Asymmetric Photocatalysis with Bis-cyclometalated Rhodium Complexes

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Asymmetric Photocatalysis with Bis-cyclometalated Rhodium Complexes Xiaoqiang Huang and Eric Meggers*

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Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein Straße 4, 35043 Marburg, Germany

CONSPECTUS: Aspects of sustainability are playing an increasingly important role for the development of new synthetic methods. In this context, the combination of asymmetric catalysis, which is considered one of the most economic strategies to generate nonracemic chiral compounds, and visible light as an abundant source of energy to induce or activate chemical reactions has recently gained much attention. Furthermore, the combination of photochemistry with asymmetric catalysis provides new opportunity for the development of mechanistically unique reaction schemes. However, the development of such asymmetric photocatalysis is very challenging and two main problems can be pinpointed to undesirable photochemical background reactions and to difficulties in controlling the stereochemistry with photochemically generated highly reactive intermediates. In this Account, we present and discuss asymmetric photocatalysis using one of the currently most versatile photoactivatable asymmetric catalysts, namely, reactive bis-cyclometalated rhodium(III) complexes. The catalysts contain two inert cyclometalating 5-(tert-butyl)-2-phenyl benzoxazole or benzothiazole ligands together with two labile acetonitriles, and the overall chirality is due to a stereogenic metal center. The bis-cyclometalated rhodium complexes serve as excellent chiral Lewis acids for substrates such as 2-acyl imidazoles and N-acyl pyrazoles, which, upon replacement of the two labile acetonitrile ligands, coordinate to the rhodium center in a 2-point fashion. These rhodium−substrate intermediates display unique photophysical and photochemical properties and are often the photoactive intermediates in the developed asymmetric photocatalysis reaction schemes. This combination of visible light excitation to generate long-lived photoexcited states and intrinsic Lewis acid reactivity opens the door for a multitude of visible-light-induced asymmetric conversions. In a first mode of reactivity, bis-cyclometalated rhodium complexes function as chiral Lewis acids to control asymmetric radical reactions of rhodium enolates with electron-deficient radicals, rhodium-coordinated enones with electron-rich radicals, or rhodium-bound radicals generated by photoinduced single electron transfer. The rhodium−substrate complexes in their ground states are key intermediates of the asymmetric catalysis, while separate photoredox cycles initiate radical generations via single electron transfer with either the rhodium−substrate complexes or additional photoactive compounds serving as the photoredox catalyst (secondary asymmetric photocatalysis). In a second mode of reactivity, the rhodium−substrate complexes serve as photoexcited intermediates within the asymmetric catalysis cycle (primary asymmetric photocatalysis) and undergo stereocontrolled chemistry either upon single electron transfer or by direct bond forming reactions out of the excited state. These multiple modes of intertwining photochemistry with asymmetric catalysis have been applied to asymmetric α- and βalkylations, α- and β-aminations, β-C−H functionalization of carbonyl compounds, [3 + 2] photocycloadditions between cyclopropanes and alkenes or alkynes, [2 + 2] photocycloadditions of enones with alkenes, dearomative [2 + 2] photocycloadditions, and [2 + 3] photocycloadditions of enones with vinyl azides. We anticipate that these reaction schemes of chiral bis-cyclometalated rhodium complexes as (photoactive) chiral Lewis acids will spur the development of new photocatalysts for visible-light-induced asymmetric catalysis.

1. INTRODUCTION

method to satisfy the growing demand for enantioenriched compounds in the chemical and pharmaceutical industries.5 However, combining photochemistry with asymmetric catalysis

Visible light has been recognized as a sustainable source of energy that facilitates a chemoselective molecular activation of chemical reactions, thereby providing ample opportunities for the design and discovery of novel chemical transformations.1−4 On the other hand, asymmetric catalysis represents an economic © XXXX American Chemical Society

Received: January 14, 2019

A

DOI: 10.1021/acs.accounts.9b00028 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Overview of bis-cyclometalated rhodium(III) complexes for asymmetric photocatalysis. (a) Design features. (b) Primary versus secondary asymmetric photocatalysis. Rh-sub = rhodium-bound substrate, Rh-int = rhodium-bound reactive intermediate, Rh-prod = rhodium-bound product; cat = catalyst, PC = additional photoredox catalyst.

followed by deprotonation). This design was inspired by the frequent use of inert bis-cyclometalated iridium complexes as standard photoredox catalysts. In contrast, bis-cyclometalated rhodium complexes were not reported as photocatalysts prior to our work. We were therefore initially surprised to discover that by just replacing iridium with rhodium, the resulting biscyclometalated rhodium complexes turned out to be extremely versatile asymmetric photocatalysts and for many applications superior to the iridium congener. The chiral rhodium catalysts contain two cyclometalating 5(tert-butyl)-2-phenylbenzo[d]oxazole (RhO)31 or 5-(tertbutyl)-2-phenylbenzo[d]thiazole (RhS)32 ligands or slight derivatives thereof, together with two labile acetonitriles and one hexafluorophosphate counteranion (Figure 1a).33 As a formal peculiarity, the metal represents the exclusive stereogenic center (only achiral ligands!)34,35 with the two cis-coordinating bidentate ligands providing helical chirality with a left-handed (Λ-enantiomer) or right-handed (Δ-enantiomer) screw sense. The stereogenic rhodium center also functions as the Lewis acid center, which, upon release of the two labile acetonitrile ligands, binds to the substrate in a bidentate fashion. Importantly, substrate binding to the rhodium catalyst not only chemically activates the substrate toward a chemical conversion but in many instances also provides the in situ assembled photoactive species for executing either primary or secondary asymmetric photocatalysis. We define secondary asymmetric photocatalysis as photochemical processes in which the photoactive species is not part of the asymmetric catalytic cycle (Figure 1b), whereas in primary asymmetric photocatalysis, the photoexcited catalyst− substrate complex is an integral part of the catalytic cycle. Primary asymmetric photocatalysis reveals some of the mechanistic advantages of single-catalyst photochemistry with sometimes unique excited state reactivity that can hardly be achieved by non-photochemical techniques, whereas in secondary asymmetric photocatalysis, the catalyst−substrate complex induces the photochemical formation of reactive intermediates, which then feed into the catalytic cycle. Alternatively, further increasing the utility of the rhodium photocatalysis system, the photochemical generation of the

poses significant challenges, which are in large parts associated with difficulties in suppressing photochemical background racemic reactions and achieving a high asymmetric induction with highly reactive intermediates such as radicals, radical ions, and photoexcited species.6−8 Most strategies in asymmetric photocatalysis are based on a dual catalysis9 approach in which an achiral photocatalyst is responsible for the photochemistry and a chiral catalyst enables the asymmetric bond formation chemistry. Remarkable progress has been made by combining photocatalysis with conventional asymmetric catalysts, such as enamine/iminium catalysis,10,11 Lewis acids,12 Brønsted acids,13,14 transition metals,15 Nheterocyclic carbenes,16 and enzymes.17 However, ideally, the visible light photochemistry and asymmetric catalysis is executed by a single catalyst.18−25 This is not only advantageous from an economic perspective but also opens new mechanistic scenarios in which the photoexcitation and asymmetric catalysis are more closely intertwined. Melchiorre and co-workers demonstrated that chiral secondary amines can perform asymmetric photocatalysis through intermediate chiral enamines (2013)18 or iminium ions (2017),19 which can be photoexcited directly or as part of electron donor−acceptor complexes and facilitate various interesting asymmetric photoreactions. In 2014, the Bach group introduced a chiral hydrogenbonding thioxanthone-based photosensitizer to trigger enantioselective [2 + 2] photocycloadditions.20 In 2016, Peters, Fu, and co-workers developed an elegant single copper/chiral phosphine ligand system for visible-light-induced asymmetric C−N cross couplings.21,22 In 2017, Xiao’s group devised an unusual thioxanthone-tailored chiral bisoxazoline ligand for the enantioselective hydroxylation of β-ketoesters with visiblelight-generated singlet oxygen.23 In 2014, our group contributed to the area of single-catalyst asymmetric photocatalysis by combining visible light induction and asymmetric catalysis with a single chiral transition metal catalyst, namely, bis-cyclometalated iridium complexes.26−30 In a key aspect of this asymmetric iridium photoredox catalysis, the visible light absorbing species is assembled in situ by coordination of the substrate to the iridium catalyst (optionally B


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Figure 2. Enantioselective α-functionalizations of ketones through radical addition to enolates. For simplicity, only Λ-configured catalysts shown. aCatalyzed by a tBu-modified iridium complex.

2. SECONDARY ASYMMETRIC PHOTOCATALYSIS IN WHICH RHODIUM−SUBSTRATE COMPLEXES REACT WITH PHOTOGENERATED REACTIVE INTERMEDIATES

reactive intermediates can instead be executed with an additional standard photocatalyst, thereby leading to conventional dual catalysis.

2.1. Rhodium Enolate Complexes React with Electrophilic Radicals

This Account summarizes the broad utility of bis-cyclometalated rhodium complexes for asymmetric photocatalysis,

In our initial studies on visible-light-induced asymmetric transition metal catalysis, we developed photoinduced enantioselective α-alkylations of 2-acyl imidazoles 1 using electrondeficient alkyl halides as radical precursors, catalyzed by single bis-cyclometalated iridium(III) complexes (Figure 2a).26−28 A computational study by Fernandez-Alvarez and Maseras36 and our mechanistic studies are consistent with a mechanism in which an in situ assembled iridium-bound enolate both acts as the photoactive species to reduce the electron-deficient alkyl halides by single electron transfer (SET) to initiate a chain reaction, and at the same time serves within the asymmetric catalysis cycle as the reaction partner for the electron-deficient carbon-centered radicals. Interestingly, when we attempted to apply this iridium-catalyzed enantioselective photoredox chemistry to the analogous enantioselective α-C−N bond formation

including asymmetric photoredox processes and stereocontrolled bond-forming reactions directly from photoexcited states. The advantages of the bis-cyclometalated rhodium over the homologous iridium system will be pointed out and can be typically traced back to much higher ligand exchange rates (higher reactivity, higher turnover frequencies) and sometimes to differences in the nature of the photoexcited state of catalyst− substrate complexes. C


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Figure 3. Proposed mechanism and steric model for the visible-light-induced rhodium-catalyzed asymmetric radical addition to enolates.

through nitrogen-centered radical intermediates, no product was observed. However, using instead the rhodium congeners as catalysts provided high yields and high enantioselectivities (Figure 2b). This initially very surprising observation was our entry point into asymmetric rhodium photoredox catalysis. Specifically, we first developed a Δ-RhO catalyzed enantioselective C−N bond formation employing (ODN)-N-functionalized carbamates 4 (ODN = 2,4-dinitrophenylsulfonyloxy) as amino radical source (Figure 2b).37 Later, we found that the related benzothiazole-derived complex (RhS) provided even better stereocontrol for the rhodium-catalyzed amination (up to >99% ee for 5a) compared to the benzoxazole-based congeners (RhO).32 This is in line with the corresponding iridiumcatalyzed alkylations and can be attributed to a higher steric congestion around the coordination sites of the benzothiazole compared to the benzoxazole complexes.38 We later expanded this chemistry to more attractive radical precursors, namely, aryl azides and α-diazo carboxylic esters 6, which feature the advantage of releasing just N2 as the sole byproduct upon reductive radical generation. In this regard, we developed a dual chiral RhS and [Ru(bpy)3](PF6)2 catalytic scheme to achieve stereocontrolled α-amination and αalkylation of 2-acyl imidazoles (Figure 2c).39,40 This work established that the rhodium complexes can also achieve synergistic effects with an additional photoredox catalyst, which provides additional options to merge chiral Lewis acid catalysis with visible light photochemistry. Mechanistically, the rhodium-catalyzed asymmetric radical addition to enolates is closely related to the iridium-catalyzed alkylation photoredox chemistry, which comprises the cooperation of a photoredox and an asymmetric catalysis cycle (Figure 3). Initially, substrate coordination and deprotonation deliver the key rhodium enolate intermediate II. Simultaneously, visible-light-induced SET reduction generates an electrondeficient radical. The addition of this radical to the electronrich enolate (intermediate II) forms a ketyl radical intermediate III. The subsequent SET oxidation of ketyl III provides the product-coordinated rhodium intermediate IV. Then, product release and recoordination of new substrate start a new catalytic cycle. The single electron donated by the ketyl intermediate III either engages in the photoredox cycle or facilitates the

reduction of radical precursors leading to radical chain propagation. In the case of the single rhodium-catalyzed αamination (Figure 2b), the rhodium enolate II serves not only as the key intermediate in the catalytic asymmetric cycle but also as photoactivated reductant to initiate and reinitiate an efficient chain process. This is supported by the UV/vis absorption spectra, which reveal greatly enhanced visible-light absorption of rhodium-bound enolate compared to the initial bis-acetonitrile catalyst, as well as a cyclic voltammogram study39 that demonstrated a significantly decreased oxidation potential after enolate formation. Alternatively, the photoactive species for the photoinduced redox cycle can be an additional photoredox catalyst in the case of dual catalysis (Figure 2c). Regarding the stereodetermining step, the asymmetric induction occurs in the course of the radical addition to the enolate intermediate II (Figure 3). X-ray crystal diffraction reveals that the Si face of the prochiral sp2 enolate carbon is sterically shielded by the tert-butyl groups of the C2 symmetric Λ-RhS.39 A computational study in collaboration with the Houk group further suggested that distortion of the rhodium-bound enolate and its cyclometalating ligand skeleton is the governing factor in enforcing enantioselectivity.41 Why are these reactions only working with the rhodium and not the related iridium complexes? Our mechanistic experiments suggest that the reason can be found in the 3−4 orders of magnitude faster ligand exchange rates in the rhodium system.37 If the ligand exchange is the rate limiting step in the catalytic cycle, the turnover frequencies of the rhodium cycle are by several orders of magnitude faster than the iridium system. High turnover frequencies are crucial for a catalytic cycle that is fed with highly reactive reaction intermediates with limited lifetimes, such as nitrogen-centered radicals, which are prone toward reduction.42 Over the past few years, we found many asymmetric photoredox reactions that only work with rhodium and not iridium, and we pinpoint this in large part to the much faster ligand exchange processes going along with much higher turnover frequencies of the rhodium catalysis. 2.2. Rhodium Enolate Complexes React with Iminium Ions

We reported a visible-light-induced Mannich-type asymmetric aminoalkylation of 2-acyl imidazoles with N-methyl anilines D


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Figure 4. Enantioselective α-aminoalkylation of ketones through trapping of metal enolates with photogenerated iminium ions.

(Figure 4).43 With air as terminal oxidant, a single chiral biscyclometalated rhodium complex, Λ-RhO, showed excellent efficiency for the dehydrogenative C−C bond formation of compound 9, while the IrO congener failed to catalyze the transformation. In this secondary photochemistry, we proposed that rhodium-bound enolates act as nucleophiles to react with oxidatively photogenerated iminium ions.44 This is supported by UV/vis absorption/emission spectra and cyclic voltammogram studies, which imply that the rhodium-bound 2-acyl imidazole can function as a visible-light-activated oxidant to initiate aerobic oxidation of the amine 8. Considering that the electronrich α-aminoalkyl radical is unfavorable to add to the electronrich enolate through the pathway described in Figure 3, a further SET oxidation is operative to give the electrophilic iminium ion intermediate. 2.3. Rhodium Enone Complexes React with Nucleophilic Radicals

Achieving highly enantioselective radical conjugate additions (Giese reactions)45 in a catalytic fashion is a formidable challenge owing to difficulties of suppressing the uncatalyzed racemic background reaction. Pioneering work by Sibi and coworkers showcased the potential of chiral Lewis acids for asymmetric induction in radical conjugate additions.46,47 Recently, we were pleased to discover that our bis-cyclometalated rhodium catalysts are surprisingly effective chiral Lewis acids to catalyze highly enantioselective radical conjugate additions at low catalyst loadings. In 2016, we reported an enantioselective alkyl radical addition to enones 10, specifically α,β-unsaturated 2-acyl imidazoles and α,β-unsaturated N-acyl pyrazoles, by combining Λ-RhS as chiral Lewis acid with an additional photoredox catalyst (Figure 5).48 We proposed that the photoredox catalyst had the task to catalyze the oxidative generation of alkyl radicals from organotrifluoroborates 11, while the rhodium complex served as the chiral Lewis acid to facilitate the stereocontrolled conjugate addition of the electronrich radical to the rhodium-bound enone (RhS-10, intermediate I). The resulting α-carbonyl radical intermediate II is then reduced to form a rhodium-bound enolate (intermediate III). The subsequent protonation and substrate/product exchange complete the asymmetric Lewis acid cycle. Considering the background reaction (without RhS, 29% yield for 12a), it is noteworthy that with only 4.0 mol % of chiral rhodium(III) catalyst, β-alkylated carbonyls 12 could be obtained in up to 97% yield and up to 99% ee. In stark contrast, the iridium congener was incompetent for this conversion. Furthermore, a competition experiment revealed that the rhodium coordination results in a strong acceleration effect on the benzylic radical addition to a typical enone substrate 10a by more than 3 × 104 times (Figure 5). This surprisingly strong acceleration of the radical conjugate addition well accounts for

Figure 5. Visible-light-activated chiral rhodium(III) catalyzed radical conjugate addition with trifluoroborates as alkyl radical precursors. aPC = [Ir(dFCF3ppy)2(bpy)]PF6; bPC = [Acr-Mes]ClO4.

the observed excellent stereoinduction in the present radical conjugate addition. Additionally, computational investigations in collaboration with the Wiest group support the experimental rate acceleration and confirm the role of the bis-cyclometalated rhodium(III) catalyst as a classic Lewis acid.49 Building on this dual chiral rhodium/photoredox catalysis method, we further developed a catalytic asymmetric C(sp3)−H bond alkylation by combining photoredox catalysis, radical translocation, and stereocontrolled radical conjugate addition (Figure 6a).50 In the key step, a 1,5-hydrogen atom transfer (1,5E


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Figure 6. Visible-light-activated chiral rhodium complexes catalyzing radical conjugate additions.

RhO(Ar), was required (Figure 6c).53 It is believed that in this system the rhodium-bound 2-acyl imidazole substrate serves as the in situ assembled photoredox mediator to initiate and reinitiate a radical chain process. Our group also verified that the photoinduced asymmetric radical conjugate addition can be executed with a single chiral rhodium catalyst. For example, we reported a photoinduced reductive decarboxylation of glycinederived N-acyloxy phthalimides 20 to generate α-aminoalkyl radicals for a subsequent asymmetric rhodium-catalyzed radical conjugate addition using Hantzsch ester as the terminal reducing agent (Figure 6d).54 This reaction provides access to biologically relevant enantioenriched γ-aminobutyric acid derivatives 21, which optionally bear fluorinated quaternary stereocenters. In related work, 4-alkyl Hantzsch esters were used as radical precursors for the rhodium-catalyzed enantioselective βalkylation of α,β-unsaturated 2-acyl imidazoles.55

HAT) of an oxygen-centered radical species, which is formed by fac-Ir(ppy)3 mediated SET reduction of N-alkoxyphthalimide 13 in the presence of Hantzsch ester as reductant, leads to the generation of a carbon-centered radical intermediate, which then adds to a rhodium-activated enone substrate in an asymmetric fashion. While this chemistry employs a transition-metal-based photoredox cocatalyst, we also demonstrated that the rhodium Lewis acid can be combined with 4,4′-difluorobenzil as an organic photoredox mediator. The visible-light-induced asymmetric three-component fluoroalkylation with perfluoroalkyl sulfinate 15, electron-rich vinyl ether 16, and α,β-unsaturated Nacyl pyrazole provided enantioenriched fluoroalkyl-containing compounds 17 with up to 98% ee (Figure 6b).51 Finally, under certain conditions, bis-cyclometalated rhodium chiral Lewis acids for radical conjugate additions can even be used in the absence of any additional photoredox cocatalyst. For example, the Kang group52 recently reported a visible-lightinduced asymmetric radical conjugate addition of tetrahydroisoquinoline-derived α-aminoalkyl radicals to α,β-unsaturated 2acyl imidazoles, for which only a single rhodium complex, Λ-

2.4. Rhodium−Substrate Complexes Engage in Redox Chemistry from Their Ground States

Lewis acid binding to a substrate activates its frontier molecular orbitals toward redox reactions. We exploited this wellestablished property of Lewis acids by triggering photoinduced F


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Figure 7. Visible-light-activated SET reduction of ground state rhodium−substrate complexes for asymmetric radical transformations.

intermediate carbamoyl radicals. Considering the electrophilic nature of these N-centered radicals, conjugate addition to electron-deficient enones is apparently unlikely, and rather a stereocontrolled coupling with electron-rich rhodium-bound βenolate radicals is supposed to be operative. It is worth pointing out that the β-alkylation with organotrifluoroborates discussed in Figure 5 is unlikely to undergo such a radical−radical recombination process, especially considering that the reduced photocatalyst [Acr-Mes]ClO4 (Ered 1/2 = −0.46 V vs SCE in CHCl3)59 is unable to reduce the rhodium-bound enones + (taking RhS-10b as an example,60 Ered 1/2 = −1.13 V vs Fc/Fc in CH2Cl2). On the contrast, SET reduction of Rh-bound αcarbonyl radical (int. II → int. III in Figure 5, for an analog of int. + III,39 Eox p = +0.078 V vs Fc/Fc in MeCN) is feasible to be mediated by the reduced [Arc-Mes]ClO4. In a third example, rhodium catalysis enabled the coupling of α,β-unsaturated N-acyl pyrazoles with allyl sulfones 27 using Hantzsch ester as reducing agent (Figure 7c).61 The reaction is proposed to proceed through a mechanism in which the rhodium-coordinated α,β-unsaturated N-acyl pyrazole is selectively reduced by photoexcited Hantzsch ester. The formed rhodium-stabilized enolate radical anion is subsequently trapped by an allyl sulfone to obtain radical β-allylation product 28 in up to 97% ee. In addition, the fragmented sulfonyl radical is utilized

single electron transfer to ground state rhodium-coordinated 2acyl imidazoles and N-acyl pyrazoles (secondary asymmetric photocatalysis). The hereby formed rhodium-bound substrate radical anion intermediates were then engaged in asymmetric radical transformations, with the metal-centered chirality of the rhodium Lewis acid controlling the stereochemistry of the bond forming reactions. In an initial example, we reported the visible-light-activated redox coupling of 2-acyl imidazoles 22 with α-silylmethylamines 23 under rhodium chiral Lewis acid and [Ru(bpy)3](PF6)2 photoredox dual catalysis to afford enantioenriched 1,2-amino alcohols 24 after desilylation (Figure 7a).56 In the proposed mechanism, [Ru(bpy)3]2+ serves as a photoinduced electron shuttle from the α-silylmethylamine to the rhodium-bound imidazolyl ketone. The intermediate rhodium-stabilized ketyl radical then undergoes a radical−radical coupling with an αaminomethyl radical. Subsequently, we demonstrated that the rhodium benzoxazole complex RhO was able to activate α,β-unsaturated 2-acyl imidazoles toward visible-light-activated stereocontrolled βaminations57 with carbamates (Figure 7b) and β-alkylations58 with N-alkyl benzamides. We proposed an iridium photoredox catalyst and a Brønsted base co-mediated proton-coupled electron transfer (PCET) pathway for the generation of G


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Figure 8. Visible-light-activated SET oxidation of excited Rh-bound enolates for asymmetric β-C−H functionalization. DABCO = 1,4diazabicyclo[2.2.2]octane.

In a first example, we reported an asymmetric β-C−H functionalization of 2-acyl imidazoles and 2-acylpyridines 30 with 1,2-dicarbonyls 31 (Figure 8).62 A simple catalytic scheme based on the single rhodium catalyst Δ-RhS(Ph) facilities the construction of two adjacent stereocenters with excellent stereoselectivities (up to >20:1 d.r., up to >99% ee). Mechanistically, an in situ formed rhodium-bound enolate (intermediate II) is proposed to be the main visible-lightabsorbing species, which after photoexcitation becomes highly reducing and triggers SET and proton transfer with dicarbonyls. The hereby formed β-enolate radical III recombines with the simultaneously generated ketyl radical IV to deliver the C−C bond formation intermediate V, which after protonation and ligand exchange gives the final product 32. Again, the rhodium complex defeats the related iridium analogue in this transformation (for an example, see 32a). In another example, we reported asymmetric [3 + 2] photocycloadditions with cyclopropyl imidazolyl ketones 33 (Figure 9).63 In this system, catalyst−substrate complexation not only lowers the LUMO of the cyclopropyl substrate but also generates a visible-light-excitable complex. For example, a representative intermediate, RhS-33a, features a greatly

to form enantioenriched sulfone 29, thereby minimizing waste generation. Cyclic voltammetry studies reveal a large decrease of the reduction potential by around 1 V upon the coordination of 10c to the rhodium(III) catalyst, which support the LUMOlowering activation for the highly selective SET reduction.

3. PRIMARY ASYMMETRIC PHOTOCATALYSIS IN WHICH PHOTOEXCITED RHODIUM−SUBSTRATE COMPLEXES ARE PART OF THE ASYMMETRIC CATALYTIC CYCLE 3.1. Redox Chemistry of Photoexcited Rhodium−Substrate Complexes

In the secondary asymmetric photocatalysis described in the previous section, the rhodium−substrate complexes underwent stereocontrolled chemistry from their electronic ground states, while the separated photoinduced redox processes were responsible for the generation of reactive reactants or enabling SET. Recently, we also developed reactions in which rhodium− substrate complexes undergo redox chemistry from their photoexcited states as part of the asymmetric catalytic cycle (primary asymmetric photocatalysis). H


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Figure 9. Visible-light-activated SET reduction of excited Rh-bound acyl cyclopropanes for [3 + 2] photocycloadditions. (a) Selected examples. (b) Cyclic voltammetry recorded in CH2Cl2 containing 0.1 M nBu4NPF6 at 22 ± 2 °C with a scan rate of 0.1 V/s. (c) Absorption spectra measured at room temperature in CH2Cl2 (33a, 0.2 mM; RhS-33a, 0.02 mM). Emission recorded at 77 K with excitation at 400 nm. DIPEA = N,Ndiisopropylethylamine.

Figure 10. Single rhodium-catalyzed direct visible-light-excited asymmetric [2 + 2] and [2 + 3] photocycloadditions. aNMR yield.

decreased reduction potential by 1.3 V (Ered p = −1.2 V vs Ag/ AgCl in CH2Cl2, Figure 9b) and can be excited by visible light

with an emission maximum at 507 nm (corresponding to 2.5 eV, Figure 9c). The photoexcited complex RhS-33a is a strong I


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Figure 11. Proposed mechanism for the [2 + 3] photocycloaddition and the different spin distributions between the excited 10d, RhS-10d, and IrS10d. The hexafluorophosphate counteranion and the other spin densities are omitted.

+ 2] photocycloadditions60 with alkenes and stereocontrolled [2 + 3] photocycloadditions64 with vinyl azides. A wide range of enantioenriched complicated cyclobutanes 36 and 1-pyrrolines 37 can be constructed in excellent diastereo- and enantioselectivities by using a simple reaction setup under very mild reaction conditions (no exclusion of air and moisture necessary). The ability of these methods to build two or three consecutive stereocenters or optionally all-carbon quaternary (spiro-) centers in a highly stereoselective way is quite unique. All these advantages render this catalytic scheme very promising for further synthetic applications. Importantly, the related iridium catalyst, IrS, enabled the transformations but without any enantioselectivity (36a and 37a). A mechanistic proposal for the [2 + 3] photocycloaddition, which is supported by experimental and computational studies, is shown in Figure 11. To begin with, rhodium-bound substrate (RhS-10 or intermediate I) forms in situ via a fast N,O-bidentate chelation. After direct visible-light excitation, the excited intermediate I* reacts directly with a vinyl azide to generate 1,4-diradical intermediate II, with the stereochemistry of this C−C bond formation controlled by the metal-centered chirality. Subsequent N2 exclusion gives 1,5-diradical species III. Then, stereospecific cyclization of intermediate III delivers the catalyst-bound product (intermediate IV). Finally, dissociation of the enantioenriched product and recoordination of an unreacted substrate initiates a new cycle. The key to the success of this scenario is the greatly enhanced visible-light absorption provided by catalyst−substrate complexation.65 For example, we determined that the molar extinction coefficient of RhS-10b at 400 nm is 169 times higher than that of the free substrate 10b.60 Hence, the chiral catalyst-bound substrate is selectively photoexcited, and all the subsequent

oxidant with a potential of +1.3 V vs Ag/AgCl in CH2Cl2. In the presence of a sacrificial electron donor as reduction initiator, the mild SET reduction of the visible-light-excited rhodium− cyclopropane complexes is able to induce the cyclopropyl ring opening to generate chiral rhodium-bound γ-enolate radical intermediates. The following stereocontrolled radical cyclization with alkenes or alkynes furnishes the enantioenriched cyclopentanes 34 and cyclopentenes 35, respectively. Excellent reaction outcomes with up to 99% yield, >20:1 d.r., and >99% ee were obtained with the single catalyst Δ-RhS at loadings of 2−8 mol %. 3.2. Stereoselective Direct Bond Formations of Photoexcited Rhodium−Substrate Complexes

Besides photoinduced SET1 and photosensitized energy transfer,2 electronically excited molecules can directly engage in bond formations without any preceding charge separation. Such primary photoreactions exploit the reactivity of photoexcited states, which are typically inaccessible by non-photochemical techniques. Furthermore, direct bond formation from photoexcited states avoids charge separation and drawbacks arising from the involvement of radical ion and free radical intermediates. However, performing such photoreactions in a catalytic and stereocontrolled fashion is underdeveloped. In this context, we introduced a simple and efficient asymmetric catalysis scheme that only relies on a single chiral catalyst, RhS, for stereocontrolled bond forming reactions directly from visible-light-excited states (Figure 10). Specifically, complexation of α,β-unsaturated carbonyl 10 with the chiral rhodium catalyst forms the key catalyst−substrate complex, RhS-10, which features greatly enhanced visible light absorption. Selective photoexcitation followed by stereoselective direct bond formation can lead to stereocontrolled [2 J


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Figure 12. Catalytic asymmetric dearomatization by visible-light-activated [2 + 2] photocycloaddition. Yields of combined regioisomers are provided. r.r. = regioisomeric ratio.

cycloadditions with internal alkenes gave products 40 with four stereocenters in perfect regioselectivities and diastereoselectivities (>20:1 r.r. and >20:1 d.r.). Interestingly, diastereoselectivities of photocycloadditions with different geometrical isomers of alkenes were proved to be temperature-dependent (40a,b), which supports the involvement of 1,4-diradical intermediates.

reactive intermediates are bound to the stereogenic rhodium center, which suppresses most of the background reactions. One interesting question remained, namely, why the iridium congener fails to provide any asymmetric induction. Computational investigations64 suggest that the spin density of excited RhS-10d is mainly localized at the CC carbons of the coordinated substrate, while in the triplet state of IrS-10d, the iridium center possesses the majority of the spin density (Figure 11). This distinct difference in the nature of the excited states could explain the different reactivity. Photoexcited IrS− substrate intermediate is unable to undergo a direct reaction with a cosubstrate but instead serves as a photosensitizer to transfer energy to the free substrate, thereby resulting in the formation of only racemic products. We further applied this strategy to the catalytic asymmetric dearomatization by a visible-light-activated [2 + 2] photocycloaddition of benzofurans and one example of a benzothiophene, which bears an N-acyl pyrazolyl moiety for catalyst coordination (Figure 12).66 A series of tricyclic structures were synthesized by a single chiral rhodium catalyst in good to excellent stereoselectivities (39, 40, >20:1 d.r., up to 99% ee). It is worth mentioning that an unusual head-to-tail regioselectivity was observed (39a), which can be ascribed to the energy differences of the corresponding 1,4-diradicals. As the calculated molecular modeling indicated, the 1,4-diradical that leads to the major head-to-tail adduct is more stable owing to the double stabilizing effect provided by the neighboring carbonyl group and the O atom. Furthermore, we found that photo-

4. SUMMARY OF REACTION MECHANISMS A summary of all discussed reaction mechanism is shown in Figure 13. The rhodium−substrate complexes, typically formed by N,O-bidentate coordination of 2-acyl imidazoles or N-acyl pyrazoles to the bis-cyclometalated rhodium catalyst, are at the center of the individual mechanisms. Diverse chemistry of these rhodium−substrate complexes ranges from ground state Lewis acid catalysis involving photochemically generated free radicals and SET processes involving ground state or photoexcited rhodium−substrate complexes to stereocontrolled direct bond formations of photoexcited rhodium−substrate complexes. The discussed catalyst−substrate intermediates can be divided into three classes, namely, rhodium enolate, rhodium enone, and rhodium ketone complexes. Rhodium enolates are very electronrich and react from their electronic ground state efficiently with photochemically generated electron-deficient radicals (pathway a)37,39,40 or electrophiles (b),43,44 achieving asymmetric αfunctionalizations of carbonyl compounds. Alternatively, upon visible light irradiation, photoexcited rhodium enolates serve as K


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Figure 13. Summary of the multiple reaction modes of the intermediate bis-cyclometalated rhodium−substrate complexes.

5. CONCLUSIONS AND OUTLOOK This Account provided an overview of the multiple options to combine visible light photochemistry with asymmetric catalysis using bis-cyclometalated rhodium catalysts as (photoactivatable) chiral Lewis acids, offering one of the currently most versatile catalytic systems for asymmetric photocatalysis. Whereas our initial work on visible-light-induced asymmetric transition metal catalysis dealt with the bis-cyclometalated iridium catalyst, we later discovered that the lighter congener rhodium is superior for many applications. This was initially surprising since according to our knowledge, rhodium complexes have not been used before in the context of photoredox catalysis. For many of our asymmetric photoreactions the advantage of rhodium over iridium can be pinpointed to much higher turnover frequencies of the catalytic cycle due to faster ligand exchange reactions in the rhodium system, which is crucial when interfacing asymmetric catalysis with short-lived radical or radical ion intermediates. Furthermore, the ligand-centered photoexcitation of rhodium-bound enone substrates allows for asymmetric [2 + 2] and [2 + 3] photocycloadditions, which has not been achieved with the related iridium catalysts. We are very interested in further expanding this mode of reactivity of photoexcited catalyst− substrate complexes. A current limitation that needs to be addressed in future work has to do with a requirement for 2-acyl imidazoles or N-acyl pyrazoles as substrates, a consequence of the preference of the rhodium Lewis acid to bind substrates in a bidentate fashion. It is interesting to note that certain reactions only work with imidazoles and others only with pyrazoles. Thus, beyond the requirement for 2-point binding, additional electronic factors in

strong reducing agents, for example for the reductive (re)initiation of radical chain processes, and this can be intertwined with the ground state reactivity of the rhodium enolates toward electron-deficient radicals addition (c).37 Photoexcited rhodium enolates can also be intermediates in the asymmetric catalysis cycle by triggering a SET followed by a proton transfer to generate intermediate rhodium enolate β-radicals, which result in an asymmetric β-C−H functionalization (d).62 Rhodium coordination to α,β-unsaturated 2-acyl imidazoles and α,βunsaturated N-acyl pyrazoles removes electron density from the double bond and results in a surprisingly strong acceleration of nucleophilic radical conjugate additions, thereby providing a tool for designing catalytic asymmetric Giese-type radical conjugate additions at low catalyst loadings (e).48−51,53−55 Enone rhodium coordination also lowers the LUMO of the coordinated enone and thereby enables selective photoinduced SET reduction to generate rhodium-bound β-enolate radical species (f).57,58,61 Alternatively, photoexcited rhodium enones can serve as photooxidants to oxidatively generate free radicals, which can be combined with rhodium-catalyzed radical conjugate additions (g).53,55 Beyond photoredox chemistry, visible-light-excited rhodium enones are capable of directly undergoing stereocontrolled bond forming reactions with alkenes and vinyl azides (h).60,64,66 Finally, ground state rhodium ketone intermediates can be reduced to ketyl intermediates (i).56 Using cyclopropyl ketones, the direct photoexcitation generates a photooxidant and upon single electron reduction provides rhodium-bound γ-enolate radical intermediates which react in [3 + 2] photocycloadditions with alkenes and even alkynes (j).63 L


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the ground and photoexcited state of the formed rhodium− substrate intermediates apparently play an important role for certain reactions. With an eye toward synthetic applications, it needs to be stressed that the imidazole and pyrazole moieties serve as auxiliaries, which can be removed in postfunctionalization reactions. While 2-acyl imidazoles require relatively harsh conditions (stoichiometric amount of trimethylsilyl trifluoromethansulfonate and base), N-acyl pyrazoles are highly attractive auxiliaries that can be converted to alcohols or other carbonyl functionalities under very mild conditions. We hope that our work on intertwining visible light photochemistry with asymmetric catalysis will further stimulate the development of novel asymmetric photocatalysis reaction schemes and the design of novel light-activatable asymmetric catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: + 49 6421 2821534. ORCID

Xiaoqiang Huang: 0000-0002-0927-4812 Eric Meggers: 0000-0002-8851-7623 Notes

The authors declare no competing financial interest. Biographies Xiaoqiang Huang was born in 1991 in Fujian, P. R. China. He received a Bachelor degree (2013) and a Master degree (2015) under the supervision of Prof. Ning Jiao at Peking University. He then joined the Meggers group at the University of Marburg and obtained a Ph.D. degree in 2019. His research interests focus on combining photochemistry and electrochemistry with asymmetric catalysis. Eric Meggers received a Diploma in Chemistry from the University of Bonn (Germany) and a Ph.D. degree in Organic Chemistry from the University of Basel (Switzerland). After postdoctoral research at the Scripps Research Institute (La Jolla, CA, USA), he started his independent career as an Assistant Professor at the University of Pennsylvania (USA). Since 2007, Eric Meggers is Full Professor in the Department of Chemistry at the University of Marburg (Germany). He held a secondary appointment as Professor at the College of Chemistry and Chemical Engineering of Xiamen University (P. R. China) from 2012 to 2016. His laboratory currently focuses on the development of chiral-at-metal complexes for asymmetric catalysis.

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ACKNOWLEDGMENTS The authors thank present and past laboratory members and collaborators for their invaluable contributions to this project. We gratefully acknowledge funding for this project from the German Research Foundation (ME1805/13-1).

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REFERENCES

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Asymmetric Photocatalysis with Bis-cyclometalated Rhodium Complexes

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