1736
Y. Luo et al.
LETTER
(3) (a) Mann, G.; Hartwig, J. F. Tetrahedron Lett. 1997, 38,
At higher temperature (170 °C), the decomposition of
starting materials was observed.
8005. (b) Mann, G.; Incarvito, C.; Rheingold, A. L.;
Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 3224.
(c) Aranyos, A.; Old, D. W.; Kiyomori, A.; Wolfe, J. P.;
Sadighi, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121,
4369.
The CuI catalyst that was presumably in ionic liquid could
be reused as depicted in entry 1a (yield shown in parenthe-
sis) in Table 2. Thus, once the products were isolated di-
rectly from reaction mixture by simple extraction, another
portion of starting materials and reagents (phenol, aryl io-
dides and NaO-t-Bu) were added, followed by heating and
stirring. Products were again extracted.11 This process
could be repeated up to three times, until the build-up of
NaI rendered the mixture solid-like at room temperature.12
From the known literature results, the reaction of p-meth-
oxyiodobenzene with a phenol catalyzed by copper(I) tri-
flate in non-ionic liquid medium, e.g. toluene, using
Cs2CO3 with the addition of ethyl acetate and equimolar
amount of aryl carboxylic acid, gave a 77% yield,2b in
comparison to the 99% yield in entry 2 of Table 2. Our
results from the identical reactions (using phenol, aryl
halides, CuI as the catalyst and NaO-t-Bu as the base)
run in toluene instead of ionic liquids provided little or no
desired products.
(4) (a) Zhu, J. Synlett 1997, 133. (b) See ref.2a
(5) (a) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68,
351. (b) Olivier, H. J. Mol. Catal. 1999, 146, 285.
(c) Welton, T. Chem. Rev. 1999, 99, 2071.
(d) Wasserscheid, P.; Keim, W. Angew. Chem. Int. Ed. 2000,
39, 3772. (e) Gordon, C. M. Appl. Catal., A 2001, 222, 101.
(f) Zhao, D.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002,
74, 157. (g) Oliver-Bourbigou, H.; Magna, L. J. Mol. Catal.
A: Chem. 2002, 182, 419. (h)Oliver-Bourbigou, H.;Magna,
L. J. Mol. Catal. A: Chem. 2002, 183, 419.
(6) Separation technologies involving the use of ionic liquids
have been demonstrated. For some references, see: (a) Dai,
S.; Ju, H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1999,
1201. (b) Visser, A. E.; Swatloski, R. P.; Rogers, R. D.
Green Chem. 2000, 2, 1. (c) Visser, A. E.; Swatloski, R. P.;
Teichert, W. M.; Rogers, R. D.; Mayton, R.; Sheff, S.;
Wierzbicki, A.; Davis, J. H. Jr. Chem. Commun. 2001, 135.
(d) Fadeev, A. G.; Meagher, M. M. Chem. Commun. 2001,
295.
In conclusion, the use of halide-based ionic liquid (bmiX)
as the reaction media for copper-catalyzed Ullmann diaryl
ether synthesis has been shown to give satisfactory results
at relatively mild condition. The main advantages of uti-
lizing ionic liquid as reaction solvent are the ease of prod-
uct isolation via extraction. The copper catalysts, which
are immobilized in ionic liquids can also be reused.
(7) (a) 1-n-Butyl-3-methylimidazolium iodide ionic liquid has
melting point of –72 °C. See: Huddleston, J. G.; Visser, A.
E.; Reichert, W. M.; Heather, D.; Willauer, H. D.; Broker, G.
A.; Rogers, R. D. Green Chem. 2001, 3, 156.
(b) Presumably, tetraalkylammonium salts, tetraalkyl-
phosphonium salts and 1-alkylpyridinium salts with melting
points below the reaction temperature can also be used.
(8) Recently the in situ formation of N-heterocyclic carbene
complexes of palladium were identified when 1-n-butyl-3-
methylimidazolium-based ionic liquids (bmiX, X = Br, BF4)
were employed in Heck reaction. See: (a) Xu, L.; Chen, W.;
Xiao, J. Organometallic 2000, 19, 1123. (b) Mathews, C. J.;
Smith, P. J.; Welton, T.; White, A. J. P.; Williams, D. J.
Organometallics 2001, 20, 3848.
Acknowledgment
The authors thank Wesleyan University for a start-up fund.
(9) Bacon, R. G. B.; Stewart, O. J. J. Chem. Soc. 1965, 4953.
(10) Sitze, M. S.; Schreiter, E. R.; Patterson, E. V.; Freeman, R.
G. Inorg. Chem. 2001, 40, 2298; and references therein.
(11) Representative Experimental Procedure: In a sealed vial,
a mixture of phenol (188.2 mg, 2 mmol), p-iodobenzonitrile
(458 mg, 2 mmol), NaO-t-Bu (192.2 mg, 2 mmol) and CuI
(38.2 mg, 0.2 mmol) in 1 mL of bmiI ionic liquid was heated
at 110 °C with vigorous stirring overnight. The reaction
mixture gave light brownish color. The product was
extracted using 2:1 hexanes/EtOAc (3×) and was purified
through a short silica gel column.
References
(1) For earlier review, see: Lindley, J. Tetrahedron 1984, 40,
1433.
(2) (a) For a review, see: Sawyer, J. S. Tetrahedron 2000, 56,
5045. (b) For a recent publication on the general copper-
catalyzed synthesis of diaryl ethers conducted in toluene,
see: Marcoux, J.-F.; Doye, S.; Buchwald, S. L. J. Am. Chem.
Soc. 1997, 119, 10539.
(12) Investigations on recylcing of bmiX ionic liquids are in
progress.
Synlett 2003, No. 11, 1734–1736 ISSN 1234-567-89 © Thieme Stuttgart · New York