Highly Recoverable Pd(II) Catalysts for the Mizoroki–Heck Reaction Based on N‑Heterocyclic Carbenes and Poly(benzyl ether) Dendrons

• Published in: Organometallics 2018-10, Vol.37 (20), p.3598-3610
• Main Authors: Ortiz, Alba, Gómez-Sal, Pilar, Flores, Juan C, de Jesús, Ernesto
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.8b00295

Two series of bis­(imidazolylidene)­palladium complexes of general formula [PdBr2(NHC)2] have been prepared. The molecular weight of the complexes was enlarged by bonding poly­(benzyl ether) dendrons of increasing size (G0 to G3) at the nitrogen atom of the NHC ligands. Complexes 1–4 contain monodentate NHC ligands coordinated in a trans mode, whereas complexes 9–12 contain a chelating ethylene-bridged bis­(NHC) ligand. The complexes were recovered from the product stream of a Mizoroki–Heck reaction by nanofiltration through thermally and chemically stable ceramic membranes. The recoverability is better for larger complexes and chelate ligands. In particular, chelate G3 complex 12 maintained a constant activity during the 13 recovery cycles performed, affording an accumulated turnover number (TON) of 13 000. On average, around 99.5% of the metal is recovered in each recovery cycle, and contamination by palladium is only 3 mg per kg of product. Several models to explain the efficiency of the recovery are discussed.