Efficient microwave-assisted preparation of squaric acid monoamides in water

Article abstract of DOI:10.1039/c3ra41369a

Squaric acid monoamides (SQAMs) were prepared via microwave-enhanced direct reaction of amines with squaric acid in yields ranging from 41 to 92%. These compounds are strong acids, with pK_a values ranging from 0.9 to 2.0 as determined by spectrophotometric titration in representative cases. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41369a

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Efficient microwave-assisted preparation of squaric acid
monoamides in water3
Cite this: DOI: 10.1039/c3ra41369a
´
Carlos Lopez, Manuel Vega, Elena Sanna, Carmen Rotger and Antoni Costa*
Received 14th December 2012,
Accepted 22nd March 2013
DOI: 10.1039/c3ra41369a
www.rsc.org/advances
Squaric acid monoamides (SQAMs) were prepared via microwave-
enhanced direct reaction of amines with squaric acid in yields
ranging from 41 to 92%. These compounds are strong acids, with
pKa values ranging from 0.9 to 2.0 as determined by spectro-
photometric titration in representative cases.
underdeveloped due, in part, to the lack of an expeditious method
of preparation.
Earlier literature reports that direct reaction of squaric acid
with amines in MeOH exclusively leads to squaraines
3
(Scheme 1). The same reaction in DMF leads to a mixture
containing the 1,3-diamino derivatives 3 and minor amounts of
squaramides 4 and 5.11 It is also known that the reaction of
squaric acid with amines in water, with12 or without10b acid
catalysis, allows for the obtention of SQAMs 5. At this point, it is
reasonable to expect that dielectric heating in water for the
synthesis of SQAMs could not only be beneficial in terms of
energetic efficiency methodology, but may also improve yields and
selectivity. In continuation of our interest in squaric acid
monoamides,13 in this work SQAMs are rapidly prepared by direct
microwave-accelerated condensation of squaric acid with primary
amines. In water, formation of compounds 3 and 4 can be
controlled to a small degree while microwave heating increases the
yield and selectivity.
Since the discovery of squaric acid in 1959, there has been
continuous interest in some of its nitrogen derivatives, and in
particular of squaric acid diamides.1 This group of molecules, with
a characteristic 1,2-cyclobutenedione ring, are 3,4-disubstituted
amide-type compounds synthesized from squaric acid diesters
and amines. The synthesis of squaramides is simple, usually
involving the stepwise reaction of commercially available squaric
acid dialkyl esters with primary or secondary amines.2 The
hydrogen bond donor–acceptor properties of squaramides have
made them very attractive for various applications in supramole-
cular chemistry, e.g. as catalysts,3 molecular sensors4 and in
molecular recognition as receptors for charged guests.5 Moreover,
there is a growing number of bioactive squaramides involved in
cancerous,6 inflammatory7 and infectious8 diseases, among
others.
Related to the above, squaric acid monoamides (SQAMs), also
named amidosquaric acids or squaramic acids, have the same
cyclobutenedione ring as above in common, but they feature a
distinctive acidic hydroxyl group, clearly different to that of related
oxamic or the unstable carbamic acids. Hence, by virtue of this
acidic hydroxyl, SQAMs are monodentate O-ligands which can
form complexes with
a
number of transition metals.9
Furthermore, the simultaneous presence of the amide-type group
allows SQAMs in their acidic or basic forms to replace carboxylic
and phosphate groups in several bioactive compounds and
nucleotide bioisosteres.10 In spite of these results, the potential
of SQAMs in supramolecular chemistry and catalysis is still clearly
´
Departament de Quımica, Universitat de les Illes Balears, Ctra. Valldemossa km 7.5,
Palma de Mallorca 07122, Spain. E-mail: antoni.costa@uib.es;
Fax: 0034 971 173426; Tel: 0034 971 173266
Electronic supplementary information (ESI) available: Experimental procedures
3
and spectroscopic data of all new compounds, pKa determinations. See DOI:
Scheme 1 Reaction of squaric acid with amines.
10.1039/c3ra41369a
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addition of two moles of sodium hydroxide per mole of squaric
acid to the starting mixture of 4-nitroaniline and squaric acid
inhibited the condensation almost entirely, yielding 5h in a
negligible amount together with unaltered amine (Table 1, entry
6). Secondly, reduction of the concentration of active ion-pairs by
the addition of tetraethylammonium chloride as competitive non-
active ion-pairs, caused a significant decrease in yield of 5h from
77 to 38% (Table 1, entry 7). Notably, an attempt to decrease the
concentration of active ion-pairs by the addition of acetic acid to
the reaction mixture as above, yielded 89% of squaraine 3h and
only 13% of target amidosquaric acid 5h (Table 1, entry 8).
In order to investigate the scope and limitations of this
method, the condensation was tested with a series of substituted
anilines, and aliphatic amines in optimal conditions of time and
temperature. Under microwave irradiation, anilines bearing both
electron donating groups such as Me2N, HO and MeO (entries 1–
6), as well as electron withdrawing groups such as I, F, NO2, CO2Et,
SO3H (Table 1, entry 5 and Table 2, entries 7–10), present at
variable positions on the aryl ring, reacted well leading to the
corresponding SQAMs 5 in moderate to excellent yields. Results
summarized in Table 2 report yields of spectroscopically pure
products. In general, SQAMs 5 were isolated by extractive workup
from aqueous solutions to remove both unreacted and excess
starting materials. We also noticed that formation of squaraines is
highly dependent on the temperature. Apart from the reactions
described in Table 1, in the set of experimental conditions
described in Table 2, 1,3-squaraine derivatives were detected only
in one case accompanying product 5k (entry 10) in 10% yield.
Remarkably, squaramides 4 were never observed in the set of
experimental conditions described here. In general, reactions with
a range of anilines proceed well leading to SQAMs 5 in 15–30 min
at 120–150 uC at molar ratios of amine to squaric acid in the range
between 2 and 0.2. The reaction of a highly deactivated aniline
such 2,4-dinitroaniline (pKa = 24.4) at a molar ratio of 0.2 gave the
corresponding SQAM 5 only in 18%, too low for practical uses. In
this case, the low yield is attributed to a combination of factors:
the weak basicity of this aniline on one side that inhibits the
formation of active ion pairs, and the very low nucleophilicity of
2,4-dinitroaniline on the other. That initial formation of ion-pairs
is not the only boundary of this reaction, which is evident using
more basic amines. Given the difference of pKa values, formation
of ion-pairs is complete in water with aliphatic amines. However,
the resulting aminium salts do not react with squaric acid in
experiments performed with conventional heating at reflux in
water for extended periods of time. With microwave assistance,
these reactions take place only using excess amine at higher
temperatures yielding SQAMs 5n to 5q albeit in moderate yields of
30–60% (Table 2, entries 13–16). Here, excess amine is necessary
to provide a suitable concentration of amine nucleophile.
Fortunately, in spite of the high temperatures required, SQAMs
were the only products observed together with unreacted starting
material. Condensations with thermally labile aliphatic amines
were unsuccessful. Attempted condensation with unprotected
amino acids at 200 uC led to inseparable mixtures of SQAMs and
other products arising from thermal decomposition of the starting
amino acids.16 The observed difference in reactivity between
Table 1 The effect of different reaction conditions on the condensation of
p-nitroaniline with squaric acida
Entry
Mol ratiob
Additive
3h (%)c
5h (%)c
1
2
3
4
5
6
7
8
2
1
—
—
—
—
87
44
22
5
,5
0
12
50
74
89
90
4
0.5
0.3
0.2
0.5
0.5
0.5
—
NaOH (2 mmol)
TEACl (1.5 mmol)
AcOH (2.5 mmol)
22
84
38
11
a
Reactions performed at 1 mmol scale with dielectric heating in a
closed system at 120 uC for 30 min, each reaction was repeated three
b
c
times. Amine/Squaric acid. Average isolated yields.
Owing to the strong acid character of squaric acid, an
intermediate anilium squarate salt 1 is the true active species
directly involved in this condensation.11 These organic salts are
ion-paired aggregates that remain suspended in the low volume of
water used as the solvent. This scenario is ideally suited to take
advantage of the highly efficient dielectric heating of microwave
irradiation as well as of the accelerating effect of ‘‘on water’’
experimental conditions.14,15
In our hands, the reaction of aniline with squaric acid at a 2 : 1
molar ratio in water for 3 h under reflux afforded the acid 5a in
fair yield (46%). In contrast, running this reaction under
microwave irradiation for 15 min at 120 uC allowed the isolation
of 5a in 83% yield after acidic workup (entry 1, Table 2).
Table 1 summarizes the effect on the outcome of the
condensation between 4-nitroaniline (pKa = 0.98) and squaric acid
(pK1 = 0.54; pK2 = 3.48), chosen as a reaction model. In aqueous
media, 3 and 5 are competitive products and their relationship
depends on the experimental conditions. However, the formation
of 3 is partially reversible.12b The existence of this equilibrium can
be demonstrated by heating 3 with one equivalent of squaric acid
in water under the same experimental conditions as above and
observing the formation of mixtures containing 5 and 3 in variable
proportions depending on the ratio of amine to squaric acid. This
is also in accordance with the beneficial effect of excess squaric
acid on the yield of 5h observed with p-nitroaniline (Table 1,
entries 1–5). Therefore, the use of squaric acid in excess over the
aniline favours not only the squarate monoanilium ion-pair and
the subsequent monocondensation product but also the displace-
ment of the competitive equilibrium leading to squaraine 3h
towards SQAM 5h. This effect is quite general and, in several
cases, there is a clear relationship between the amine/squaric acid
ratio and the yields of SQAMs 5 (Table 2, entries 4, 5, 9 and 12).
Besides the reported condensation of previously isolated solid
anilium squarate salts at 200 uC,9b additional evidence supports
that the condensation takes place starting from tight anilium
squarate ion-pairs. First, destruction of the ion-pair by the
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Table 2 Microwave-assisted synthesis of SQAMs 5 in watera
Entry
Amine; R=
Time (min)
Temp. (uC)
SQAM
Mol ratiob
Yield (%)c
1
2
3
C6H5
C10H9
p-Me2NC6H4
15
30
20
120
120
150
5a
5b
5c
2
0.5
2
0.5
2
0.5
0.2
2
1
0.5
1
2
2
2
1
2
0.5
0.3
0.5
2
0.8
0.6
2
83
64
66
56
55
74
82
48
53
78
95
63
58
37
27
55
73
83
70
40
85
92
60
26
51
36
58
30
4
p-HOC6H4
15
130
5d
5e
5
o-HOC6H4
15
130
6
7
8
9
8-AQd
2-MeO-5-MeC6H3
p-IC6H4
15
30
20
40
120
120
120
120
5f
5g
5i
5j
C6F5
10
p-EtO2CC6H4
20
120
5ke
11
12
p-HO3SC6H4
p-H2NCH2C6H4
30
20
120
120
5ld
5m
13
14
C6H5CH2
20
20
200
200
5n
5o
1
2
1
2
p-IC6H5CH2
15
16
3,4,5-triMeOC6H5CH2
nBu
50
60
200
200
5p
5q
3
a
b
c
d
e
See experimental. Amine/Squaric acid. Isolated yield of spectroscopically pure products. 8-Aminoquinoline. The corresponding
squaraine 3 was also obtained in 10% yield, see text.
aromatic and aliphatic amines allowed chemoselective condensa-
tions to take place. For example, 4-aminomethylaniline was
converted into the corresponding amidosquaric 5m in excellent
yield (Table 2, entry 12). In this case, the aliphatic amine remains
fully blocked by in situ protonation while the aromatic amine
group acts as the nucleophile. A similar preferential functionaliza-
tion of aromatic over aliphatic amine groups occurred in a
microwave-assisted amination of chloropyrimidines with 4-ami-
nomethylaniline.17
SQAMs have OH and NH groups that can interact with acceptor
groups, i.e. as supramolecular catalysts. In addition, SQAMs are
relatively strong acids and potentially useful as ligands in metal
complexes. In both cases, knowledge of the ionization constants is
the key for a better understanding of these properties. However, so
far the ionization constants of SQAMs are unknown. To fill this
gap, the Brønsted acid–base behaviour of representative SQAMs in
aqueous solution was investigated using a spectrophotometric
method. An intense characteristic band (log e = 4.2–4.6) in the
278–288 nm region is observed in the UV/Vis spectra of SQAMs
obtained for the aliphatic amines 5n to 5q. This long-wavelength
CT absorption band is red-shifted due to extended p conjugation
of the aryl ring with the squaric ring with SQAMs arising from
aromatic amines. In addition, the UV/Vis of SQAMs displayed
negative solvatochromism evidenced by the hypochromic shift (Dl
= 211 to 235 nm) of the long-wavelength charge-transfer band
increasing the polarity of the solvent from MeCN to H2O.
Acid–base titrations of representative SQAMs in water covering
a range from 1 to 13 showed the existence of two or three buffer
intervals that were analyzed with HypSpec.18 The pKa values
obtained are shown in Table 3.
Somewhat as expected, SQAMs are strong acids, and their first
ionization values compare well to those of strong acids such as
squaric (pKa1 = 0.6), oxalic (pKa1 = 1.38) or diphenylphosphoric
(pKa = 1.85) acids.19 In all of the cases studied, the development of
a proton-transfer band was also evident at the basic side of the pH
scale (pH . 10). These are diagnostic bands corresponding to a
proton-transfer process from the squaramide NHs.20 The mea-
sured pKa values corresponding to ionization of the squaramide
NHs are in the range between 11.5 and 13.0. These values, lower
than those typically found in secondary amides (,15), can be
attributed to the aromatic character of the resulting squarate
dianion. Additional acidic groups on the aromatic ring convert
SQAMs into triprotic organic acids. The pKa values of zwitterionic
dimethylamino derivatives 5c13a and of 5e (Table 3, entries 2 and
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for 15 min. The resulting slurry was partitioned between a solution
of 1 M NaOH and Et2O to remove unreacted squaric acid. After
acidification to pH 1, the aqueous layer was saturated with NaCl
and extracted with AcOEt (3 6 15 mL). Finally, the combined
AcOEt extracts were evaporated in vacuo to afford 5a in 83% yield.
Table 3 pKa values of representative SQAMs determined by spectrophotometric
titrations at 25 uCa
d
Entry SQAM
1
pKOH
pKX
pKNH
5a 1.55(1)
—
12.90(1)
Acknowledgements
Financial support from the Spanish Ministry of Economy and
Competitiveness (CTQ2011-27512/BQU and CONSOLIDER-
INGENIO 2010 CSD2010-00065, FEDER funds) and the
2
3
5c 0.89(1) 5.33(1)b 11.54(1)
5e 1.31(1) 9.18(1)c 13.00(3)
´
`
"Direccio General de Recerca, Desenvolupament Tecnologic i
´
Innovacio del Govern Balear" (CAIB, project 23/2011, FEDER
funds) are gratefully acknowledged. E. S. and C. L. thank CAIB
for a predoctoral fellowship.
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