To Bind or Not to Bind: Mechanistic Insights into C–CO<sub>2</sub> Bond Formation with Late Transition Metals
Diego García‐López, Ljiljana Pavlovic, Kathrin H. Hopmann
Abstract
In transition metal-mediated carboxylation reactions, CO<sub>2</sub> inserts into a metal–nucleophile bond. At the carboxylation transition state (TS), CO<sub>2</sub> may interact with the metal (<i>inner</i>-sphere path) or may insert without being activated by the metal (<i>outer</i>-sphere path). Currently, there is no consensus as to which path prevails. In order to establish general predictions for the insertion of CO<sub>2</sub> into metal–carbon bonds, we computationally analyze a series of experimentally reported Cu, Rh, and Pd complexes. Our focus is on carboxylation of aromatic substrates, including C<sub>sp3</sub><i>benzyl</i> and C<sub>sp2</sub><i>aryl</i> and <i>alkeny</i>l nucleophiles. We observe clear trends, where the nature of the nucleophile determines the preferred path: benzylic C<sub>sp3</sub> nucleophiles favor <i>outer</i>-sphere and C<sub>sp2</sub> systems favor <i>inner</i>-sphere CO<sub>2</sub> insertion into the metal–carbon bond. An exception are Cu–benzyl bonds, where <i>inner</i>- and <i>outer</i>-sphere CO<sub>2</sub> insertions are found to be competitive, highlighting the need to include both paths in mechanistic studies and in the rationalization of experimental results. For insertion into Pd–C<sub>sp2</sub> bonds, we find that the metal–CO<sub>2</sub> interactions at the TS are weak and may be beyond 3 Å for sterically congested ligands. Nonetheless, on the basis of a comparison to other TSs, we argue that the CO<sub>2</sub> insertion into Pd–C<sub>sp2</sub> bonds should be classified as <i>inner</i>-sphere.