Pd/MO_x Materials Synthesized by Sol-Gel Coprecipitation as Catalysts for Carbon–Carbon Coupling Reactions of Aryl Bromides and Chlorides

Abstract

In order to control the interaction of palladium species with oxide supports, a sol-gel coprecipitation synthesis has been developed for Pd/MO_x. The resulting Pd/MO_x catalysts are characterized by a rather strong interaction between the highly dispersed Pd²⁺ species and the alumina and silica lattices. During coupling reactions of the Heck type the catalytically active species is generated by partial dissolution of Pd from the support surface. Due to the strong bonding of Pd to the support this dissolution occurs to significant extent only at comparatively high temperatures (≥160 °C). These temperatures are exactly necessary for the activation of less-reactive substrates like bromobenzene and aryl chlorides. In this way we are able to synthesize supported catalysts that facilitate a controlled release of soluble Pd species under reaction conditions that are required for the activation of aryl bromides and chlorides. The Pd/MO_x catalysts show excellent activity in the Heck reaction of bromobenzene (TON up to 10000; TOF up to 5000 h^–1) and 4-chloroacetophenone (TON up to 9300; TOF up to 1550 h^–1).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)