discovery and functionalization of a wide variety of organic salts and functional materials.

Full text of DOI:10.1055/s-0037-1611783

SYNLETT
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1
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0
9
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©
Georg Thieme Verlag Stuttgart · New York
2019, 30, A–I
letter
A
en
Synlett
B. Hu et al.
Letter
General and Greener Synthesis of Diverse Functional Organic Salts
through Schiff Base Chemistry
Baoping Hua
Qingrong Shia
Feipeng Lua
Pengcheng Zhanga
Panpan Penga
Chaofeng Zhaoa
Yao Dua
NH2
R1
H
mild conditions
H
N
NH2
NH2
R1
+
2
H O
+
N
H2N
R2 general and greener
N
NH2
O
H
Advantages:
Free of metal salts
Abundant structures
Water sole byproduct
Access to polyanion salts
Easy modulation
Broad substrate scope (>70 examples)
Hui Sua
Shenghua Li*a
Siping Pang*a
Fudie Nieb
CHO
OHC
CHO
OHC
1
CHO
COO
R CHO
eg:
eg:
NO2
Yield range: 56–97%
CN
N
N
R2
N
CN
N
NO2
a
School of Materials Science and Engineering, Beijing Institute
of Technology, South Street No. 5, Zhongguancun, Haidian
District, Beijing, 100081, P. R. of China
lishenghua@bit.edu.cn
pangsp@bit.edu.cn
Institute of Chemical Materials, China Academy of Engineering
b
Physics (CAEP), Mianyang, Sichuan 621900, P. R. of China
–
–
Received: 20.02.2019
Accepted after revision: 14.03.2019
Published online: 16.04.2019
with conventional inorganic anions such as Cl , NO , or
3
–
BF , organic anions contain several carbon atoms that can
4
DOI: 10.1055/s-0037-1611783; Art ID: st-2019-l0104-l
be functionalized, thereby facilitating the expansion of the
range of structures of these salts or tuning of their proper-
Abstract We report a greener and more-general organic method for
the synthesis of functional organic salts containing organic anions
through a Schiff base reaction between readily available aldehydes and
simple aminoguanidinium salts. This reaction is operationally simple,
free of metal salts, and forms water as the sole byproduct. The broad
scope and good functional-group compatibility of this method permit
its use to provide ready access to a library of more than 70 distinct or-
ganic salts, including those of heterocyclic anions, complex pharmaceu-
tical anions, and polyanions, which are difficult to obtain through classi-
cal inorganic methods. Moreover, choosing different aldehydes and
organic anions provides a convenient method for modulating or im-
proving the functional properties of the designed organic salts, such as
their melting points, fluorescence, and energetic properties. We there-
fore expect that this method will open new opportunities for the dis-
covery and functionalization of a wide variety of organic salts and func-
tional materials.
(a) Structures of organic salts
Limited structures
1
. Organic cation + inorganic anion
R -A+
1
Cl ,NO3 ,BF4
–
–
–
Difficult modulation
Easy modulation
2
. Organic cation + inorganic anion
R1-A+
B–-R2 (e.g N
N
)
Expanded functions: pharmaceuticals,
medicines, and energetic materials
(
b) Traditional inorganic methods for synthesis of organic anion-based salts
. Acid–based neutralization
R1-A + B-R2
R -A+B–-R2
1
Limited substrates
1
2
.Metathetical reaction
Limited substrates
Numerous byproducts
Difficult to scale up
1
+
2
+ –
1
+
–
+
-
2
R -A B-R + M X
R -A X + M B R
+
–
(M X such as BaSO4 or AgCl)
(c) This work: Synthesis and modulation of the functions of organic anion-based
salts through a Schiff base reaction
Key words organic salts, Schiff base reaction, functional materials
NH2
R1
H
H
N
NH2
R1
2
+ H O
+
mild conditions
N
H2N
R2
N
NH2
O
general and greener
H
NH2
Organic salts are a versatile and important class of mol-
ecules that typically consist of organic cations and inorgan-
ic or organic anions. Their unique nature, in that they con-
sist entirely of ions, renders them useful various fields in-
cluding organic synthesis (e.g., aryldiazonium salts), ionic
dyes, ionic conductivity, and ionic liquids. Recently, or-
ganic salts containing organic anions such as alkyl, arene,
heterocyclic (e.g., azolate anions), or pharmaceutical anions
have attracted growing interest because, in comparison
Free of metal salts
Water sole byproduct
Broad substrate scope (>70 examples)
Abundant structures
Access polyanion salts
Easy modulation
Figure 1 (a) Comparison of organic salts containing organic anions
with analogues containing inorganic anions. (b) Conventional inorganic
methods for synthesizing organic salts containing organic anions.
(c) The proposed organic method for synthesizing organic salts contain-
ing organic anions and for modulating their functions. R , R = alkyl,
aryl, hetaryl, etc.
1–8
1
2
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B
Synlett
B. Hu et al.
Letter
ties through the introduction of various functional groups
that by choosing different aldehydes (with a functional unit
9,10
1
2
[
Figure 1(a)]. Moreover, such salts have found new appli-
R ) and organic anions (with a functional unit R ), the pro-
posed method might be useful for preparing libraries of di-
verse organic salts, essential for structure–property rela-
tionship studies or for the modulation of their functions or
properties. We hoped that, by taking advantage of the wide
abundance of aldehydes and simple organic salts, the pro-
posed organic reaction would be broadly useful and orthog-
onal to classical inorganic reactions.
cations as pharmaceuticals, medicines, nonlinear optical
11–21
materials, lubricants, or energetic materials.
Despite
the importance of these organic salts, a broadly applicable
methodological approach to their synthesis remains elu-
sive.
Interestingly, most organic salts containing organic an-
ions are usually prepared by classical inorganic-type reac-
tions such as acid–base neutralization or metathesis reac-
To test the validity of our hypothesis, our initial investi-
gations focused on the reaction of benzaldehyde (1), which
contains a few carbon atoms that can be functionalized, as a
model substrate for reaction with various simple dinitropy-
razolide salts containing various amino-group containing
cations (Table 1). The reaction of benzaldehyde (1) with
ammonium 3,5-dinitropyrazolide (2a) did not proceed at
room temperature or at 85 °C (Table 1, entries 1 and 2); in-
stead, we simply recovered the starting materials. Guani-
dinium 3,5-dinitropyrazolide (2b), which contains more
amino groups, was then tested, and this too showed no re-
action (entries 3 and 4). It is appeared that the amino
groups in these substrates were not sufficiently reactive.
Hydrazinium 3,5-dinitropyrazolide (2c), which contains
more-reactive amino groups, was then used, but this gave a
stoichiometric amount of the covalent product PhCH=N-
N=CHPh (3) through coupling of benzaldehyde and hydra-
zine with loss of the 3,5-dinitropyrazolide anion (entry 5);
it therefore failed to give the desired product. It is possible
that the positive charge resides on the reactive amino group
of the hydrazinium cation; when the condensation reaction
occurs, the positive charge is removed and the correspond-
ing counteranion is lost.
11–21
tions [Figure 1(b)].
In the former case, the substrates
are often limited to strong organic acids or bases such as
hydrazine hydrate and phosphonium hydroxide. The me-
tathesis reaction depends mainly on a difference in solubili-
ty between the reactants and the products. Consequently,
this reaction is limited to substrates that are more soluble
in the reaction solvent than are their corresponding prod-
ucts. Moreover, this reaction often results in the production
of at least stoichiometric amounts of waste products such
as AgCl or BaSO ; it therefore has poor atom economy and
4
suffers from difficulties in scalability and environmental
pollution. Furthermore, although these classical methods
readily give simple organic salts containing organic anions
2
2–24
such as phosphonium,
ammonium, hydrazinium, or
their structural diversity is
13–21
aminoguanidinium ions,
relatively limited. The synthesis of organic salts with a vari-
ety of structures and versatile functions remains a signifi-
cant challenge. This challenge can be attributed to the rela-
tively low reactivity of inorganic-type reactions toward or-
ganic salts. Moreover, the larger sizes of organic anions
compared with inorganic anions can introduce steric hin-
drance, preventing the formation of the corresponding or-
ganic salts.
On the basis of the above results, a salt with an amino-
group-containing cation in which the positive charge is
separated from the reactive amino group would be expect-
ed to react in the desired manner. We therefore designed
and synthesized the aminoguanidinium 3,5-dinitropyra-
zolide (2d), in which the positive charge is separated from
the reactive hydrazine group and resides on another amino
group. Gratifyingly, we found that 2d reacted smoothly
with 1 to form the desired salt 4 in 82% yield at room tem-
perature, with water as the only byproduct (Table 1, entry
6). Note that the disubstituted organic salt 5 was not ob-
tained, even when the reaction was performed at 65 °C for
seven hours (entry 7). It is possible that the hydrazine
group of the aminoguanidinium cation is more reactive
than the amino group in 2d, which facilitates condensa-
tion.33 According to a Gaussian calculation, the lengths of
the C1–N2 (1.33 Å), C1–N4 (1.34 Å), and C1–N7 (1.35 Å)
bonds are between the values for C–N (1.47 Å) and C=N
(1.22 Å) bonds, showing that electron delocalization occurs
in the amino group of the aminoguanidinium cation. This
makes the amino group more stable than a hydrazine
group34 (see Supporting Information). The optimized con-
In considering ways to address the challenges associated
with devising a more-efficient, greener, and more-general
method for the synthesis of organic salts containing organic
anions, our attention was drawn to the well-established
25
Schiff base reaction. This organic reaction involves con-
densation between an amine and aldehyde. It exhibits high
reactivity, operational simplicity, mild reaction conditions,
and, more importantly, water is the only byproduct. The
Schiff base reaction has been widely used for the synthesis
of various covalent compounds, including natural products,
pharmaceuticals, agrochemicals, functional materials, and
26–32
some salts containing inorganic anions.
On the basis of the attractive characteristics of the Schiff
base reaction, we hypothesized that this classical organic
reaction might be developed as a novel method for the syn-
thesis of organic salts containing organic anions if the anion
contains a suitable amino-containing cation was present in
a simple organic salt precursor containing an organic anion;
this amino group would have to be able to condense with
an aldehyde to form a new cation while retaining the origi-
nal organic anion [Figure 1(c)]. Moreover, we envisioned
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Synlett
B. Hu et al.
Letter
Table 1 Evaluation of Dinitropyrazolide Salts Containing Various Amino-Group-Containing Cations 2 for the Condensation Reaction with Benzalde-
a
hyde (1)
NO2
O
N
N
conditions
A+
Products
H
NO2
1
2
Entry
1
Reactant
Conditions
Productsb
NO2
N
NH4
N
EtOH, 25 °C, 12 h
EtOH, 85 °C, 12 h
–
NO2
NO2
2a
N
N
2
3
NH4
–
–
NO2
a
2
NO2
NH2
N
EtOH, 25 °C, 12 h
N
H2N
NH2
NO2
NO2
2b
NH2
N
N
4
5
6
EtOH, 85 °C, 1 h
EtOH, 25 °C, 12 h
EtOH, 25 °C, 7 h
–
H2N
NH2
NO2
2
b
NO2
N
N
N
NH2NH3
N
NO2
N
3, 95.2%
2
c
NO2
NO2
NO2
NO2
NH2
NH2
N
N
NH2
H2N
N
N
N
NH2
H
2
d
4, 82%
NO2
NH2
N
N
N
NH2
, 92%
NO2
NO2
NH2
4
NO2
N
N
NH2
+
7
EtOH, 65 °C, 7 h
H2N
N
H
NO2
NO2
NH2
2d
N
N
N
N
H
N
5
, 0%
a
Standard conditions: PhCHO (1.0 equiv), dinitropyrazolide salt (1.0 equiv), 1 mmol scale.
Isolated yields are reported.
b
ditions were eventually identified as follows. Ethanol is the
With the optimized conditions in hand, we attempted
to explore the generality of this condensation protocol to
enrich the range of structures of the organic salts. First, we
investigated the scope of aldehyde substrate and we found
optimal the solvent, due to the high solubility of both the
aminoguanidinium salts and aldehydes in this solvent, and
the reaction is carried out at 65 °C for seven hours.
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Scheme 1 Substrate scope. Reaction conditions: aldehyde (1.0 equiv), aminoguanidinium salt (1.0 equiv), 65 °C, 7 h (1 mmol scale). Isolated yields are
reported.
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Letter
that a variety of aldehydes could be successfully trans-
formed into the corresponding products in modest to good
yields (Scheme 1). The reaction showed good tolerance to-
ward electron-donating groups such as amide, ether, or
methyl, and to electron-withdrawing groups such as sulfo-
nyl, nitro, cyano, or trifluoromethyl. Aryl halides were also
compatible with the new transformation (9–11). It is inter-
esting to note that the reaction even tolerated an unpro-
tected OH group (15). The reaction with fused-ring alde-
hydes also proceeded smoothly (18 and 19).
isolation (e.g., by filtration). In addition, 42 and 46 were
prepared on a gram scale under the standard reaction con-
ditions in yields similar to those obtained from 1 mmol
scale experiments, demonstrating both the scalability and
practicality of this method. In contrast, attempts to prepare
several products, including 34 and 46, by classical metathe-
sis reactions of their hydrochlorides with the corresponding
metal salts were unsuccessful. It is likely that the organic
anions of these compounds contain a few nitrogen or oxy-
gen atoms that can readily coordinate to transition metal
+
Furthermore, aliphatic aldehydes also reacted to give
reasonable quantities of the corresponding condensation
products 20–22. We were pleased to find that aldehydes
containing various hetaryl moieties, such as pyrrolyl, imid-
azolyl, or thienyl groups, gave the desired products 23–29
in good yields with no byproducts from the elimination of
the 3,5-dinitropyrazolide counteranion to form a covalent
product. Ferrocene derivatives are an important class of or-
ganometallic compounds that exhibit a wide range of bio-
ion (e.g., Ag ) to form an insoluble complex, resulting in dif-
ficulties in the reactions (see Supporting Information). The
above features indicate that the present condensation
method is complementary to classical reactions for the syn-
thesis of organic salts with a wide range of structures.
The promising functional-group compatibility and
broad substrate scope of this protocol encouraged us to
evaluate this condensation method for complex pharma-
ceutical anions containing aminoguanidinium salts. The
preparation of active pharmaceutical ingredients in an an-
ionic form would permit an improvement in their physico-
35–37
logical and electrochemical activities.
Ferrocenecarbal-
dehyde also afforded the desired product 30 in good yield.
Furthermore, the reaction also tolerates a significant degree
of steric hindrance; the reaction of 2d with 2,4,6-trime-
thoxybenzaldehyde gave 17 in good yield. Interestingly, the
condensation reaction was also expanded to a series of ke-
tones at elevated temperatures. For example, acetone and
acetophenone were readily transformed into afford the cor-
responding products in good yields (see Supporting Infor-
mation).
chemical and biological properties, such as their solubility,
lipophilicity, or toxicity.1
1,39
The nalidixic anion, a well-
known antibiotic ingredient used in the treatment of uri-
nary-tract infections, when tested under the standard con-
ditions gave the target product 58 in 86% yield. Moreover,
the naproxen anion, a widely used nonsteroidal antiinflam-
matory ingredient, was amenable to this condensation re-
action. Other active pharmaceutical ingredients, including
indomethacin and camphorsulfonic acid salts, were also vi-
able substrates. This transformation therefore offers good
potential for the discovery of interesting new biologically
active organic salts.
Having studied the scope of the aldehyde substrate, we
turned our attention to various organic anions containing
simple aminoguanidinium salts by using benzaldehyde (1)
as a model substrate (Scheme 1). The ready availability of
these simple aminoguanidinium salts, either from commer-
cial suppliers or by synthesis in a single step by methods
described in literature, allowed us to perform the reaction
on a synthetically useful 1 mmol scale. We found that the
reaction was effective for a variety of benzoate (31–37) and
benzenesulfonate anions (41–44). A wide range of func-
tional groups, including ether, methyl, amino, nitro, cyano,
chloro, and bromo were tolerated under mild reaction con-
ditions. Furthermore, alkanoic acid anions were also suit-
able for this condensation reaction (38–40). Heterocyclic
anions were also investigated, with the knowledge that or-
ganic salts containing heterocyclic anions are widely used
as energetic materials, gas absorbents, and ionic liquids.1
In addition to the pyrazolide anion, other heterocyclic an-
ions such as imidazolide, triazolide, tetrazolide, and ben-
zotriazolide also reacted smoothly (45–55). Thiophenecar-
boxylate and furancarboxylate anions were also compatible
with the reaction.
Having demonstrated the broad scope of this new reac-
tion, we next sought to examine its application in modulat-
ing the functional properties of the new organic salts by se-
1
2
lecting different aldehydes (R ) and organic anions (R ) con-
taining aminoguanidinium salts (Scheme 1). For example,
the melting point is a fundamental physical property of or-
ganic salts, and its modulation is important for gaining in-
sight into the correlation between structure and melting
point, and for various applications.7
,8,14
By simply changing
the alkyl chain of the aldehyde substrate, the melting points
of the corresponding product was modulated (Figure 2). For
example, when the aldehyde substrate was changed from
butanal to octanal, thermogravimetric/differential scanning
calorimetry (TG/DSC) analysis unambiguously showed that
the melting point of the corresponding products with the
same anion decreased from 139 °C (62) to 77 °C (63). Note
that the melting point of the organic salt 63 was 77 °C,
whereas its decomposition temperature was 217 °C. This
temperature difference of more than 100 °C between the
melting point and the decomposition temperature suggests
that 63 is potential useful as an ionic liquid.
3–24
The structures of seven as-synthesized organic salts, in-
cluding 4 and 46, were unambiguously confirmed by single-
38
crystal X-ray diffraction analysis. Most products were ob-
tained in sufficiently pure form to permit straightforward
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B. Hu et al.
Letter
In addition to organic salts containing single organic an-
ions, complex organic salts containing polyanions also ex-
hibited promising antimicrobial, and energetic properties,
13,41
as polyanions usually enhance their unique properties.
The preparation of such compounds has relied mainly on
inorganic metathesis reactions; however, the need to intro-
duce several large organic anions into organic salts make
this reaction more difficult, and it usually suffers from low
yields and a limited substrate scope, thereby hindering the
discovery of promising new scaffolds. To highlight the utili-
ty of our method, we attempt to employ this reaction to
synthesize various organic salts containing polyanions with
polyaldehydes as substrates. For example, glyoxal con-
densed smoothly with simple aminoguanidinium salts un-
der our standard conditions to give the corresponding prod-
ucts 68–70 in high yields (Figure 4). Similarly, terephthalal-
dehyde and pyridine-2,6-dicarbaldehyde also worked well.
The nitrogen-rich organic salts based on glyoxal as a
substrate 68–70 exhibited acceptable energetic properties
(Table 2). In particular, organic salt 70 possesses a high de-
composition temperature (241 °C), good detonation prop-
erties [detonation velocity (D): 8840 m/s; detonation pres-
sure (P): 31.0 GPa] and extremely low sensitivities (impact
sensitivity: >40 J; friction sensitivity: >360 N). Its detona-
tion velocity and pressure are comparable to those of RDX
(D, 8861 m/s; P, 34.5 GPa). Thus, the organic salt 70 might
Figure 2 DSC curves of organic salts 62–64
In addition to aldehydes, various organic anions can be
also used to tune the properties of the products (Figure 3).
Ionic fluorescent organic materials have attracted signifi-
cant attention because of their various applications, for ex-
ample in photovoltaics, ionic organic light-emitting diodes,
4,40
and ionic liquid crystals.
When pyrene-1-carbaldehyde
was treated with simple aminoguanidinium salts contain-
ing various organic anions, such as 4,5-dicyanoimidazolide
or 3,3-dibromotriazolide, a series of ionic fluorescent salts
were obtained in high yields. Interestingly, the 4,5-dicyano-
imidazolide pyrene salt 65 and the 3,3-dibromotriazolide
pyrene salt 66 both exhibited higher fluorescent intensities
than that of the corresponding inorganic nitrate anion-
based analogue 67 at the same concentration. It is possible
that these organic anions have larger -conjugated systems
than inorganic anions, which could contribute to their en-
hanced fluorescence. Therefore, this approach provides a
useful tool for the modulation or improvement of the prop-
erties of organic salts.
Table 2 Physical Properties of As-Synthesized Organic Salts and RDX
Compound 68
a
69
1.69
70
1.68
71
1.57
RDX
1.81
1.66
b
T
d
257
282
241
246
210
37.8
-21.6
7.5
N%c
63.0
-32.0
16
65.1
-33.5
4.0
58.3
-22.2
>40
47.3
-21.6
>40
d
Ω
CO
ISe
FSf
>360
>360
>360
>360
120
Δ
Δ
Δ
f
f
f
H–g
H+h
Hi
112.8
1811.6
578.3
23.0
9.8
536.8
1811.6
1448.1
31.0
–307.9
1811.6
–307.9
19.3
–307.9
–307.9
70.3
1811.6
390.7
22.8
Pj
34.5
Dk
7694
7981
8840
7441
8861
a
Density measured with a gas pycnometer (g/cm).
Decomposition temperature (°C; onset).
Nitrogen content.
Oxygen balance.
Impact sensitivity (J).
b
c
d
e
f
Friction sensitivity (N).
g
h
i
Heat of formation of anion.
Heat of formation of cation.
Heat of formation of salt.
Detonation pressure.
Figure 3 Fluorescence spectra of organic salts 65–67. Excitation at
j
280 nm in MeOH (1 mmol/L).
k
Detonation velocity. The detonation properties were calculated by using
EXPLO5 v6.01. The properties of RDX are taken from the work of Klapötke
42
et al.
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B. Hu et al.
Letter
NH2
HN
N
NH
P2+
=
H2N
NH2
N
H2N
Dianion organic salts
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
P2+
NO2
P2+
NHNO2
N
P2+
NO3- P2+ NO3-
NO2
8, 85%
NHNO2
NO2
NO2
6
69, 80%
NH
70, 86%
71, 76%
H
N
H
N
N
NH2
HN
NH2
H2N
N
N
N
Q2+
NH2
R2+
=
=
H2N
NH2
NH2
CN
N
H2N
CN
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NO2 Q2+
NO2
CF3 Q2+
73, 72%
CF3
CN Q2+
74, 83%
CN
N
N
N
N
72, 76%
N
N
N
N
N
N
Br
Q2+
Br
Br
Br
75, 75%
CN
CN
70
R2+
N
N
CN
CN
N
N
76, 81%
72
Figure 4 Preparation of polyanion salts through the condensation reaction. Reaction conditions: aldehyde (1.0 equiv), aminoguanidinium salt (2.0
equiv), 65 °C, 7 h.
be potentially useful as an insensitive energetic material.
Moreover, these organic salts also display higher nitrogen
contents, greater densities, higher heats of formation, and
better detonation properties than those of their corre-
sponding nitrate salt 71. Remarkably, the heat of formation
and detonation velocity of 70 are, respectively, more than
vantages including freedom from metal salts, the formation
of water as the sole byproduct, and a remarkably broad
scope that includes salts of heterocyclic anions, complex
pharmaceutical anions, and polyanions. The usefulness of
this method is also demonstrated by the fact that choosing
different aldehydes and organic anions permits convenient
modulation or improvement of the functional properties of
the products, including their melting points, fluorescence,
and energetic properties. In particular, the difference be-
tween the melting point and the decomposition tempera-
ture of organic salt 63 is more than 100 °C, making it poten-
tially useful as an ionic liquid. Moreover, the nitrogen-rich
organic salt 70 has a high decomposition temperature
(241 °C), good detonation properties (D: 8840 m/s; P: 31.0
GPa), and extremely low sensitivities (impact sensitivity:
>40 J; friction sensitivity: >360 N), and it might be useful as
an insensitive energetic material. We therefore expect that
our new method could open new opportunities for the dis-
covery and functionalization of a wide variety of organic
salts and functional materials, such as pharmaceuticals,
1700 kJ/mol and more than 1400 m/s higher than the corre-
sponding values for 71 (1448.1 kJ/mol and 8840 m/s, re-
spectively, for 70 versus –357.5 kJ/mol and 7441 m/s for
71). It is possible that the nitrogen-rich anions of 68–70
have higher nitrogen contents, greater densities, and higher
heats of formation than those of the inorganic nitrate anion
–
(
NO ), resulting in better detonation properties. Therefore,
3
our transformation might offer the potential for the discov-
ery of promising functional organic salts containing
polyanions.
In conclusion, we have developed a greener and more-
general protocol for the synthesis of organic salts through a
Schiff base reaction between simple aminoguanidinium
43
salts and aldehydes. Compared with conventional inor-
ganic methods, this condensation method has several ad-
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–I

H
Synlett
B. Hu et al.
Letter
medicines, nonlinear optical materials, and energetic mate-
rials, in particular those that are difficult to access by con-
ventional inorganic methods.
(18) Xu, Z.; Cheng, G.; Yang, H.; Ju, X.; Yin, P.; Zhang, J.; Shreeve, J. M.
Angew. Chem. Int. Ed. 2017, 56, 5877.
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Stierstorfer, J.; Witkowski, D. T. G. Angew. Chem. Int. Ed. 2016,
55, 16132.
(
20) Zhao, G.; He, C.; Yin, P.; Imler, G. H.; Parrish, D. A.; Shreeve, J. M.
Funding Information
J. Am. Chem. Soc. 2018, 140, 3560.
The authors acknowledge financial support from NSAF (U1530262),
the National Natural Science Foundation of China (21875021 and
(21) Fischer, D.; Klapötke, T. M.; Stierstorfer, J. Angew. Chem. Int. Ed.
2015, 54, 10299.
2
1576026), and the Fundamental Research Funds for the Central Uni-
(22) Tao, D.-J.; Chen, F.-F.; Tian, Z.-Q.; Huang, K.; Mahurin, S. M.;
Jiang, D.-e.; Dai, S. Angew. Chem. Int. Ed. 2017, 56, 6843.
versities.
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ati
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a
(2
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7
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6)Nati
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(
23) Wang, C.; Cui, G.; Luo, X.; Xu, Y.; Li, H.; Dai, S. J. Am. Chem. Soc.
011, 133, 11916.
(24) Chen, K.; Shi, G.; Zhou, X.; Li, H.; Wang, C. Angew. Chem. Int. Ed.
016, 55, 14364.
2
Acknowledgment
2
The authors declare no competing financial interest.
B.P.H. performed the experiment. Q.R.S, F.P.L., P.C.Z., P.P.P., C.F.Z., Y.D.,
and H.S provided some help. S.H.L. and S.P.P. conceived the idea and
provided funding support. S.H.L. and B.P.H. analyzed the data and
wrote the manuscript.
(25) Schiff, H. Justus Liebigs Ann. Chem. 1864, 131, 118.
(26) Das, S.; Heasman, P.; Ben, T.; Qiu, S. Chem. Rev. 2017, 117, 1515.
(27) Chu, S.; Münster, N.; Balan, T.; Smith, M. D. Angew. Chem. Int. Ed.
2016, 55, 14306.
(28) Jiao, T.; Chen, L.; Yang, D.; Li, X.; Wu, G.; Zeng, P.; Zhou, A.; Yin,
Q.; Pan, Y.; Wu, B.; Hong, X.; Kong, X.; Lynch, V. M.; Sessler, J. L.;
Li, H. Angew. Chem. Int. Ed. 2017, 56, 14545.
Supporting Information
(
(
29) Peng, W.; Paulson, J. C. J. Am. Chem. Soc. 2017, 139, 12450.
30) Rizzuto, F. J.; Wood, D. M.; Ronson, T. K.; Nitschkte, J. R. J. Am.
Chem. Soc. 2017, 139, 11008.
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611783.
S
u
p
p
orti
n
g Inform ati
o
n
S
u
p
p
orit
n
g Inform ati
o
n
(
(
(
31) Tamura, T.; Song, Z.; Amaike, K.; Lee, S.; Yin, S.; Kiyonaka, S.;
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References and Notes
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graphic data for compounds 4, 24, 39, 46, 58, 70, and 72, respec-
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Crystallographic
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A 50 mL round-bottomed flask was charged with PhCHO (0.106 g,
1
mmol) and EtOH (6 mL) A solution of aminoguanidinium 3,5-
dinitropyrazolide (0.232 g, 1 mmol) in EtOH (20 mL) was added
dropwise to the flask and the resulting mixture was stirred for 7
h at 65 °C. The solvent was then slowly removed under a
vacuum, and the residue was crystallized from H O to give
2
(17) Tang, Y.; Mitchell, L. A.; Imler, G. H.; Parrish, D. A.; Shreeve, J. M.
light-yellow needle crystals; yield: 0.294 g (92%); mp 174.32–
Angew. Chem. Int. Ed. 2017, 56, 5894.
174.96 °C.
IR (KBr): 759, 834, 1010, 1318, 1351, 486, 541, 1622, 1670, 3255
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–I

I
Synlett
B. Hu et al.
Letter
cm–1. H NMR (600 MHz, DMSO-d ):  = 7.35 (s, 1 H, CH), 7.45 (s,
1
(s), 155.57 (s), 156.81 (s). MS (ESI-MS): m/z = 163.05 (C H11N4+,
6
8
–
3
H, NH), 7.72 (t, J = 7.5 Hz, 3 H, CH), 7.87 (m, 2 H, CH), 8.22 (s, 1
H, CH,), 11.59 (s, 1 H, NH). C NMR (150 MHz, DMSO-d ):  =
cation); 156.95 (C HN O , anion). Anal. Calcd: C 42.25, N 34.99,
3
4
4
13
H 3.78. Found: C 42.82, N 34.52, H 3.96.
6
98.89 (s), 128.07 (s,), 129.13 (s), 130.98 (s), 133.85 (s), 147.75
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–I