Safer conditions for the Curtius rearrangement of 1,3,5- benzenetricarboxylic acid

Article abstract of DOI:10.1080/00397910701555766

A rapid method to prepare 1,3,5-triaminobenzene and its derivatives is the tris-Curtius rearrangment of 1,3,5-benzenetricarboxylic acid. The hazards associated with the acyl azide route are minimized by using 1,2-dichloroethane as solvent. A second method that avoids acyl azide preparation uses diphenylphosphoryl azide to yield the tricarbamate in one step. Copyright Taylor & Francis Group, LLC.

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Safer Conditions for the Curtius
Rearrangement of 1,3,5‐Benzenetricarboxylic
Acid
Matthew C. Davis a
a Chemistry and Materials Division, Michelson Laboratory , Naval Air Warfare
Center , China Lake, California, USA
Published online: 05 Oct 2007.
To cite this article: Matthew C. Davis (2007) Safer Conditions for the Curtius Rearrangement of
1,3,5‐Benzenetricarboxylic Acid, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:20, 3519-3528, DOI: 10.1080/00397910701555766
To link to this article: http://dx.doi.org/10.1080/00397910701555766
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Synthetic Communicationsw, 37: 3519–3528, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701555766
Safer Conditions for the Curtius
Rearrangement of 1,3,5-
Benzenetricarboxylic Acid
Matthew C. Davis
Chemistry and Materials Division, Michelson Laboratory, Naval Air
Warfare Center, China Lake, California, USA
Abstract: A rapid method to prepare 1,3,5-triaminobenzene and its derivatives is the
tris-Curtius rearrangment of 1,3,5-benzenetricarboxylic acid. The hazards associated
with the acyl azide route are minimized by using 1,2-dichloroethane as solvent. A
second method that avoids acyl azide preparation uses diphenylphosphoryl azide to
yield the tricarbamate in one step.
Keywords: Curtius rearrangement, 1,2-dichloroethane, diphenylphosphoryl azide,
1,3,5-triaminobenzene, 1,3,5-triisocyanatobenzene, trimesic acid
Curtius was the first to describe the preparation and rearrangement of 1,3,
5-benzenetricarbonyl triazide (2), and, because the reaction was conducted
in ethanol, the product he obtained was the triethyltricarbamate of 1,3,5-tria-
minobenzene.[1,2] This reaction might have wider application if the intermedi-
ate 1,3,5-triisocyanatobenzene (3)[3] could be isolated, since isocyanates
undergo many different reactions apart from carbamate formation
(Scheme 1). However, there is one major issue making the process treacher-
ous, which is the isolation of 2. For this synthetic route to be considered for
scale up, the energetic hazard associated with 2 must be minimized or elimi-
nated if possible.
Received in the USA February 23, 2007
Address correspondence to Matthew C. Davis, Chemistry and Materials Division,
Michelson Laboratory, Naval Air Warfare Center, China Lake, CA 93555, USA.
E-mail: matthew.davis@navy.mil
3519

3520
M. C. Davis
Scheme 1. Reagents and conditions: a) NaN3, H2O, 1,2-dichloroethane, 08C to rt;
b) anhydrous MgSO4, 1,2-dichloroethane; c) reflux, 1,2-dichloroethane.
A convenient method to prepare 2 is the reaction of sodium azide and 1,3,
5-benzenetricarbonyl chloride (1) in a biphasic medium, as seen in the recent lit-
erature.[4–6] In all these reports, 2 precipitates during the reaction and must be
extracted with an organic solvent. The presence of 2 in the solid state at any
time during the reaction, or its workup, is extremely dangerous because 2 is a
friction/shock-sensitive explosive. (The author sustained injuries after a ꢀ2-g
sample of 2 exploded while removing the solid from a glass flask with a steel
spatula.) A small amount of 2 was prepared using the conditions of van Gorp[5]
but with diethyl ether extraction rather than toluene. It was found that 2 has
low solubility at room temperature in several organic solvents (acetonitrile, tetra-
hydrofuran, methylene chloride, ethanol, and dioxane) including those report-
edly[1,4–6] employed in the extraction process (toluene, diethyl ether).
However, 2 showed acceptable solubility at room temperature in chloroform
(116 mg/0.4 mL) and 1,2-dichloroethane (114 mg/1 mL). 1,2-Dichloroethane
was selected over chloroform for the azide displacement reaction, since chloro-
form is known to react with azide ions to generate the explosive triazido-
methane.[7] Given the boiling point of 1,2-dichloroethane (838C), a second
consideration was the possibility that the rearrangement of 2 could also be
conducted in this solvent.[8]
Compound 1 was dissolved in 25% excess 1,2-dichloroethane, which is
necessary to dissolve the equivalent amount of 2. This was done to ensure that
compound 2 remained in solution throughout the reaction. The solution of
1 was added dropwise to a cold solution of sodium azide in water. Only a 0.1
molar equivalent excess of sodium azide gave complete consumption of 1. The
1
displacement reaction was conveniently monitored by H NMR by analyzing
an aliqout of the organic layer in deuteriochloroform (Fig. 1). Each of the reson-
ances for 1, monoacyl azide, diacyl diazide, and 2 were clearly observed. Several
runs showed that the displacement reaction was very fast, typically completing in
less than one hour. In all instances, 2 remained in solution during the reaction.
Next, the 1,2-dichloroethane solution of 2 was dried over anhydrous
magnesium sulfate to prevent hydrolysis during rearrangement (Scheme 1).
The drying agent was removed by gravity filtration through fluted filter
paper into a boiling flask. This helped to minimize any evaporation of

1,3,5-Benzenetricarboxylic Acid
3521
Figure 1. 1H NMR spectral analysis during preparation of 1,3,5-benzenetricarbonyl
triazide (deuteriochloroform).
solvent and consequent dry crystalline films of 2. The solution was refluxed
1
and monitored by H NMR in the same manner as before. The tris-Curtius
rearrangement ran smoothly in this solvent. The resonances for 2, monoiso-
cyanate, diisocyanate, and 3 were clearly observed (Fig. 2). Once all acyl
azide was exhausted, the solution was considered safe for removal from
Figure 2. 1H NMR spectral analysis during Curtius rearrangement of 1,3,5-benzene-
tricarbonyl triazide (deuteriochloroform).

3522
M. C. Davis
Scheme 2. Reagents and conditions: a) conc. HCl; b) 28% NH4OH, heat; c) MeOH or
EtOH, heat.
behind blast shields and evaporated at the benchtop. The yield over the two-
step sequence was 97%, and crude 3 had excellent purity by NMR.
The chemistry of 3 was briefly explored (Scheme 2). Decarboxylation of
3 by the action of warm concentrated hydrochloric acid gave the trihy-
drochloride salt of 1,3,5-triaminobenzene (4) in quantitive yield.[9,10] Next,
1,3,5-triureidobenzene (5)[11,12] was made by reaction of 3 with concentrated
ammonium hydroxide. Last, refluxing 3 with methanol or ethanol gave the
corresponding trimethyl- or triethyl-carbamates (6 and 7). The methyl
carbamate (6) appears to be a new compound. Curtius reported 7 with a
melting point of 110–1118C from a mixture of ethanol and water.[1] The
procedure described here gave 7 melting at 185–1888C, which is in agreement
with Gill et al.[3] The sample of 7 was recrystallized under several conditions
from ethanol and water, and in all cases no lower melting form was found.
Perhaps the safest method to perform the Curtius rearrangement described
here would be to avoid generating 2 entirely. Diphenylphosphoryl azide
(DPPA) is known to convert carboxylic acids into carbamates.[13–15]
Reaction of 1,3,5-benzenetricarboxylic acid with DPPA, tert-butanol,
Scheme 3. Reagents and conditions: a) diphenylphosphoryl azide, t-BuOH, TEA,
toluene, reflux.

1,3,5-Benzenetricarboxylic Acid
3523
triethylamine, and toluene at reflux gave a solid that was initially thought to be
the tri-tert-butyl ester of 1,3,5-benzenetricarboxylic acid (Scheme 3).
However, NMR spectroscopy in deuteriochloroform and methylsulfoxide-d6
showed the product was in fact tri-tert-butyl N0,N00,N000-1,3,5-triaminobenze-
netricarbamate (8). The yield of the reaction was 50%. During analysis of
8, the reports of Taylor and Puapaiboon were discovered, which may be the
first description of the rearrangement of 1,3,5-benzenetricarboxylic acid into
tricarbamate by DPPA.[16,17]
In conclusion, a safer method for the preparation of 3 through Curtius
rearrangement of 2 is described. The key modification is the use of
1,2-dichloroethane as solvent for both the azide displacement reaction and
the Curtius rearrangement. In this way, the friction/shock-sensitive 2 is main-
tained in solution and never isolated. In addition, the rediscovery of the
reaction between DPPA with 1,3,5-benzenetricarboxylic acid in the
presence of alcohol is noteworthy. This may represent both the safest and
quickest method to prepare the 1,3,5-triaminobenzene nucleus.[11,18,19] It
stands repeating that 2 is a friction/shock-sensitive explosive that under no
circumstances should be handled in a pure state. Apart from the valuable
Curtius rearrangement described here, there may be no reason to prepare 2.
EXPERIMENTAL
Melting points were collected with a Mel-Temp II from Laboratory Devices
(Holliston, MA) and are not corrected. NMR data were obtained on a
Bru¨ker Avance-300 spectrometer (1H at 300 MHz, 13C at 75 MHz, and 31P
at 121 MHz). NMR data were processed using NUTS software from Acorn
NMR, Inc. (Livermore, CA). Spectra were referenced to solvent or tetra-
methylsilane. For 31P NMR, 85% H3PO4 was used as internal standard.
1,3,5-Benzenetricarboxylic acid (trimesic acid) was obtained from Amoco
Chemical Company (Chicago). Diphenylphosphoryl chloride was obtained
from Aldrich Chemical Company (Milwaukee). All other reagents were
obtained commercially and used as received. The synthesis of DPPA
described here is based on the method of Wolff and Waldvogel.[20]
1,3,5-Benzenetricarbonyl Trichloride (1)
A 2-L round-bottomed flask equipped with magnetic stirbar and reflux
condenser was charged with 250 g of 1,3,5-benzenetricarboxylic acid
(1.19 mol) and 500 mL of SOCl2. A dry N2 purge line was setup at the top
of the condenser to neutralize the exhaust gases (HCl and SO2). The
exhaust tubing was connected to two aqueous NaOH traps (2 L H2O/80 g
NaOH each) in series. Ten drops of anhydrous DMF were added, and the
mixture was refluxed for 18 h. After this time, the reaction was a golden
yellow solution. A stillhead was equipped, and the excess SOCl2 was

3524
M. C. Davis
removed by reduced pressure distillation (15 torr). A clean receiver was
attached, the cooling water was shut off, and the product was distilled (1
torr). The pale yellowish oil slowly solidified in the receiver (299.7 g, 95%).
dH (CDCl3): 9.11 (s); dC (CDCl3): 166.31, 138.36, 135.77. Elemental
analysis calculated for C9H3Cl3O3: C, 40.72; H, 1.14. Found: C, 40.93; H, 0.96.
1,3,5-Triisocyanatobenzene (3)
A 100-mL round-bottomed flask equipped with magnetic stirbar was charged
with 7 mL of H2O and 715 mg of NaN3 (0.011 mol, 1.1 equiv). The mixture
was allowed to stir at rt until all the solids dissolved. The flask was stirred
in an ice bath while a solution of 2.64 g of 1 (0.01 mol) in 30 mL of
1,2-dichloroethane was added by addition funnel over 10 min. The cooling
bath was removed, and the mixture was stirred at rt. The reaction was
monitored by 1H NMR of aliquots dissolved in CDCl3. The two intermediates,
5-azidocarbonylisophthaloyl dichloride [dH (CDCl3): 9.03–9.0 (m, 3H)] and
3,5-bis(azidocarbonyl)benzoyl chloride [dH (CDCl3): 8.95 (d, J ¼ 1.6 Hz,
1H), 8.93 (t, J ¼ 1.7 Hz, 2H)], were both observed. After 1 h, the formation
of 1,3,5-benzenetricarbonyl triazide (2) [dH (CDCl3): 8.86(s); dC (CDCl3):
170.77, 135.14, 132.51] was complete. The organic phase was separated
with a polyethylene separatory funnel. The aqueous phase was washed once
with 5 mL of 1,2-dichloroethane. The organic phase was dried over 5 g of
anhydrous MgSO4 for 15 min. The drying agent was removed by filtration
through fluted filter paper into a 250-mL round-bottomed flask equipped
with a magnetic stirbar. A heating mantle and reflux condensor were
equipped, and the mixture was refluxed behind a blast shield. The reaction
1
was monitored by H NMR of aliquots dissolved in CDCl3. The two inter-
mediates, 1-isocyanato-3,5-benzenedicarbonyl diazide [dH (CDCl3): 8.47
(t, J ¼ 1.5 Hz, 1H), 7.98 (d, J ¼ 1.5 Hz, 2H)] and 1,3-diisocyanato-
5-benzenecarbonyl azide [dH (CDCl3): 7.59 (d, J ¼ 1.7 Hz, 2H), 7.06
(t, J ¼ 1.9 Hz, 1H)], were observed. After 1.5 h, the formation of the title
compound was complete. The solvent was rotary evaporated, leaving 1.9 g
(97%) of a white solid that was pure by 1H NMR. The crude was recrystallized
from n-heptane to give the title compound as white needles (1.1 g, 60%
recovery). A small amount of orange residue was generated in the process.
Mp 80–828C (lit.[3] 84–858C). dH (CDCl3): 6.67 (s, 3H); dH (C6D6): 5.87
(s, 3H); dC (CDCl3): 135.92, 125.71, 118.86. Elemental analysis calculated
for C9H3N3O3: C, 53.74; H, 1.50; N, 20.89. Found: C, 53.74; H, 1.66; N, 20.82.
1,3,5-Triaminobenzene Trihydrochloride (4)
A 100-mL Erlenmeyer flask equipped with magnetic stirbar was charged with
15 mL of conc. HCl. The solution was stirred at rt while 2 g of 3 (0.01 mol)

1,3,5-Benzenetricarboxylic Acid
3525
was added in one portion. The mixture was heated in a 508C water bath. The
mixture was stirred until evolution of CO2 was finished (ꢀ2 h). Methanol
(15 mL) was added, and the mixture was filtered on a coarse-fritted funnel.
The solid was placed in a vacuum oven (10 torr, 508C) overnight to remove
residual solvent to give the title compound as a white powder (2.12 g,
99%); it did not melt in temperatures up to 3008C. dH (D2O): 7.13 (s); dH
(DMSO): (bs, 8.75, NH); dC (D2O): 136.45, 114.23; dC (DMSO): 139.25,
.
105.89. Elemental analysis calculated for C6H9N3 3HCl: C, 30.99; H, 5.20;
N, 18.07. Found: C, 31.00; H, 5.15; N, 17.38. Attempts to recrystallize the
compound were unsuccessful.
1,3,5-Triureidobenzene (5)
A 100-mL Erlenmeyer flask equipped with magnetic stirbar was charged with
15 mL of conc. NH4OH. The solution was stirred in an ice bath while 500 mg
of 3 (0.002 mol) were added in portions within 5 min. The cooling bath was
removed, and the mixture was heated to 508C for 10 min. The mixture was
cooled to rt, and the white precipitate was filtered on a medium-porosity
glass frit. The white microcrystalline powder was dried in a vacuum oven
(10 torr, 508C), and further purification was not necessary (440 mg, 70%). It
did not melt in temperatures up to 3008C (lit.[3,11,12] . 3508C). dH
(DMSO): 8.45 (s, 3H), 7.16 (s, 3H), 5.73 (s, 6H); dC (DMSO): 156.24,
140.89, 100.31. Elemental analysis calculated for C9H12N6O3: C, 42.86; H,
4.80; N, 33.32. Found: C, 42.43; H, 4.85; N, 32.92.
Trimethyl N0,N00,N000-1,3,5-Triaminobenzenetricarbamate (6)
A 250-mL Erlenmeyer flask equipped with magnetic stirbar was charged
with 50 mL of anhydrous MeOH. The solution was stirred, and 1.0 g of
3 (0.005 mol) was added in one portion. The mixture instantly boiled and
then cooled back down. The mixture was refluxed for 10 min and filtered
through fluted filter paper to remove some insolubles. The solvent was
rotary evaporated, leaving 1.2 g (80%) of a white solid. The solid was recrys-
tallized from a mixture of DMSO/H2O to give the title compound as white
microcrystals. Mp 215–2178C. dH (DMSO): 9.54 (s, 3H), 7.30 (s, 3H), 3.63
(s, 9H); dH (CDCl3): 7.26 (s, 3H), 6.67 (s, 3H), 3.76 (s, 9H); dC (DMSO):
153.95, 139.63, 103.76, 51.50. Elemental analysis calculated for
C12H15N3O6: C, 48.48; H, 5.09; N, 14.14. Found: C, 48.23; H, 5.05; N, 14.09.
Triethyl N0,N00,N000-1,3,5-Triaminobenzenetricarbamate (7)
A similar procedure to 6 was used here. From 1 g of 3 (0.005 mol) and 50 mL
of EtOH, 1.1 g of crude white solid (66%) was obtained. The solid was

3526
M. C. Davis
recrystallized from 50% aqueous EtOH to give the title compound as white
needles. Mp 185–1888C (lit. 110–1118C,[1] 191–1928C[3]). dH (DMSO):
9.49 (s, 3H), 7.28 (s, 3H), 4.10 (q, J ¼ 7.2 Hz, 6H), 1.23 (t, J ¼ 7.2 Hz,
9H); dH (CDCl3): 7.29 (s, 3H), 6.78 (s, 3H), 4.21 (q, J ¼ 7.1 Hz, 6H), 1.31
(t, J ¼ 7.3 Hz, 9H); dC (DMSO): 153.46, 139.58, 103.81, 59.97, 14.53; dH
(CDCl3): 153.96, 139.62, 103.65, 61.51, 14.74. Elemental analysis calculated
for C15H21N3O6: C, 53.09; H, 6.24; N, 12.38. Found: C, 52.88; H, 6.31; N,
12.09.
Diphenylphosphoryl Azide (DPPA)
A 500-mL round-bottomed flask equipped with magnetic stirbar was charged
with 38.6 g of diphenylphosphoryl chloride (0.14 mol) and 250 mL of
acetone. Over 1 h, 11.23 g of NaN3 (0.17 mol, 1.2 equiv) was added to the
mixture, and stirring continued for 18 h. The solids were filtered away with
a medium-porosity glass frit. The solvent was rotary evaporated, leaving an
oil. Behind a blast shield, the oil was short-path distilled (1 torr) to give the
title compound as a colorless liquid (35 g, 90%). dH (CDCl3): 7.42–7.32
(m, 4H), 7.29–7.19 (m, 6H); dC (CDCl3): 149.49 (d, JC,P ¼ 10 Hz), 130.14
(d, JC,P ¼ 1.1 Hz), 126.21 (d, JC,P ¼ 1.7 Hz), 120.29 (d, JC,P ¼ 5 Hz); dP
(CDCl3): 29.9 (s).
Tri-tert-Butyl N0,N00,N000-1,3,5-Triaminobenzenetricarbamate (8)
A 500-mL round-bottomed flask equipped with magnetic stirbar was charged
with 5 g of 1,3,5-benzenetricarboxylic acid (0.024 mol), 20.95 g of DPPA
(0.076 mol, 3.2 equiv), 7.68 g of TEA (0.076 mol, 3.2 equiv), 25 mL of
t-BuOH, and 25 mL of toluene and refluxed for 18 h. The resulting solution
was rotary evaporated, leaving a semisolid. The residue was partitioned
between 200 mL of H2O and 200 mL of CHCl3. The organic phase was
separated and washed with 100 mL of 5% Na2CO3 and then 100 mL of
brine. The solvent was evaporated, leaving an oil. The oil was dissolved in
a mixture of hexanes/EtOAc and filtered through a short plug of 100 g of
˚
SiO2 (32–63 mm, 60 A). The solvent was evaporated, leaving a white
crystalline solid. The solid was recrystallized from a mixture of toluene/
cyclohexane to obtain white microcrystals (5 g, 50%). Mp 163–1668C. dH
(CDCl3): 7.21 (s, 3H), 6.75 (s, 3H), 1.48 (s, 27H); dH (DMSO): 9.16,
7.24, 1.46; dC (CDCl3): 152.88, 139.65, 129.98, 103.29, 28.42; dC
(DMSO): 152.72, 139.69, 103.68, 78.59, 28.14. Elemental analysis
calculated for C21H33N3O6: C, 59.56; H, 7.85; N, 9.92. Found: C, 59.86; H,
8.07; N, 9.45.

1,3,5-Benzenetricarboxylic Acid
ACKNOWLEDGMENTS
3527
The generous financial support of this research by the Office of Naval Research
is gratefully acknowledged. Support from Barbara Lupei, Cynthia Kitchens, and
the librarians of the Technical Library (NAWCWD) for collecting references 1
and 11 is appreciated. Thanks go to Brian Zentner (NAWCWD) for supplying
the trimesic acid, Andrew P. Chafin (NAWCWD) for suggestions and correc-
tions in preparation of this manuscript, and Ms. Chrissy Everett (NAWCWD)
for help collecting the combustion analyses.
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