Mechanistic Insights into Pincer-Ligated Palladium-Catalyzed Arylation of Azoles with Aryl Iodides: Evidence of a Pd-II-Pd-IV-Pd-II Pathway

摘要

Pincer-based ((POCNR,2)-P-R2)PdCl complexes, along with CuI cocatalyst catalyze the arylation of azoles with aryl Iodides to give the 2-arylated azole products:Herein; we report an extensive mechanistic investigation for the direct arylation of azoles involving a well-defined and highly efficient ((POCNEt2)-P-iPr2)PdCl (2a) catalyst, which emphasizes a rare Pd-II-Pd-IV-Pd-II redox catalytic pathway. Kinetic studies and deuterium labeling experiments indicate that the C-H bond cleavage on azoles occurs via two distinct routes in, a reversible manner. Controlled reactivity of the catalyst 2a underlines the iodo derivative ((POCNEt2)-P-iPr2)PdI (3a) to be the resting state of the catalyst. The intermediate species ((POCNEt2)-P-iPr2)Pd-benzothiazolyl (4a) has been isolated and structurally, characterized. A determination of reaction rates of compound 4a with electronically different aryl iodides has revealed the kinetic significance of the oxidative addition of the C(sp(2))-X electrophile, aryl iodide, to complex 4a. Furthermore, the reactivity behavior of 4a suggests that the arylation of benzothiazole proceeds via an oxidative addition/reductive elimination pathway involving a ((POCNEt2)-P-iPr2)Pd-IV(benzothiazolyl)(Ar)I species, which is strongly supported by DFT calculations.