Shining Light on Copper: Unique Opportunities for Visible-Light

Article pubs.acs.org/accounts

Shining Light on Copper: Unique Opportunities for Visible-LightCatalyzed Atom Transfer Radical Addition Reactions and Related Processes Published as part of the Accounts of Chemical Research special issue “Photoredox Catalysis in Organic Chemistry”. Oliver Reiser* Institut für Organische Chemie, Universität Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany

CONSPECTUS: Visible-light photoredox catalysis offers exciting opportunities to achieve challenging carbon−carbon bond formations under mild and ecologically benign conditions. Desired features of photoredox catalysts are photostability, long excited-state lifetimes, strong absorption in the visible region, and high reduction or oxidation potentials to achieve electron transfer to substrates, thus generating radicals that can undergo synthetic organic transformations. These requirements are met in a convincing way by RuII(phenanthroline)3- and IrIII(phenylpyridine)3-type complexes and, as a low-cost alternative, by organic dyes that offer a metal-free catalyst but suffer in general from lower photostability. CuI(phenanthroline)2 complexes have been recognized for more than 30 years as photoresponsive compounds with highly negative Cu(I)* → Cu(II) oxidation potentials, but nevertheless, they have not been widely considered as suitable photoredox catalysts, mainly because their excited lifetimes are shorter by a factor of 5 to 10 compared with Ru(II) and Ir(III) complexes, their absorption in the visible region is weak, and their low Cu(II) → Cu(I) reduction potentials might impede the closure of a catalytic cycle for a given process. Contrasting again with RuIIL3 and IrIIIL3 complexes, CuIL2 assemblies undergo more rapid ligand exchange in solution, thus potentially reducing the concentration of the photoactive species. Focusing on atom transfer radical addition (ATRA) reactions and related processes, we highlight recent developments that show the utility of CuI(phenanthroline)2 complexes as photoredox catalysts, demonstrating that despite their short excited-state lifetimes and weak absorption such complexes are efficient at low catalyst loadings. Moreover, some of the inherent disadvantages stated above can even be turned to advantages: (1) the low Cu(II) → Cu(I) reduction potential might efficiently promote reactions via a radical chain pathway, and (2) the tendency for ligand exchange in CuIL2 assemblies allows the efficient synthesis of heteroleptic CuILL′ complexes to tune the steric and electronic properties and also might coordinate and thus activate substrates in the course of a reaction in addition to electron transfer. Moreover, new photoredox cycles have also been discovered beyond the visible-light-induced Cu(I)* → Cu(II) electron transfer that is arguably best known: examples of the Cu(II)* → Cu(I) and Cu(I)* → Cu(0) transitions have been realized, greatly broadening the potential for copper-based photoredox-catalyzed transformations. Finally, a number of organic transformations that are unique to Cu(I) photoredox catalysts have been discovered.

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MacMillan, Stephenson, and Yoon.1−3 Most broadly applied as catalysts for such transformation have been metal complexes based on ruthenium and iridium, featuring strong absorption in the visible-light region (>400 nm), sufficiently long lifetimes

INTRODUCTION

Visible-light photoredox catalysis has developed into a powerful tool in organic synthesis, dating back to the pioneering work from the 1980s and early 1990s by Deronzier, Fukuzumi, Kellogg, Okada, Pac, Pandey, Tanaka, Tomioka, and Willner and more recently by many research groups, starting with spectacular applications developed by the research groups of © XXXX American Chemical Society

Received: June 14, 2016

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DOI: 10.1021/acs.accounts.6b00296 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Ligands and copper(I) complexes derived therefrom used as visible-light photoredox catalysts.

(>1 μs), and high oxidation and reduction potentials of their excited states, which allow efficient electron transfer to a great number of organic compounds as a starting point for radical transformations. As a metal-free alternative, organic dyes have also proved to be both exceptional photooxidants or -reductants, for which even multiphoton excitation has been demonstrated to achieve the activation of stable bonds by electron transfer.4 In contrast, copper complexes have been used scarcely as visible-light photoredox catalysts for organic transformations, although early work by the groups of Kutal and Mitani and more recent work by Fu pointed to exciting possibilities to achieve cycloaddition, atom transfer radical addition (ATRA), or coupling reactions by combining copper(I) salts with UV irradiation.5 During the last years, a fast-growing number of copper(I) complexes (Figure 1) have been evaluated as visiblelight photocatalysts, impressively showing that such complexes have special features beyond their role as economically attractive alternatives to ruthenium or iridium photocatalysts to promote electron transfer to organic substrates, thus opening unique opportunities for visible-light-mediated synthetic transformations. Seminal work by Sauvage and co-workers in 1987 demonstrated that [Cu(dap)2]Cl (5e) (dap = 2,9-bis(4anisyl)-1,10-phenanthroline) efficiently catalyzes the reductive coupling of 4-nitrobenzyl bromide (10a) to 11 in the presence of the sacrificial electron donor triethylamine (Scheme 1).6 This transformation is remarkable from a number of aspects: 5e is obviously able to photochemically activate 10a, despite the

drastically shorter lifetime of its photoexcited state (≤270 ns) compared with those of commonly used photocatalysts such as [Ru(bpy)3]Cl2 (bpy = bipyridyl) (1100 ns) or fac-Ir(ppy)3 (ppy = 2-phenylpyridyl) (1900 ns). It was proposed that electron transfer from 5e to 10a generates the benzyl radical, which dimerizes to give the bisbenzil coupling product 11 (Scheme 1, left). It is assumed that the role of triethylamine is to regenerate Cu(I) from Cu(II) that is formed upon reduction of 10a, and thus, the photochemical key step follows the socalled oxidative quenching cycle. Indeed, [Cu(dap)2]Cl is a stronger reductant (*Cu+/Cu2+ = −1.43 V) than [Ru(bpy)3]Cl2 (*Ru2+/Ru3+ = −0.86 V; Ru+/Ru2+ = −1.33 V), regardless of whether the latter is utilized in the oxidative or reductive quenching cycle, but a weaker one than fac-Ir(ppy)3 (*Ir3+/Ir4+ = −1.73 V; Ir2+/Ir3+ = −2.19 V). It is important to note that excited Cu(I)* in 5e is not reduced to Cu(0) by triethylamine and thus that a reductive quenching cycle analogous to that known from ruthenium(II) or iridium(III) complexes does not take place, although this reaction mode has now also been realized employing complex 9.7 Interestingly, Mitani et al.8 concluded already in 1981 in the CuCl/UV-catalyzed ATRA reaction of halides to alkenes that the reaction might not follow a radical pathway on the basis of the fact that radical inhibitors do not shut down such transformations. As a possible alternative, they considered a photochemically promoted oxidative addition of Cu(I)* to the alkyl halide giving rise to a Cu(III) intermediate. Following this rationale, such an intermediate could then undergo coupling with 10a to generate the product and a Cu(III) halide, which in turn could be reduced by a sacrificial electron donor such as NEt3 to regenerate Cu(I) (Scheme 1, right). Applying this proposal for the transformation of 10a to 11 would also be fully consistent with the experimental results and moreover would not call for a radical dimerization. In line with this mechanistic picture, CuIL2 complexes are prone to undergo structural reorganization and ligand exchange, contrasting with octahedral RuIIL3 or IrIIIL3 assemblies. Indeed, there is growing evidence that copper photoredox catalysts are acting beyond their ability to promote electron transfer and that copper−substrate species are involved in the product formation. Notably, the thermal CuBr2-catalyzed trifluoromethylhalogenation of alkenes using Togni’s reagent/SOX2 was very recently reported, for which also the involvement of Cu(III) is proposed, being generated by single-electron transfer (SET) from the copper(II) halide.9

Scheme 1. Reductive Coupling of 4-Nitrobenzyl Bromide (10a) Photocatalyzed by [Cu(dap)2]Cl (5e)

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Scheme 2. [Cu(dap)2]Cl-Catalyzed ATRA Reactions between Organohalides 12 and Alkenes 13 or Silyl Enol Ethers 15

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CU(I)-PHOTOCATALYZED ATRA AND RELATED PROCESSES The findings above suggested the feasibility of using [Cu(dap)2]Cl (5e) to promote photochemical ATRA reactions, which was indeed realized in a series of publications. Starting in 2012, our research group showed that such processes can be carried out using activated organohalides 12 such as CBr4, αhalocarbonyl compounds, or nitro-substituted benzyl halides (Scheme 2). The reactions proceed through the oxidative quenching cycle, so no sacrificial electron donor is necessary. ATRA products obtained from 2-nitrobenzyl halides (i.e., 14e, 16a, 16b) can undergo facile conversion into tetrahydroquinolines upon reduction.10,11 Tang and Dolbier12 showed that fluorinated sulfonyl chlorides undergo ATRA-like reactions under SO2 extrusion with electron-def icient alkenes, allowing the introduction of not only trifluoro- but also mono- and difluoromethyl groups along with chloride across the C−C double bond. Complementary to this, our studies13 showed that upon catalysis with 5e, ATRA reactions between trifluoromethylsulfonyl chloride and electronneutral alkenes lead to trifluoromethylchlorosulfonylation (and thus, SO2 extrusion does not occur), while changing to Ru(II)based photocatalysts13,14 under otherwise unchanged reaction conditions again leads exclusively to trifluoromethylchlorination. Representative examples are shown in Scheme 3. These results raise a number of issues pointing to unique features of copper(I) photocatalysts that go beyond their ability to promote electron transfer (Scheme 4). In all cases, there is good evidencemainly because the reactions are not stereospecificthat the reactions are initiated by electron transfer from the photocatalyst to 17, thus generating a CF3· radical that adds to the alkene. A common mechanistic pathway that is depicted for many photoredox-catalyzed processes is the oxidation of the resulting intermediate 22/24 to a cation 26, thus regenerating the catalyst. Cismesia and Yoon15 pointed out in an insightful study that radical chain propagation is nevertheless the dominating process for many photocatalyzed ATRA reactions, in agreement with thermally induced

Scheme 3. Trifluoromethylchlorination and Trifluoromethylchlorosulfonylation of Alkenes

processes of this kind. Nevertheless, it has been argued12 that radical 22 is too electrophilic to carry a radical chain with 17, and instead, the reaction with a Cu(II) species such as 21a, which delivers a chlorine with concurrent regeneration of Cu(I), is proposed. A radical chain process between the more nucleophilic radical 24 and 17 seems feasible, but such a pathway would not explain the formation of different products 25 and 27 depending on the catalyst employed (Cu(I) and Ru(II), respectively). Therefore, the formation of 25 from 24 was suggested to proceed by involving a species like 21b. Indeed, structural reorganization and ligand exchange in CuLn complexes is feasible, in contrast to RuL3 assemblies. An alternative explanation would involve the low oxidation power of Cu(II) in contrast to that of Ru(III): while with the latter the cation 26 might be on the reaction pathway and then could combine with chloride to afford 27, with the former this oxidation might not be possible and therefore a radical chain might be started, which would however call for attack of 24 on sulfur in 17. The involvement of copper−ligand species in the transfer of the SO2Cl group into the product was also corroborated by the C


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Scheme 4. Mechanistic Possibilities for Trifluoromethylchlorination and Trifluoromethylchlorosulfonylation of Alkenes

Scheme 6. Synthesis of β-Hydroxysulfones by Sequential Photocatalysis

different outcomes of the reaction when different copper-based photocatalysts are employed (Scheme 5). It was observed that Scheme 5. Steric Influence of Copper(I) Complexes on Trifluoromethylchlorosulfonylation and Trifluoromethylchlorination

applications (Scheme 7). Azidinations of styrenes can be achieved with the Zhdankin reagent 35 catalyzed by 5e. A Scheme 7. Azidination of Styrene Catalyzed by [Cu(dap)2] Cl (5e) sterically more demanding compounds lead to increased amounts of chlorination at the expense of chlorosulfonylation. In turn, when sterically less demanding copper(I) photocatalysts are employed, the preference for chlorosulfonylation increases. For example, the 29:30 ratio increased from 3:1 to 9:1 with a sterically less demanding heteroleptic copper(I) complex having a bis(isonitrile) ligand and a phenanthrene ligand. Again, employing [Ru(bpy)]3Cl2 for the reaction of 28 exclusively leads to chlorinated 30. The unusual trifluoromethylchlorosulfonylation of alkenes allowed the combination of two photoredox-catalyzed processes in sequence (Scheme 6).16 Alkenes 31 were first converted to sulfonyl chlorides 32 catalyzed by [Cu(dap)2]Cl as described above. While this catalyst is not able to further activate the sulfonyl chloride by electron transfer, this was possible with fac[Ir(ppy)3], and the resulting sulfonyl radicals could be trapped by styrenes, giving rise to 33. Insightful studies by Greaney and co-workers further contributed to the discussion of whether radical chain pathways are operative in photoredox-catalyzed ATRA-type processes and at the same time disclosed very useful synthetic

double azidination giving rise to 34 is observed when the reaction is carried out in the dark, while under visible-light irradiation methoxy azides 37 are obtained.17 The results have been rationalized by oxidation of radical intermediate 38 (obtained by azide radical addition to styrene 36) under visiblelight irradiation to give cation 39, which is then trapped by methanol to afford 37. Since this oxidation step is not observed D


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Accounts of Chemical Research in the dark, it appears to be plausible that the oxidant is excited *[Cu(dap)2]2+, a species that has not been implicated in the photoredox cycle of the copper−dap system so far. The observed double azidination to afford 34 in the dark has been rationalized by arguing that radical 38 has a sufficient lifetime to abstract an azide radical from 35. However, in this scenario it is not obvious either how Cu(I) is regenerated to close the catalytic cycle, which is necessary to again generate an azide radical by electron transfer from Cu(I) to 35, or how a radical chain would be maintained. Mechanistically more straightforward appears to be the [Cu(dap)2]Cl-catalyzed azidination of benzylic C−H bonds disclosed by the same authors (Scheme 8).18 The role of

Scheme 9. Visible-Light-Mediated Photoredox Cyclization Catalyzed by in Situ-Generated Cu(dmp-Xantphos) Complex 6b

Scheme 8. Azidination of Benzylic C−H Bonds Catalyzed by [Cu(dap)2]Cl (5e)

The weak oxidation power of the Cu(II) → Cu(I) process in the [Cu(dap)2] system was also recently utilized for the selective generation of aryl-stabilized α-amino radicals.21 Even more intriguingly, Cu(I) complex 9 was found to be efficient in promoting the photooxidation of N-aryl tetraisoquinolines (Cu(I)/Cu(0) cycle).7 Clearly, besides being potent photoreductants, Cu(I) complexes are being increasingly recognized to promote visible-light-activated processes that involve oxidation as the key step. The benefit of heteroleptic copper(I) complexes was also demonstrated for visible-light-mediated allylations. While 5e proved to be efficient only in couplings with allyltributyltin,10 (phenanthroline)bis(isonitrile)copper(I) complex 8 (cf. Figure 1) also showed good activity for couplings with allylsilanes (Scheme 10).22 While 8 has a considerably longer excited-state lifetime (17 000 ns) and stronger reductive power (Cu(II)/ Cu(I)* = −1.88 V), the increased activity of 8 is nevertheless not convincingly rationalized on the basis of initial radical formation by electron transfer to the α-bromocarbonyl compounds 42 and bromide extrusion, since both complexes

[Cu(dap)2]Cl is seen here solely to initiate a radical chain by electron transfer to 35, which is possible also in the dark but proceeds in better yields when the reaction is carried out under visible-light irradiation. It it important to note that both reactions (Schemes 7 and 8) are not catalyzed by [Ru(bpy)3]Cl2 and Ir(ppy)3 and especially that the latter is a more potent reductant than [Cu(dap)2]Cl (5e) and thus should be clearly capable of promoting SET to 35 to initiate the reaction. The ease with which heteroleptic copper(I) complexes are formed allows the tuning of their steric and electronic properties, aiming at the development of strong photoreductants and -oxidants. Increasing the steric bulk of the ligands hinders excited-state structural relaxation and hence exciplex quenching, thus prolonging the excited-state lifetime of the complex as well as its photostability. Most prominent are mixed copper(I) phenanthroline phosphine complexes of type 6 (Figure 1), which were introduced by McMillin19 and have now been widely used especially for light-emitting diodes. More recently, such complexes have also found an exciting application in the organic synthesis disclosed by Collins and co-workers, allowing the synthesis of helicenes by photoredoxinitiated electrocyclic processes with unprecedented efficiency (Scheme 9).20 Notably, the key step is proposed to be the oxidation of 39 to 40, which can be initiated either by an iodine radical generated via reduction of iodine by photoexcited *Cu(I) or by the resulting Cu(II) species. As a further benefit, the possibility to generate the heteroleptic copper(I) complex 6b in situ in this process was recognized, and scale-up of the reaction was achieved by transferring the batch process to flow.

Scheme 10. Cu(I)-Catalyzed Photoallylation with Allyltributylstannane and Allylsilanes

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Accounts of Chemical Research 5e and 8 are capable photocatalysts for this step. Also not obvious is the formation of 44e-2 when crotyltrimethylsilane is used, which is not in line with a SN2′ mechanism but might point toward the formation of allyl radicals during the process. Alternatively, 1,3-transposition of the silyl group in the allyltrimethylsilane under the visible-light photoredox conditions might occur, in contrast to their stability for such processes under thermal conditions. Complementary to this, allylations of aryl radicals derived from diaryliodonium salts 46 with allyl tosylates 47 were reported by Fensterbank, Goddard, and Ollivier using [Cu(dpp)2]PF6 (5c) as the photocatalyst (Scheme 11).23

Scheme 12. Ir(III) Photoredox Generation of Vinyl Radicals and Coupling with Heteroarenes

Scheme 11. Allylation between Diaryliodonium Salts 46 and Allyl Tosylates 47 with [Cu(dpp)2]PF6 (5c) as the Photoredox Catalyst

Scheme 13. Ir(III) Photoredox Generation of Vinyl Radicals and Coupling with Alkenes

The allylations shown differ in an important detail: The couplings with allyltin and allylsilane (Scheme 10) were carried out in the absence of a sacrificial electron donor, which would require either propagation of the reaction by a radical chain mechanism or catalyst regeneration by the formation of trialkyltin or trialkylsilyl bromide with concurrent back electron transfer (Cu(II) to Cu(I)). In the reaction between diaryliodonium salts and allyl tosylates (Scheme 11), the stoichiometric amount of DIPEA is assumed to be necessary for the reduction of Cu(II) to Cu(I) in order to close the catalytic cycle. In the examples shown so far, the ability of Cu(I)* catalysts to initiate organic ATRA-like transformations proved to be equal or even superior to those of ruthenium- or iridium-based complexes as a result of the strong reducing power of the former to transfer an electron to a suitable organic substrate. However, the weak oxidation power of Cu(II) back to Cu(I) to regenerate the catalyst also appears to be an important factor that might be responsible for the success or failure of a given process. For example, α-bromocinnamates 49 can be efficiently coupled with heteroarenes or alkenes to afford tri- and tetracyclic scaffolds 51 by [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 (Scheme 12).24 In contrast, no reaction occurs in the presence of [Cu(dap)2]Cl (5e), although the latter is a much more potent photoreductant in the oxidative quenching cycle than the former (Cu(I)*/Cu(II) = −1.43 V vs Ir(III)*/Ir(IV) = −0.89 V). A possible explanation could be that the necessary oxidation of a radical intermediate along the reaction pathway to regenerate the catalyst is ineffective using 5e (Cu(II)/Cu(I) = +0.62 V) while this step is facile with [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 (Ir(IV)/Ir(III) = +1.69 V). Similarly, trapping of 50 with alkenes 53 resulted in either allylation to give 54, Heck-type coupling to form 55, or dihydronaphthalene annelation to afford 56 depending on the substitution pattern of 53 (Scheme 13). Again, only iridium photocatalysts were suitable to promote the reaction, while with [Cu(dap)2]Cl no reaction took place.25

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SUMMARY In summary, copper(I)−phenanthroline-based complexes have proved to be powerful visible-light photocatalysts for ATRA reactions and related processes. Besides their most notable feature, i.e., being strong photoreductants utilizing the transition from Cu(I)* to Cu(II), the possibility of photooxidation utilizing the Cu(II)* to Cu(I) or Cu(I)* to Cu(0) transition is emerging as well. The excited-state lifetime can be greatly prolonged from nano- to microseconds by moving from simple Cu(phenanthroline) 2 -type to heteroleptic Cu(phenanthroline)(phosphine) or Cu(phenanthroline)(bis(isonitrile)) complexes. Nevertheless, the former, most notably Sauvage’s complex [Cu(dap)2]Cl (5e), are also most capable of promoting ATRA and ATRA-like reactions. Overall, copper complexes seem to have developed into a class of photocatalysts complementing well-established ruthenium or iridium complexes not only from an economical point of view.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. F


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Accounts of Chemical Research Biography

(13) Bagal, D. B.; Kachkovskyi, G.; Knorn, M.; Rawner, T.; Bhanage, B. M.; Reiser, O. Trifluoromethylsulfonylation of Alkenes: Evidence for an Inner-Sphere Mechanism by a copper Phenanthroline Photoredox Catalyst. Angew. Chem., Int. Ed. 2015, 54, 6999−7002. (14) Oh, S. H.; Malpani, Y. R.; Ha, N.; Jung, Y.-S.; Han, S. B. Vicinal Difunctionalization of Alkenes: Chlorotrifluoromethylation with CF3SO2Cl by Photoredox Catalysis. Org. Lett. 2014, 16, 1310−1313. (15) Cismesia, M. A.; Yoon, T. P. Characterizing chain processes in visible light photoredox catalysis. Chem. Sci. 2015, 6, 5426−5434. (16) Pagire, S. K.; Paria, S.; Reiser, O. Synthesis of βHydroxysulfones from Sulfonyl Chlorides and Alkenes Utilizing Visible Light Photocatalytic Sequences. Org. Lett. 2016, 18, 2106− 2109. (17) Fumagalli, G.; Rabet, P. T. G.; Boyd, S.; Greaney, M. F. ThreeComponent Azidation of Styrene-Type Double Bonds: Light-Switchable Behavior of a Copper Photoredox Catalyst. Angew. Chem., Int. Ed. 2015, 54, 11481−11484. (18) Rabet, P. T. G.; Fumagalli, G.; Boyd, S.; Greaney, M. F. Benzylic C−H Azidation Using the Zhdankin Reagent and a Copper Photoredox Catalyst. Org. Lett. 2016, 18, 1646−1649. (19) Cuttell, D. G.; Kuang, S.-M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A. Simple Cu(I) Complexes with Unprecedented ExcitedState Lifetimes. J. Am. Chem. Soc. 2002, 124, 6−7. (20) Hernandez-Perez, A. C.; Vlassova, A.; Collins, S. K. Toward a Visible Light Meditated Photocyclization: Cu-Based Sensitizers for the Synthesis of [5]Helicene. Org. Lett. 2012, 14, 2988−2991. (21) Nicholls, T. P.; Constable, G. E.; Robertson, J. C.; Gardiner, M. G.; Bissember, A. C. Brønsted Acid Cocatalysis in Copper(I)Photocatalyzed α-Amino C−H Bond Functionalization. ACS Catal. 2016, 6, 451−457. (22) Knorn, M.; Rawner, T.; Czerwieniec, R.; Reiser, O. [Copper(phenanthroline) (bisisonitrile]+-Complexes for the Visible-LightMediated Atom Transfer Radical Addition and Allylation Reactions. ACS Catal. 2015, 5, 5186−5193. (23) Baralle, A.; Fensterbank, L.; Goddard, J. P.; Ollivier, C. Aryl Radical Formation by Copper(I) Photocatalyzed Reduction of Diaryliodonium Salts: NMR Evidence for a CuII/CuI Mechanism. Chem. - Eur. J. 2013, 19, 10809−10813. (24) Paria, S.; Reiser, O. Visible Light Photoredox Catalyzed Cascade Cyclizations of α-Bromochalcones or α-Bromocinnamates with Heteroarenes. Adv. Synth. Catal. 2014, 356, 557−562. (25) Paria, S.; Kais, V.; Reiser, O. Visible Light-Mediated Coupling of α-Bromochalcones with Alkenes. Adv. Synth. Catal. 2014, 356, 2853− 2858.

Oliver Reiser studied at the Universities of Hamburg and Jerusalem and at UCLA and received his Ph.D. under the supervision of Armin de Meijere. After postdoctoral reseach at IBM Research Center with Robert D. Miller and Harvard University with David A. Evans, he began his independent career at the University of Göttingen. After moving to Stuttgart University as an associate professor, he became professor of organic chemistry in 1997 at the University of Regensburg. His research interests center around synthesis and catalysis, including the development of new photo- and magnetic nanoparticle-supported catalysts.

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ACKNOWLEDGMENTS Most of all I thank my co-workers who have contributed to the projects from our group that are described in this Account as well as to related ones in which visible light was the key but copper would not deliver. Moreover, I had the pleasure to meet with many colleagues working in the field and benefited from their insightful discussions, which stimulated our work. Our work was financially supported by the DFG Graduiertenkolleg 1626 “Photocatalysis”, which is also gratefully acknowledged.

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REFERENCES

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Shining Light on Copper: Unique Opportunities for Visible-Light

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