ORGANIC
LETTERS
2004
Vol. 6, No. 25
4679-4681
N-Heterocyclic Carbenes as Highly
Efficient Catalysts for the
Cyclotrimerization of Isocyanates
Hung A. Duong, Michael J. Cross, and Janis Louie*
Department of Chemistry, UniVersity of Utah, 315 South 1400 East,
Salt Lake City, Utah 84112
Received September 3, 2004
ABSTRACT
A series of N-heterocyclic carbenes (NHCs) were evaluated as potential catalysts for the cyclotrimerization of isocyanates to afford isocyanurates.
1,3-Bis-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) was found to be a highly efficient catalyst for the cyclotrimerization of a
variety of isocyanates.
Isocyanurates, the aromatic products arising from cyclotri-
merizing isocyanates, are used to enhance the physical
properties of a wide variety of polyurethanes and coating
materials.1 Polymeric blends of isocyanurates show increased
thermal resistance, flame retardation, chemical resistance, and
film-forming characteristics.2 For example, triaryl isocya-
nurates are often used as activators for the polymerization
and postpolymerization of ꢀ-caprolactam in the production
of nylons with high melt viscosities.3 Triallyl isocyanurate
has been used in the preparation of flame-retardant laminating
materials for electrical devices as well as in the preparation
of copolymer resins that are water-resistant, transparent, and
impact-resistant.4
catalysts have been discovered that facilitate this reaction.5
Lewis base catalysts include phosphines,6 amines,7 NO,8 and
alkoxyalkenes,9 as well as salts containing p-toluenesulfi-
nate,10 cyanate,11 fluoride,12 or carbamate anions.13 Organo-
metallic compounds, which in some cases may proceed
(4) (a) Misawa, H.; Koseki, T. JP 03,166,255, 1991. (b) Hirabayashi,
M.; Ozawa, S. JP 0309,907, 1991. (c) Sato, F.; Tateyama, M.; Kurokawa,
S. JP 63,122,748, 1988. (d) Ishikawa, T.; Tanaka, S.; Shidara, M.; Hatsurtori,
I.; Takagame, H.; Mashita, K. JP 0187,651, 1989. (e) Nagai, H.; Komya,
T. JP 0187,649, 1989. (f) Osturek, R.; Wingler, F.; Geyev, O. DE 3,729,-
457, 1989.
(5) Frisch, K. C.; Rumao, L. P. J. Macromol. Sci. ReV. Macromol. Chem.
1970, C5, 103.
(6) (a) Tang, J.; Mohan, T.; Verkade, J. G. J. Org. Chem. 1994, 59,
4931. (b) Laas, H.-J.; Halpaap, R.; Pedain, J.; Konig, K. U.S. 5,750,629,
1998.
(7) (a) Disteldorf, M. J.; Huebel, R. W.; Schmitz, G. K. U.S. 4,912,210,
1990. (b) Tagachi, Y.; Shibaya, I.; Yasumoto, Y.; Ynoemoto, K. Bull. Chem.
Soc. Jpn. 1990, 63, 3486. (c) Richter, R.; Ulrich, H. Synthesis 1975, 463.
(d) Wong, S.; Frisch, K. C. J. Polym. Sci. Polym. Chem. Ed. 1986, 24,
2877. (e) Kogon, I. C. J. Am. Chem. Soc. 1956, 78, 4911. (f) Wallis, A. F.
A.; Wearne, R. H. Eur. Polym. J. 1990, 26, 1217.
The commercial importance of isocyanurates has led to
considerable effort in developing effective methods for their
synthesis through isocyanate cyclotrimerization. Numerous
(1) Lin, I. S.; Kresta, J. E.; Frisch, K. C. Reaction Injection Molding
and Fast Polymerization Reactions; Plenum Publishing: New York, 1982;
p 147.
(8) Villa, J. F.; Powell, H. B. Synth. React. Metal-Org. Chem. 1976, 6,
59.
(2) (a) Wirpsza, Z. Polyurethanes: Chemistry, Technology and Applica-
tion, Ellis Horwood: London, 1993. (b) Zitinkina, A. K.; Sibanova, N. A.;
Tarakanov, O. G. Russ. Chem. ReV. 1985, 54, 1866. (c) Nawata, T.; Kresta,
J. E.; Frisch, K. C. J. Cell. Plast. 1975, 267. (d) Nicholas, L.; Gmitter, G.
R. J. Cell. Plast. 1965, 85.
(3) (a) Bukac, Z.; Sebenda, J. Chem. Prum. 1985, 35, 361. (b) Horsky,
J.; Kubanek, U.; Marick, J.; Kralicek, J. Chem. Prum. 1982, 32, 599.
(9) Mizura, J.; Yokozawa, T.; Endo, T. J. Polym. Sci., Polym. Chem.
1991, 29, 1544.
(10) Moghaddam, F. M.; Dekamin, M. G.; Khajavi, M. S.; Jalili, S. Bull.
Chem. Soc. Jpn. 2002, 75, 851.
(11) Tanimoto, F.; Tanka, T.; Kitano, H.; Fukui, K. Bull. Chem. Soc.
Jpn. 1996, 39, 1922.
(12) Nambu, Y.; Endo, T. J. Org. Chem. 1993, 58, 1932.
10.1021/ol048211m CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/12/2004