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Suzuki Cross-Coupling Reactions Catalyzed by an Aliphatic Phosphine-Based Pincer Complex of Palladium: Evidence for a Molecular Mechanism



A new phosphine-based pincer complex with an adamantyl core, [{C10H13-1,3-(CH2PCy2)2}Pd(Cl)] (1), has proved to be an excellent Suzuki catalyst. Catalyst 1 enables the quantitative coupling of a wide variety of electronically activated, deactivated, and/or sterically hindered and highly functionalized aryl bromides with phenylboronic acid in pure water with NaOH as base within very short reaction times and low catalyst loadings and without the need for exclusion of air. Hydrophobic substrates, which lead to inefficient conversions in aqueous solution, are efficiently and quantitatively coupled by 1 in toluene with K3PO4 as base. Mechanistic investigations indicate that palladium nanoparticles are probably not the catalytically active form of 1. Experimental results strongly indicate that the phenyl pincer complex [{C10H13-1,3-(CH2PCy2)2}Pd(Ph)] (3) is a key intermediate in the catalytic cycle of the Suzuki reaction in both toluene and aqueous solution. Treatment of 1 with phenylboronic acid exclusively yields 3 under catalytic reaction conditions. Moreover, stoichiometric reactions of 3 with aryl bromides lead to the exclusive formation of [{C10H13-1,3-(CH2PCy2)2}Pd(Br)] (2) and the corresponding biaryl, thus indicating that biaryl formation occurs either by oxidative addition of aryl bromides to 3, to form neutral hexacoordinated pincer-type PdIV intermediates with the general formula of [{C10H13-1,3-(CH2PCy2)2}Pd(Ar)(Br)(Ph)], followed by reductive elimination of the coupling products or by direct biaryl formation on the PdII center of 3, via a four-centered transition state.