Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
ORGANIC
LETTERS
2004
Vol. 6, No. 24
4435-4438
η
2-dba Complexes of Pd(0): The
Substituent Effect in Suzuki
Coupling
−Miyaura
Ian J. S. Fairlamb,* Anant R. Kapdi, and Adam F. Lee
Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K.
Received August 11, 2004 (Revised Manuscript Received October 15, 2004)
ABSTRACT
The influence of aryl substituents in dibenzylidene acetone (dba) ligands, for Pd(0) complexes, has been evaluated for Suzuki−Miyaura cross-
coupling reactions. Electron-withdrawing substituents such as NO2 or CF3 deactivate the catalyst species whereas strongly donating substituents
such as OMe increase catalytic activity over that of unsubstituted dba ligands.
Palladium-catalyzed cross-coupling reactions constitute some
of the most important and applied transformations in natural
product synthesis, pharmaceutical targets, and conjugated
materials.1 Carbon-carbon and carbon-heteroatom bond-
forming processes, catalyzed by palladium, such as Heck,
Hartwig-Buchwald, Negishi, Sonogashira, Suzuki-Miyaura,
and Stille reactions, are being used increasingly throughout
the synthetic community. There has been a recent flurry of
activity in the design of highly reactive Pd-catalysts for these
processes,2 particularly for Suzuki-Miyaura coupling.3
Palladium(0) or palladium(II) precursors, in the presence
of donor ligands, facilitate efficient cross-coupling of
deactivated and sterically hindered aryl chlorides with various
organometallic reagents.Without question, the most routinely
employed phosphine-free Pd(0) precursor is the air-stable
complex Pd2dba3 (dba ) trans,trans-dibenzylidene acetone).4
The importance attached to this complex has been highlighted
in the seminal mechanistic studies conducted by Amatore
and Jutand.5 No matter what the nature of the phosphine
ligands used, the most reactive Pd(0) species in the oxidative
addition with iodobenzene is the lowest ligated complex
“PdLn”. Ligation by dba, via η2-alkene coordination, dimin-
ishes the concentration of this species, affecting the overall
reactivity.5a Therefore, in consideration of the classic catalytic
cycle, dba ligand dissociation is required prior to oxidative
addition.
The strength of Pd-η2-dba coordination could be affected
by the π-electron-accepting ability6 of the dba ligand.
(3) (a) Suzuki, A. In Metal-Catalyzed Cross-Coupling Reactions; Dieder-
ich, F., Stang, P. J., Eds.; VCH: Weinheim, 1998; p 49. (b) Littke, A. F.;
Fu, G. C. Angew. Chem., Int. Ed. 1998, 37, 3387. (c) Old, D. W.; Wolfe,
J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 9722. (d) Wolfe, J. P.;
Buchwald, S. L. Angew. Chem., Int. Ed. 1999, 38, 2413. (e) Zhang, C.;
Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org. Chem. 1999, 64, 3804. (f)
Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int. Ed. 2000, 39, 4153.
(g) Andreu, M. G.; Zapf, A.; Beller, M. Chem. Commun. 2000, 2475. (h)
Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020. (i) Li,
G. Y.; Zheng, G.; Noonan, A. F. J. Org. Chem. 2001, 66, 8677. (j) Li, G.
Y. J. Org. Chem. 2002, 67, 3643. (k) Bedford, R. B.; Cazin, C. S. J.; Coles,
S. J.; Gelbrich, T.; Horton, P. N.; Hursthouse, M. B.; Light, M. E.
Organometallics 2003, 22, 987. (l) Navarro, O.; Kelly, R. A., III; Nolan,
S. P. J. Am. Chem. Soc. 2003, 125, 16194.
* Corresponding author. Fax: 0044(0)1904 432515. Tel: 0044(0)1904
434091.
(1) (a) Heck, R. F. In Palladium Reagents in Organic Syntheses;
Academic Press: New York, 1985. (b) Handbook of Organopalladium
Chemistry for Organic Synthesis; Negishi, E., Ed.; John Wiley & Sons:
Hoboken, NJ, 2002.
(4) Amatore, C.; Jutand, A. Coordin. Chem. ReV. 1998, 178-180, 511
and references therein.
(2) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.
10.1021/ol048413i CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/27/2004