Photoredox catalysis harvesting multiple photon or electrochemical energies

Abstract

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

“…The excited-state reduction potential of 4a 2+ compares favorably with those of most known photocatalysts, ,, and it seems that only the 3-cyano-1-methylquinolinium ion, which does not absorb visible light, is a significantly stronger photooxidant, considering only purely photo- and not electrophotochemical approaches. Moreover, the oxidative power of 4a 2+ is comparable to what is obtained with two-photon excitation schemes ,, or via electrophotochemistry ,, systems as mentioned in the Introduction (Figure ).…”

Photoinduced Single‐Electron Reduction of Alkenes.

“…This strategy enabled the reduction of alkenes via a multiphoton tandem photoredox cycle. [28,29] A wide array of terminal and internal alkenes worked well in the reaction and provided hydrogenation products in good to excellent yields. Furthermore, this strategy was also applied to hydrofunctionalization of alkenes with weakly electrophilic aliphatic ketones.…”

Photo- and electro-chemical strategies for the activations of strong chemical bonds

“…Efforts have been made towards the activation of energy demanding organic compounds under mild conditions. 165 Except for designing new neutral visible light photocatalysts ( PC s), 166,167 the consecutive photoinduced electron transfer (conPET) strategy has successfully been employed for super reductions, 10,168 in which the energies of two photons are harvested to promote the desired process. In a typical example, a neutral state PC is photoexcited to *PC which can be easily quenched by an electron donor, such as triethylamine, to afford the one-electron-reduced form of PC ( PC˙ − ) – the ‘active’ photocatalyst.…”