Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins

Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an alpha-diimine as ligand in the presence of CS2CO3 as base was tested using various substrates. The Pd(OAC)(2)/DAB-Cy (1, DAB-Cy = 1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acaC)(2) (acac = acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of " Bu4NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac)(2) and Pd(acaC)(2)/DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance. (C) 2003 Elsevier Science B.V. All rights reserved.