Fast cyclotrimerization of a wide range of isocyanates to isocyanurates over acid/base conjugates under bulk conditions

Article abstract of DOI:10.1016/j.catcom.2020.106097

An array of organic bases DMAP (4-dimethylaminopyridine), DBU (1, 8-diazabicyclo [5.4.0] undec-7-ene), TBD (1, 5, 7-triazabicyclo [4.4.0] dec-5-ene), and their base/acid conjugate organocatalyst systems were evaluated in the trimerization of various isocyanates. The performance depended greatly on the combination of the catalyst systems, and the [HTBD][OAc] (acetic acid) catalyst systems were considerably the most active in contrast to the corresponding DMAP and DBU counterparts. The [HTBD][OAc] catalyst system was capable of providing isocyanurates from the cyclotrimerization of various isocyanate substrates in excellent yields in seconds even under bulk conditions. A bifunctional catalytic mechanism over [HTBD][OAc] was proposed.

Full text of DOI:10.1016/j.catcom.2020.106097

CatalysisCommunications145(2020)106097
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Short communication
Fast cyclotrimerization of a wide range of isocyanates to isocyanurates over
acid/base conjugates under bulk conditions
Li Wua,1, Wei Liub,1, Jinxing Yeb, Ruihua Chenga,⁎
a College of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
b Engineering Research Center of Pharmaceutical Process Chemistry, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cyclotrimerization
Isocyanate
Isocyanurate
Acid/base conjugates
An array of organic bases DMAP (4-dimethylaminopyridine), DBU (1, 8-diazabicyclo [5.4.0] undec-7-ene), TBD
(1, 5, 7-triazabicyclo [4.4.0] dec-5-ene), and their base/acid conjugate organocatalyst systems were evaluated in
the trimerization of various isocyanates. The performance depended greatly on the combination of the catalyst
systems, and the [HTBD][OAc] (acetic acid) catalyst systems were considerably the most active in contrast to the
corresponding DMAP and DBU counterparts. The [HTBD][OAc] catalyst system was capable of providing iso-
cyanurates from the cyclotrimerization of various isocyanate substrates in excellent yields in seconds even under
bulk conditions. A bifunctional catalytic mechanism over [HTBD][OAc] was proposed.
1. Introduction
and cost, and ability to work under air- and water-tolerating reaction
conditions. In the trimerization of isocyanates, Verkade et al. [1] de-
Isocyanurates produced by the trimerization of isocyanates can be
used to enhance the chemical stability and physical mechanical prop-
erties of polyurethanes. Substituted aryl-functionalized isocyanurate is
a useful activator for anionic polymerization of ε-caprolactams to
Nylon-6. Additionally, it could substantially enhance the stability of
polyurethane networks and coating materials with respect to thermal
resistance, flame retardancy, chemical resistance, and film-forming
characteristics [1]. It has been applied as a key component in chiral
identification, microporous materials [2], coating materials [3], drug
delivery, and as a crossing-linking agent for plastic. As a result, efficient
and green isocyanurate synthesis is currently of commercial im-
portance, especially in the production of rigid urethane-modified iso-
cyanurate foams [2].
The common route for the preparation of isocyanurates is the cy-
clotrimerization of isocyanates. To promote the reaction, the catalyst
systems including a broad range of compounds have been studied for
many years. Metal catalysts have been reported such as organotin
compounds, palladium(0) systems [4], hemilabile aluminumpyridyl-bis
(iminophenolate) complex [5], as well as rare earth metal catalysts [6].
Recently, organocatalysis has proven to be powerful and attractive
in organic synthesis. Compared to the organometallic catalysts, the
main attribute of metal-free organocatalyst including Brønsted/Lewis
base and/or acid was its versatility, high selectivity, reduced toxicity
veloped a method for efficient trimerization of isocyanates using
0.0033 mol% ZP(MeNCH2CH2)3N (Z = lone pair, proazaphosphatrane)
catalyst, which could be recycled six times without lost activity; how-
ever, the synthesis process was complicated and expensive. Giuglio
et al. [7] proved that TDAE (tetrakis (dimethylamino) ethylene) had
good activity under ambient conditions with low catalyst loading;
however, the synthetic product had a strong odor in the final product
[8]. High yields of isocyanurates could be obtained with a N-hetero-
cyclic olefin catalyst loading as low as 0.005% [9].
N-heterocyclic carbene was also widely studied as an efficient or-
ganic catalyst. Duong et al. [10] found that isocyanurates could be
obtained in excellent yields only with 0.1 mol% 1, 3-bis- (2, 6-diiso-
propylphenyl) -4, 5-dihydroimidazol-2-ylidene at room temperature. Li
et al. [11] reported that amino-NHC (amine-linked N-heterocyclic
carbene) was used in the trimerization of isocyanates with 10 mol%
catalyst loading at room temperature for 3 h. However, this kind of
catalyst system has the disadvantage of high toxicity and difficult se-
paration.
In parallel, multi-component organocatalysts have proven useful in
the generation of oligomers and polymers. Lewis pair was originally
used to catalyze the hydrogenations of small molecular compounds
[12] and to catalyze polymerization [13]. In the ring opening poly-
merization (ROP) of lactones, the trade-off between the selectivity and
⁎ Corresponding author.
E-mail address: rhcheng@ecust.edu.cn (R. Cheng).
1 Author Contributions, L. W and W. L. contributed equally.
https://doi.org/10.1016/j.catcom.2020.106097
Received 9 March 2020; Received in revised form 22 June 2020; Accepted 22 June 2020
Availableonline25June2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

L. Wu, et al.
CatalysisCommunications145(2020)106097
reactivity was solved by the multi-component system. The acid/base
complexes could control the targeted molecular weights and narrow
polydispersities by avoiding the transesterifiction [14,15]. Further, Lin
and Waymouth found that the polymerization activities even span 6
orders of magnitude if the H-bond donors derived from ureas or
thioureas combined with the basic cocatalysts [16]. Zhang and cow-
orkers summarized the effect of acidity, basicity, and steric hindrance of
Lewis pairs on polymerization of polar vinyl monomers [17]. At the
same time, the strategies of the combined catalyst system were also
effective in the isocyanurates trimerization. Among tetra-
butylammonium halides, only the fluoride had catalytic activity for the
reaction, while it was difficult to accurately control trimerization and
generate dimerization products [18]. In the presence of potassium
sulfate, tetrabutylammonium bromide also had the capability to tri-
merize isocyanates at high temperature in a solvent-free system, and
effectively shortened the reaction time [19]. It has also been found that
the addition of TBAPINO (tetrabutylammonium phthalimide-N-oxyl)
and TEACB (tetraethylammonium 2- (carbamoyl)benzoate) has a great
effect on the trimerization of isocyanates [20]. Besides the work on
tetrabutylammonium halides, less attention has been focused on the
multi-component organocatalysts.
Inspired by the above work by bifunctional catalyst, we report
herein a synthesis of acid/base mixtures and their application to cata-
lyze the trimerization of isocyanates. Among the successful organoca-
talysts, DMAP (4-dimethylaminopyridine), DBU (1, 8-diazabicyclo
[5.4.0] undec-7-ene), and TBD (1, 5, 7-triazabicyclo [4.4.0] dec-5-ene)
have been extensively studied. These base/acid conjugate organocata-
lyst systems from organobases and appropriate Brønsted acids were
prepared and surveyed in trimerization of isocyanates. Under the op-
timal conditions, the reaction generality was evaluated and scale-up
experiments was demonstrated.
synthesized according to the reported procedures, respectively.
2.4. Cyclotrimerization of isocyanate over [HTBD][OAc] catalyst
In an oven-dried 10 mL round bottom flask, [HTBD][OAc] (10 mg,
0.5 mol%) and isocyanates (10 mmol) were added vigorously in air to
form isocyanurates (a-i). For the liquid isocyanate, the mixture was
vigorously stirred at room temperature, while the solid isocyanate was
stirred at 50 °C for a few seconds. During this process, the reaction time
was recorded with a stopwatch. The obtained white solid was cooled to
room temperature and then ground into powder, which was stirred for
10 min in 4 mL cold ether. The solid was separated by vacuum filtration
and washed with additional water to remove the catalyst. After filtra-
tion, recrystallization from ethanol gave the desired isocyanurate.
2.5. Scaled-up experiment
In an oven-dried 100 mL round bottom flask, [HTBD][OAc]
(150 mg, 0.75 mmol) and p-tolyl isocyanate (20 g, 150 mmol) were
added and the mixture was stirred vigorously at room temperature.
During this process, the reaction time was recorded with a stopwatch.
The solid was cooled to room temperature and ground into powder. A
quantity of 40 mL of cold diethyl ether was added and stirred for
10 min. The resulting solid was separated by vacuum filtration and
washed with additional water to remove the catalyst. After filtration,
recrystallization from ethanol gave the desired p-tolyl isocyanurate
(19.2 g, 96% yield).
3. Result and discussion
3.1. Catalyst screen in the isocyanate trimerization
2. Experimental section
Organocatalysts (Fig. 1) derived from DMAP, DBU, and TBD have
been widely adopted in the polymerization of cyclic esters, including LA
(L-lactide), VL (δ-valerolactone), and CL (ε-caprolactone) [24]. In this
regard, their performances in the trimerization of p-tolyl isocyanate
were surveyed in the absence of solvents.
2.1. Materials
1, 8-diazabicyclo [5.4.0] undec-7-ene, 1, 5, 7-triazabicyclo [4.4.0]
non-5-ene, 4-dimethylamino-pyridine, and various types of isocyanates
and other reagents were commercially available and used as received.
All solvents were domestic analytical pure reagents and were not
treated before use.
In Table 1, the DMAP was almost inactive for the reaction, and only
a small amount of cyclotrimerization products were generated at 70 °C
for 24 h (Entry 1). For DBU, the yield was only 17% (Entry 2) after
reaction for 1.5 h. Upon testing the TBD, the reaction mixture was so-
lidified within seconds at room temperature. The basicity values in AN
(acetonitrile) of DMAP, DBU, and TBD are 11.43, 24.34, and 26.03,
respectively [25]. That is, the insufficiently basic DMAP with low pKa
was unfavorable for the trimerization. The increased basicity had a
beneficial effect on the activity. Unfortunately, the product of the re-
action over TBD was found to be a mixture of dimers and trimers with
the ratio of 1:1. Thus, the preliminary experiments indicated that the
desired products were not obtained in an acceptable yield by the three
single organocatalysts.
2.2. Experimental instruments and methods
Nuclear magnetic resonance apparatus: Bruke DRX 400, based on
the chemical shift of TMS (tetramethylsilane), in ppm. High resolution
mass spectrometry: Waters Micromass GCT Detector. Melting point
instrument: XT3A micro melting point tester. Column chromatography
silica gel and thin layer chromatography silica gel plate: Yantai
Jiangyou Silicone Development Co., Ltd.
Consequently, to tune the reactivity and improve the thermal sta-
bility of organocatalysts, a versatile pairing of the organic superbasic
proton acceptors with organic acids was explored [23]. Protic ionic li-
quids were synthesized via proton transferfrom acids to organic bases
[26]. Organocatalysts based on ionic mixtures of acids and bases were
successfully applied in the polymerization of trimethylene carbonate
with MTBD (7-methyl-1, 5, 7-triazabicyclo [4.4.0] dec-5-ene)/TFA
(trifluoromethanesulfonic acid) [27]. DMAP/HOTf (triflic acid) dis-
played an outstanding catalytic activity to produce poly (L-lactide)
(PLLA) [28]. Such conjugates may be less active than the organocata-
lytic bases, but the polymerization appeared to be more controlled. In
the presence of DBU/BA (benzoic acid), the dimerization of ethyle-
neglycol was proven effective [29]. TBD/MSA (methanesulfonic acid)
was also demonstrated in PET (polyethylene terephthalate) depoly-
merization [30].
2.3. Synthesis of [HTBD][OAc] catalyst
1, 5, 7-Triazabicyclo [4.4.0] non-5-ene (500 mg, 3.59 mmol) was
dissolved in 2 mL ethanol and stirred at 0 °C, and then acetic acid was
slowly added (216 mg, 3.59 mmol). The mixture was stirred at room
temperature for 1 h and concentrated under reduced pressure to re-
move all ethanol, and then transferred to a vacuum oven at 80 °C
overnight. The catalyst [HTBD][OAc] (acetic acid) was a white crys-
talline solid after cooling (694.5 mg, 97% yield).
Other catalysts in the article can be readily prepared by neutralizing
an organic base with carboxylic acid, ketone, and azole. [HDMAP]
[PhCO2] (benzoic acid), [HDBU][OAc], and [HTBD][DioxoCy] (1, 3-
cyclohexanedione) were synthesized according to the reported proce-
dure [21]. [HTBD][TFE] (2, 2, 2-trifluoroethanol) and [HTBD][Benlm]
(benzimidazole) [22], [HDBU][TFE] and [HDBU][PhCO2] [23] were
Nine conjugates with stoichiometry mixtures of the above bases and
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L. Wu, et al.
CatalysisCommunications145(2020)106097
Fig. 1. Chemical structures of DAMP, DBU, TBD, and their derived salts.
reaction time compared with the neutral DMAP. Similarly, the [HDBU]
[OAc], [HDBU][TFE] (2, 2, 2-trifluoroethanol), and [HDBU][PhCO2]
presented much higher catalytic activity than the original base catalyst.
The selectivity increased significantly for the five TBD-derived
conjugates. Varying acidity and structure of the conjugate acid could
modulate the H-bond donating ability of the catalysts. The [HTBD]
[OAc] presented the highest yield in 35 s. While, the [HTBD][PhCO2],
or [HTBD][TFE] resulted in comparable yields of 90% varied the re-
action time from 2.5 to 21 min, which was longer than the single TBD.
[HTBD][DioxoCy] (1, 3-Cyclohexanedione) showed the lowest yield of
86% at higher temperature (Entry 11), while the yield could increase to
91% using [HTBD][BenIm] at 50 °C (Entry 12). Generally, the activity
over the TBD-based conjugates increased with the increase of pKa, ex-
cept [HTBD][DioxoCy] and [HTBD][BenIm] with large steric hin-
drance. [HTBD][OAc], with the weakest conjugate acid, was the most
favorable of the trimerization of p-tolyl isocyanate.
Table 1
Catalytic activity of various organocatalysts and their derived salts on the cy-
clotrimerization of p-tolyl isocyanate.
Entry
Cat.
T (°C)
Time
Yield (%)a,b,c
1
2
3
4
5
6
7
8
DMAP
DBU
TBD
70
70
25
70
70
70
70
25
50
25
50
50
24 h
1.5 h
40 s
5 h
2 h
3 h
2 h
35 s
21 min
2.5 min
7.5 min
6 min
6
17
45
13
51
41
53
93
90
91
86
91
[HDMAP][PhCO2]
[HDBU][OAc]
[HDBU][TFE]
[HDBU][PhCO2]
[HTBD][OAc]
[HTBD][TFE]
[HTBD][PhCO2]
[HTBD][DioxoCy]
[HTBD][BenIm]
3.2. Trimerization of isocyanate over [HTBD][OAc] catalyst
Based on the above analysis, [HTBD][OAc] was chosen in the next
optimization experiments in terms of the catalyst loadings from 1.0% to
0.05%. In Table 2, the highest 95% yield was obtained (Entry 2) with
0.5 mol% vs. monomer in 52 s. With the decrease of [HTBD][OAc]
loadings, the reaction time increased and the yield decreased sig-
nificantly. The yield was only 84% over 0.05 mol% [HTBD][OAc] with
the reaction time extended to 32 min. The reactions under air or ni-
trogen represented similar yields, indicating that the catalyst was ef-
fectively resistant to air and humidity. Meanwhile, the catalyst could be
stored for several months without loss of performance.
The organocatalysts are often tolerant to a variety of solvents and
even a solvent-free condition, which facilitates the separation of the
product. Considering the solubility of [HTBD][OAc], as well as the
solvent-dependent pKa values in various solvents in the reaction, seven
solvents were examined and summarized in Table 3. Generally, the
presence of solvent in the reaction led to a longer reaction time and
decreased the yield regardless of the polarity of the solvent. The pro-
duct yield could reach 95% within 52 s (Entry 1) at the solvent-free
9
10
11
12
a
Reaction conditions: 10 mmol of p-tolyl isocyanate, 0.1 mmol Cat.
Reactions were performed under air.
Isolated yields (average of three runs).
b
c
acids in Fig. 1 were conveniently synthesized in high yield. The tested
Brønsted acids are benzimidazole, CH3COOH (acetic acid), PhCOOH
(benzoic acid), DioxoCy (1,3-cyclohexanedione), and CF3COOH (tri-
fluoroacetic acid), and their pKa values in DMSO (dimethyl sulfoxide)
of the proton donors are 16.4, 12.3, 11.0, 10.3, and 3.4, respectively
[31].The pKa difference between the acids and the bases correlated
with the stability of the mixtures [32]. The results of trimerization of p-
tolyl isocyanate over these conjugates were presented in Table 1. The
activity of [HDAMP][PhCO2] improved from 6% to 13% with a shorter
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L. Wu, et al.
CatalysisCommunications145(2020)106097
Table 2
[HTBD][OAc] catalyst loadings on the trimerization of isocyanate.
Entry
Mol.(%)
Time
Yield (%)a,b,d
1
2
3
4
1.00
0.50
0.25
0.10
0.05
0.50
35 s
52 s
2.5 min
8 min
32 min
57 s
93
95
89
83
84
95
5
6c
a
Reaction conditions: 10 mmol of p-tolyl isocyanate.
Reactions were performed under air.
Reactions were performed under nitrogen.
Isolated yields. (average of three runs).
b
c
d
condition, while only the diethyl ether realized the best yield 88%
(Entry 3) among these solvents. Accordingly, the optimal condition for
the trimerization reaction was 0.5 mol% [HTBD][OAc] in a bulk
system, avoiding the use of toxic solvents.
accelerated the reaction with 94% isolated yields in 31 s. The decrease
[1,10] or even failure [33] over various catalysts in the reactivity due to
the electron-dense substrates was not found in the case of [HTBD][OAc]
bifunctional catalyst. For the aryl isocyanates containing various elec-
tron-withdrawing substituents, 4-bromophenyl isocyanate was im-
mediately cyclotrimerized at room temperature within 26 s in a yield of
95% (f). As the 4-chlorophenyl isocyanate, 3,4-dichlorophenyl iso-
cyanate, and 4-nitrophenyl isocyanate were solid at room temperature,
the isocyanates melted at 50 °C and then trimerized with the addition of
catalyst under vigorous stirring to ensure a homogeneous reaction
condition. Neither the type nor the position of the substitutes of the
isocyanates influenced the reaction, and the yields were 97%, 95%, and
93%, respectively (d, e, g). Furthermore, [HTBD][OAc] was also ef-
fective for sterically hindered isocyanates such as 1-naphthyl iso-
cyanate, and the product (i) was in the yield of 89%. However, benzyl
isocyanate took about 21 min to synthesize the expected product in the
yield of 81% (h). Here, cyclohexyl isocyanate was chosen, but no
3.3. Versatility of aryl isocyanates cyclotrimerization over [HTBD][OAc]
catalyst
With the optimized conditions in hand, the scope and efficiency of
the process were investigated. The results of a broad range of structu-
rally diverse aryl isocyanates bearing electron-donating or electron-
withdrawing substituents were summarized in Scheme 1. Aromatic
isocyanates were well tolerated as substrates. For the substituents with
methyl groups in the benzene rings, the isocyanurate with the methyl
group at either para- or meta- position presented > 92% isolated yields
within 1 min. The position of the substituents had little effect on the
performance. Even the much stronger 4-methoxypheny moiety
Table 3
Solvent effect in isocyanate trimerization.
Entry
Solvent
Time
Yield (%)a,b,c,d
1
2
3
4
5
6
7
8
None
THF
Et2O
n-Hexane
CH3CN
CH2Cl2
Toluene
EtOAc
52 s
12 h
20 min
18 min
30 min
12 h
95
23
88
79
42
25
27
31
12 h
12 h
a
Reaction conditions: 10 mmol of p-tolyl isocyanate, 0.05 mmol [HTBD][OAc].
Reactions were performed under air at room temperature.
Reaction was performed with 1 mL of solvent.
b
c
d
Isolated yields. (average of three runs).
4

L. Wu, et al.
CatalysisCommunications145(2020)106097
Scheme 1. Various aryl isocyanates trimerization over [HTBD][OAc] catalyst.
desired product was obtained in our catalytic system even after a long
reaction time, indicating that the catalyst [HTBD][OAc] was not ef-
fective for catalytic alkyl isocyanate trimerization. The alkyl isocyanate
trimerization was difficult to achieve, and few catalysts presented the
capability [1]. The performance of cyclotrimerization of alkyl and
aromatic isocyanates is greatly dependent on the catalysts. Aromatic
isocyanurates were well synthesized by catalysts such as Pd (0) com-
plexes [4], while m-terphenyl Mn(II) and Fe(II) complexes showed ex-
clusive reactivity towards the cyclotrimerization of primary aliphatic
isocyanates in toluene [34]. Furthermore, a few catalysts, such as
hemilabile aluminum pyridyl-bis (iminophenolate) complex [5] and N-
heterocyclic catalyst [9], were valid for both alkyl and aromatic iso-
cyanates cyclotrimerization. Besides the electron-withdrawing and
electron-donating effect of the substituent ligand on the nitrogen atoms,
more recently, computational NOB (natural bond orbital) investigations
showed that the resonance orbital interaction between the phenyl ring
isocyanate molecule and the NCO moiety in aromatic isocyanates pro-
vided more significant resonance stabilization than that in the alkyl-
substituted derivatives in the reaction [35].
[a]
Reactions were performed under air at 50 °C.
3.4. Scaled-up production of isocyanate cyclotrimerization by [HTBD]
[OAc]
In order to test the potential application of the isocyanate trimer-
ization catalyst over [HTBD][OAc], a scaled-up experiment was carried
out (Scheme 2).
The reaction was scaled up under optimal conditions with 0.5 mol%
amount of catalyst [HTBD][OAc]. With 20 g p-tolyl isocyanate, the
separation yield was 96% within 49 s. Compared with the small scale
reaction (Entry 1 in Table 3), the yield improved with the similar re-
action time, showing that [HTBD][OAc] was also excellent at a level of
5

L. Wu, et al.
CatalysisCommunications145(2020)106097
4. Conclusions
In summary, we have developed a new, mild, and efficient con-
jugates catalyst for the rapid and green synthesis of various aryl iso-
cyanurates. The compacted counterparts of organocatalysis bases and
the Brønsted acids, the amount of catalyst, and the solvent were tested
in the trimerization reaction. The catalytic activity mainly depends on
the basic strength of base conjugate, and the acid can tune the reaction
performance. Under the optimal reaction conditions with 0.5 mol%
[HTBD][OAc], the trimerization of aryl isocyanate can achieve an ex-
cellent yield within few seconds or minutes under the solvent-free
condition at room temperature. This catalytic system is compatible for
isocyanates with various substrates. A twenty-gram experiment on the
trimerization of p-tolyl isocyanate demonstrated the scalability and
potential applicability. A possible reaction mechanism of isocyanate
trimerization was proposed based on the bifunctional in the conjugates.
The study provided an eco-friendly synthetic method to access aryl
isocyanurates.
Scheme 2. Scaled-up production of isocyanate trimerization.
ten grams.
3.5. Catalytic mechanism of isocyanate cyclotrimerization over [HTBD]
[OAc]
It is common for isocyanate trimerization to proceed via nucleo-
philic attack on the isocyanate carbon. In combination with previous
DFT studies on the catalytic trimerization of isocyanates catalysts
[5,35], we hypothesized a potential synergistic H-bonding functional
catalytic mechanism with repeated coordination–insertion of iso-
cyanate. As shown in Scheme 3, the catalyst was able to activate the
-NCO moiety that the oxygen atom with more electronegativity on the
isocyanate functional group. Combined with an additional H-bonding
in reaction of catalyst [HTBD][OAc] and isocyanate, a nucleophilic
attack of the catalyst on the carbon was formed. The -NCO moiety then
became sufficiently nucleophilic to attack another isocyanates. The
catalyst [HTBD][OAc] could activate the isocyanate through its nega-
tively charged oxygen atom (I-II), and the obtained anion cyclized with
another activated isocyanate (III-IV). Finally, the cycle of isocyanate
ring trimerization was completed to obtain the isocyanurate.
Credit author statement
L. W. and W. L. contributed to materials synthesis, and catalytic
evaluation; J. Y. and R. C. contributed to the conception, design and
results interpretation; L. W. wrote the original draft. Finally, R. C. re-
viewed, edited, and submitted the manuscript in the final form. All
authors contributed to the discussion, read and approved the final
version of the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Scheme 3. Proposed catalytic cycle for the isocyanate trimerization over [HTBD][OAc] catalyst.
6

L. Wu, et al.
CatalysisCommunications145(2020)106097
Appendix A. Supplementary data
[18] Y. Nambu, T. Endo, J. Organomet. Chem. 58 (1993) 1932–1934.
[19] M.G. Dekamin, S. Mallakpour, M. Ghassemi, Synth. Commun. 35 (2005) 427–434.
[20] M.G. Dekamin, K. Varmira, M. Farahmand, S. Sagheb-Asl, Z. Karimi, Catal.
Commun. 12 (2010) 226–230.
[21] M. Hulla, S.M.A. Chamam, G. Laurenczy, S. Das, P.J. Dyson, Angew. Chem. Int. Ed.
129 (2017) 10695–10699.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2020.106097.
[22] C. Wang, H. Luo, D. Jiang, H. Li, S. Dai, Angew. Chem. Int. Ed. 49 (2010)
5978–5981.
References
[23] J. Qiu, Y. Zhao, Z. Li, H. Wang, M. Fan, J. Wang, ChemSusChem. 10 (2017)
1120–1127.
[1] J. Tang, T. Mohan, J.G. Verkade, J. Organomet. Chem. 59 (1994) 4931–4938.
[2] Y. Zhang, S.N. Riduan, J. Ying, Chem. Eur. J. 15 (2009) 1077–1081.
[3] A. Nebioglu, M.D. Soucek, JCT Res. 3 (2006) 61–68.
[4] F. Paul, S. Moulin, O. Piechaczyk, P.L. Floch, J.A. Osborn, J. Am. Chem. Soc. 129
(2007) 7294–7304.
[5] M.A. Bahili, E.C. Stokes, R.C. Amesbury, B. Christo, R.J. Horne, B.M. Kariuki,
J.A. Stewart, R.L. Taylor, P.A. Williams, M.D. Jones, K.D.M. Harris, B.D. Ward,
Chem. Commun. 55 (2019) 7679–7682.
[6] X. Zhu, J. Fan, Y. Wu, S. Wang, L. Zhang, G. Yang, Y. Wei, C. Yin, H. Zhu, S. Wu,
H. Zhang, Organometallics. 28 (2009) 3882–3888.
[24] N.E. Kamber, W. Jeong, R.M. Waymouth, R.C. Pratt, B.G.G. Lohmeijer, J.L. Hedrick,
Chem. Rev. 107 (2007) 5813–5840.
[25] I. Kaljurand, A. Kütt, L. Sooväli, T. Rodima, V. Mäemets, I. Leito, I.A. Koppel, J.
Organomet. Chem. 70 (2005) 1019–1028.
[26] H. Luo, G.A. Baker, J.S. Lee, R.M. Pagni, S. Dai, J. Phys. Chem. B 113 (2009)
4181–4183.
[27] X. Wang, S. Cui, Z. Li, S. Kan, Q. Zhang, C. Zhao, H. Wu, J. Liu, W. Wu, K. Guo,
Polym. Chem. 5 (2014) 6051–6059.
[28] J. Kadota, D. Pavlovic, H. Hirano, A. Okada, Y. Agari, B. Bibal, A. Deffieux,
F. Peruch, RSC Adv. 4 (2014) 14725–14732.
[29] I. Flores, J. Demarteau, A.J. Müller, A. Etxeberria, L. Irusta, F. Bergman, C. Koning,
H. Sardon, Eur. Polym. J. 104 (2018) 170–176.
[30] C. Jehanno, I. Flores, A.P. Dove, A.J. Müller, F. Ruipérez, H. Sardon, Green Chem.
20 (2018) 1205–1212.
[31] F.G. Bordwell, Acc. Chem. Res. 21 (1988) 456–463.
[32] D.R. MacFarlane, M. Kar, J.M. Pringle, Fundamentals of Ionic Liquids: from
Chemistry to Applications, John Wiley & Sons, 2017, p. 81, https://doi.org/10.
1002/9783527340033.
[33] J.C.W. Chien, T. Nozaki, J. Polym. Sci., Part A: Polym. Chem. 29 (1991) 505–514.
[34] H.R. Sharpe, A.M. Geer, H.E.L. Williams, T.J. Blundell, W. Lewis, A.J. Blake,
D.L. Kays, Chem. Commun. 53 (2017) 937–940.
[7] A.G. Giuglio-Tonolo, C. Spitz, T. Terme, P. Vanelle, Tetrahedron Lett. 55 (2014)
2700–2702.
[8] S. Kohlstruk, M. Kreczinski, United States Patent Application, (2003) 20030009054.
[9] C. Li, W. Zhao, J. He, Y. Zhang, Chem. Commun. 55 (2019) 12563–12566.
[10] H.A. Duong, M.J. Cross, Louie J, Org. Lett. 6 (2004) 4679–4681.
[11] C.Y. Li, Y.Y. Kuo, J.H. Tsai, G.P.A. Yap, T.G. Ong, Chem. Asian J. 6 (2011)
1520–1524.
[12] W. Meng, X. Feng, H. Du, Acc. Chem. Res. 51 (2018) 191–201.
[13] M. Hong, J. Chen, E.Y.X. Chen, Chem. Rev. 118 (2018) 10551–10616.
[14] D.J. Coady, A.C. Engler, H.W. Horn, K.M. Bajjuri, K. Fukushima, G.O. Jones,
A. Neison, J.E. Rice, J.L. Hedrick, ACS Macro Lett. 1 (2012) 19–22.
[15] D.J. Coady, K. Fukushima, H.W. Horn, J.E. Rice, J.L. Hedrick, Chem. Commun. 47
(2011) 3105–3107.
[35] T. Uchimaru, S. Yamane, J. Mizukado, S. Tsuzuki, RSC Adv. 10 (2020)
15955–15965.
[16] B. Lin, R.M. Waymouth, Macromolecules. 51 (2018) 2932–2938.
[17] W. Zhao, J. He, Y. Zhang, Sci. Bull. 64 (2019) 1830–1840.
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