Flexible Steric Bulky Bis(Imino)acenaphthene (BIAN)-Supported N-Heterocyclic Carbene Palladium Precatalysts: Catalytic Application in Buchwald–Hartwig Amination in Air

Abstract

To achieve efficient palladium-catalyzed cross-coupling reaction under mild reaction conditions with the flexible steric bulk strategy, a series of Pd-PEPPSI (PEPPSI: pyridine-enhanced precatalyst preparation, stabilization, and initiation) complexes C1-C6 were synthesized and characterized, in which unsymmetric flexible steric bulk was introduced on the N-aryl of ancenaphthyl skeleton. These well-defined palladium complexes were found to be excellent precatalysts for Buchwald-Hartwig amination of aryl chlorides with amines in air. The electronic effect of the Pd-PEPPSI complexes and the effect of ancillary pyridine ligands were evaluated, among which complex C3 exhibited the most efficiency. It was demonstrated that the cross-coupling products were obtained in excellent yields in the presence of 0.5-0.1 mol % palladium loading. A wide range of aryl- and heteroaryl chlorides as well as various amines were compatible. The oxidative addition of aryl chlorides is revealed to be the rate-determining step in the catalytic cycle. The catalytic activity can be enhanced by introducing electron-donating groups to the Pd-PEPPSI complexes. This type of Pd-PEPPSI precatalyst showed the most efficiency reported to date for the challenging C-N cross-coupling reactions requiring no anhydrous and inert atmosphere protections, suggesting flexible steric bulk as a promising catalyst design strategy.

Accessing Low‐Valent Inorganic Cations by Using an Extremely Bulky N‐Heterocyclic Carbene

“…This thermally stable species is a versatile “all‐in‐one” transmetalation/ligation reagent and was used to generate new low‐coordinate main group cations (as was exemplified by the successful synthesis of a [GeCl] + cation). Future work will involve modification of the umbrella‐shaped CPh 3 , including the formation of structurally flexible alkyl analogues, and the introduction of the ITr ligand scaffold to the domain of metal‐mediated catalysis …”

Rate and Computational Studies for Pd‐NHC‐Catalyzed Amination with Primary Alkylamines and Secondary Anilines: Rationalizing Selectivity for Monoarylation versus Diarylation with NHC Ligands

“…To further complicate matters, the arylation of primary amines can give either desired secondary (mono‐coupled) or undesired tertiary (di‐coupled) aniline products, thus selectivity is of paramount importance in these reactions. Although we and others have presented kinetic data for amination using secondary amines catalyzed by Pd‐NHC complexes, there are no reports on the kinetics and selectivity of amination of primary aliphatic amines using the same or related catalysts. Herein, we present kinetic data to examine the effect of ligand structure on the overall rate and selectivity of palladium‐catalyzed amination of primary alkylamines using Pd‐PEPPSI (PEPPSI=pyridine‐enhanced precatalyst preparation, stabilization, and initiation) precatalysts (Figure ).…”

The Highly Efficient Suzuki–Miyaura Cross‐Coupling of (Hetero)aryl Chlorides and (Hetero)arylboronic Acids Catalyzed by “Bulky‐yet‐Flexible” Palladium–PEPPSI Complexes in Air

“…The α‐diimine compounds were readily synthesized by a one‐pot model reaction by using ZnCl 2 /HOAc, and previously published procedures for the synthesis of the corresponding N‐heterocarbene salts and Pd–PEPPSI complexes (i.e., C1 – C5 ) were followed ,. All of the prepared palladium complexes were found to be air and moisture stable.…”