Oxidative Addition, Transmetalation, and Reductive Elimination at a 2,2′-Bipyridyl-Ligated Gold Center

Abstract

Three-coordinate bipyridyl complexes of gold, [(κ-bipy)Au(η-CH)][NTf], are readily accessed by direct reaction of 2,2'-bipyridine (bipy), or its derivatives, with the homoleptic gold ethylene complex [Au(CH)][NTf]. The cheap and readily available bipyridyl ligands facilitate oxidative addition of aryl iodides to the Au(I) center to give [(κ-bipy)Au(Ar)I][NTf], which undergo first aryl-zinc transmetalation and second C-C reductive elimination to produce biaryl products. The products of each distinct step have been characterized. Computational techniques are used to probe the mechanism of the oxidative addition step, offering insight into both the origin of the reversibility of this process and the observation that electron-rich aryl iodides add faster than electron-poor substrates. Thus, for the first time, all steps that are characteristic of a conventional intermolecular Pd(0)-catalyzed biaryl synthesis are demonstrated from a common monometallic Au complex and in the absence of directing groups.

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

“…Significant recent progress has been made in demonstrating elementary cross‐coupling steps at Au centers . Bourissou, Amgoune, and co‐workers and our group have shown that Au I systems modified with appropriate ligands are effective for hitherto unknown Ar−X (X=I, Br) oxidative additions . In addition to these thermal pathways, significant progress has also been made in oxidative addition at gold using photochemically activated routes .…”

“…Catalysts that can overcome this are significant because they dispense with the highly reactive external or internal oxidants typically required in Au‐mediated cross‐couplings. Indeed, using Ar−X oxidative addition as a platform, redox neutral Au‐based aryl C−H arylations and Negishi‐like cross‐couplings have now been achieved . In these studies, reversible Ar−X oxidative addition was faster for more electron rich arenes, offering a counterpoint to the electronic trends typically seen in Pd‐based processes .…”

“…Indeed, using Ar−X oxidative addition as a platform, redox neutral Au‐based aryl C−H arylations and Negishi‐like cross‐couplings have now been achieved . In these studies, reversible Ar−X oxidative addition was faster for more electron rich arenes, offering a counterpoint to the electronic trends typically seen in Pd‐based processes . Accordingly, new Au‐mediated C−X oxidative additions are not merely of interest from a reactivity viewpoint but also have significant potential for addressing broader selectivity challenges in cross‐coupling chemistry.…”

“…Fundamental to the endeavors outlined above is the detailed insight into the oxidative addition step provided by stoichiometric studies (Scheme A). The underpinning feature of Au systems that are effective for Ar−X oxidative addition is the use of bidentate ligands, namely a carboranyl diphosphine (Bourissou & Amgoune), a hemilabile P,N system (Bourissou & Amgoune), and 2,2′‐bipyridine (our group, Scheme B) . These enforce sufficient changes to the properties of the Au center to allow Ar−X oxidative addition.…”

“…Our studies to explore the feasibility of alkenyl C( sp 2 )−I oxidative addition began by examining 5,5′‐difluoro‐2,2′‐bipyridyl gold(I) ethylene complex 1 (Scheme A) . As evidenced by spectroscopic analysis of the ethylene unit, the narrow bite angle (75°) bipyridyl ligand leads to significant back donation to the co‐ligand, a feature that is desirable for promoting oxidative addition.…”