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Alkene Insertions into a Ru–PR2 Bond

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posted on 01.12.2016, 00:00 by Krista M. E. Burton, Dimitrios A. Pantazis, Roman G. Belli, Robert McDonald, Lisa Rosenberg
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.