Date of Award
Doctor of Philosophy (PhD)
This project has been working on introducing new type of method to induce chemoselectivity into electrochemical reactions by modifying the surface of an electrode and then confining chemical reactions to the region of the reaction close to that surface. The goal is to translate the extremely high levels of site-selectivity seen in chemical reactions on a microelectrode array into a tool for preparative synthetic reactions.As a proof-of-principle experiment, Pt-anode was coated with a diblock copolymer that is known to have an affinity for pyrene groups. The electrode is then used in the oxidation of two alcohol substrates: one having the pyrene group and one not having a pyrene group. The oxidation reactions are mediated by a TEMPO-derived oxidant. The oxidant can be confined to the region immediately surrounding the electrode with the use of methyl-α-D-glucopyranoside which is known to undergo fast oxidations with the TEMPO derived oxidant and is water soluble for easy removal following the reaction. In the experiment, it was found that a competition study between the two substrate led to a 1:1 mixture of the two aldehydes when a bare Pt-electrode was used, a 2:1 mixture (pyrene aldehyde: 4-nitrobenzyl aldehyde) of the two aldehydes when the Pt electrode was modified with the diblock copolymer, and a 11:1 ratio (pyrene aldehyde: 4-nitrobenzyl aldehyde) of the two aldehydes when the modified electrode was used in conjunction with the confining agent. From this experiment, it was clear that a molecular recognition event on the surface of the electrode could be used to control the selectivity of the reaction when the reaction was confined to the surface of the electrode. With the competition experiment established, a series of benzylic alcohols were employed as substrates in order to probe the generality of the observation. We found that the surface confined reactions favor electron-rich aryl rings, and that the selectivity of the reaction could be optimized using the same principles used to guide the development of site-selective reactions on an array. The origin of the selectivity was investigated by varying the polymer on the surface of the electrode and through computational efforts done in collaboration with the group of Professor Matt Neurock at the University of Minnesota. It appears that the observed selectivity is a result of positively charged pockets of aromatic rings on the surface of the anode (there is no selectivity in the absence of aromatic rings in the polymer coating the electrode or the use of a graphite-based anode). Recent events have provided evidence that the direction of the selectivity can be reversed by using the modified electrodes as cathodes for a reductive process, a result that provides support for our working model of the electrode surface. The next step that this project is working on is to expand the scope of the reactions that can be controlled by confinement to an electrode surface. This work is targeting an opportunity to introduce new types of selectivity into transition metal catalyzed cross coupling reactions.
Chair and Committee
Kevin K. Moeller John-Stephen Taylor
Jonathan Barnes, Vladimir Birman, Zhen (Jason) He,
Jing, Qiwei, "New Methods of Selectivity using Electrochemistry" (2023). Arts & Sciences Electronic Theses and Dissertations. 2868.