Oxidatively Induced Reductive Elimination: Exploring the Scope and Catalyst Systems with Ir, Rh, and Ru Complexes

Abstract

Direct conversion of C–H bonds into C–C bonds is a promising alternative to the conventional cross-coupling reactions, thus giving rise to a wide range of efficient catalytic C–H functionalization reactions. Among the elementary stages in the catalytic C–C bond formation, reductive elimination constitutes a key step of the catalytic cycle, and, therefore, extensive studies have been made to facilitate this process. In this regard, oxidation on the metal center of a post-transmetalation intermediate would be an appealing approach. Herein, we have explored the substrate scope, catalyst systems, and oxidation tools to prove that the oxidatively induced reductive elimination (ORE) plays a critical role in the product-releasing C–C bond formation. Notably, we have demonstrated that ORE broadly operates with a series of half-sandwich d6 Ir­(III)-, Rh­(III)-, and Ru­(II)-aryl complexes. We have described that the metal center oxidation of the isolable post-transmetalation intermediates by means of chemical- or electro-oxidation can readily deliver the desired arylated products upon reductive elimination even at ambient temperature. Computational studies delineated the thermodynamics of the reductive elimination, where the activation barriers are shown to be significantly reduced upon increasing the oxidation states of the intermediates. We were also successful in corroborating this ORE in the corresponding Rh-methyl complex. In addition, catalytic conditions were optimized to incorporate this mechanistic understanding into the Ir-, Rh-, and Ru-catalyzed C–C bond formations under mild conditions.

Ruthenium‐Catalyzed meta‐CAr–H Bond Difluoroalkylation of 2‐Phenoxypyridines

“…Finally, a competitive difluoroalkylation between 2‐(4‐methoxyphenyl)pyridine, bearing an electron‐donating group, and 2‐[4‐(trifluoromethyl)phenyl]pyridine, bearing an electron‐withdrawing group, was conducted to study the impact of substrate electronic properties on the transformation. The result indicated that electron‐rich substrates were better suited to this reaction than electron‐deficient substrates, supporting the classification of Ru III ‐catalyzed difluoroalkylation as a base‐assisted electrophilic C–H activation process (Scheme c) …”

Sustainable Knowledge‐Driven Approaches in Transition‐Metal‐Catalyzed Transformations

“…Inspiredb yt hese results, Chang and co-workerse xtended their knowledge-driven approacht ot he development of TMcatalyzed CÀHa rylationr eactions by using arylboronic esters as transmetalating agents.I nt his work, apart from iridium systems, they also explored the potential of oxidative-induced reductive elimination pathways from well-defined Rh and Ru complexes,t op romote carbon-carbon bond-forming reactions. [29]…”

Aren‐freie Ruthenium(II/IV)‐katalysierte gegabelte Arylierungen für oxidative C‐H/C‐H‐Funktionalisierungen

“…[18] Dabei wurden unabhängig von der Art der verwendeten Substrate 1 und 2 vergleichbare Resultate erhalten (Abbildungen S-4-S-6 in den Hintergrundinformationen). [21] Der Einsatz von 2-Brombenzonitril als Oxidationsmittel reduziert die fürd ie oxidative Addition bençtigte Energie um 3.8 kcal mol À1 (Abbildung S-10). Während keine Abhängigkeit in Bezug auf die Arylhalogenid-Konzentration beobachtet wurde (Abbildung 4b), zeigte sich eine positive Reaktionsordnung bezüglich der Konzentration des Katalysators (Abbildung 4c).…”

“…Im Anschluss wird Ruthenium(IV)-Intermediat H über geschwindigkeitsbestimmende oxidative Addition des Arylhalogenids gebildet, was auf einen Oxidations-induzierten reduktiven Eliminierungsmechanismus schließen lässt. [21] Der Einsatz von 2-Brombenzonitril als Oxidationsmittel reduziert die fürd ie oxidative Addition bençtigte Energie um 3.8 kcal mol À1 (Abbildung S-10). Ein konkurrierender Ruthenium(II/0/II)-Mechanismus und das Auftreten hçherer Spinzustände konnten ausgeschlossen werden .…”