Organocatalytic, rapid and facile cyclotrimerization of isocyanates using tetrabutylammonium phthalimide-N-oxyl and tetraethylammonium 2-(carbamoyl)benzoate under solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2010.08.018

Tetrabutylammonium phthalimide-N-oxyl and tetraethylammonium 2-(carbamoyl)benzoate were found to be effective and easily accessible organocatalysts for selective cylotrimerization of aryl and alkyl isocyanates under mild reaction conditions. The reaction proceeds smoothly using very low catalyst loadings of these metal-free organocatalysts (0.025 and 0.25 mol%, respectively) under solvent-free conditions at room temperature within a very short reaction time.

Full text of DOI:10.1016/j.catcom.2010.08.018

Catalysis Communications 12 (2010) 226–230
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Catalysis Communications
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Short Communication
Organocatalytic, rapid and facile cyclotrimerization of isocyanates using
tetrabutylammonium phthalimide-N-oxyl and tetraethylammonium 2-(carbamoyl)
benzoate under solvent-free conditions
Mohammad G. Dekamin ,1, Kambiz Varmira, Mehdi Farahmand, Solmaz Sagheb-Asl, Zahra Karimi
⁎
Pharmaceutical and Biologically-Active Compounds Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran 16846-13114, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 April 2010
Received in revised form 31 July 2010
Accepted 26 August 2010
Available online 14 October 2010
Tetrabutylammonium phthalimide-N-oxyl and tetraethylammonium 2-(carbamoyl)benzoate were found to
be effective and easily accessible organocatalysts for selective cylotrimerization of aryl and alkyl isocyanates
under mild reaction conditions. The reaction proceeds smoothly using very low catalyst loadings of these
metal-free organocatalysts (0.025 and 0.25 mol%, respectively) under solvent-free conditions at room
temperature within a very short reaction time.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Isocyanate cylotrimerization
Polyurethanes
Coating materials
Organocatalysis
Tetrabutylammonium phthalimide-N-oxyl
Tetraethylammonium 2-(carbamoyl)benzoate
1. Introduction
academic studies [1–14]. Cyclotrimerization of isocyanates has been
shown to be promoted by catalytic systems such as neutral trisamino-
The most important reaction between isocyanates is cyclotrimer-
ization to form the heteroaromatic isocyanurate (1,3,5-trisubstituted
hexahydro-s-triazinetrione) ring. This heterocycle is thermally very
stable and is used to enhance the physical properties of a wide variety
of polyurethanes and other coating materials such as polyureas in
commercial systems. Polymeric blends of isocyanurates demonstrate
increased thermal resistance, flame retardation, chemical resistance,
and film-forming characteristics [1–4]. Furthermore, different deri-
vatives of isocyanurates have recently received considerable attention
in other areas such as heterogeneous catalysis [5], periodic mesopor-
ous organosilicas (PMOs) with uniformly distributed organic and
organometallic groups within the silica framework [6,7], cross-linker
and glass coating agent [8–10], drug delivery [11,12] selective ion
bonding [13] and chiral discrimination [14].
The general route for the preparation of isocyanurates is the catalytic
cyclotrimerization of the corresponding isocyanates. However, isocya-
nates are very active agents which produce different products such as
trimer, dimer, carbodiimide, urethane or allophanate depending on the
reaction conditions [3,15]. Therefore, developing of catalysts which
promote cyclotrimerization selectively is of great importance due to
wide application of isocyanurate structure in both industry and
phosphines, trialkyl arsenic oxide, tertiary amines, and anions such as
carboxylates, cyanate, phenoxide [3], fluoride [16], sulfite [17] or sulfate
[18]. Catalysts containing metal species such as organotin compounds,
arene manganese tricarbonyl, and nickel halides have also been
reported [3]. Recently, metallic catalysts such as complexes of Yb, Eu,
Sm, Nd, Yb [19], Sn, Ge [20,21], and Pd [22] or zirconacyclopentanes [23]
have been employed. On the other hand, organocatalysis has received
great attention due to the mildness of the reaction conditions,
operational simplicity, the potential for the development of large scale
versions of the reaction, the ready availability, and environmental
friendliness in therecentyears [24]. Alongthis line, organocatalysts such
as N-heterocyclic carbenes (NHCs) [25], calcium carbene with chelating
iminophosphorane substituents [26], an electron-rich trisaminopho-
sphine [27], potassium or sodium salts of piperidinedithiocarbamate
[28], p-toluenesulfinate [29], saccharin [30], phthalimide [31], and
phthalimide-N-oxyl [32] have also been introduced.
In commercial systems, alkali metal carboxylates such as potassium
acetate in combination with ethylene glycol or its oligomers and
potassium 2-ethylhexanoate can be considered as standard catalysts.
Furthermore, the quaternary ammonium carboxylates demonstrate
high performance catalytic activity that can significantly assist in
processing and/or physical property modification [33]. We have recently
introduced tetrabutylammonium phthalimide-N-oxyl (TBAPINO) [34]
and tetraethylammonium 2-(carbamoyl)benzoate (TEACB) [24] as
efficient and metal-free organocatalysts for cyanosilylation of carbonyl
⁎
Corresponding author. Tel.: +98 21 7724 0284; fax: +98 21 7322 2703.
E-mail address: mdekamin@iust.ac.ir (M.G. Dekamin).
Previous name: Mohammad G. Dakamin.
1
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.08.018

M.G. Dekamin et al. / Catalysis Communications 12 (2010) 226–230
227
prepared according to a previously described procedure [30]. The
catalysts were powdered and dried at 70 °C for 1 h under reduced
pressure. Dry ether was freshly distilled over Na and benzophenone.
All reactions were protected from air moisture using a CaCl2 guard
tube. Analytical TLC was carried out using Merck 0.2 mm silica gel 60
F-254 Al-plates. All compounds were well characterized using melting
points, IR and NMR spectral data as compared with those obtained
from authentic samples or reported in the literature [3,16,21,25,28].
Caution: For safety reasons, all the experiments and operations were
performed in an efficient fume hood and necessary precautions were
considered in order to avoid any contact with the isocyanates.
Scheme 1. Cyclotrimerization of different isocyanates catalyzed by tetrabutylammo-
nium phthalimide-N-oxyl (TBAPINO) 1 or tetraethylammonium 2-(carbamoyl)benzo-
ate (TEACB) 2.
2.2. General procedure for cyclotrimerization of isocyanates catalyzed by
TBAPINO 1 or TEACB 2
compounds. In continuation of our interest on the catalytic applications
of both TBAPINO and TEACB for various organic transformations, we
herein disclose cyclotrimerization of aryl and alkyl isocyanates catalyzed
by TBAPINO 1 and TEACB 2 as Lewis basic and bifunctional organoca-
talysts, respectively (Scheme 1).
2.2.1. Cyclotrimerization of aryl isocyanates
Aryl isocyanate (3a–g) (20 mmol) was added to TBAPINO 1 (2 mg,
0.025 mol%) or TEACB 2 (14.7 mg, 0.25 mol%) in an oven-dried 25 mL
round-bottomed flask equipped with a condenser. The mixture was
vigorously stirred at room temperature for liquid isocyanates. In the
case of solid isocyanates, the mixture was vigorously stirred at 50 °C.
After complete solidification of the reaction mixture, it was crushed
and 5 mL of dry Et2O was added with stirring for 10 min. The mixture
was filtered and the obtained solids were suspended in 5 mL of water
to extract the catalyst. The mixture was filtered and the obtained
solids were recrystallized from EtOH/EtOAc to afford the desired
triaryl isocyanurates (4a–g).
2. Experimental
2.1. General
Melting points were determined by open capillaries on an
electrothermal apparatus and are uncorrected. FT IR spectra were
recorded as KBr pellets on a Shimadzu FT IR-8400S spectrometer. 1H
NMR (500 MHz) and 13C NMR (125 MHz) spectra were obtained
using a Bruker DRX-500 AVANCE spectrometer. All NMR spectra were
determined in CDCl3 or Acetone-d6 at ambient temperature. All
chemicals were purchased from Merck, Fluka or Alfa Aesar and used
as received except for 4-methoxyphenyl isocyanate which was
2.2.2. Cyclotrimerization of alkyl isocyanates
Allyl or ethyl isocyanate (3i–h) (20 mmol) was added to TBAPINO
1 (20 mg, 0.25 mol%) or TEACB 2 (14.7 mg, 0.25 mol%) in an oven-
Table 1
Cyclotrimerization of aryl and alkyl isocyanates catalyzed by TBAPINO 1.
Entry
Isocyanate (R)
Mol
(%)
T
(°C)
Time
(min)
Product
Yielda
(%)
TON
TOF
m.p.
(°C) [Lit.]
(h−1
)
1
2
3
4
5
6
7
8
Phenyl (3a)
Phenyl (3a)
Phenyl (3a)
Phenyl (3a)
0.1
0.05
r.t.
r.t.
r.t.
r.t.
50
50
50
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
13
14
14
35
8
0.5
0.5
30
50
13
16
540
1200
4a
4a
4a
4a
4a
4b
4c
4d
4e
4f
99
98
98
81
70
98
99
92
98
98
94
88
86
990
1960
3920
6480
2800
3920
3960
3680
3920
3920
3760
352
4569
8400
279–281 [3,23]
279–281 [3,23]
279–281 [3,23]
279–281 [3,23]
279–281 [3,23]
317–318 [23]
277–279 [18]
262–264 [16]
178–180 [25]
261–262 [3,25]
339–341 [23]
24–26 [3,25]
0.025
0.0125
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.25
16,800
11,109
21,000
470,400
475,200
7360
Phenyl (3a)
4-Chlorophenyl (3b)b
3,4-Dichlorophenyl (3c)b
4-Methylphenyl (3d)
2-Methylphenyl (3e)
4-Methoxyphenyl (3f)
1-Naphthyl (3g)
Allyl (3i)
9
4704
10
11
12c
13c
18,092
14,100
39
4g
4i
4h
Ethyl (3h)
0.25
348
17
93–95 [3,23]
a
Isolated yields (average of at least two runs).
Solid substrate.
Performed with 0.25 mol% of TBAPINO loading.
b
c

228
M.G. Dekamin et al. / Catalysis Communications 12 (2010) 226–230
Table 2
Cyclotrimerization of aryl and alkyl isocyanates catalyzed by TEACB 2.
O
Et4N+
O
NH2
O
2 (0.25 mol%)
R
O
R
O
O
N
N
R
N
C
O
solvent-free conditions
N
R
4
3
a
Entry
Isocyanate (R)
Mol
(%)
Temp
(°C)
Time
(min)
Yield
(%)
Product
TON
TOF
(h−1
)
1
2
3
4
5
6
7
8
Phenyl (3a)
Phenyl (3a)
Phenyl (3a)
Phenyl (3a)
1
0.5
r.t.
r.t.
r.t.
r.t.
50
50
r.t.
r.t.
r.t.
50
50
0.25
0.5
97
96
98
85
98
97
96
92
96
95
87
4a
4a
4a
4a
4b
4c
4d
4e
4g
4h
4i
97
192
392
680
392
388
384
368
384
380
348
23,280
23,040
28,337
34,872
138,353
280,482
17,723
7360
0.25
0.125
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.83
1.17
0.17
0.083
1.3
3
4
360
900
4-Chlorophenyl (3b)b
3,4-Dichlorophenyl (3c)b
4-Methylphenyl (3d)
2-Methylphenyl (3e)
1-Naphthyl (3g)
Allyl (3h)
9
10
11
5760
63
23
Ethyl (3i)
a
Isolated yields (average of at least two runs).
Solid substrate.
b
dried 20 mL round-bottomed flask equipped with a condenser. The
mixture was vigorously stirred at room temperature or 50 °C for the
time indicated in Table 1 or 2. After cooling to room temperature,
15 mL of dry ether was added and the mixture was stirred for 10 min.
On adding of ether, any urea formed by hydrolysis of the substrates is
separated as white crystals. The mixture was filtered on a Buchner
funnel and the crystals washed with additional 5 mL ether. The filtrate
was evaporated at reduced pressure to afford the desired isocyanu-
rates 4i–h. Further purification of triethyl isocyanurate (4h) could be
performed by its recrystallization from EtOH.
2.2.3.4. 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)trione (Triallyl iso-
cyanurate, 4i).
2.2.3. Selected spectroscopic data
2.2.3.1. 1,3,5-Triphenyl-1,3,5-triazine-2,4,6(1H,3H,5H)trione (Triphenyl
isocyanurate, 4a). m.p. 279–281 °C; IR (KBr): υ 3065, 1709, 1491,
1415 cm−1 1H NMR (500 MHz, CDCl3): δ 7.44–7.52 (m, 15H); 13C
;
NMR (125 MHz, CDCl3): δ 128.2, 128.6, 129.0, 133.3, 148.7 [3,23].
m.p. 24–26 °C; IR (KBr): υ 3085, 2987, 2958, 1690, 1457, 1413, 1318, 993,
3
a
933 cm−1
;
1H NMR (500 MHz, CDCl3): δ 4.46–4.47(td, JH Hd=6.0 Hz,
b
c
3
a
b c
4JH d=4JH Hd=1.2 Hz, 6Hd), 5.20–5.23 (dd, JH Hc=10.2 Hz, 2JH
=
H
H
2.2.3.2. 1,3,5-Tris-(4-chlorophenyl)-1,3,5-triazine-2,4,6(1H,3H,5H)
1.1 Hz, 3Hc), 5.26–5.30 (dd, 3JH Hb=17.1 Hz, 2JH c=1.1 Hz, 3Hb), 5.81–
a
b
H
a
trione (Tris-(4-chlorophenyl) isocyanurate, 4b). m.p. 317–318 °C; IR
5.89 (tdd, JH Hb=17.1 Hz, JH Hc=10.2 Hz, JH Hd=6.0 Hz, 3Ha); 13C
NMR (125 MHz, Acetone-d6): δ 45.2, 119.2, 131.1, 148.5 [3,25].
3
a
3
a
3
(KBr): υ 3277, 1720, 1692, 1593, 1491, 1422, 1089, 812 cm−1 1H
;
NMR (500 MHz, CDCl3): δ 7.42–7.44 (d, J=8.8, 6H), 7.58–7.60 (d,
J=8.8, 6H); 13C NMR (125 MHz, CDCl3): δ 129.9, 130.0, 131.9, 135.8,
148.2 [23,27].
3. Results and discussion
At first, phenyl isocyanate 3a was used as a model substrate and the
effect of different catalyst loading of TBAPINO was studied from 0.1 to
0.025 mol% under solvent-free conditions at room temperature. After
completion of the reaction (a crystalline white solid precipitated out of
the reaction mixture and monitored by TLC), a simple work up afforded
the desired product in excellent yields. The reaction proceeded very
cleanly and the obtained solid was free of any side products such as
2.2.3.3. 1,3,5-Tris-(4-methoxyphenyl)-1,3,5-triazine-2,4,6(1H,3H,5H)
trione (Tris-(4-methoxyphenyl) isocyanurate, 4f). m.p. 261–262 °C; IR
(KBr): υ 3030, 2963, 2841, 1770, 1715, 1695, 1609, 1420, 1175 cm−1; 1H
NMR (500 MHz, Acetone-d6): δ 3.83 (s, 9H), 7.00–7.02 (d, J=9.0 Hz,
6H), 7.33–7.35 (d, J=9.0 Hz, 6H); 13C NMR (125 MHz, Acetone-d6): δ
55.8, 114.9, 126.6, 129.7, 149.4, 160.1 [3,25].

M.G. Dekamin et al. / Catalysis Communications 12 (2010) 226–230
229
R
Et4N+
O
R
N
C
O
N
-
(3)
O
H
C
N
H
O
O
O
O
O
C
O
(I)
+
-
Et4N
O
Et4N+
O
N
NH2
H
H
-
N
R
O
(2)
R
O
R
O
N
N
O
(II)
N
R
N
R
C
O
O
O
C
O
(3)
N
O
H
O
O
N
(4)
R
O
H
R
Et4N+
O
H
H
-
N
N
C
N
O
R
-
N
Et4N+
N
O
O
R
(III)
R
N
R
C
O
(IV)
(3)
Scheme 2. Plausible mechanism for cyclotrimerization of isocyanates catalyzed by TEACB.
dimer [25] oreven catalyst (entries 1–4, Table 1). Interestingly, TBAPINO
is more soluble in water comparing to solvents with low to medium
polarity such as CH2Cl2, Et2O and EtOAc. Therefore, it substantially
partitions in the aqueous phase and this allows very convenient
separation of the catalyst from the reaction mixture. Furthermore,
TBAPINO can be easily prepared from N-hydroxyphthalimide and
tetrabutylammonium hydroxide. It has good lifetime and is resistant
to moisture and air. TBAPINO could be used for months without loss of
performance [34,35]. It was found that the best result could be obtained
with catalyst loading of 0.025 mol% (entry 3). Encouraged by these
results, differentisocyanates were subjected to cyclotrimerization under
the optimized reaction conditions (TBAPINO; 0.025 mol%, solvent-free
conditions, r.t.). Table 1 shows the scope of the reaction using a number
of representative isocyanates wherein high to quantitative yields of the
corresponding isocyanurates (4a–h) were obtained within short
reaction time in all the cases studied (entries 6–13, Table 1). It is
noteworthy that allyl isocyanate 3i afforded its corresponding iso-
cyanurate (4i) as a commercially important compound [25].
showed relatively strong effects on the reaction time (entries 4, 6–13).
On the other hand, isocyanates 3b and 3c (entries 6 and 7) are solid at
room temperature. Since the reaction takes place under solvent-free
conditions, the reactivity of these isocyanates was studied at 50 °C. At
this temperature, these isocyanates melt and the catalysts dissolve in
the obtained melts. Consequently, these substrates involved in the
reaction conditions very well.
These encouraging findings prompted us to further investigate the
use of a more efficient organocatalyst for cyclotrimerization of
isocyanates. This tactic would be more useful for isocyanates such as
allyl isocyanate which require longer reaction time. Therefore, we
decided to investigate the use of tetraethylammonium 2-(carbamoyl)
benzoate (TEACB) which demonstrated higher catalytic activity
comparing to TBAPINO in the cyanosilylation of carbonyl compounds
[24,34]. The results of this remarkably accelerated isocyanate cyclo-
trimerization methodology have been summarized in Table 2. This
improved catalytic activity can be attributed to an additional hydrogen
bonding interaction of TEACB with the oxygen atom of the isocyanate
functionality in the complex or intermediates (I–IV), comparing to
TBAPINO, whereas the later catalyst activates isocyanates only by its
negatively charged oxygen atom (Scheme 2).
As shown in Table 1, aryl isocyanates react much faster under
similar reaction conditions comparing to allyl and alkyl derivatives.
Furthermore, the nature of the substituents on the aromatic ring
Table 3
Comparison of some of the results obtained by cyclotrimerization of isocyanates in the presence of TBAPINO (1) and TEACB (2), with some of those reported by proazaphosphatrane
(3), 1,3-Bis-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr, 4), tetrasulfido complexes of Sn (5) and palladium(0) (dibenzylideneacetone) complex bearing 1,10-
phenanthroline (6).
Entry
Substrate
Method [temperature (°C)/TOF (h−1)]
1a
2a
3
4b
5a
6c
[3]
[25]
[21]
[22]
1
2
3
4
5
PhNCO
r.t./16,800 50/21,000
50/470,400
r.t./7360
r.t./18,092
r.t./39
r.t./28,377
50/138,353
r.t./17,723
–
r.t./8545a
–
r.t./980
–
r.t./238
–
r.t./238
–
4-ClPhNCO
4-MePhNCO
4-MeOPhNCO
Allyl
–
–
–
r.t./0.23
r.t./0.52
–
r.t./3418a
r.t./850
1.0/980
r.t./407
–
50/63
r.t./3b
a
The reactions were carried out under solvent-free conditions.
The reactions were carried out in THF as solvent.
The reaction was carried out in PhNO2 as solvent.
b
c

230
M.G. Dekamin et al. / Catalysis Communications 12 (2010) 226–230
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722–732.
shown to compare our results with those reported in the literature
[3,21,22,25].
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4. Conclusion
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[18] M.G. Dekamin, S. Mallakpour, M. Ghassemi, J. Chem. Res. S (2005) 177–179.
[19] X.C. Zhu, J.X. Fan, Y.J. Wu, S.W. Wang, L.J. Zhang, G.S. Yang, Y. Wei, C.W. Yin, H. Zhu,
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(2007) 7294–7304.
In summary, we have developed new, mild and highly efficient
protocols for the synthesis of a wide range of aryl and alkyl
isocyanurates catalyzed by tetrabutylammonium phthalimide-N-
oxyl (TBAPINO) or tetraethylammonium 2-(carbamoyl) benzoate
(TEACB). The reported procedures clearly demonstrated that TBA-
PINO and especially TEACB are suitable metal-free organocatalysts for
the preparation of isocyanurates. The important features of our
method are: the mild reaction conditions, low catalysts loading, high
to quantitative yields of the products, chemical stability and the
simple preparation of the catalysts or their removal from the reaction
mixture.
[23] Y.Z. Li, H. Matsumura, M. Yamanaka, T. Takahashi, Tetrahedron 60 (2004)
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[24] M.G. Dekamin, S. Sagheb-Asl, M.R. Naimi-Jamal, Tetrahedron Lett. 50 (2009)
4063–4066.
[25] H.A. Duong, M.J. Cross, J. Louie, Org. Lett. 6 (2004) 4679–4681.
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Acknowledgement
We are grateful for the financial support from the Research Council
of Iran University of Science and Technology (IUST), Iran (Grant No.
160/5059).
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