Lithium dibenzylamide, a simple, selective and highly efficient catalyst for isocyanate cyclotrimerization to isocyanurate

Article abstract of DOI:10.1055/s-0030-1260825

Lithium dibenzylamide catalyzed the rapid and essentially quantitative cyclotrimerization of a variety of isocyanates under mild conditions and could be recycled for six times in high yield. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1260825

LETTER
1937
Lithium Dibenzylamide, a Simple, Selective and Highly Efficient Catalyst for
Isocyanate Cyclotrimerization to Isocyanurate
Lithium
D
i
ibenzyl
n
amide Catal
g
yze
d
Cyclotr
c
imerization
h
of Isocyana
a
tes o Shi,a Zhiqiang Guo,b Xuehong Wei,*a,b Diansheng Liu,*b Michael F. Lappertc
a
School of Chemistry and Chemical Engneering, Shanxi University, 92 Wucheng Road, Taiyuan 030006, P. R. of China
Institute of Applied Chemistry, Shanxi University, 92 Wucheng Road, Taiyuan 030006, P. R. of China
Fax +86(351)7011688; E-mail: xhwei@sxu.edu.cn; dsliu@sxu.edu.cn
b
c
Department of Chemistry, University of Sussex, Brighton, BN1 9QJ, UK
Received 25 April 2011
catalyst for the cyclotrimerization of both aryl and alkyl
isocyanat. 8
Abstract: Lithium dibenzylamide catalyzed the rapid and essen-
tially quantitative cyclotrimerization of a variety of isocyanates un-
der mild conditions and could be recycled for six times in high
yield.
In this paper, we report a facile synthetic method for the
synthesis of isocyanurates by the cyclotrimerization of an
Key words: lithium amides, catalysis, isocyanates cyclotrimeriza- appropriate isocyanate catalyzed by a simple lithium, so-
tion, isocyanurates
dium, or potassium amide.
Our initial objective was to prepare metal complexes con-
taining a k-N,O-chelated ureido ligand from a lithium
Aromatic isocyanurates, manufactured by cyclotrimeriz-
amide and an isocyanate. In most cases, isocyanurates
ing corresponding isocyanates, have been shown to en-
were obtained as predominant products. This led us to in-
hance the uses of polyurethanes as coating materials.1,2
vestigate a new kind of catalyst for the cyclotrimerization
Polymers, such as polyurethanes incorporated with isocy-
of an isocyanate to an isocyanurate. Thus, phenyl isocyan-
anurates, can improve their flame retardation characteris-
ate (Table 1) was selected as a model substrate and a vari-
ety of lithium amides, as well as Na or K amides or a tert-
tics, film-formation, and thermal and chemical
resistance.3–6 Isocyanurates are also used in the synthesis
amine were screened as potential catalysts, and the results
of co-polymer resins to enhance their water-resistance,
are summarized in Table 1. Tertiary amine (entry 12) had
transparency, and impact resistance.7,8 An optically active
no catalytic activity for the cyclotrimerization of phenyl
isocyanurate having peripheral amino acid units served
isocyanate and lithium primary amides (entries 6 and 7)
for the chiral recognition of racemic binaphthol.9 Trialkyl
exhibited very low catalytic activities, but lithium second-
isocyanurates have been employed in the preparation of
ary amides, except for lithium bis(trimethylsilyl)amide
flame-retardant laminating materials for electrical de-
(entry 3), were very effective. Sodium or potassium
vices,10 and, because of their selective microbial and low
dibenzylamide also catalyzed the reaction efficiently (en-
mammalian toxicity, are used for drug delivery and re-
tries 9 and 10). Lithium dibenzylamide at 0.1 mol% load-
lease.11,12 Triaryl isocyanurates are activators in the con-
ing in diethyl ether in two minutes at room temperature
(entry 8) promoted the quantitative cyclotrimerization of
phenyl isocyanate to the corresponding isocyanurate; and
lithium diethylamide or diisopropylamide catalyzed this
tinuous anionic co-polymerization of e-caprolactam to
nylon-6 possessing low monomer content and high melt
viscosities.13
reaction under the same conditions to give an isolated
product yield of >90% (entries 1 and 2). Lithium bis(tri-
methylsilyl)amide catalyzed the reaction in only moderate
yield, which may have been due to its high steric hin-
drance. In each case, IR spectral examination of the crude
product showed that the isocyanurate was the exclusive
product of the reaction, free of any dimer.2 The isolation
and purification of the product was straightforward, e.g.
the insoluble triphenyl isocyanurate was obtained by sim-
ply filtering and washing the precipitate from the reaction
mixture at the end of the reaction.
The various industrial and commercial applications of iso-
cyanurates have attracted much attention in developing
more effective methods for isocyanate cyclotrimerization.
Development of efficient catalysts is crucial. A number of
catalysts for isocyanate trimerization have been reported,
including phosphines,7,8,14 N-heterocyclic carbenes,2,15
calcium carbene complexes,16,17 amines,18 NO,19 fluoride
anions,20 alkoxyalkenes,21 or p-toluenesulfinate,13
cyanate22 or carbamate23 anions. Organometallic catalysts
include organotin24 and zirconium25 compounds, orga-
nozinc halides and alkoxides,26 copper and nickel ha-
lides,19 rare earth metal complexes27,28 and palladium(0)
systems.29 Very recently, an electron-rich, sterically hin-
dered proazaphosphatrane was reported as an effective
In view of the above results, a variety of isocyanate sub-
strates was examined, using lithium dibenzylamide as the
catalyst (Table 2). Even though only 0.05 mol% or 0.01
mol% catalyst loadings were employed, phenyl isocyan-
ate was readily cyclotrimerized in two minutes at room
temperature in 98% and 96% isolated yields of product,
respectively (entry 1). Compared to the reported good cat-
SYNLETT 2011, No. 13, pp 1937–1939
x
x
.x
x
.2
0
1
1
Advanced online publication: 28.06.2011
DOI: 10.1055/s-0030-1260825; Art ID: W09911ST
© Georg Thieme Verlag Stuttgart · New York

1938
J. Shi et al.
LETTER
Table 1 Phenyl Isocyanate Cyclotrimerization Catalyzed by Vari-
Table 2 Lithium Dibenzylamide Catalyzed Cyclotrimerization of
ous Lithium (or Na or K) Amidesa
Isocyanatesa
Ph
R
O
N
O
O
R
N
O
O
R
cat. (0.1 mol%)
Et2O, r.t.
lithium dibenzylamide (0.1 mol%)
Et2O, r.t., 2 min
PhN=C=O
RN=C=O
N
N
N
N
Ph
Ph
O
Entry
1
Catalyst
Et2NLi
Yield (%)b
Entry
1
Isocyanatesb
Product
Yield (%)c
91
99
97c
1
98d
96e
NCO
2
3
i-Pr2NLi
97
2
3
4
5
2
3
4
5
98(lit. 93)21
98(lit. 91)15
98(lit. 96)8
(Me3Si)2NLi
50
Cl
NCO
56c
4
5
pyrrolididolithium
piperidinatolithium
86
NCO
90c
83
MeO
NCO
88c
32
6
7
Ph(H)NLi
t-Bu(H)Nli
Bn2NLi
9
6
89f (lit. 99)2
NCO
a Reaction conditions: lithium dibenzylamide (0.1 mol%), isocyanate
(30 mmol), Et2O (30 mL), r.t., 2 min.
8
99
98
97
97
0
b Isocyanates were degassed and dried prior to use.
c Isolated yield (average of three runs).
9
Bn2NNa
d A 0.05 mol%-loading of lithium dibenzylamide was used
e A 0.01 mol%-loading of lithium dibenzylamide was used.
f A 0.1 mol%-loading of sodium dibenzylamide was used as catalyst,
r.t., 30 min.
10
11
12
13
Bn2NK
Ph2NLi
N,N-dimethylaniline
none
We propose that the mechanism for the formation of iso-
cyanurate follows a route similar to that for a variety of
Lewis base catalyzed reactions (Scheme 1).2,7,17,27 How-
ever, the formation of isocyanurates, particularly, alkyl
isocyanurates catalyzed by previous Lewis bases was typ-
ically quite slow (taking many hours) even at elevated
temperatures17 and commonly gave a mixture of both
dimerization and trimerization products in low yields.2,27
In this system, the stronger Lewis base, sodium dibenzyl-
amide initiated the cyclotrimerization rapidly and in good
yield.
0
a Reaction conditions: catalyst (0.1 mol%), phenyl isocyanate (30
mmol, degassed and dried prior to use), Et2O (30 mL), r.t., 2 min.
b Isolated yield (average of three runs).
c Reaction time: 30 min.
alysts such as proazaphosphatrane7,8 or N-heterocyclic2
carbenes, similar isolated yields with lower catalyst ratio
or shorter reaction time were achieved. 4-Chlorophenyl
isocyanate, or 4-methylphenyl isocyanate, or 4-methox-
yphenyl isocyanate was converted successfully to its re-
spective isocyanurate in two minutes in 98% isolated
yield (entries 2–4). Such electron-rich substrates have typ-
ically exhibited low reactivities in the isocyanate cyclotri-
merization reactions and afforded low isolated yields by
using other catalyst systems.8,15,21
Table 3 Recyclability of Lithium Dibenzylamidea
Entry
Cycle
Time (min)
Yield (%)b
1
2
3
4
5
6
1
2
3
4
5
6
2
2
2
2
2
2
99
99
97
94
94
89
Unexpectedly, the alkyl compound cyclohexyl isocyanate
was cyclotrimerized to its isocyanurate in only low isolat-
ed yield (entry 5); good yield was obtained when sodium
dibenzylamide was employed in diethyl ether albeit long-
er reaction time (30 min) was required.
The high catalytic performance of lithium dibenzylamide
encouraged us to test its recyclability in the PhNCO–Et2O
system as summarized in Table 3. Thus, the isolated yield
of isocyanurate on the sixth run had only decreased by ca.
10% compared to the initial run.
a Reaction conditions: lithium dibenzylamide (0.1 mol%), phenyl iso-
cyanate (15 mmol), Et2O (15 mL), r.t.
b Isolated yield (average of two runs).
Synlett 2011, No. 13, 1937–1939 © Thieme Stuttgart · New York

LETTER
Lithium Dibenzylamide Catalyzed Cyclotrimerization of Isocyanates
1939
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R1
N
O
O
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N
N
R1
R1
O
NR2
R1
N C O
2
R1
N
C
N
O
O
C
R1
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N
R1
R1
O
N
C
O
C
NR2
O
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2
NR2
2
R1
N C
Scheme 1
In conclusion, alkaline metal (Li, Na, and K) amides have
been used for the first time in cyclotrimerization of isocy-
anates to isocyanurates.30,31 Aryl isocyanate, ArNCO [Ar
= Ph, or 4-XC6H4 (X = Cl, Me or MeO)] is rapidly, selec-
tively and essentially quantitatively cyclotrimerized by
treating a diethyl ether solution with 0.1 mol% of lithium
dibenzylamide, while alkyl isocyanate, C6H11NCO was
converted to the corresponding isocyanurate in good yield
by using sodium dibenzylamide as catalyst.
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5503.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Primary Data for this article are available online at http://
www.thieme-connect.com/ejournals/toc/synlett and can be cited
using the following DOI: 10.4125/pd0013th.
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3882.
Acknowledgment
We thank the NSFC (No. 20572065; 20772074), the SXNSFC
(2008011021; 2008012013-2) and the Research Project Supported
by Shanxi Scholarship Council of China for financial support.
(29) Paul, F.; Moulin, S.; Piechaczyk, O.; Le Floch, P.; Osborn,
J. A. J. Am. Chem. Soc. 2007, 129, 7294.
(30) Representative Procedure: An oven-dried Schlenk tube
under nitrogen was charged with phenyl isocyanate, lithium
dibenzylamide (0.1 mol%), and Et2O (30 mL). The reaction
mixture was stirred at r.t. for 2 min. The resulting precipitate
was filtered off, washed with Et2O (3 × 3 mL), and dried
under reduced pressure to give the isocyanurate 1,3,5-
triphenyl-1,3,5-triazinane-2,4,6-trione (1) as a white solid.
(31) Procedure for Recycling Experiment of Lithium
Dibenzylamide: Phenyl isocyanate was added to a solution
of lithium dibenzylamide (0.1 mol%) in Et2O (30 mL) with
stirring. The reaction mixture was stirred at r.t. for the
allotted time and filtered via a canula filter. The Et2O filtrate
was subjected to a second reaction cycle. This procedure was
repeated six times (for details see Table 3).
References and Notes
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Synlett 2011, No. 13, 1937–1939 © Thieme Stuttgart · New York