To Bind or Not to Bind: Mechanistic Insights into C–CO2 Bond Formation with Late Transition Metals

Abstract

In transition metal-mediated carboxylation reactions, CO₂ inserts into a metal–nucleophile bond. At the carboxylation transition state (TS), CO₂ 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 CO₂ 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_sp3<i>benzyl</i> and C_sp2<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_sp3 nucleophiles favor <i>outer</i>-sphere and C_sp2 systems favor <i>inner</i>-sphere CO₂ insertion into the metal–carbon bond. An exception are Cu–benzyl bonds, where <i>inner</i>- and <i>outer</i>-sphere CO₂ 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_sp2 bonds, we find that the metal–CO₂ 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 CO₂ insertion into Pd–C_sp2 bonds should be classified as <i>inner</i>-sphere.