Rate-Limiting Step of the Rh-Catalyzed Carboacylation of Alkenes:C-C Bond Activation or Migratory Insertion?

摘要

Rhodium-catalyzed intramolecular carboacylation of alkenes, achieved using quinolinyl ketones containing tethered alkenes, proceeds via the activation and functionaliza-tion of a carbon-carbon single bond. This transformation has been demonstrated using RhCl(PPh_3)_3 and [Rh(C_2H_4)_2Cl]_2 catalysts. Mechanistic investigations of these systems, including determination of the rate law and kinetic isotope effects, were utilized to identify a change in mechanism with substrate. With each catalyst, the transformation occurs via rate-limiting carbon-carbon bond activation for species with minimal alkene substitution, but alkene insertion becomes rate-limiting for more sterically encumbered substrates. Hammett studies and analysis of a series of substituted analogues provide additional insight into the nature of these turnover-limiting elementary steps of catalysis and the relative energies of the carbon-carbon bond activation and alkene insertion steps.