ORGANIC
LETTERS
2002
Vol. 4, No. 13
2229-2231
An Air-Stable Palladium/N-Heterocyclic
Carbene Complex and Its Reactivity in
Aryl Amination
Mihai S. Viciu, Rebecca M. Kissling, Edwin D. Stevens, and Steven P. Nolan*
Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148
Received April 26, 2002
ABSTRACT
The synthesis and characterization of [Pd(IPr)Cl2]2 (1), an air- and moisture-stable complex, is reported. The utilization of 1 as a catalyst for
amination of aryl chlorides and bromides with a variety of amine coupling partners under mild conditions is described. The amination reactions
with 1 show a remarkable insensitivity to oxygen and water, and thus the amination reactions could be performed in air on the benchtop with
undried reagent grade solvents and substrates with small effects on reaction times and conversions.
Palladium-catalyzed coupling reactions have garnered enor-
mous interest over the past few decades,1 with great strides
achieved in amination of aryl halides.2 Catalysts based on
strongly donating ligands and late transition metals have led
to the recent successes in coupling reactions using unacti-
vated aryl chlorides3 with various transmetalating reagents.1
Most reported active species are formed in situ from the
metal source and (generally) air-sensitive phosphine ligands
rather than use of an isolated metal-ligand complex.4,5 Easily
synthesized, highly active, yet air- and “shelf”-stable pal-
ladium complexes could be welcome additions to the existing
arsenal of cross-coupling catalysts.6
N-Heterocyclic carbenes (NHC) such as N,N′-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene (IPr) can benefit late
metal catalysis systems by virtue of their strong electron-
donating properties and their hedge-like steric bulk. The
ligand electron-donating nature surely helps promote the aryl
halide (or pseudo-halide) oxidative addition, the ligand bulk
presumably facilitates the reductive elimination step. Our
previous studies on palladium/NHC system revealed the Pd-
carbene bond to be robust over time and tolerant of arduous
conditions (heat), indicating such a system might not require
excess ligand to compensate for ligand-metal bond lability.7
In studies involving a number of cross-coupling systems, the
(1) Mitchell, T. N. In Metal-Catalyzed Cross-Couplings Reactions;
Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998. (b) Collman,
J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications
of Organotransition Metal Chemistry; University Science: Mill Valley, CA,
1987. (c) Suzuki, A. In Metal-Catalyzed Cross-Couplings Reactions;
Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998; pp 49-
97. (d) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic
Press: New York, 1985.
(5) Recently palladium tertiary phosphine complexes have been used as
catalyst precursors in cross-coupling and related chemistry. See for
example: Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719-2724.
(6) For recent reports of defined catalysts for Suzuki coupling, see: (a)
Bedford, R. B.; Cazin, C. S. J. Chem. Commun. 2001, 1540-1541. (b)
Andreu, M. G.; Zapf, A.; Beller, M. Chem. Commun. 2000, 2475-2476.
(7) (a) Huang, J.; Grasa, A.; Nolan, S. P. Org. Lett. 1999, 1, 2053-
2055 (b) Grasa, G. A.; Viciu, M. S.; Huang, J.; Nolan, S. P. J. Org. Chem.
2001, 66, 7729-7737. (c) Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999,
121, 9889-9890. (d) Zhang, C.; Huang, J.; Trudell, M. T.; Nolan, S. P. J.
Org. Chem. 1999, 64, 3804-3805 (e) Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W.
K.; Weskamp, T.; Herrmann, W. A. J. Organomet. Chem. 2000, 595, 186-
190. (f) Lee, H. M.; Nolan, S. P. Org. Lett. 2000, 2, 1307-1309. (g) Gradel,
B.; Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron Lett. 2001, 42, 5689-
5692. (h) Desmarets, C.; Schneider, R.; Fort, Y. J. Org. Chem. 2002, 67,
3029-3036.
(2) See for example: (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.;
Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805-818. (b) Hartwig, J. F.
Acc. Chem. Res. 1998, 31, 852-860.
(3) (a) Wolfe, J. P.; Tomori, H.; Sadighi, Y. P.; Yin, J.; Buchwald, S. L.
J. Org. Chem. 2000, 65, 1158-1174. (b) Stauffer, S. R.; Lee, S.; Stambuli,
J. P.; Hauck, S. I.; Hartwig, J. F. Org. Lett. 2000, 2, 1423-1426. (c) Littke,
A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1999, 38, 2411-2413 (d) Littke,
A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1998, 37, 338-3388. (e) Littke,
A. F.; Fu, G. C. J. Org. Chem. 1999, 64, 10-11. (f) Bei, X.; Guram, A. S.;
Turner, H. W.; Weinburg, W. H. Tetrahedron Lett. 1999, 40, 3855-3858.
(g) Indolese, A. F. Tetrahedron Lett 1997, 38, 3513-3516.
(4) For an in situ catalyst generation, see for example: Harris, M. C.;
Buchwald, S. L. J. Org. Chem. 2000, 65, 5327-5333.
10.1021/ol0260831 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/30/2002