Efficient synthesis of 2-substituted phthalimides from phthalic acids in one step

Article abstract of DOI:10.1002/ejoc.201300952

Efficient procedures for synthesizing 2-substituted phthalimide (isoindole-1,3-dione) analogues starting from phthalic acids have been developed by using experimental design. The phthalimide central fragment frequently appears in biologically active compounds, materials, catalysts, and fluorescent probes, and therefore the development of general, fast, and convenient synthetic methods to this scaffold under neutral, acidic, and basic conditions would be attractive. After an initial screening, the use of acetonitrile, acetic acid, or pyridine in combination with microwave heating proved most promising. Experimental design was applied to these conditions to optimize the time, temperature, and concentration. This strategy has successfully generated synthetic methods that have been used to synthesize a series of phthalimides from phthalic acids and various amines or anilines in excellent yields. The developed methods have proven to be general, fast, convenient, and economic, and thus are expected to have broad utility to efficiently construct novel compounds for future biological and chemical applications. Efficient procedures for synthesizing 2-substituted phthalimide (isoindole-1,3-dione) analogues starting from phthalic acids have been developed by using experimental design. The developed methods are fast, general, high-yielding, do not use additives, and are based on 1:1 molar ratios.

Full text of DOI:10.1002/ejoc.201300952

FULL PAPER
DOI: 10.1002/ejoc.201300952
Efficient Synthesis of 2-Substituted Phthalimides from Phthalic Acids in One
Step
Erik Chorell*[a] and Elin Chorell*[b,c]
Keywords: Oxygen heterocycles / Nitrogen heterocycles / Experimental design / Microwave chemistry
Efficient procedures for synthesizing 2-substituted phthal-
imide (isoindole-1,3-dione) analogues starting from phthalic
acids have been developed by using experimental design.
The phthalimide central fragment frequently appears in bio-
logically active compounds, materials, catalysts, and fluores-
cent probes, and therefore the development of general, fast,
and convenient synthetic methods to this scaffold under neu-
tral, acidic, and basic conditions would be attractive. After
an initial screening, the use of acetonitrile, acetic acid, or
pyridine in combination with microwave heating proved
most promising. Experimental design was applied to these
conditions to optimize the time, temperature, and concentra-
tion. This strategy has successfully generated synthetic
methods that have been used to synthesize a series of phthal-
imides from phthalic acids and various amines or anilines in
excellent yields. The developed methods have proven to be
general, fast, convenient, and economic, and thus are ex-
pected to have broad utility to efficiently construct novel
compounds for future biological and chemical applications.
crowave irradiation (MWI) as the heating source, which has
proven to be a valuable tool for high-temperature reactions
as it enables fast heating allied to good temperature and
time control.
Introduction
The phthalimide core structure is found in commercial
drugs and in a wide range of precursors for different bio-
logical targets.[1–3] In addition, phthalimides have also suc-
cessfully been used as key components in materials and
polymers,[4–7] as catalysts,[8,9] and as fluorescent
probes.[10–12] Because of this great interest in phthalimides,
the development of convenient, economic, and general syn-
thetic procedures to generate substituted phthalimides di-
rectly from phthalic acids would be valuable. Phthalimides
are normally synthesized from phthalic acids in more than
one step by using high temperatures and long reaction times
or by using various additives such as coupling reagents or
catalysts, which often require additional purification
steps.[1,13–16] A straightforward protocol with a 1:1 molar
ratio of phthalic acid and amine or aniline that could be
performed in high concentration and without any additives
would be both economic and simplify the purification of
the target compounds. Herein, we describe the development
and optimization of such synthetic procedures by using mi-
Results and Discussion
In this work we used MWI to optimize the synthesis of
phthalimide 3 by using phthalic acid (1) and 4-amino-
benzoic acid (2) as substrates (Table 1). 4-Aminobenzoic
acid (2) was chosen, because it previously resulted in the
formation of byproducts, which we aimed to eliminate in
Table 1. Evaluating the suitability of different solvents.
Entry
Solvent
Yield [%][a]
[a] Department of Chemistry, Umeå University,
90187 Umeå, Sweden
1
7
5
4
8
3
6
2
9
1,4-dioxane
1,2-dichloroethane
DMF
25
12
55
29
64
78
58
44
26
E-mail: erik.chorell@chem.umu.se
www.chemistry.umu.se
[b] Department of Public Health and Clinical Medicine, Umeå
University,
DMSO
EtOAc
MeCN
NMP
THF
toluene
90187 Umeå, Sweden
E-mail: elin.chorell@medicin.umu.se
www.phmed.umu.se
[c] Computational Life Science Cluster (CLiC), Umeå University,
90187 Umeå, Sweden
www.clicumu.se
[a] Calculated by HPLC–MS using biphenyl as the internal stan-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300952.
dard (standard curve R2 = 98%).
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Substituted Phthalimides from Phthalic Acids
the improved procedure. The reaction conditions are of key
Next, statistical experimental design[17–21] was employed
importance in the introduction of more delicate functional to optimize the reaction under different conditions. Aceto-
groups, and we thus decided to develop and optimize acidic, nitrile appeared to require higher temperatures for full con-
basic, and neutral conditions. First, different solvents were version compared with acetic acid and pyridine, which was
screened, and the results are shown in Table 1.
corrected for in the design. No general correlation between
The choice of solvent proved to be highly important for pressure and yield had been observed up to this point and –
the reaction outcome, with acetonitrile giving the best yield because the pressure is dependent upon the temperature –
of 78% after microwave heating at 150 °C for 30 min. Vari- it was not included as a separate factor in the experimental
ous acids (acetic acid, p-toluenesulfonic acid, trifluoroacetic design. Consequently, by varying the temperature, reaction
acid, and pyridinium p-toluenesulfonate) were next evalu- time, and concentration, measuring the yield as response,
ated in different amounts and solvents. The use of trifluoro- three fractional factorial multilevel designs[22] with 12 ex-
acetic acid in acetonitrile gave high conversion, but unfortu- perimental runs (i.e., nine different experiments and three
nately also led to the formation of byproducts, whereas the repeats for each solvent for reproducibility) were created by
use of acetic acid as solvent proved to be the most promis- using MODDE software 9.1.0.0.[23] The results are shown
ing of the evaluated acidic conditions, and pyridine as sol- in Table 2 and Figure 1.
vent was found to provide the most effective basic condi-
All three designs successfully generated efficient condi-
tions following an evaluation of different concentrations of tions that gave the target compound in close to quantitative
1,4-diazabicyclo[2.2.2]octane, triethylamine, 4-(dimethyl- yields (Table 2 and Figure 1). Furthermore, the reactions
amino)pyridine, N,N-diisopropylethylamine, pyridine, and could be performed at high concentrations with relatively
1,8-diazabicyclo[5.4.0]undec-7-ene in different solvents. short reaction times. In all three designs, the temperature
Hence, the solvent of choice for the neutral conditions was proved to be the most influential factor on the yield. The
acetonitrile, the acid of choice for the reaction under acidic reaction neutral conditions needed slightly higher tempera-
conditions was acetic acid, and the base of choice for the tures (180 °C) for a yield over 95% in comparison with re-
basic conditions was pyridine. Mixtures of either acetic acid actions performed under acidic and basic conditions
or pyridine in acetonitrile led to lower yields.
(150 °C). The reaction time (30 min, 1 h, 1.5 h) showed only
Figure 1. Response surface plots from the fractional factorial designs in (A) acetonitrile, (B) acetic acid, and (C) pyridine at different
concentrations (0.5, 1, and 2 mmol/mL). Colors represent variation in yield (low yield in green and high yield in red). The temperature
has the most influence on the yield; the reaction seemed very insensitive to time and concentration at the tested intervals.
Eur. J. Org. Chem. 2013, 7512–7516
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. Chorell, E. Chorell
FULL PAPER
Table 2. Three different fractional factorial designs to optimize
conditions in acetic acid (A1–A12), pyridine (B1–B12), and aceto-
nitrile (N1–N12).
Table 3. Additional experiments to study the effect of shorter reac-
tion times and lower concentrations under the three different con-
ditions (acidic: A13–A15; basic: B13–B15; neutral: N13–N18).
Exp. T [°C] t [min] Conc. [mmol/mL] Solvent Yield [%][a]
A13
A14
A15
B13
B14
B15
N13
N14
N15
N16
N17
N18
150
150
150
150
150
150
150
180
150
150
180
180
15
30
90
15
30
90
15
15
30
90
30
90
1
AcOH
AcOH
AcOH
80
58
74
78
38
73
50
94
57
80
80
97
0.1
0.1
1
0.1
0.1
1
pyridine
pyridine
pyridine
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Exp. T [°C] t [min] Conc.[mmol/mL] Solvent Yield [%][a]
1
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
A11
A12
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
B12
N1
N2
N3
N4
N5
N6
N7
N8
N9
N10
N11
N12
50
50
50
30
60
90
30
60
90
30
60
90
60
60
60
30
60
90
30
60
90
30
60
90
60
60
60
30
60
90
30
60
90
30
60
90
60
60
60
0.5
1
2
1
2
0.5
2
0.5
1
1
1
1
0.5
1
2
1
2
0.5
2
0.5
1
1
1
1
0.5
1
2
1
2
0.5
2
0.5
1
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
0
0
0
0.1
0.1
0.1
0.1
100
100
100
150
150
150
100
100
100
50
10
17
27
90
96
91
28
28
27
0
[a] Calculated by HPLC–MS using biphenyl as the internal stan-
dard (standard curve R2 = 98%).
AcOH
AcOH
AcOH
To verify that the optimized conditions were applicable
to other substrates, a series of eight phthalimides (3–10)
with varying substituents were synthesized from the corre-
sponding phthalic acids and amines or anilines by using the
three optimized procedures (Figure 2). Overall, the opti-
mized conditions performed well and gave the desired com-
pounds in high yields. For the sterically more hindered
alkyl-substituted amines (used in the synthesis of 6 and 7),
a prolonged reaction time of 90 min was needed to secure
high yields.
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
50
50
0
0
5
100
100
100
150
150
150
100
100
100
120
120
120
150
150
150
180
180
180
150
150
150
23
15
90
99
99
10
10
12
6
41
61
78
94
88
98
99
100
93
89
89
Conclusions
By using experimental design we have successfully gener-
ated synthetic methods for synthesizing phthalimides from
phthalic acids under neutral, acidic, or basic conditions in
excellent yields. The solvent proved to be very important in
the reaction, and acetonitrile gave the highest yield of the
nine tested solvents. Acetic acid and pyridine proved to be
best for reactions performed under acidic and basic condi-
tions, respectively. All conditions were optimized for tem-
perature, time, and concentration by using experimental de-
sign. This resulted in fast and convenient methods that
could be applied to the synthesis of a panel of eight dif-
ferently substituted phthalimides in high yields. The reac-
1
1
1
[a] Calculated by HPLC–MS using biphenyl as the internal stan-
dard (standard curve R2 = 98%).
a small influence on the yield. However, when additional tions were performed without any additives and in 1:1 mo-
reactions were performed with shorter reaction times of lar ratios, and the pure compounds could thus be isolated
15 min, the yield also started to drop, which indicates that after simple workup without need for column chromatog-
reaction times of 30 min or more are needed to obtain a raphy. Combined with the possibility of using both anilines
high yield (Table 3, A13, B13, N13, N14). The reaction was and sterically hindered amines together with the insensitiv-
not sensitive to the concentration within the tested range, ity of the methods to high concentration, we conclude that
and even the lowest concentration tested in the design the developed methods are general, fast, convenient, and
(0.5 mmol/mL) gave excellent yields. Nevertheless, reducing economic. In light of this, the developed methods are ex-
the concentration to 0.1 mmol/mL did reduce the yields, pected to have broad utility for the efficient construction of
which suggests that the concentration should be 0.5 mmol/ novel compounds for future biological and chemical appli-
mL or higher (Table 3, A14, A15, B14, B15, N15–N18).
cations.
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Eur. J. Org. Chem. 2013, 7512–7516

Substituted Phthalimides from Phthalic Acids
Figure 2. Synthesis of phthalimides from the corresponding phthalic acids and amines or anilines by using the optimized conditions with
MWI. Isolated yields are shown. Conditions A: Acetic acid, 150 °C, 30 min, 1 mmol/mL. Conditions B: Pyridine, 150 °C, 30 min, 1 mmol/
mL. Conditions N: acetonitrile, 180 °C, 30 min, 1 mmol/mL. Conditions *A: Acetic acid, 150 °C, 90 min. Conditions *B: Pyridine, 150 °C,
90 min. Conditions *C: Acetonitrile, 180 °C, 90 min.
The data for compounds 3, 4, and 6–8 are in agreement with pub-
Experimental Section
lished data.[24–28]
General Synthesis: Unless otherwise stated, all reagents and sol-
vents were used as received from commercial suppliers. All reac-
tions were carried out under an inert atmosphere. All microwave
4-[(1,3-Dioxoisoindolin-2-yl)methyl]benzoic Acid (5): Prepared ac-
cording to the general procedure by using acetonitrile as solvent,
phthalic acid (332 mg, 2 mmol) and 4-(aminomethyl)benzoic acid
reactions were carried out in a Biotage® Initiator+ monomode reac-
(302 mg, 2 mmol) gave 5 as a white solid (540 mg, 96%). M.p. 267–
tor by using sealed 2–5 mL Biotage® process vials. Reaction times
270 °C. IR: ν = 1768, 1678, 1609, 1428, 1391, 1319, 1281, 1169,
˜
refer to irradiation time at the target temperature, not the total
irradiation time. The temperature was measured with an IR sensor.
1079, 938 cm–1. 1H NMR (400 MHz, [D6]DMSO): δ = 4.83 (s, 2
H), 7.42 (d, J = 8.3 Hz, 2 H), 7.78–7.88 (m, 4 H), 7.91 (d, J =
8.2 Hz, 2 H), 12.95 (s, 1 H) ppm. 13C NMR (100 MHz, [D6]-
DMSO): δ = 40.7, 123.3 (2 C), 127.5 (2 C), 129.7 (2 C), 130.0,
131.6 (2 C), 134.6 (2 C), 141.5, 167.1, 167.7 (2 C) ppm. HRMS
(ES+): calcd. for C16H11NNaO4 304.0586 [M + Na]+; found
304.0592.
1
The H and 13C NMR spectra were recorded with a Bruker DRX-
400 spectrometer at 298 K in [D6]DMSO [residual DMSO (δH
=
2.50 ppm) or DMSO (δC = 40.0 ppm) as internal standard]. IR
spectra were recorded with a Bruker Equinox 55 spectrometer
equipped with an ATR device. HRMS data were acquired by using
a Micromass Q-Tof Ultima™ mass spectrometer with electrospray
ionization (ES+); sodium formate was used as calibration com-
pound. LCMS was conducted with a Waters® system by using a
Waters® 2487 Dual Absorbance Detector connected to a Waters®
Micromass® ZQ mass spectrometer with ES+ ionization.
4-(5-Hydroxy-1,3-dioxoisoindolin-2-yl)benzoic Acid (9): Prepared
according to the general procedure by using acetonitrile as solvent,
4-hydroxyphthalic acid (360 mg, 2 mmol) and 4-aminobenzoic acid
(274 mg, 2 mmol) gave 9 as a white solid (538 mg, 95%). M.p. 368–
372 °C. IR: ν = 3299, 1774, 1686, 1596, 1463, 1374, 1263, 1121,
˜
General Synthesis of Phthalimides from Phthalic Acids and Amines
or Anilines: Solvent [acetic acid for acidic conditions (A), pyridine
for basic conditions (B), and acetonitrile for neutral conditions (N);
2 mL] was added to phthalic acid (2 mmol) and aniline or amine
(2 mmol) in a 2–5 mL microwave vial. The vial was flushed with
nitrogen, sealed, and heated to the target temperature (150 °C for
acidic and basic conditions and 180 °C for neutral conditions) by
microwave irradiation. The mixture was heated for 30–90 min be-
fore being cooled (many products precipitated upon cooling and
were simply filtered and washed with water to give the pure prod-
uct). The reaction mixture was diluted with dichloromethane and
washed with water. The organic phase was dried with Na2SO4, fil-
tered, and concentrated to give the pure product.
1087, 894 cm–1. 1H NMR (400 MHz, [D6]DMSO): δ = 7.12–7.24
(m, 2 H), 7.58 (d, J = 8.0 Hz, 2 H), 8.08 (d, J = 7.9 Hz, 2 H), 7.91
(d, J = 8.2 Hz, 2 H), 11.28 (br. s, 1 H), 12.90 (br. s, 1 H) ppm. 13C
NMR (100 MHz, [D6]DMSO): δ = 110.5, 121.4, 121.9, 126.2, 127.2
(2 C), 130.2, 130.3 (2 C), 134.6, 136.5, 164.1, 166.78, 166.85,
167.2 ppm. HRMS (ES+): calcd. for C15H9NNaO5 306.0378 [M +
Na]+; found 306.0383.
4-(5-Nitro-1,3-dioxoisoindolin-2-yl)benzoic Acid (10): Prepared ac-
cording to the general procedure by using acetonitrile as solvent,
4-nitrophthalic acid (420 mg, 2 mmol) and 4-aminobenzoic acid
(274 mg, 2 mmol) gave 10 as a yellow solid (599 mg, 96%). M.p.
335–339 °C. IR: ν = 1779, 1725, 1686, 1607, 1534, 1421, 1381, 1346,
˜
Eur. J. Org. Chem. 2013, 7512–7516
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. Chorell, E. Chorell
FULL PAPER
1290, 1217, 1116, 1101, 923 cm–1. 1H NMR (400 MHz, [D6]-
DMSO): δ = 7.62 (d, J = 8.3 Hz, 2 H), 8.11 (d, J = 8.3 Hz, 2 H),
8.23 (d, J = 8.2 Hz, 1 H), 8.57–8.61 (m, 1 H), 8.66–8.71 (m, 1 H),
13.15 (br. s, 1 H) ppm. 13C NMR (100 MHz, [D6]DMSO): δ =
118.7, 125.5, 127.4 (2 C), 130.36, 130.41 (2 C), 131.3, 133.4, 135.7,
136.7, 152.0, 165.4, 165.6, 167.3 ppm. HRMS (ES+): calcd. for
C15H8N2NaO6 335.0280 [M + Na]+; found 335.0285.
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Published Online: September 25, 2013
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Eur. J. Org. Chem. 2013, 7512–7516