Synthesis and Electronic Structure of Bis(imino)pyridine Iron Metallacyclic Intermediates in Iron-Catalyzed Cyclization Reactions

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• 作者：Jordan M. Hoyt ; Kevin T. Sylvester ; Scott P. Semproni ; Paul J. Chirik
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：March 27, 2013
• 年：2013
• 卷：135
• 期：12
• 页码：4862-4877
• 全文大小：754K
• 年卷期：v.135,no.12(March 27, 2013)
• ISSN：1520-5126

文摘

The bis(imino)pyridine iron dinitrogen compound, (^iPr(TB)PDI)Fe(N₂)₂ (^iPr(TB)PDI = 2,6-(2,6-ⁱPr₂-C₆H₃-N鈺怌-(CH₂)₃)₂(C₅H₁N)) is an effective precatalyst for the [2蟺 + 2蟺] cycloaddition of diallyl amines as well as the hydrogenative cyclization of N-tosylated enynes and diynes. Addition of stoichiometric quantities of amino-substituted enyne and diyne substrates to (^iPr(TB)PDI)Fe(N₂)₂ resulted in isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and M枚ssbauer spectroscopic and computational studies established S = 1 iron compounds that are best described as intermediate-spin iron(III) (S_Fe = 3/2) antiferromagnetically coupled to a chelate radical anion (S_PDI = 1/2). Catalytically competent bis(imino)pyridine iron diene and metallacycles relevant to the [2蟺 + 2蟺] cycloaddition were also isolated and structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe(I)鈥揊e(III) catalytic cycle where the bis(imino)pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active bis(imino)pyridine chelate for enabling catalytic cyclization chemistry with iron.