Catalysis of Methyl Transfer Reactions by Oriented External Electric Fields: Are Gold-Thiolate Linkers Innocent?

Oriented external electric fields (OEEFs) are potent effectors of chemical change and control. We show that the Menshutkin reaction, between substituted pyridines and methyl iodide, can be catalyzed/inhibited at will, by just flipping the orientation of the EEF (F_Z) along the "reaction axis" (Z), N - -C - -I. A theoretical analysis shows that catalysis/inhibition obey the Bell-Evans-Polanyi principle. Significant catalysis is predicted also for EEFs oriented off the reaction axis. Hence, the observation of catalysis can be scaled up and may not require orienting the reactants vis-à-vis the field. It is further predicted that EEFs can also catalyze the front-side nucleophilic displacement reaction, thus violating the Walden-inversion paradigm. Finally, we considered the impact of gold-thiolate linkers, used experimentally to deliver the EEF stimuli, on the Menshutkin reaction. A few linkers were tested and proved not to be innocent. In the presence of F_Z, the linkers participate in the electronic reorganization of the molecular system. In so doing, these linkers induce local electric fields, which map the effects of the EEF and induce catalysis/inhibition at will, as in the pristine reaction. However, as the EEF becomes more negative than -0.1 V/Å, an excited charge transfer state (CTS), which involves one-electron transfer from the 5p lone pair of iodine to an antibonding orbital of the gold cluster, crosses below the closed-shell state of the Menshutkin reaction and causes a mechanistic crossover. This CTS catalyzes nucleophilic displacement of iodine radical from the CH₃I^•+ radical cation. The above predictions and others discussed in the text are testable.