Cobalt-Catalyzed [2蟺 + 2蟺] Cycloadditions of Alkenes: Scope, Mechanism, and Elucidation of Electronic Structure of Catalytic Intermediates

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• 作者：Valerie A. Schmidt ; Jordan M. Hoyt ; Grant W. Margulieux ; Paul J. Chirik
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：June 24, 2015
• 年：2015
• 卷：137
• 期：24
• 页码：7903-7914
• 全文大小：737K
• ISSN：1520-5126

文摘

Aryl-substituted bis(imino)pyridine cobalt dinitrogen compounds, (^RPDI)CoN₂, are effective precatalysts for the intramolecular [2蟺 + 2蟺] cycloaddition of 伪,蠅-dienes to yield the corresponding bicyclo[3.2.0]heptane derivatives. The reactions proceed under mild thermal conditions with unactivated alkenes, tolerating both amine and ether functional groups. The overall second order rate law for the reaction, first order with respect to both the cobalt precatalyst and the substrate, in combination with electron paramagnetic resonance (EPR) spectroscopic studies established the catalyst resting state as dependent on the identity of the precatalyst and diene substrate. Planar S = ¹/₂ 魏³-bis(imino)pyridine cobalt alkene and tetrahedral 魏²-bis(imino)pyridine cobalt diene complexes were observed by EPR spectroscopy and in the latter case structurally characterized. The hemilabile chelate facilitates conversion of a principally ligand-based singly occupied molecular orbital (SOMO) in the cobalt dinitrogen and alkene compounds to a metal-based SOMO in the diene intermediates, promoting C鈥揅 bond-forming oxidative cyclization. Structure鈥揳ctivity relationships on bis(imino)pyridine substitution were also established with 2,4,6-tricyclopentyl-substituted aryl groups, resulting in optimized catalytic [2蟺 + 2蟺] cycloaddition. The cyclopentyl groups provide a sufficiently open metal coordination sphere that encourages substrate coordination while remaining large enough to promote a challenging, turnover-limiting C(sp³)鈥揅(sp³) reductive elimination.