Palladium(II)-exchanged hydroxyapatite-catalyzed Suzuki–Miyaura-type cross-coupling reactions with potassium aryltrifluoroborates

Abstract

Palladium(II)-exchanged hydroxyapatite (PdHAP; Pd/Ca ratio = ca. 1/200) was prepared by ion-exchanging with Ca 2+ from calcined hydroxyapatite (2.0 mmol) and Pd(NO 3) 2 (0.10 mmol) in water (150 mL) at 70 °C for 24 h. The PdHAP (1 mol% Pd to aryl bromides) functioned as a catalyst for the Suzuki–Miyaura-type cross-coupling reaction of aryl bromides and potassium aryltrifluoroborates with triphenylphosphine (1 mol%) and excess potassium carbonate at 50 °C in methanol under air. This cross-coupling reaction was found to proceed in water, while the rate was slow compared with that in methanol. The heterogeneous PdHAP catalyst was used ten times repeatedly at 50 °C in methanol under air, though its catalytic activity gradually declined. XRD and XRF analyses and TEM images revealed that most of palladium species would still be supported as isolated ions on the repeatedly used PdHAP, in spite of the changes of the structure and composition of PdHAP surface at the late stages in the repetitive uses and the partial leaching of Pd species at the initial stages in the repetitive uses. The gradual decline of the catalytic activity might be dependent on either the changes of PdHAP surface at the late stages or the partial leaching of Pd species at the initial stages. Still, heterogeneous PdHAP served as a reusable catalyst for Suzuki–Miyaura-type cross-coupling reaction with potassium aryltrifluoroborates.