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Electrochemistry and Photoredox Catalysis: a Comparative Evaluation in Organic Synthesis

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Electrochemistry and Photoredox Catalysis: a Comparative Evaluation in Organic Synthesis molecules Review Electrochemistry and Photoredox Catalysis: A Comparative Evaluation in Organic Synthesis Rik H. Verschueren and Wim M. De Borggraeve * Department of Chemistry, Molecular Design and Synthesis, KU Leuven, Celestijnenlaan 200F, box 2404, 3001 Leuven, Belgium; [email protected] * Correspondence: [email protected]; Tel.: +32-16-32-7693 Received: 30 March 2019; Accepted: 23 May 2019; Published: 5 June 2019 Abstract: This review provides an overview of synthetic transformations that have been performed by both electro- and photoredox catalysis. Both toolboxes are evaluated and compared in their ability to enable said transformations. Analogies and distinctions are formulated to obtain a better understanding in both research areas. This knowledge can be used to conceptualize new methodological strategies for either of both approaches starting from the other. It was attempted to extract key components that can be used as guidelines to refine, complement and innovate these two disciplines of organic synthesis. Keywords: electrosynthesis; electrocatalysis; photocatalysis; photochemistry; electron transfer; redox catalysis; radical chemistry; organic synthesis; green chemistry 1. Introduction Both electrochemistry as well as photoredox catalysis have gone through a recent renaissance, bringing forth a whole range of both improved and new transformations previously thought impossible. In their growth, inspiration was found in older established radical chemistry, as well as from cross-pollination between the two toolboxes. In scientific discussion, photoredox catalysis and electrochemistry are often mentioned alongside each other. Nonetheless, no review has attempted a comparative evaluation of both fields in organic synthesis. Both research areas use electrons as reagents to generate open-shell radical intermediates. Because of the similar modes of action, many transformations have been translated from electrochemical to photoredox methodology and vice versa. This reciprocity is very interesting as it provides the ideal study ground for analysis and comparison of both fields. In this review, we look where those research domains interlock and where they differ. Analogies will be drawn between both methods to obtain a better understanding in both areas of research. In an effort to do so, this review will be selective. Transformations were selected which had reports of great similarity to ease comparison. This comparative evaluation can serve as a guidance tutorial review and as an introduction to the fields of electrochemistry and photoredox catalysis in organic synthesis. More rigorous reviews of the literature can be found in the following accounts and references therein [1–12]. The present study serves to provide a summary of synthetic transformations that can be performed by electrochemistry as well as photoredox catalysis to gain a better understanding of both. Key features are extracted that can be used as guidelines to formulate new ideas. In the following section we will display 10 synthetic transformations that have been performed by both electro- and photoredox catalysis and one photoelectrochemical method integrating these two. Schematic presentations of the conditions, the respective mechanisms (as proposed in literature) and a selected scope will assist in a side by side comparison of the two synthetic methodologies for the same transformation. Molecules 2019, 24, 2122; doi:10.3390/molecules24112122 www.mdpi.com/journal/molecules Molecules 2019, 24, 2122 2 of 38 2. SideMolecules by Side 2019, Comparison24, x FOR PEER REVIEW of Synthetic Methodologies 2 of 38 2.1. Dehydrogenative2. Side by Side Comparison Lactonization of ofSynthetic C(sp2/sp Methodologies3)-H Bonds The2.1. Dehydrogenative lactonization Lactonization of carboxylic of C(sp acids2/sp has3)-H evolvedBonds as a very convenient approach to integrate lactones asThe structural lactonization subunits of carboxylic in molecules acids has of evolved medicinal as a andvery biologicalconvenient interestapproach [ 13to– integrate18]. Among the establishedlactones as structural approaches, subunits direct in dehydrogenative molecules of medicinal C-H /andO-H biological oxidative interest coupling [13–18]. has Among gained the special interestestablished due to itsapproaches, high atom direct economy dehydrogenative [19–28]. Pioneering C-H/O-H work oxidativ wase reportedcoupling has in 2013 gained by Martinspecial [19], Wanginterest [20] and due Gevorgyan to its high atom [21], whoeconomy independently [19–28]. Pionee developedring work the was intramolecular reported in 2013 oxidative by Martin coupling [19], of C(sp2Wang)-H bonds [20] and of aromaticGevorgyan carboxylic [21], who acidsindependently using Cu- developed and Pd-catalysis. the intramolecular In 2015, oxidative the Xu group coupling reported 2 a mild,of silverC(sp )-H catalyzed bonds of version aromatic of C(spcarboxylic2)-H lactonizationacids using Cu- [23 and]. Nonetheless, Pd-catalysis. mostIn 2015, of thesethe Xu methods group use reported a mild, silver catalyzed version of C(sp2)-H lactonization [23]. Nonetheless, most of these high temperatures, transition-metal catalysts and strong oxidants. In this comparative segment we methods use high temperatures, transition-metal catalysts and strong oxidants. In this comparative will analyze how photoredox catalysis and electrochemistry perform regarding the dehydrogenative segment we will analyze how photoredox catalysis and electrochemistry perform regarding the 2 3 lactonizationdehydrogenative of C(sp lactonization/sp )-H bonds of C(sp (Figure2/sp31)-H). bonds (Figure 1). FigureFigure 1. Side 1. Side by side,by side, a comparative a comparative visualization visualization of the the photoredox photoredox catalyzed catalyzed and and electrochemical electrochemical dehydrogenativedehydrogenative lactonization lactonization of C-H C-H bonds bonds regard regardinging reaction reaction conditions, conditions, mechanismmechanism and product and productscope. scope. Scope Scopeexamples examples shown to shown work with towork both methods with both in black, methods solely in shown black, by solely photoredox shown by + photoredoxcatalysis catalysisin blue and in blueby electrosynthesis and by electrosynthesis in red. CCE in= constant red. CCE current= constant electrolysis, current Mes-Acr electrolysis, = 9-mesityl-10-methylacridinium. Mes-Acr+ = 9-mesityl-10-methylacridinium. Gonzalez-Gomez was the first to employ a photoredox catalyst to facilitate the transformation Gonzalez-Gomez was the first to employ a photoredox catalyst to facilitate the transformation in 2015 [24]. In 2017, Luo disclosed a very similar method using photoredox cobalt co-catalysis and in 2015the [inclusion24]. In of 2017, C(sp Luo2)-X (X disclosed = F, Cl, Br, a OMe, very SMe) similar bond method cleavage using which photoredox also yields the cobalt lactonization co-catalysis 2 and theadducts inclusion [27]. In of 1938, C(sp Fichter)-X (X and= coworkersF, Cl, Br, desc OMe,ribed SMe) the bondfirst anodic cleavage lactonization which also[29]. yieldsThey the lactonizationreported adductsthe electrosynthesis [27]. In 1938, of 5-methyl Fichter andphthalide coworkers from described2,4-dimethyl the benzoic first anodic acid. Later, lactonization in 2017, [29]. TheyLam reported and coworkersthe electrosynthesis reported another of 5-methylelectrosynthesis phthalide of phthalides from 2,4-dimethyl from aromatic benzoic carboxylic acid. acids Later, in 2017, Lam and coworkers reported another electrosynthesis of phthalides from aromatic carboxylic acids in combination with aliphatic carboxylic acids [30]. The first dehydrogenative lactonization of Molecules 2019, 24, 2122 3 of 38 biphenyl-2-carboxylic acids by anodic oxidation was reported by Tylor and coworkers in 1957 [31]; however, the electrochemical approach was not the main focus. In 2017 Zeng [28], closely followed in 2018 by Xu [26] and Luo [25], independently reported more elaborate electrochemical dehydrogenative lactonizations of C(sp2/sp3)-H Bonds. For both strategies we will mainly discuss and compare the work of Gonzalez-Gomez and Zeng in more detail (Figure1). Both the photoredox and electrochemical approach omit the use of transition metal catalysts as well as high temperatures. Most of the earlier, transition metal catalyzed procedures use temperatures above 75 ◦C to overcome the large HOMO-LUMO energy gap needed for the reaction to take place [19–21,23]. As can be seen in Figure1, both the photoredox catalyzed and electrochemical reaction mechanistically operate via single electron oxidation-radical cyclization. The 9-mesityl-10-methylacridinium (Acr-Mes+) cation contains one of the most oxidizing triplet states among the organophotocatalysts. Upon irradiation with visible light, the excited photocatalyst generates a carboxylate radical via single electron transfer (SET). The catalyst turnover occurs via reoxidation with a persulfate as the terminal oxidant. In the electrochemical pathway, the same single electron oxidation occurs at the anode by anodic oxidation. At the cathode, hydrogen gas is generated at the same time forming methoxide which can deprotonate the carboxylic acid prior to its oxidation. Following carboxylate radical generation, the lactonization of the C(sp2)-H bond happens by way of a 6-endo-trig cyclization. Finally, rearomatization by another single electron oxidation occurs. The final oxidant for the photoredox catalyzed reaction is the sulfate radical-anion [SO4•−] originating
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