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

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 5; no. 47; pp. 17625 - 17634
• Main Authors: Fazekas, Eszter, Jenkins, David T, Forbes, Andrew A, Gallagher, Brendan, Rosair, Georgina M, McIntosh, Ruaraidh D
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 07.12.2021
• Subjects: Amines; Amino acids; Bromides; Catalysts; Chemical reactions; Coupling; Dealkylation; Ligands; Optimization; Palladium; Pyridines; Racemization
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d1dt03068j

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 (LH 2 ) 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 (NR 3 ) afforded a mixture of LPdNR 3 and LPdNHR 2 species. The latter was shown to form via an ambient-temperature C-N cleavage involving unstable Pd(OAc) 2 (NHR 2 ) 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. Palladium complexes of amino acid-derived bisphenolate ligands show unprecedented pincer-like coordination and activity in C-C coupling reactions.