Synthetic Photoelectrochemistry

Abstract

Photoredox catalysis (PRC) and synthetic organic electrochemistry (SOE) are often considered competing technologies in organic synthesis. Their fusion has been largely overlooked. We review state-of-the-art synthetic organic photoelectrochemistry, grouping examples into three categories: 1) electrochemically mediated photoredox catalysis (e-PRC), 2) decoupled photoelectrochemistry (dPEC), and 3) interfacial photoelectrochemistry (iPEC). Such synergies prove beneficial not only for synthetic "greenness" and chemical selectivity, but also in the accumulation of energy for accessing super-oxidizing or -reducing single electron transfer (SET) agents. Opportunities and challenges in this emerging and exciting field are discussed.

Keywords: electrochemistry; mediators; photoelectrochemistry; photoelectrodes; photoredox catalysis.

Figure 1

A) Comparison of photoredox catalysis (PRC) and synthetic organic electrochemistry (SOE). B) Types and benefits of synthetic photoelectrochemistry.

Figure 2

A) Conceptual redox energy level diagram for the photoexcitation of electrochemically generated ions in e‐PRC. B) SOMO–HOMO inversion concept for two electromediated photoredox catalysts (e‐PRCs).

Figure 3

Early reports of photoelectrochemistry in organic synthesis: the photoexcitation of electrochemically generated ions.

Figure 4

A) Oxidation of unactivated arenes under PRC using DDQ. B) Proposed mechanism. C) Examples from the substrate scope. D) Nucleophilic or arene partners that reacted with ground‐state DDQ.

Figure 5

A) Direct oxidation of unactivated arenes by e‐PRC using photoexcited electrogenerated trisaminocyclopropenium dications. B) Proposed mechanism. C) Example scope.

Figure 6

A) Direct reduction of electron‐rich chloroarenes by e‐PRC using photoexcited electrogenerated dicyanoanthracene radical anions. B) Proposed mechanism. C) Example scope.

Figure 7

Comparison of net‐oxidative PRC and e‐PRC using anodic current for electrorecycling of the photocatalyst.

Figure 8

A) S_NAr reactions of unactivated aryl fluorides at ambient temperature and without base under e‐PRC. B) Proposed mechanism. C) Example scope.

Figure 9

A) S_NAr reactions of unactivated aryl fluorides at ambient temperature and without base under e‐PRC. B) Proposed mechanism. C) Example scope.

Figure 10

A) Photoelectrochemical 1,4‐addition of acyl groups under dPEC. B) Proposed mechanism.

Figure 11

A) Hofmann–Löffler–Freytag amination of C(sp³)−H bonds under dPEC. B) Proposed mechanism. C) Example scope.

Figure 12

Schematic of a photoanode used for oxidation of organic compounds. RHE: relative Hydrogen electrode; E _AP: applied potential; E _F: Fermi level; CB: conduction band; VB: valence band.

Figure 13

A) iPEC C−H amination of electron‐rich arenes with a hematite photoanode. B) Proposed mechanism. C) Example scope.

Figure 14

A) iPEC oxidation of simple organic compounds with a BiVO₄ photoanode. B) Proposed mechanism. C) Benzyl alcohol iPEC oxidation. D) Cyclohexene iPEC oxidation.

Figure 15

A) iPEC oxidative dimethoxylation of furan mediated by bromide ions using a BiVO₄/WO₃ photoanode. B) Proposed mechanism.

Figure 16

Typical custom‐built batch photoelectrochemical cells.

Figure 17

Continuous‐flow photoelectrochemical formic acid oxidation to CO₂.

Figure 18

Conceptual photoelectrochemical flow reactor.

Figure 19

Herein proposed precomplexation of radical dication TAC^⋅2+/benzene, photoexcitation, and quenching by ultrafast inner‐sphere SET.

Figure 20

Redox potential scale showing current limitations of PRC and direct electrolysis technologies and opportunities for e‐PRC.86