Alkene Insertions into a Ru–PR2 Bond

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• 作者：Krista M. E. Burton ; Dimitrios A. Pantazis ; Roman G. Belli ; Robert McDonald ; Lisa Rosenberg
• 刊名：Organometallics
• 出版年：2016
• 出版时间：December 12, 2016
• 年：2016
• 卷：35
• 期：23
• 页码：3970-3980
• 全文大小：615K
• ISSN：1520-6041

文摘

An unusually broad series of discrete alkene insertion reactions has provided the opportunity to examine the mechanism(s) of this fundamental carbon–heteroatom bond-forming process. Ethylene, electron-rich and electron-poor (activated) alkenes all react with the Ru–P double bond in Ru(η⁵-indenyl)(PCy₂)(PPh₃) to form κ²-ruthenaphosphacyclobutanes. Thermal decomposition of these metallacycles in solution, via alkene deinsertion and β-hydride elimination, is particularly favored for electron-rich alkenes, and hydride-containing decomposition products are implicit intermediates in the observed isomerization of 1-hexene. Kinetic studies, including a Hammett analysis of the insertion reactions of para-substituted styrenes, suggest that two distinct inner-sphere pathways operate for the insertion of electron-rich versus activated alkenes. DFT analyses have identified one pathway involving simple cycloaddition via a four-centered transition state and another that proceeds through an η²-alkene intermediate. Such an intermediate was observed spectroscopically during formation of the ethylene metallacycle, but not for substituted alkenes. We propose that “pre-polarized”, activated alkenes participate in direct cycloaddition, while rate-determining η²-adduct formation is necessary for the activation of electron-rich alkenes toward migratory insertion into the Ru–P bond.