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9,10-Phenanthrenedione As Visible-Light Photoredox Catalyst

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9,10-Phenanthrenedione As Visible-Light Photoredox Catalyst catalysts Article 9,10-Phenanthrenedione as Visible-Light Photoredox Catalyst: A Green Methodology for the Functionalization of 3,4-Dihydro-1,4-Benzoxazin-2- Ones through a Friedel-Crafts Reaction Jaume Rostoll-Berenguer, Gonzalo Blay , José R. Pedro * and Carlos Vila * Departament de Química Orgànica, Facultat de Química, Universitat de València, Dr. Moliner 50, 46100 Burjassot, València, Spain; [email protected] (J.R.-B.); [email protected] (G.B.) * Correspondence: [email protected] (J.R.P.); [email protected] (C.V.); Tel.: +34-9-6354-4329 (J.R.P.); +34-9-6354-3043 (C.V.) Received: 12 November 2018; Accepted: 6 December 2018; Published: 12 December 2018 Abstract: A visible-light photoredox functionalization of 3,4-dihydro-1,4-benzoxazin-2-ones through a Friedel-Crafts reaction with indoles using an inexpensive organophotoredox catalyst is described. The reaction uses a dual catalytic system that is formed by a photocatalyst simple and cheap, 9,10-phenanthrenedione, and a Lewis acid, Zn(OTf)2. 5W white LEDs are used as visible-light source and oxygen from air as a terminal oxidant, obtaining the corresponding products with good yields. The reaction can be extended to other electron-rich arenes. Our methodology represents one of the most valuable and sustainable approach for the functionalization of 3,4-dihydro-1,4-benzoxazin-2-ones, as compared to the reported procedures. Furthermore, several transformations were carried out, such as the synthesis of the natural product cephalandole A and a tryptophol derivative. Keywords: visible-light photocatalysis; organophotoredox catalysis; Friedel-Crafts reaction; indoles; 1,4-benzoxazin-2-ones 1. Introduction Visible-light (sunlight) is a safe, renewable, abundant, inexpensive, and non-polluting source of energy, which means that sunlight is the most “green” energy source that we can use. Therefore, the development of methodologies using visible-light has become one of the greatest challenges in the scientific community in the last century [1,2]. In this context, the development of methodologies to increase the use of visible-light to control chemical reactivity and achieve molecular complexity with higher levels of efficiency have become a hot topic in the last years and many challenging organic reactions have been described [3–9]. For this purpose, intensive research has been devoted to develop photoredox catalysts that are capable of absorbing visible light and transfer this energy to the organic molecules. Many elegant works on photocatalysis have been reported using transition metal ruthenium or iridium polypyridyl complexes as efficient photosensitizers [10–13]. However, these transition metals are expensive and they have potential toxicity that has limited their usefulness. Therefore, for the development of more sustainable visible-light photoredox methodologies the use of organic dyes is more convenient due to the low cost, high availability, and low toxicity that offer this kind of catalyst. However, some of the organophotoredox catalysts are expensive, such as pyrilium [14–18] or acridinium [19–24] salts (Figure1). Organic dyes, such as Rose Bengal and Eosin Y, are more convenient due to their lower cost [25–31]. Nevertheless, the development of new methodologies using simpler organophotoredox catalysts that improve the sustainability of the “green” chemical Catalysts 2018, 8, 653; doi:10.3390/catal8120653 www.mdpi.com/journal/catalysts Catalysts 2018,, 8,, 653x FOR PEER REVIEW 22 of of 2122 methodologies using simpler organophotoredox catalysts that improve the sustainability of the process“green” ischemical highly desirable. process Inis thishighly context, desirable.α-diketones In this represent context, a classα-diketones of compounds represent that cana class exhibit of absorptioncompounds bands that can in the exhibit visible absorption range and bands that have in the been visible used forrange photochemical and that have processes been used [32– 34for]. Forphotochemical example, 9,10-phenanthrenedione processes [32–34]. For isexample, an inexpensive 9,10-phenanthren organic compoundedione is withan inexpensive very low molecular organic weightcompound (Figure with1 )very when low compared molecular withweight other (Figure organophotoredox 1) when compared catalysts. with other This organophotoredoxα-diketone has absorptioncatalysts. This bands α-diketone in the visible has absorption region (412 bands and 505 innm the invisible acetonitrile, region see(412 Supplementary and 505 nm in Materials acetonitrile, for further35, see supplementary details) and therefore materials could for befurther excited details) by visible-light. and therefore However, could be it hasexcited been by rarely visible-light. used in visible-lightHowever, it photochemicalhas been rarely processesused in visible-light [35–37]. photochemical processes [35–37]. Figure 1. ComparisonComparison ofof commercially commercially available available common common visible-light visible-light photoredox photoredox catalysts catalysts and 9,10- and 9,10-phenanthrenedionephenanthrenedione (source: (source: Sigma-Aldrich Sigma-Aldrich (2018)). (2018)). On the other hand, tertiary amines represent an important class of compounds in organic synthesis, wherewhere functionalization functionalization is ofis greatof great interest interest for the for chemical the chemical community, community, medicinal medicinal chemistry, pharmaceutical,chemistry, pharmaceutical, and agrochemical and agrochemical industry. In indust this context,ry. In thethis combinationcontext, the ofcombination visible-light of catalysis visible- andlight C-Hcatalysis bond and functionalization C-H bond functionalization adjacent to a tertiaryadjacent amine to a tertiary has been amine successfully has been developed successfully in 3 thedeveloped last years in [the38– last41]. years Normally, [38–41]. this Normally, sp -C-H functionalizationthis sp3-C-H functionalization involves the oxidationinvolves the of theoxidation amine toof the iminium amine ion,to iminium which canion, bewhich attacked can be by attacked various by kind various of nucleophiles. kind of nucleophiles. Nonetheless, Nonetheless, the major the numbermajor number of examples of examples are regarded are rega torded the functionalization to the functionalization of N-aryl of tetrahydroisoquinolines N-aryl tetrahydroisoquinolines [42–52], N[42–52],,N-dimethylanilines N,N-dimethylanilines [53–57], [53–57], and N and-aryl N-aryl glycine glycine derivatives derivatives [58– [58–662].2]. Hence, Hence, exploring exploring other substrates is highly desirable.desirable. In In this this context, 1,4-dibenzoxazinone skeleton skeleton is is present in a wide number of compounds with biological activities and its functionalization could be significantsignificant and 3 interesting for medicinal chemistry [[63–69].63–69]. Very recently, HuoHuo describeddescribed thethe ironiron catalyzedcatalyzed spsp3-C-H functionalization of 3,4-dihydro-1,4-benzoxazin-2-ones3,4-dihydro-1,4-benzoxazin-2-ones [70,71] [70,71] using as a terminal oxidant tert-Butyl hydroperoxide (TBHP)(TBHP) [ 70[70]] or or 2,3-dichloro-5,6-dicyanobenzoquinone 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) (DDQ) [71 ].[71]. We envisionedWe envisioned that thisthat functionalizationthis functionalization could becould achieved be achieved by a visible-light by a visible-light photochemical photochemical process. Herein, process. continuing Herein, withcontinuing our interest with our in interest the synthesis in the ofsynthesis multisubstituted of multisubstituted 1,4-dihydrobenzoxazin-2-ones 1,4-dihydrobenzoxazin-2-ones [72] and [72] the Friedel-Craftsand the Friedel-Crafts reactions withreactions indoles with [73 –indoles75], we [73–75], described we the described visible-light the photoredoxvisible-light Friedel-Crafts photoredox reactionFriedel-Crafts of indoles reaction with of benzoxazin-2-ones indoles with benzoxazin-2-ones using as catalyst using a simple as catalyst and cheap a simple diketone and such cheap as thediketone 9,10-phenanthrenedione, such as the 9,10-phenanthrenedione, and oxygen as terminal and oxidant.oxygen Duringas terminal our experimental oxidant. During work andour theexperimental preparation work of manuscript, and the preparation a photoredox of functionalization manuscript, a photoredox of 3,4-dihydro-1,4-benzoxazin-2-ones functionalization of 3,4- wasdihydro-1,4-benzoxazin-2-ones reported [76,77]. In both cases, was thereported expensive [76,77]. Ru(bpy) In both2Cl cases,2 was usedthe expensive as photocatalyst. Ru(bpy)2 Besides,Cl2 was unlikeused as the photocatalyst. photoredox catalyticBesides, systemunlike describedthe photor earlieredox [catalytic76], the resultssystem that described were obtained earlier with[76], ourthe methodresults that are were not affected obtained by with the stericour method hindrance are no aroundt affected the C3by carbonthe steric atom hindrance of the indole around skeleton. the C3 Thecarbon second atom paperof the [indole77] deals skeleton. with theThe functionalization second paper [77] of deals 3,4-dihydro-1,4-quinoxalin-2(1 with the functionalization Hof)-one 3,4- skeletondihydro-1,4-quinoxalin-2(1 and only one exampleH)-one skeleton of 3,4-dihydro-1,4-benzoxazin-2-ones and only one example of 3,4-dihydro-1,4-benzoxazin-2- was reported with low yieldones was (44%). reported with low yield (44%). Catalysts 2018, 8, x FOR PEER REVIEW 3 of 22 2. Results Initially, we choose the Friedel-Crafts reaction between indole 1a and 4-benzyl-3,4-dihydro-2H-
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