Mechanistic Studies of Alkene Isomerization Catalyzed by CCC-Pincer Complexes of Iridium

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• 作者：Spring Melody M. Knapp ; Sarah E. Shaner ; Daniel Kim ; Dimitar Y. Shopov ; Jennifer A. Tendler ; David M. Pudalov ; Anthony R. Chianese
• 刊名：Organometallics
• 出版年：2014
• 出版时间：January 27, 2014
• 年：2014
• 卷：33
• 期：2
• 页码：473-484
• 全文大小：500K
• ISSN：1520-6041

文摘

Iridium complexes containing CCC-pincer m-phenylene-bridged N-heterocyclic carbene ligands were examined as catalysts for alkene isomerization. Complexes containing either mesityl or adamantyl side groups were found to catalyze the isomerization of a number of alkenes to the internal isomers, including 1-octene, vinylcyclohexane, and allylbenzene. Mechanistic studies indicate a surprising dichotomy, apparently caused by ligand steric effects. For the mesityl-substituted catalyst, several lines of evidence provide strong support for isomerization via an iridium allyl hydride intermediate: (1) H鈥揇 crossover experiments indicate that 1,3-hydrogen migration is exclusively intramolecular, (2) the catalyst resting state, a 蟺-allyl hydride species, was isolated and serves as a kinetically competent catalyst, (3) NMR experiments indicate that the 蟺-allyl hydride resting state undergoes reversible C鈥揌 reductive elimination that is rapid relative to catalytic turnover, and (4) kinetic studies indicate that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. H鈥揇 crossover experiments for alkene isomerization catalyzed by the adamantyl-substituted complex show selectivity for a 1,3-deuterium shift, as well as the intermolecular transfer of hydrogen. These results are consistent with an insertion/elimination mechanism proceeding selectively through a secondary metal鈥揳lkyl or with a 蟺-allyl-type mechanism with an unknown pathway for intermolecular hydrogen crossover.