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 (inner-sphere path) or may insert without being activated by the metal (outer-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 Csp3 benzyl and Csp2 aryl and alkenyl nucleophiles. We observe clear trends, where the nature of the nucleophile determines the preferred path: benzylic Csp3 nucleophiles favor outer-sphere and Csp2 systems favor inner-sphere CO2 insertion into the metal–carbon bond. An exception are Cu–benzyl bonds, where inner- and outer-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–Csp2 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–Csp2 bonds should be classified as inner-sphere.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

“…19 Given the rapid rate of insertion into R PBP-ligated complexes, we were able to complete a rare kinetic study on CO 2 insertion into a metal alkyl complex. Our results demonstrate that the solvent parameter that is best for predicting the rate of CO 2 insertion is the Dimroth-Reichardt E T (30) parameter. This is analogous to what we have observed for transition metal hydrides, 5b but the dependence of the insertion rate on the solvent is signicantly less pronounced in the case of the metal alkyl complex.…”

“…Similar ligand effects were previously observed for NHC-Cu complexes. 30 The t Bu PCP-supported complex, ( t Bu PCP)Pd(CH 3 ), also shows a preference for the S E 2 pathway, with a computed barrier of 24.2 kcal mol À1 (TS_Out, Fig. S48 3).…”

“…It is likely that E T ( 30) is a better predictor of CO 2 insertion rates because it explicitly considers specic solute/solvent interactions, which are not accounted for when the solvent is modelled using either dielectric constant or AN. 5b Unfortunately, we were unable to extend the correlation between the rate of CO 2 insertion into ( t Bu PBP)Pd(CH 3 ) and E T (30) to more polar solvents. This is a These values are the average of two trials and the errors are AE10%.…”

Computational and Experimental Insights into Asymmetric Rh‐Catalyzed Hydrocarboxylation with CO2

“…Moreover, it was shown that benzylic substrates display an unusual η 6 ‐coordination mode, with the nucleophilic carbon positioned up to 3.6 Å away from rhodium [5] . The same substrate binding mode and preference for an outer sphere CO 2 insertion were found computationally for the chiral Rh‐( S )‐SEGPHOS catalyst [6] . This raises the question how the enantioselectivity is controlled in systems where CO 2 is not constrained through interactions with the metal.…”

“…[5] The same substrate binding mode and preference for an outer sphere CO 2 insertion were found computationally for the chiral Rh-(S)-SEGPHOS catalyst. [6] This raises the question how the enantioselectivity is controlled in systems where CO 2 is not constrained through interactions with the metal. Although CO 2 preferably is positioned in the outer sphere, it may still be affected by repulsive and attractive nonbonding interactions with the ligand.…”