852 Bull. Chem. Soc. Jpn., 75, No. 4 (2002)
© 2002 The Chemical Society of Japan
AC spectrometer. IR spectra were performed on a Mattson 1000
spectrometer. Melting points were determined on a Mettler FPS,
and are uncorrected. All products were known, and their spectro-
scopic data were simillar to those reported in the litrature.7
General Procedure for Trimerization of Isocyanates: In a
20 mL flask equipped with a condenser was placed isocyanate (15
mmol), sodium p-toluenesulfinate 0.0053 g (0.2 mol%), and tet-
rabutylammonium iodide 0.0054 g (0.1 mol%) under dry condi-
tions. The reaction mixture was stirred and heated in an oil bath at
70 °C. After the required times (see Table 2) dry diethyl ether (5
mL) was added and filtered. Further isolation of the product was
carried out as previously reported.1,10 The sodium p-toluenesulfi-
nate used in this work was supplied as a monohydrate from Lan-
caster, England, and was dried after powdering in vacuum at 130
°C for 2 h.
Scheme 1.
fied by the alpha effect from the sulfur atom, having a pair of
nonbonding electrons, which can attack isocyanates as hard
heterophiles.13b A similar behavior has been observed in the
nitrite ion13a as well. The trimerization process is, therefore,
proceeded by an initial attack of (1) on the first isocyanate
molecule, followed by attacks of the resulting intermediates on
two isocyanate molecules, leading to an isocyanurate as (1)
leaves (Scheme 1).
References
1
2
Y. Nambu, and T. Endo, J. Org. Chem., 58, 1932 (1993).
J. Tang, T. Mohan, and J. G. Verkade, J. Org. Chem., 59,
4931 (1994), and references cited there in.
a) J. G. Verkade, and J. Tang, U. S. US 5,260,436; Chem.
3
Abstr., 120, 218836v (1994). b) Z. Bukac, and J. Sebenda, Czech.
CS 227,247,1985; Chem. Abstr., 105, 173226r (1986). c) Z.
Bukac, and J. Sebenda, Chem. Prum., 35, 361 (1985); Chem. Ab-
str., 103, 123978c (1985).
In order to enhance the solubility of sodium p-toluenesulfi-
nate and to increase its nucleophilicity, a catalytic amount of a
quaternary ammonium halide was added as a phase-transfer
catalyst. The synthesis of isocyanurates, catalyzed by tetrabu-
tylammonium flouride, was previously reported, while no cata-
lytic activity was observed when using quaternary ammonium
salts of other halides.1,9b Based on these data, the efficiency of
sulfinate salt in the presence of tetrabutylammonium iodide
was investigated. Among quaternary ammonium salts of ha-
lides the iodide salt has the softest anion; interestingly, it was
found that the catalyst–tetrabutylammonium iodide system has
a better catalytic activity than sulfinate salt. Table 1 gives the
optimized conditions in the presence of ammonium salt (Table
1, entry 12). It can be deduced that when ammonium salt is
added to the reaction medium, the amount of sulfinate salt,
time, and temperature are reduced by a factor of 1/6, 1/2, and
1/3, respectively. The optimal conditions were also applied for
a number of aryl and alkyl isocyanates except for ethyl isocy-
anate, in which the reaction was performed near its boiling
point. The results are summarized in Table 2. To the best of
our knowledge, a few catalysts are able to catalyse the trimer-
ization of alkyl isocyanates.2,9b Interestingly, it was found that
alkyl isocyanurates can also be prepared under our reaction
conditions (Table 2, entries 9–11).
4
a) H. Takahashi, Y. Murakami, M. Yoshimoto, K. Kido, Y.
Iwane, N. Tanaka, and K. Nishimoto, Mokuzai Gakkaishi, 31, 504
(1985); Chem. Abstr., 104, 16499w (1986). b) E. Moehring and P.
Roessler, Ger. Offen. 2,273,119; Chem. Abstr., 90, 121666p
(1979).
5
a) H. Sugimoto, Y. Yamane, and S. Inoue, Tetrahedron:
Asymmetry., 11, 2067 (2000). b) P. K. Thallapally, K.
Chakraborty, H. L. Carrell, S. Kotha, and G. R. Desiraju, Tetrahe-
dron, 56, 6721 (2000).
6
7
K. Tanaka, and F. Toda, Chem. Rev., 100, 1025 (2000).
a) S. Wong, and K. C. Frisch, J. Polym. Sci., Polym. Chem.
Ed., 24, 2877 (1986). b) Y. Taguchi, M. Shibuya, T. Tsuchia, and
K. Yonemoto, Bull. Chem. Soc. Jpn., 63, 3486 (1990). c) J. I.
Jones and N. G. Savill, J. Chem. Soc., 1957, 4392. d) I. C. Kogon,
J. Am. Chem. Soc., 78 4911 (1956). e) A. F. A. Wallis and R. H.
Wearne, Eur. Polym. J., 26, 1217 (1990). f) I. Wakeshima and I.
Kijima, Bull. Chem. Soc. Jpn., 48, 953 (1975).
8
a) I. C. Kogon, J. Org. Chem., 24, 83 (1959). b) D. W.
Kaiser, U. S. Pat. 2536849; Chem. Abstr., 45, 5726I (1951).
a) S. Herbstman, J. Org. Chem., 30, 1259 (1965). b) F.
9
Marc, A. Saux, M. Ratier, J. G. Duboudin, and G. Daude, Poly-
mer, 35, 5146 (1994). c) I. Wakeshima, H. Suzuki, and I. Kijima,
Bull. Chem. Soc. Jpn., 48, 1069 (1975). d) A. J. Bloodworth, and
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10 M. S. Khajavi, M. G. Dakamin, and H. Hazarkhani, J.
Chem. Res. (S), 2000, 145.
In conclusion, sodium p-toluenesulfinate, either alone or in
the presence of tetrabutylammonium iodide, is an efficient cat-
alyst for the selective and convenient trimerization of aryl and
alkyl isocyanates under solvent-free and environmentally
friendly conditions. Further applications of this methodology
will be presented in due course.
11 G. Solladie, and F. Matloubi Moghaddam, J. Org. Chem.,
47, 91 (1982), and references cited there in.
12 a) F. A. Carey and R. J. Sundberg, in “Advanced Organic
Chemistry,” 3rd ed, Plenum Press, New York (1993), Part A,
Chap. 1, p. 19 and Chap. 5, p. 288. b) N. S. Isaacs, in “Physical
Organic Chemistry,” Longman Scientific & Technical, England
(1987), p. 240.
Experimental
The experiments were caried out under a dry-nitrogen atmos-
phere. The isocyanates were purchsed from Merck and Aldrich
companies. 1H-NMR spectra were recorded on a Bruker FT-80