Calix[4]arene-Supported N-Heterocyclic Carbene Ligands as Catalysts for Suzuki Cross-Coupling Reactions of Chlorotoluene

Abstract

Stable N-heterocyclic carbenes are generated starting from 5,17-(bis-imidazolium)-substituted calix[4]arenes. Addition of Pd(OAc) 2 to these carbenes leads to macrocyclic cis-palladium chelate complexes in which the distal bridging of the upper rim leads to a strong distortion of the calix[4]arene skeleton which was proven by X-ray crystal structure determination. Using an in-situ catalytic system consisting of a calixarene-imidazolium salt, Cs2CO3 as a base and a source of palladium, a species is formed which can be used to catalyse the Suzuki cross-coupling of 4-chloro toluene. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.