Interrogation of 2,2'-Bipyrimidines as Low-Potential Two-Electron Electrolytes.

As utilization of renewable energy sources continues to expand, the need for new grid energy storage technologies such as redox flow batteries (RFBs) will be vital. Ultimately, the energy density of a RFB will be dependent on the redox potentials of the respective electrolytes, their solubility, and the number of electrons stored per molecule. With prior literature reports demonstrating the propensity of nitrogen-containing heterocycles to undergo multielectron reduction at low potentials, we focused on the development of a novel electrolyte scaffold based upon a 2,2'-bipyrimidine skeleton. This scaffold is capable of storing two electrons per molecule while also exhibiting a low (∼-2.0 V vs Fc/Fc+) reduction potential. A library of 24 potential bipyrimidine anolytes were synthesized and systematically evaluated to unveil structure-function relationships through computational evaluation. Through analysis of these relationships, it was unveiled that steric interactions disrupting the planarity of the system in the reduced state could be responsible for higher levels of degradation in certain anolytes. The major decomposition pathway was ultimately determined to be protonation of the dianion by solvent, which could be reversed by electrochemical or chemical oxidation. To validate the hypothesis of strain-induced decomposition, two new electrolytes with minimal steric encumbrance were synthesized, evaluated, and found to indeed exhibit higher stability than their sterically hindered counterparts.

Homobenzylic Oxygenation Enabled by Dual Organic Photoredox and Cobalt Catalysis.

Activation of aliphatic C(sp3)-H bonds in the presence of more activated benzylic C(sp3)-H bonds is often a nontrivial, if not impossible task. Herein we show that leveraging the reactivity of benzylic C(sp3)-H bonds to achieve reactivity at the homobenzylic position can be accomplished using dual organic photoredox/cobalt catalysis. Through a two-part catalytic system, alkyl arenes undergo dehydrogenation followed by an anti-Markovnikov Wacker-type oxidation to grant benzyl ketone products. This formal homobenzylic oxidation is accomplished with high atom economy without the use of directing groups, achieving valuable reactivity that traditionally would require multiple chemical transformations.

Reversing the Regioselectivity of Halofunctionalization Reactions through Cooperative Photoredox and Copper Catalysis.

Halofunctionalization of alkenes is a classical method for olefin difunctionalization. It gives rise to adducts which are found in many natural products and biologically active molecules, and offers a synthetic handle for further manipulation. Classically, this reaction is performed with an electrophilic halogen source and leads to regioselective formation of the halofunctionalized adducts. Herein, we demonstrate a reversal of the native regioselectivity for alkene halofunctionalization through the use of an acridinium photooxidant in conjunction with a copper cocatalyst.

Hydrodecarboxylation of Carboxylic and Malonic Acid Derivatives via Organic Photoredox Catalysis: Substrate Scope and Mechanistic Insight.

A direct, catalytic hydrodecarboxylation of primary, secondary, and tertiary carboxylic acids is reported. The catalytic system consists of a Fukuzumi acridinium photooxidant with phenyldisulfide acting as a redox-active cocatalyst. Substoichiometric quantities of Hünig's base are used to reveal the carboxylate. Use of trifluoroethanol as a solvent allowed for significant improvements in substrate compatibilities, as the method reported is not limited to carboxylic acids bearing α heteroatoms or phenyl substitution. This method has been applied to the direct double decarboxylation of malonic acid derivatives, which allows for the convenient use of dimethyl malonate as a methylene synthon. Kinetic analysis of the reaction is presented showing a lack of a kinetic isotope effect when generating deuterothiophenol in situ as a hydrogen atom donor. Further kinetic analysis demonstrated first-order kinetics with respect to the carboxylate, while the reaction is zero-order in acridinium catalyst, consistent with another finding suggesting the reaction is light limiting and carboxylate oxidation is likely turnover limiting. Stern-Volmer analysis was carried out in order to determine the efficiency for the carboxylates to quench the acridinium excited state.