Palladium-guanidine complex immobilized on SBA-16: a highly active and recyclable catalyst for Suzuki coupling and alcohol oxidation

Abstract

By immobilizing a mixture of a palladium-guanidine complex and guanidine on the mesoporous cage-like material SBA-16 via a one-pot silylation, we have successfully prepared a new solid catalyst for Suzuki coupling and the aerobic oxidation of alcohols. The structure and composition of the solid catalyst were characterized with N₂ sorption, XRD, TEM, FT-IR and solid state NMR. The coordination state of the palladium-guanidine complex supported on the solid catalyst was preliminarily investigated with XPS and UV-vis spectroscopy. This catalyst afforded fast conversions for the Suzuki coupling of various aryl bromides and arylboronic acids in a 50% aqueous ethanol solution at 50 °C even at loadings of 0.01–0.001 mol%, and the TOF (turn-over frequency) for the Suzuki coupling of bromobenzene and arylboronic acids could be up to 45 700 h⁻¹ under mild conditions. A satisfactory yield was obtained even after the catalyst was reused several times. The developed solid catalyst combined with the mild conditions investigated represents one of the most efficient heterogeneous systems for the Suzuki coupling of bromides. Furthermore, it was found that the solid catalyst could also work well for the aerobic oxidation of benzylic alcohols and cinnamyl alcohol, affording over 99% selectivity. The catalyst could be reused 15 times without a significant decrease in activity and selectivity.