Suzuki cross-coupling reaction catalyzed by sulfur-containing palladacycles: Formation of palladium active species

Air-stable-cyclopalladated compounds derived from the ortho-metalation of benzylic tert-butyl thioethers efficiently promote C-C coupling reactions and were studied mechanistically in the Suzuki reaction. The catalytic reaction was monitored by F-19 NMR and CC-MS and the decomposition of the S-palladacycle was studied under stoichiometric conditions, showing that these palladacycles serve as a reservoir of zerovalent palladium species. The formation of these species is initiated by attack of the arylboronic acid on the palladacycle resulting in the formation of an arylated palladacycle. Thereafter, this species undergoes a reductive elimination to form the active Pd(O) species. As a minor pathway, the Pd(O) species could also be generated in a pathway involving a reduction process that leads to the decomposition of the palladacycle to Pd(O) affording the ortho hydrogenated thioether. Poisoning studies and TEM analysis indicated the presence of palladium nanoparticles. Competitive experiments showed that electron-withdrawing substituents on the aryl halide and electron-donating substituents on the arylboronic acid facilitated the reaction. They also provide evidence that the soluble species formed from the oxidative addition, that are active for Heck and Suzuki reactions, are the same species. (c) 2008 Elsevier B.V. All rights reserved.