Bio-Inspired Cu/Pd Bimetallic Catalysts Facilitating One-Pot Relay Catalysis of Homo-Couplings of Arylboronic Acids

Abstract

Inspired by the multienzymatic relay catalysis in nature, herein, we reported a pioneering bio-inspired Cu/Pd bimetallic composite microspheres (Cu/CMC@Pd/C) as a simple and efficient relay catalyst for the homo-coupling of arylboronic acids. In this composite, Pd/C was encapsulated by Cu(II)-crosslinking carboxymethyl cellulose (Cu/CMC). Cu/CMC@Pd/C microspheres were easily achieved by adding a mixture solution of Pd/C and carboxymethyl cellulose into a Cu(II) solution through an ion exchange reaction between carboxylate groups of CMC and Cu(II) ions. The morphology and composition of the as-prepared Cu/CMC@Pd/C microspheres were well characterized using various analytical tools such as FT-IR, XRD, XPS, SEM, TEM, elemental mapping, EDS, and ICP-AES. The analysis indicated that the composite microspheres have a porous structure and Pd/C is uniformly distributed within the interior of the composite microspheres. The porous structure facilitates mass transfer of the substrate and increases the contact area of the catalyst, thus improving reaction efficiency. The performance of Cu/CMC@Pd/C microspheres was assessed in the homo-coupling of arylboronic acid for the synthesis of symmetric biaryl without the need for an aryl halide as a partner. This process mimics a multienzyme catalysis through a relay Cu(I)-catalyzed halogenation of one portion of arylboronic acid and a Pd(0)-catalyzed Suzuki–Miyaura cross-coupling between the in situ generated aryl halide and another portion of arylboronic acid. The results showed that this bio-inspired Cu/CMC@Pd/C has excellent relay catalytic activity with tolerance of broad functional groups, producing the desired symmetrical biaryls in excellent yields within a short reaction time under mild conditions. Moreover, the catalyst can be easily recovered through simple filtration and still has good catalytic activity after six cycles. Consequently, this innovative pseudo-homogeneous Cu/Pd relay catalyst emerges as a convenient and efficient alternative for the synthesis of asymmetric biaryls.