Mimics of Pincer Ligands: An Accessible Phosphine-Free N-(Pyrimidin-2-yl)-1,2-azole-3-carboxamide Framework for Binuclear Pd(II) Complexes and High-Turnover Catalysis in Water

Reusable Proline-Containing Bimetallic Pd–Ni(Co, Cu, Fe)–Pro/Al2O3 Catalysts for Suzuki Reaction in Water

Abstract

Bimetallic proline-containing composites Pd–Ni(Co, Cu, Fe)–Pro/Al2O3, due to the synergistic effect, exhibit high catalytic activity in the Suzuki reaction in aqueous media, which makes it possible to reduce the amount of expensive Pd to 10–2 mol %. New catalysts are easily regenerated from the reaction mixture and can be reused up to 5 times without losing activity.

Amino acid-derived bisphenolate palladium complexes as C-C coupling catalysts

A series of amine bisphenol (ABP) pro-ligands featuring amino acid ester pendant arms were prepared. Optimisation of the synthetic method allowed the facile incorporation of naturally occurring, chiral amino acids into the ABP scaffold with minimal racemisation. Reaction of the pro-ligands (LH₂) with Pd(OAc)₂, in the presence of amines, led to the formation of complexes with an unprecedented pincer-like O,N,O coordination mode around the Pd^II centre. The complexations in the presence of trialkylamines (NR₃) afforded a mixture of LPdNR₃ and LPdNHR₂ species. The latter was shown to form via an ambient-temperature C-N cleavage involving unstable Pd(OAc)₂(NHR₂)₂ intermediates. Using pyridine as base eliminated this dealkylation and resulted in the exclusive formation of LPd(py) complexes in high yields. In total, seven novel Pd^II ABP complexes were prepared, exhibiting distorted square-planar geometries with the asymmetric ligand moieties orientated towards the metal centre. The air- and moisture-stable LPd(py) complexes were successfully employed as catalysts in two types of C-C coupling reactions. The Suzuki-Miyaura coupling of 4'-bromoacetophenone and phenylboronic acid reached high yields (up to 81%), while a scope of further alkyl bromides was also efficiently converted using low catalyst loadings (1 mol%) and mild temperatures (40 °C). Furthermore, a Pd-pyridine complex achieved high activity in the Mizoroki-Heck coupling of styrene and 4'-bromoacetophenone.