(191c) Unraveling the Effects of Reaction Microenvironment on Catalytic Turnovers

Within the context of transition state theory, turnovers of a catalytic step reflect the activation free energy. Modulating this free energy difference, either by lowering the free energy of the transition state or by increasing that of the reactant state would decrease the activation free energy and in turn promote rates. Here, we illustrate several examples on the strategies in modulating the reaction microenvironment, defined as the local structures and molecular assembly around the reacting species, that leads to marked catalytic consequences. Specific examples are: (i) C-O bond scission catalysis in alcohols, ethers, and trioxanes on Brønsted acid catalysts over a wide range of solvent, where solvent molecules solvate the Brønsted acid sites and mediate catalytic turnovers, by either promoting or inhibiting rates; (ii) C-H bond scission rates in the deuteration reaction of phenol, also an acid catalyzed reaction, are highly sensitive to the identity of the solvent molecules within the solvation shell and the local hydrogen bonding network; (iii) the formation of interfacial protons and their involvement in attacking the C=O bond during hydrogenation of carbonyls or ketones on transition metal surfaces. We quantitatively capture these marked effects of rates, interpreting their effects as the excess activation free energies and connect these rate observations, departure from those of the reference conditions (without the presence of local microenvironment) via a thermochemical construct. This work illustrates how we could exploit the microenvironment and leverage excess free energies in thermodynamically non-ideal environment to drive catalysis.