Homogeneous and heterogeneous catalysts of organopalladium functionalized-polyhedral oligomeric silsesquioxanes for Suzuki-Miyaura reaction

Abstract

© 2015 Elsevier Inc. The mononuclear T8-Pd and multinuclear T10-Pd catalysts (3 and 6) were prepared from a reaction between Pd(COD)Cl2(COD = 1,5-cyclooctadiene) and the corresponding octameric T8and decameric T10silsesquioxane cages, functionalized with pyridine-triazole ligands. The T8-Pd complex 3 featuring one Pd(II) center was employed as a homogeneous catalyst for Suzuki-Miyaura cross coupling in a 1:1 EtOH:H2O solvent. On the other hand, the multinuclear T10-Pd catalyst 6 (ca. 4.6 Pd for each T10cage) was obtained as amorphous insoluble materials with exceptionally high molecular Pd loading (1.61 mmol Pd g-1). Under the same catalytic conditions, the homogeneous catalysts 3 exhibited slightly higher activity than the heterogeneous catalyst 6 (initial TOFs = 870 and 690 h-1, respectively). Furthermore, for 6, 4-(MeO)C6H4Br and PhB(OH)2substrates were catalyzed in the presence of a low catalyst loading of 3.6 × 10-3mol% Pd under aerobic conditions to afford the coupled product in 91% yield (i.e., turnover number (TON) = 2.5 × 104). The silsesquioxane-supported Pd catalyst 6 was recovered by simple centrifugation and reused for at least five catalytic cycles without a loss in activities.