Overcoming the Potential Window-Limited Functional Group Compatibility by Alternating Current Electrolysis

Effect of Alternating Polarity in Electrochemical Olefin Hydrocarboxylation

The electrochemical generation of radical anions from feedstock olefins offers a selective and efficient route for synthesizing commodity chemicals and pharmaceutical precursors via hydrofunctionalization. Traditional methods for electrochemical olefin hydrofunctionalization, for example, hydrocarboxylation, rely on anion intermediates and follow an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism involving olefin reduction, carboxylation, further reduction, and protonation. Enhancing terminal carboxylate selectivity often requires a proton source, reducing functional group tolerance and favoring proton reduction over olefin reduction. Alternating polarity, a nascent technique in organic electrochemistry, can improve product selectivity by influencing electron transfer rates and electrode surface species. Herein, we report the use of alternating polarity to selectively generate radical anions from styrene derivatives, using electrochemical hydrocarboxylation as a model. This approach shifts the mechanism to an electrochemical-chemical-chemical (ECC) pathway, where the final step involves hydrogen atom transfer. We showcase how alternating polarity modulates product selectivity, yield, and material decomposition, offering new insights into how alternating polarity can advance olefin functionalization by enabling more controlled and selective reaction pathways.

Enantioselective Reduction of 1-Naphthamides by Electrochemical Reduction and Catalytic Asymmetric Hydrogenation in Tandem

Chiral 1-tetrahydronaphthamides are the core structures of many bioactive molecules, yet their efficient asymmetric synthesis from a simple feedstock remains a challenge. Herein, we present a one-pot synthesis strategy that combines electrochemical reduction and ruthenium-catalyzed asymmetric hydrogenation to achieve the enantioselective reduction of 1-naphthalenamides to chiral 1-tetrahydronaphthamides. The protocol provides a practical platform for selectively constructing high-value chiral tetrahydronaphthenes from readily available naphthalene feedstock, thereby expanding the scope of asymmetric hydrogenation. The synthetic utility of this protocol is further demonstrated through the synthesis of bioactive molecules.

Electrochemical Denitrative Cyclization Driven by Alternating Polarity

Alternating current electrolysis has emerged as a promising technique for addressing challenging redox reactions that are otherwise difficult or impossible for direct current electrolysis. Under mild and transition-metal-free reaction conditions, a general electrochemical denitrative cyclization of nitroarenes was developed to access various cyclic sulfone-containing derivatives of biological significance. The key to success lies in the facile manipulation of multiple redox events upon rapid alternating polarity switching to enhance the selectivity and efficiency.

A General Hydrotrifluoromethylation of Unactivated Olefins Enabled by Voltage-Gated Electrosynthesis

Here we present the first successful hydrotrifluoromethylation of unactivated olefins under electrochemical conditions. Commercially available trifluoromethyl thianthrenium salt (TT+-CF3BF4 -, Ep/2=-0.85 V vs Fc/Fc+) undergoes electrochemical reduction to generate CF3 radicals which add to olefins with exclusive chemoselectivity. The resulting carbon centered radical undergoes a second cathodic reduction, instead of a classical HAT process, to generate a carbanion that can be terminated by protonation from solvent. The use of MgBr2 (+0.20 V onset oxidation potential) plays a key role as an enabling sacrificial reductant for the reaction to operate in an undivided cell. Guided by cyclic voltammetry (CV) studies, fine-tuning the solvent system, trifluoromethylating reagent's counteranion and careful selection of redox processes, this work led to the development of a voltage-gated electrosynthesis by pairing two redox processes with a narrow potential difference (ΔE≈1.00 V) allowing the reaction to proceed with two important advances: (a) high reactivity and selectivity towards hydrotrifluoromethylation over undesired dibromination, and (b) an unprecedented functional group tolerance, including aniline, phenols, unprotected alcohol, epoxide, trialkyl amine, and several redox sensitive heterocycles.

Physicochemical Principles of AC Electrosynthesis: Reversible Reactions

Electrolysis integrates renewable energy into chemical manufacturing and is key to sustainable chemistry. Controlling the waveform beyond direct current (DC) addresses the long-standing obstacle of chemoselectivity, yet it also expands the parameter set to optimize, creating a demand for theoretical predictions. Here, we report the first analytical theory for predicting chemoselectivity in an alternating current (AC) electrosynthesis. The mechanism is a selective reversal of the unwanted redox reaction during periods of opposite polarity, reflected in the final reaction outcome as a time-averaged effect. In the ideal scenario of all redox reactions being reversible, square AC waveform biases the outcome toward more overoxidation/over-reduction, whereas sine AC waveform exhibits the opposite effect. However, in a more realistic scenario of some redox reactions being quasi-reversible, sine AC may behave mostly like square AC. These predictions are in numerical agreement with model experiments employing acetophenone and align qualitatively with the literature precedent. Collectively, this study provides theoretical proof for a growing trend that promotes changing waveforms to overcome limitations challenging to address by varying canonical electrochemical parameters.

Electrochemical Generation of Aryl Radicals from Organoboron Reagents Enabled by Pulsed Electrosynthesis

Aryl radicals play a pivotal role as reactive intermediates in chemical synthesis, commonly arising from aryl halides and aryl diazo compounds. Expanding the repertoire of sources for aryl radical generation to include abundant and stable organoboron reagents would significantly advance radical chemistry and broaden their reactivity profile. While traditional approaches utilize stoichiometric oxidants or photocatalysis to generate aryl radicals from these reagents, electrochemical conditions have been largely underexplored. Through rigorous mechanistic investigations, we identified fundamental challenges hindering aryl radical generation. In addition to the high oxidation potentials of aromatic organoboron compounds, electrode passivation through radical grafting, homocoupling of aryl radicals, and decomposition issues were identified. We demonstrate that pulsed electrosynthesis enables selective and efficient aryl radical generation by mitigating the fundamental challenges. Our discoveries facilitated the development of the first electrochemical conversion of aryl potassium trifluoroborate salts into aryl C-P bonds. This sustainable and straightforward oxidative electrochemical approach exhibited a broad substrate scope, accommodating various heterocycles and aryl chlorides, typical substrates in transition-metal catalyzed cross-coupling reactions. Furthermore, we extended this methodology to form aryl C-Se, C-Te, and C-S bonds, showcasing its versatility and potential in bond formation processes.

Revisiting Alternating Current Electrolysis for Organic Synthesis

This review summarizes the recent advancements in alternating current (AC)-driven electroorganic synthesis since 2021 and discusses the reactivities AC electrolysis provides to achieve new and unique organic transformations.