Electrocatalytic C–H/N–H functionalization mediated by ferrocene derivatives
- UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
- Amy L. Waldbusser (Creator)
- Institution
- The University of North Carolina at Greensboro (UNCG )
- Web Site: http://library.uncg.edu/
- Advisor
- Shabnam Hematian
Abstract: The functionalization of carbon- or nitrogen-centered radical species is an important process in many chemical reactions aimed at turning simple chemical feedstocks, such as those in petroleum products, into chemicals of higher complexity and value. Although there exist a variety of methods to perform these reactions, electrochemistry stands out as a powerful approach, due to its mild, cost effective, and tunable features. Electrochemical oxidation of a substrate offers more control over the process by isolating the produced radical species, allowing further functionalization reactions to occur, and lowering the chance for unwanted side reactions. Additionally, a redox mediator can be utilized to eliminate issues that can occur with direct substrate oxidation, particularly that of primary amines, such as fouling of the electrode surface or degradation of the substrate. Ferrocene (Fc) and its analogs are ideal compounds for this role, as their well-known one-electron redox couple going between ferrocene and ferricenium (Fc+) is very reversible. Additionally, their redox potentials are in the appropriate range for the oxidation of amines and can be easily tuned by altering the nature of the substituents on the cyclopentadienyl (Cp) rings. We begin with a thorough electrochemical study of ferrocene and nine of its derivatives with either electron-donating or electron-withdrawing substituents in three different solvents and with two different supporting electrolytes (Chapter 1). This investigation gave us a variety of possible redox mediators that we could use to perform the electrocatalytic anaerobic oxidation of benzylic amine substrates (Chapter 2). Chapter 1 details a facile and effective strategy for the preparation of a series of ferricenium complexes bearing either electron-donating or electron-withdrawing substituents with weakly coordinating anions such as [B(C6F5)4]- or SbF6-. These systems were thoroughly investigated for their ground state electronic structures in both solution and solid states using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies as well as single crystal X-ray crystallography. We also present the redox behavior of the corresponding ferricenium/ferrocene redox couples including potential values (E1/2), peak-to-peak separation (?E1/2), anodic/cathodic peak current ratios (ipa/ipc) and diffusion coefficients (D) of the redox active species in order to provide a better understanding of our library of ferrocene redox mediators in deferent media. Our electrochemical studies involved three different solvents and two tetra-n-butylammonium supporting electrolytes with a more traditional anion, hexafluorophosphate (PF6-), as well as pentafluoro substituted tetra-phenylborate, [B(C6F5)4]-. Notably, our findings point to the significant effect of ion pairing in lowering the energy necessary for reduction of the ferricenium ion and the overall half-wave potential values in lower-polarity media. Chapter 2 describes the anaerobic electrocatalytic oxidation of two benzylic primary amines (i.e., benzylamine and 2-picolylamine) in the presence of an electron deficient ferrocene derivative as a redox mediator. The use of the appropriate redox mediator prevented fouling of the electrode surface, which is dominant during the direct electrochemical oxidation, as well as decreased the half-wave potential at which the catalytic oxidation reaction occurred (Ecat/2). Cyclic voltammetry studies revealed an ErCi’ catalytic process between the ferrocene derivative and both substrates. Through anaerobic controlled-potential electrolysis, we have demonstrated a method that utilized 90% of electrons removed from the system towards forming the desired coupled imine product of benzylamine oxidation while avoiding an excess of problematic hydrolysis and other side reactions. The major and minor products obtained from bulk electrolysis experiments were characterized through IR, 1H- and 13C-NMR spectroscopies and proposed mechanistic steps were laid out for the electrocatalytic process. Our results can guide the development of new electrocatalytic systems aimed at oxidizing and transforming simple compounds into chemicals of higher complexity and value.
Electrocatalytic C–H/N–H functionalization mediated by ferrocene derivatives
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Created on 5/1/2020
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Additional Information
- Publication
- Thesis
- Language: English
- Date: 2020
- Keywords
- Catalysis, Electrochemistry, Ferrocene, Oxidation, Redox Mediator
- Subjects
- Electrochemistry
- Ferrocene
- Oxidation-reduction reaction
- Catalysis