Experimental Study of the Mechanism of the Palladium-Catalyzed Aryl鈥揂lkyl Negishi Coupling Using Hybrid Phosphine鈥揈lectron-Withdrawing Olefin Ligands

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• 作者：Estefan铆a Gioria ; Jes煤s M. Mart铆nez-Ilarduya ; Pablo Espinet
• 刊名：Organometallics
• 出版年：2014
• 出版时间：September 8, 2014
• 年：2014
• 卷：33
• 期：17
• 页码：4394-4400
• 全文大小：353K
• ISSN：1520-6041

文摘

A detailed study of the Negishi cross-coupling reaction of ArI (Ar = 2-C₆H₄CO₂Et) and ZnEt₂ with palladium catalysts containing conventional phosphines versus one using a chelating hybrid phosphine鈥揺lectron-withdrawing olefin (P-EWO) ligand reveals that for conventional phosphines (e.g., PPh₃) 尾-H elimination from intermediate [PdArEt(PPh₃)₂] is competitive with Ar鈥揈t reductive elimination and is responsible for part of the undesired reduction product ArH. In contrast, with the EWO phosphine, the 尾-H elimination from intermediate [PdArEt(P-EWO)] is slow compared to the fast Ar鈥揈t reductive elimination, and the undesired reduction product ArH observed proceeds in this case of hydrolysis of ZnArEt, formed in transmetalations where Ar is transferred from Pd to Zn. The rate of these transmetalations is comparable to the rate of reductive eliminations. Consequently, undesired transmetalations affording [PdEt₂(P-EWO)] and ZnArEt are more effective at early stages of the reactions and less effective when the ethylating agent becomes poorer in ZnEt₂ and richer in ZnEtX (X = I), as the reaction proceeds. Careful analysis of the experiments reveals the detailed changing evolution of the reaction, not only providing the main features of the catalytic cycle but also deducing how the reagents in the system change with time and what the effects on the products of these changes are.