All-electron numerical density functional theory calculations with scalar relativistic corrections have been utilized to examine the mechanism of the intramolecular rhodium-catalyzed hydroacylation reaction. The gas-phase results reveal a key branch point early in the reaction at the oxidative addition step wherein the two important pathways evolve through five-coordinate Rh(III) intermediates characterized by an apical acyl group and an equatorial hydride, orientations seemingly counter to trans influence arguments. These pathways account for the gross features of the experimental product distribution as well as the isotope labeling outcomes observed by previous investigators in this area. A greatly simplified approximation to modeling the reaction environment was applied that focused on redressing the coordinative unsaturation prevalent during certain steps of the catalytic process by including an explicit molecule of solvent or an additional molecule of substrate. Such an approach allowed us to explain the catalytic deactivation, substrate inhibition and dependence of the reaction rate on this coordinated ligand. Importantly, the application of a popular QM/MM method was unable to locate some of the key stationary points along the reaction path.

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A theoretical analysis of the C-S and C-H bond-activation pathways involving thiophene and Cp*Rh(PMe 3 ) is presented in which B3LYP density-functional theory is utilized. In addition to the traditional pathway which connects the η 2 -coordinated intermediate with the C-H bond activation products, a new pathway is discussed which connects the η 1 S-bound intermediate with these products. Calculations are performed in the basis of molecular orbitals of the interacting fragments for the reactive η 1 -and η 2 -coordinated intermediates to examine the orbital interactions and density transfer between the fragments. The calculated binding energy of the η 1 -coordinated intermediate is 15.5 kcal/mol weaker than that for the η 2 -coordinated intermediate due to the increased energy separation and reduced density transfer between the molecular orbitals of the interacting fragments. The structure of the transition state connecting the η 1 -coordinated intermediate with the C-S bondactivated product involves the strong overlap of the HOMO of the metal fragment with the LUMO of thiophene which results in the close proximity of the distal carbons on thiophene with the methyl groups of the Cp* ligand. Substitution of bulky groups at the 3 and 4 positions on thiophene may result in a significant steric component to the C-S activation barrier. Substitution of bulky groups on all carbons results in a significant thermodynamic component to the instability of the ring-opened product. Preliminary results indicate that the lack of reactivity toward C-S bond activation in Cp*Re(CO) 2 is due to the participation of the electron withdrawing carbonyl groups in the transfer of density between the interacting fragments.

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A comprehensive mechanistic examination of an asymmetric palladium-catalyzed Tsuji−Trost allylation reaction that identifies the enantioselective step was completed utilizing DFT computational tools and the nudged elastic band method. Key components of the study include (a) plausible reaction pathways for the full interconversion of a square-planar palladium allyl enolate intermediate with low barriers relative to the subsequent enantioselectivity-determining reductive C−C coupling step, thereby disputing the previously identified mechanism, (b) a detailed analysis of the factors influencing the stereochemical control involved in forming the preferred configuration via the reductive C−C coupling step, (c) a comprehensive examination of the competing outer-sphere mechanism that includes a metal counterion as an escort to the nucleophile in order to modulate the effects of modeling the reaction step of oppositely charged species, and (d) examination of the possible role water plays in stabilizing a keto-coordinated adduct of Pd II -η 1 -allyl, formed early in the catalytic cycle, relative to a carboxylate-coordinated adduct, the known resting state of the reaction. Barrier energies for the enantioselective C−C coupling are investigated with several levels of theory, and together they support a reaction mechanism consistent with the preferred formation of the correct enantiomer on the basis of the enantiomer of the ligand selected.