Chemistry glows green with photoredox catalysis

Abstract

Can organic chemistry mimic nature in efficiency and sustainability? Not yet, but recent developments in photoredox catalysis animated the synthetic chemistry field, providing greener opportunities for industry and academia. Light on sustainability Nature is the main inspiration for scientists when it comes to sustainability. In biology, plants use photosynthesis to convert raw materials (CO 2 and water) into chemical energy (carbohydrates), exploiting the energy of solar photons. Photosynthesis is the quintessence of sustainable chemical reactivity, and the pinnacle that Green Chemistry aims to reach. Perhaps, one step forward in this direction has been made by recent progress in photochemical methods, particularly photoredox catalysis 1. Working as aspiring leaves, these strategies generate reactive radicals by using the ability of coloured catalysts (transition metal complexes or organic dyes) to absorb visible light radiation and activate stable low-energy organic molecules through single-electron processes (oxidation or reduction). These open-shell intermediates have been used to design a variety of reactions, which are unattainable with classical ionic chemistry triggered by thermal activation. Importantly, photoredox catalysis has revitalised other areas of synthesis, such as radical chemistry and photochemistry, providing opportunities for reaction invention and improvement. This chemistry has been used to tackle longstanding challenges in medicinal chemistry 2 , natural product synthesis 3 , and more broadly, across the spectrum of organic chemistry and catalysis 1,4. Along with its synthetic advantages, photoredox catalysis has clear benefits for sustainability, fulfilling several principles of Green Chemistry 5. Light radiation is its primary energy source. Light is free, non-hazardous, and environmentally friendly (energy efficiency). Photons provide enough energy to achieve the desired reactivity, without the high temperatures or harsh conditions often required by thermal activation. The light-absorbing species (photocatalysts) can be used in low catalytic amounts (use of catalytic reagents). By reaching an electronically excited state, they trigger single-electron transfer (SET) events to or from inactive/stable substrates. This generates highly reactive species in a mild and controlled manner. This has two positive impacts on sustainability. First, one can use less reactive low-energy reagents, allowing less hazardous and safer synthetic routes and easier disposal of less toxic or polluting by-products. Second, photoredox catalysis can activate generally poor reactive moieties within molecules (e.g. C-H bonds), while showing heightened functional group tolerance. This makes photoredox catalysis invaluable for designing shorter synthetic routes with enhanced atom economy, using renewable feedstock materials. Revived opportunities for drug discovery All these features have attracted the attention of the chemical industry, which recognised the potential of photoredox strategies to achieve efficient an...

A coating from nature

“…Global production of acrylate esters exceeds 3.5 million metric tons a year (7), and it is evident that sustainable alternatives, maintaining the favorable properties of acrylate-based materials, can be highly transformative in tomorrow's chemistry and materials. Despite progress toward the use of renewable feedstock for fuels (8), polymers (9)(10)(11), and specialty chemicals (12,13), as well as recent advances in photoredox (14) and electrochemical conversions (15,16) aiming at the design of benign and low waste transformations (17,18), direct biomassderived alternatives for acrylate-based coatings remain largely unexplored (18)(19)(20)(21)(22)(23). Although bio-based polyesters are now well established (9), limited reports on (co)polymerization of acrylate analogs did not reveal materials function (24) or indicated large differences in reactivity compared with common acrylates (25).…”

Advances in asymmetric organocatalysis over the last 10 years

“…This scenario was overturned in 2008 after MacMillan and co-workers reported the inventive merging of enamine-mediated covalent catalysis and photoredox catalysis. In 2016, Melchiorre 14 , 15 applied iminium activation in photocatalysis to deliver β , β -disubstituted cyclic enones 2 with very high enantiopurity in the asymmetric construction of quaternary carbon stereocenters (Fig. 1a , reaction i) 16 .…”

Photoinduced regioselective olefination of arenes at proximal and distal sites

“…This rede ning of chemistry is forged either by a modulated oxidation state or in situ generation of an excited catalytic species through energy transfer 23 . Despite the economic and operational bene ts 24 , its application in C-H activation with palladium catalysis remains elusive 25 . Converging the appealing properties and complementary reactivities of palladium catalysis with photocatalysis 26,27 has the potential of creating diverse C-H activation manifolds in a sustainable manner, and this combination ts aptly in the current context of regioselective Fujiwara-Moritani reaction (Fig.…”