The isolation of sigma-alkylpalladium Heck intermediates, possible when beta-hydride elimination is inhibited, is a rather rare event. Performing intramolecular Heck reactions on N-allyl-2-halobenzylamines in the presence of [Pd(PPh(3))(4)], we isolated and characterized a series of stable bridged palladacycles containing an iodine or bromine atom on the palladium atom. Indolyl substrates were also tested for isolation of the corresponding complexes. X-ray crystallographic analysis of one of the indolyl derivatives revealed the presence of a five-membered palladacycle with the metal center bearing a PPh(3) ligand and an iodine atom in a cis position with respect to the nitrogen atom. The stability of the sigma-alkylpalladium complexes is probably a consequence of the strong constraint resulting from the bridged junction that hampers the cisoid conformation essential for beta-hydride elimination. Subsequently, the thus obtained bridged five-membered palladacycles were proven to be effective precatalysts in Heck reactions as well as in cross-coupling processes such as Suzuki and Stille reactions.

sigma-Alkylpalladium Intermediates in Intramolecular Heck Reactions: Isolation and Catalytic Activity

BRENNA, STEFANO;GALLI, SIMONA;BROGGINI, GIANLUIGI
2010

Abstract

The isolation of sigma-alkylpalladium Heck intermediates, possible when beta-hydride elimination is inhibited, is a rather rare event. Performing intramolecular Heck reactions on N-allyl-2-halobenzylamines in the presence of [Pd(PPh(3))(4)], we isolated and characterized a series of stable bridged palladacycles containing an iodine or bromine atom on the palladium atom. Indolyl substrates were also tested for isolation of the corresponding complexes. X-ray crystallographic analysis of one of the indolyl derivatives revealed the presence of a five-membered palladacycle with the metal center bearing a PPh(3) ligand and an iodine atom in a cis position with respect to the nitrogen atom. The stability of the sigma-alkylpalladium complexes is probably a consequence of the strong constraint resulting from the bridged junction that hampers the cisoid conformation essential for beta-hydride elimination. Subsequently, the thus obtained bridged five-membered palladacycles were proven to be effective precatalysts in Heck reactions as well as in cross-coupling processes such as Suzuki and Stille reactions.
CHEMISTRY-A EUROPEAN JOURNAL
Heck reaction, homogeneous catalysis, metallacycles, palladium, X-ray diffraction
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11383/1717033
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