Mechanistic Insights into Pincer-Ligated Palladium-Catalyzed Arylation of Azoles with Aryl Iodides: Evidence of a PdII–PdIV–PdII Pathway

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• 作者：Shrikant M. Khake ; Rahul A. Jagtap ; Yuvraj B. Dangat ; Rajesh G. Gonnade ; Kumar Vanka ; Benudhar Punji
• 刊名：Organometallics
• 出版年：2016
• 出版时间：March 28, 2016
• 年：2016
• 卷：35
• 期：6
• 页码：875-886
• 全文大小：792K
• ISSN：1520-6041

文摘

Pincer-based (^R2POCN^R′2)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 (^iPr2POCN^Et2)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 (^iPr2POCN^Et2)PdI (3a) to be the resting state of the catalyst. The intermediate species (^iPr2POCN^Et2)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(sp²)–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 (^iPr2POCN^Et2)Pd^IV(benzothiazolyl)(Ar)I species, which is strongly supported by DFT calculations.