Copper(II)-Catalyzed Asymmetric Photoredox Reactions:Enantioselective Alkylation of Imines Driven by Visible LightYanjun Li, Kexu Zhou, Zhaorui Wen, Shi Cao, Xiang Shen, Meng Lei, and Lei Gong*
Key Laboratory of Chemical Biology of Fujian Province, iChEM, College of Chemistry and Chemical Engineering, XiamenUniversity, Xiamen, Fujian 361005, China
*S Supporting Information
ABSTRACT: Asymmetric photoredox catalysis offers excitingopportunities to develop new synthetic approaches to chiralmolecules through novel reaction pathways. Employing thefirst-row transition metal complexes as the chiral photoredoxcatalysts remains, however, a formidable challenge, althoughthese complexes are economic, environmentally friendly, andoften exhibit special reactivities. We report in this Article thedevelopment of one class of highly efficient asymmetric/photoredox bifunctional catalysts based on the copper(II)bisoxazoline complexes (CuII−BOX) for the light-inducedenantioselective alkylation of imines. The reactions proceedunder very mild conditions and without a need for any other photosensitizer. The simple catalytic system and readily tunablechiral ligands enable a significantly high level of enantioselectivity for the formation of chiral amine products bearing atetrasubstituted carbon stereocenter (36 examples, up to 98% ee). Overall, the CuII−BOX catalysts initiate the radicalgeneration, and also govern the subsequent stereoselective transformations. This strategy utilizing chiral complexes comprisedof a first-row transition metal and a flexible chiral ligand as the asymmetric photoredox catalysts provides an effective platformfor the development of green asymmetric synthetic methods.
■ INTRODUCTION
Visible-light photoredox catalysis has been developed into apowerful tool to construct carbon−carbon or carbon−heteroatom bonds in organic synthesis.1 Through high-energyintermediates such as radicals and radical ions, uniquereactions that are unavailable under thermal conditions canbe accessed. Significant advances have been achieved in thisfield by employing ruthenium(II), iridium(III) complexes, ororganic dyes as photoredox catalysts.2
In contrast, the use of first-row transition metal complexessuch as copper species has been much less frequentlyreported.3−7 Beside the aspects of relatively shorter excited-state lifetimes and weaker visible-light absorption, an inherentdrawback of copper complexes as photoredox catalysts is thatthe low reduction potentials of CuII → CuI might impede theclosure of a photocatalytic cycle.4 Very recently, an appealingstrategy involving light-accelerated homolysis (CuII−X →CuII−Y → CuI + Y•) has been developed to address thisproblem, and opens new avenues for copper-based photo-catalysis.8
On the other hand, asymmetric catalysis promoted by visiblelight is emerging as an attractive synthetic strategy for chiralorganic molecules. However, achieving a high level ofenantioselectivity in the photoredox reactions remains aremarkable challenge.9 In the limited success that has beenreported, the solutions typically rely on the dual catalysisinvolving a ruthenium or iridium-based photocatalyst and a
chiral cocatalyst, which often contains one or more preciousmetals.10 Bifunctional catalysis employing a chiral-at-iridium orrhodium complex as both the asymmetric catalyst and thephotocatalyst has also been developed by Meggers and othergroups recently.11,12 Although use of a single chiral complex ofa first-row transition metal as the asymmetric photocatalystwould be economically attractive and environmentally friendly,it has been scarcely explored.13−15 An impressive pioneeringwork by Fu et al. disclosed light-promoted enantioselective C−N cross-couplings by a copper(I) catalyst containing a chiralphosphine ligand.13 Even more recently, a nickel(II)-DBFOX-catalyzed enantioselective photoredox reaction of α,β-unsatu-rated carbonyl compounds and α-silylamines was developed inour laboratory.14 These studies revealed that chiral complexesof first-row transition metal exhibit special features andreactivities in photochemical reactions beyond the role asinexpensive alternatives to ruthenium or iridium-based photo-catalysts. Therefore, the development of new chiral photo-catalysts or applications based on the first-row transitionmetals would be of great value and in high demand.Herein, we disclose the development of one class of
copper(II) bisoxazoline complexes (CuII−BOX) as asymmet-ric/photoredox bifunctional catalysts in the light-inducedenantioselective alkylation of imines (Scheme 1). Without
Received: August 28, 2018Published: October 16, 2018
Article
pubs.acs.org/JACSCite This: J. Am. Chem. Soc. 2018, 140, 15850−15858
the need of any other photosensitizer and under very mildconditions, the copper-catalyzed photochemical reactionsafforded a range of chiral amine products containing atetrasubstituted carbon stereocenter with good to excellentenantioselectivities.
■ RESULTS AND DISCUSSION
Initial Experiments. In our primary studies on catalystselection, we found that several chiral copper(II) bisoxazolinecomplexes (CuII−BOX) exhibited obvious absorption in thevisible-light region. In particular, absorption in the range of400−550 nm, attributed to metal−ligand-charge-transfer(MLCT), was one of the key features required for a visible-light photocatalyst. In combination with their readily tunablechiral environment, redox-active metal center, and theestablished photoactivities of their reduced states−copper(I)species,4 we assumed that these chiral copper(II) complexesmight be potential candidates for the development ofasymmetric photoredox catalysis based on the first-rowtransition metals.Accordingly, we developed a model catalytic system using in
situ generated copper(II) bisoxazoline complexes as thecatalyst, benzyl trifluoroborate 2a as the radical precursor,and an α-carbonyl imine 1a as the coupling partner.16 The α-carbonyl adjacent to the CN bond of the imine substratewas thought to be able to act as a directing group for theasymmetric induction.17 Such a photochemical reaction wouldlead to the development of an inexpensive method for radical-based enantioselective alkylation of imines under mild andconvenient conditions.18
In an initial experiment, the mixture of substrate 1a, 2a,premixed copper salt Cu(BF4)2·H2O (10 mol %), and ligandL1 (11 mol %) in chloroform was stirred at 25 °C in argonunder irradiation with a 24 W blue LEDs lamp. The desiredproduct (3a) was produced in quantitative yield and with 74%ee (Table 1, entry 1). In the absence of the blue LEDs lamp,stirring in the dark at room temperature (entry 2) or 80 °C(entry 3), removing the ligand (entry 4), or the copper salt(entry 5), the reaction failed to proceed. Addition of anotherphotosensitizer such as [Ru(bpy)3](PF6)2 dramatically accel-erated the reaction (2 h, 100% conversion, entries 6 and 7),implying a light-induced pathway. Water was well-tolerated inthe system (entry 8), while protic additive such as methanolled to the lower reaction rate and slightly reducedenantioselectivity (entry 9).
Other metal salts were also tested in this reaction, and it wasfound that the cuprous salt Cu(MeCN)4BF4 provided similarresults (entry 11), while Ni(OTf)2 and Fe(OTf)3 failed ingenerating the product (entries 12 and 13). Ligand screeningexperiments showed that the chiral BOX ligands with differentsubstituents R1, R2, and R3 remarkably affected the conversionand enantioselectivity (entries 14−19). For example, L2 (R1 =iPr) only provided trace amounts of the product, while L3 (R1
= Bn) resulted in 37% conversion and 50% ee (entries 14 and15). Ligands L4−6 with two side chains (R2, R3) gave a slightlyimproved the enantioselectivity (entries 16−18). Such side-chain effects of chiral BOX ligands have been intensivelyinvestigated by Tang et al. and others.19 According to theseresults, a sterically more demanding ligand (L7) withadamantyl-modification was designed and used in the reaction.This gave the best result, a quantitative conversion in 6 h, with85% ee (entry 19). Other oxazoline-type ligands L8−10 led toboth lower reaction rates and lower enantioselectivities (entries20−22). The chiral diphosphine ligand (L11) failed to deliverany product (entry 23). In all cases, the catalytic outcomeswere strongly dependent on the light source. The reactionunder irradiation with a 30 W red LEDs lamp did not proceed(entry 24), while the same process with a 15 W UV lightprovided lower conversion (entry 25). Ultimately, theenantioselectivity was further improved to 89% ee byincreasing the concentration of 1a to 0.030 M and reducingthe temperature to −20 °C (entries 26−29).
Substrate Scope. With the best catalyst and reactionconditions in hand, we next evaluated the reaction scope. Arange of alkyl trifluoroborates bearing substituents withdifferent electronic and steric properties were examined(Scheme 2). Primary benzyl trifluoroborates with electron-withdrawing substituents on the phenyl ring gave products3a−i with better enantioselectivity (87−94% ee), while thosewith an electron-donating substituent resulted in products 3k,lwith reduced ee values. Halogen and ester substituents werewell tolerated, offering the opportunity for further functional-ization of the chiral sultam products. Naphthalen-2-ylmethyltrifluoroborate (product 3m) and thiophen-3-ylmethyl tri-fluoroborate (product 3n) were also compatible under thestandard conditions. Moreover, a secondary benzyl trifluor-oborate provided product 3o as a 1:1 diastereomeric mixturewith moderate ee values (59% and 59% ee). The twodiastereomers could be readily separated by silica gelchromatography. A tertiary alkyl trifluoroborate also afforded
Scheme 1. Strategy of Using a Single CuII−BOX Complex as the Chiral Photoredox Bifunctional Catalyst
the product (3p) in a good yield but with a lowenantioselectivity of 24% ee.N-Sulfonylimines containing a distinct α-carbonyl group
adjacent to the imino moiety (products 3q−s), or those withan electron-donating substituent (products 3t, 3u), anelectron-withdrawing substituent within the benzenesulfonylring (products 3v−x), or a fused naphthalenesulfonyl moiety(products 3y, 3z), were all tested (Scheme 3). The chiralsultams containing a tetrasubstituted carbon stereocenter20
were obtained in 92−98% yield and with 82−94% ee. Thereaction of a six-membered N-sulfonylimine also proceeded
smoothly, producing 3za in 95% yield and with 80% ee.However, replacement of the α-carbonyl group next to theimine moiety by a methyl group led to zero product formation(product 3zb).Other α-carbonyl imines were examined in this system in an
attempt to evaluate the generality of the method. To our mostinterest, acyclic imines such as isatin-derived ketimines workedwell under the standard conditions, and ligand L9 wasidentified as the best ligand (Scheme 4). These reactionstypically needed an extended irradiation time (32−47 h) andproduced the chiral 3-amino-3-alkyl oxindole (5a−h), a
Table 1. Initial Experiments and Optimization of Reaction Conditionsa
entry metal salt ligand T (°C) light source t (h) conv. (%)b ee (%)c
1 Cu(BF4)2·H2O L1 25 blue LEDs 6 quant. 742 Cu(BF4)2·H2O L1 25 none 12 0 n.a.3 Cu(BF4)2·H2O L1 80 none 12 0 n.a.4 Cu(BF4)2·H2O none 25 blue LEDs 12 0 n.a.5 none L1 25 blue LEDs 12 0 n.a.6d Cu(BF4)2·H2O L1 25 blue LEDs 2 quant. 737d none none 25 blue LEDs 12 22 n.a.8e Cu(BF4)2·H2O L1 25 blue LEDs 6 quant. 739f Cu(BF4)2·H2O L1 25 blue LEDs 15 93 6810 Cu(OTf)2 L1 25 blue LEDs 6 92 6911 Cu(MeCN)4BF4 L1 25 blue LEDs 6 quant. 7012 Ni(OTf)2 L1 25 blue LEDs 21 0 n.a.13 Fe(OTf)3 L1 25 blue LEDs 21 0 n.a.14 Cu(BF4)2·H2O L2 25 blue LEDs 6 <5 n.d.15 Cu(BF4)2·H2O L3 25 blue LEDs 6 37 5016 Cu(BF4)2·H2O L4 25 blue LEDs 6 76 7717 Cu(BF4)2·H2O L5 25 blue LEDs 6 quant. 7818 Cu(BF4)2·H2O L6 25 blue LEDs 6 quant. 8419 Cu(BF4)2·H2O L7 25 blue LEDs 6 quant. 8520 Cu(BF4)2·H2O L8 25 blue LEDs 6 52 6221 Cu(BF4)2·H2O L9 25 blue LEDs 6 19 −5722 Cu(BF4)2·H2O L10 25 blue LEDs 6 <5 n.d.23 Cu(BF4)2·H2O L11 25 blue LEDs 6 0 n.a.24 Cu(BF4)2·H2O L7 25 red LEDs 6 0 n.a.25 Cu(BF4)2·H2O L7 25 UV lamps 6 33 8426g Cu(BF4)2·H2O L7 25 blue LEDs 3 quant. 8527g Cu(BF4)2·H2O L7 0 blue LEDs 3 quant. 8728g Cu(BF4)2·H2O L7 −20 blue LEDs 12 quant. 8929g Cu(BF4)2·H2O L7 −40 blue LEDs 21 99 89
aReaction conditions: 1a (0.10 mmol), 2a (0.15 mmol), metal salt (10 mol %), ligand (11 mol %), CHCl3 (5 mL), indicated temperature,indicated light source, under argon; see more details for the screening of solvent and light source in the Supporting Information. bConversiondetermined by 1H NMR. cThe ee value determined by chiral HPLC. dIn the presence of 2 mol % [Ru(bpy)3](PF6)2.
eIn the presence of 10 equivof H2O.
fIn the presence of 10 equiv of MeOH. gReaction performed with a concentration of 0.030 M (based on 1a). n.d. = not determined, n.a. =not applicable.
structure commonly found in nature products,21 with higherenantiomer excess (96−98% ee).Probing the Radical Pathway. Several control experi-
ments were conducted in an effort to confirm the radicalpathway (Scheme 5). First, introduction of air to the copper-catalyzed reaction 1a + 2d → 3d was found to completelyinhibit the transformation to 3d, instead affording an aldehydeside-product (6a) in 52% yield (Scheme 5a). Addition ofexcess of the radical quencher 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) resulted in a radical coupling product 6b in62% yield (Scheme 5b). Interference with a radical acceptorethyl 2-((phenylsulfonyl)methyl)acrylate led to the formation
of a mixture of 3d (31%), 6c (44%), and a self-couplingproduct 6d (5%) (Scheme 5c). These outcomes stronglyindicate that an alkyl radical is involved in the catalytic process.The reaction 1a + 2d → 3d was monitored by electronparamagnetic resonance spectroscopy (EPR) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the correspond-ing HRMS analysis gave results that further strengthen theconjecture (Scheme 5d).
UV−Vis Spectra. UV−vis spectra were recorded toevaluate the light absorption of the reaction components(Figure 1). The individual substrates 1a, 2a, chiral ligand L7,and copper salts Cu(BF4)2·H2O and Cu(MeCN)4BF4 showedno obvious absorption in the visible-light region. However,complexes CuII−L7, CuII−L7−1a, CuI−L7, and CuI−L7−1a,which potentially exist in the catalytic system, exhibitedsignificant absorption enhancement in the range of 400−550nm. The broad signals at 600−800 nm in the UV/visabsorption spectra of CuII−L7 and CuII−L7−1a are attributedto d−d transitions of copper(II).22 These results demonstratedthat the combination of the copper with the BOX ligand leadsto the formation of species potentially active in visible light.
Cyclic Voltammetry Analysis. The cyclic voltammetry ofsubstrates 1a, 2a, chiral ligand L7, and copper complex CuI−L7 was measured under argon to estimate their redox ability.As illustrated in Figure 2, the N-sulfonylimine (1a) exhibited areversible reduction/oxidation peak at E1/2 = −0.49 V(Ered(1a/1a
•−)), while the alkyl trifluoroborate 2a showed anirreversible oxidation peak at +1.34 V (Eox(2a
•+/2a)). Thechiral ligand (L7) did not provide any obvious reduction/oxidation signals, revealing that the ligand itself was redox-inertin the catalytic cycles. Moreover, reversible reduction/oxidation signals were observed at E1/2 = +0.81 V for thecopper complex CuI−L7 (Ered (Cu
II−L7/CuI−L7)), suggest-ing that direct oxidation of 2a by the copper(II) complex isthermodynamically unfavorable. Therefore, it was assumedthat there must be some other pathway to achieve single-electron oxidation of the alkyl trifluoroborate to generate thecorresponding alkyl radical.
Scheme 2. Reaction Scope of Alkyl Trifluoroboratesa
aReaction 1a + 2o → 3o was performed at 25 °C due to the lowreaction rate.
Scheme 3. Reaction Scope of N-Sulfonylimines
Scheme 4. Application of the Reaction to EnantioselectiveBenzylation of Isatin-Derived Ketimines
Investigations of Single-Electron Oxidation of AlkylTrifluoroborates by CuII. Inspired by a recent report fromRehbein, Reiser et al.,8 we assumed that a ligand exchange/light-accelerated homolysis of the copper(II) species might beinvolved in the oxidation process. Accordingly, the chiralcopper catalyst CuII−L undergoes fast ligand exchange withalkyl trifluoroborate, generating the alkyl copper(II) inter-mediate, perhaps through the hydroxide complex. Underirradiation with visible light, the alkyl copper(II) intermediatewas excited. Ligand−metal-charge-transfer (LMCT) andhomolysis led to the formation of the CuI species and alkylradical (Scheme 6).To confirm this speculation, we carefully analyzed the
catalytic system. First, a crystal form of the copper hydroxidedimeric complex was obtained by mixing Cu(BF4)2·H2O withBOX ligand in a 1:1 ratio in chloroform. This demonstratedthe ligand exchange process perhaps going through the similarhydroxide intermediate (Figure 3). In addition, the putative
alkyl copper(II) intermediate was observed in the HRMSanalysis of a mixture of Cu(BF4)2·H2O, ligand L7, and alkyltrifluoroborate 2d in chloroform.A model reaction of alkyl trifluoroborate 2d and the catalyst
CuII−L7 in chloroform under irradiation of a blue LEDs lampprovided the self-coupling product 6d in 24% yield. Thereaction failed to proceed in the absence of the ligand or light,revealing that oxidation of alkyl trifluoroborates to thecorresponding alkyl radicals must go through a light-inducedpathway. In addition, a time-course UV−vis absorptionanalysis showed that the broad signal at 600−800 nmattributed to d−d transitions of CuII was significantly reducedunder irradiation with a blue LEDs lamp for only 0.5 h,suggesting consumption of the copper(II) species in thephotochemical reaction (Scheme 7).
Mechanistic Proposal. With this better understanding ofthe key step for the light-initiated radical generation, wepropose a plausible mechanism of the copper(II)-catalyzedasymmetric photochemical reaction (Scheme 8). CatalystCuII−L oxidizes the trifluoroborate substrate 2 to the radicalA through a ligand exchange/light accelerated homolysisprocess. On the other hand, imine substrate 1 or 4 undergoesfast ligand exchange with CuII−L and affords the intermediatecomplex B. The nucleophilic alkyl radical A proceeds withradical addition to the CN double bond of complex B in anenantioselective fashion and transformation to radical C.23
Such copper(II)-stabilized N radical species have beenreported by the group of Liu.24 Reduction of C by CuI−Laffords monocationic complex D, followed by protonation andligand exchange to release chiral product 3 or 5 andregenerated intermediate complex B. The effective asymmetricinduction can be explained by radical attack from the stericallyless hindered side of the copper-coordinated imine (Figure4).25,26 The proposed transition state is consistent with anobserved R-configuration in the products 3a−za. Overall, thecopper catalyst serves as a bifunctional catalyst, which isinvolved in both the photoredox process and the asymmetriccatalysis.
Scheme 5. Control Experiments To Probe the Existence ofthe Alkyl Radical
Figure 1. UV−vis spectra of the individual substrates 1a, 2a, chiralligand L7, copper salts Cu(BF4)2·H2O, Cu(MeCN)4BF4, coppercomplexes CuI−L7 (generated in situ by stirring a 1:1 mixture ofCu(MeCN)4BF4 and chiral ligand L7), CuI−L7−1a (generated insitu by stirring a 1:1:1 mixture of Cu(MeCN)4BF4, chiral ligand L7,and substrate 1a), CuII−L7 (generated in situ by stirring a 1:1mixture of Cu(BF4)2·H2O and chiral ligand L7), and CuII−L7−1a(generated in situ by stirring a 1:1:1 mixture of Cu(BF4)2·H2O, chiralligand L7, and substrate 1a). All of the samples were prepared as a 3.0mM solution and used fresh for the measurement.
Alternative pathways to initiate the radical generation canalso be considered. For example, pentacoordinated copper(II)bisoxazline complexes are also well-known.27 It is possible thata pentacoordinated copper(II) complex of the type L−CuII(imine)−R forms, which undergoes light-acceleratedhomolysis to afford R radical followed by attack on thecoordinated imine. In the radical transformation process, theradical chain propagation pathway can be excluded by thequantum yield estimation (Φ = 0.27).Transformation of the Products. Finally, we investigated
the further conversion of the products (Scheme 9). Forexample, product 3i (90% ee) can be readily converted into apolycyclic benzosultam compound 7 with retention of theenantiomeric excess through a copper-catalyzed intramolecularcross coupling of the C−N bond. The crystal structure of 7revealed its absolute configuration to be R. Reduction ofproduct 3g (94% ee) followed by treatment with trichlor-omethyl carbonochloridate and trimethylamine in THF led tothe formation of compound 8 in 62% yield and with 93% ee.
These polycyclic chiral sultams are important building blockspresent in many bioactive compounds and syntheticintermediates for some nature product synthesis.28
■ CONCLUSIONSIn summary, we have developed a highly efficient asymmetricphotoredox catalysis based on readily available copper(II)-
Figure 2. Cyclic voltammograms of 1a, 2a, L7, and CuI−L7 (generated in situ by stirring a 1:1 mixture of Cu(MeCN)4BF4 and chiral ligand L7) inMeCN (0.030 M) containing 0.1 M nBu4NBF4. Scan rate: 0.1 V/s. 1a, E1/2 = −0.49 V; 2a, Ep = +1.34 V; CuI−L7, E1/2 = +0.81 V.
Scheme 6. Proposed Pathway for Light-Induced Single-Electron Oxidation of the Alkyl Trifluoroborates by theCopper(II) Catalyst
Figure 3. ORTEP drawing of a CuII−BOX−OH dimeric complex[L6-Cu(H2O)μ-(OH)]2(BF4)2 with 50% probability thermal ellip-soids.
BOX complexes. The copper catalysts were employed in theenantioselective alkylation of imines driven by visible light andexhibited high catalytic efficiency for both the photoactivation
and the asymmetric induction. The simple catalytic system andfine-tunable chiral ligands enabled a significantly high level ofenantioselectivity for the formation of a range of chiral aminescontaining a tetrasubstituted carbon stereocenter (36 exam-ples, up to 98% ee). The mechanistic studies revealed that aligand exchange/light-accelerated homolysis pathway might beengaged to overcome the low oxidizability of the Cu(II)complexes. The strategy of utilizing chiral complexescomprised of a first-row transition metal and flexible chiralligand as bifunctional chiral photocatalysts provides aneffective platform for the development of green asymmetricsynthetic methods. Further investigations on reaction mecha-nisms and applications are in progress in the laboratory.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.8b09251.
X-ray crystallographic data for copper complex [L6-Cu(H2O)μ-(OH)]2(BF4)2 (CIF)X-ray crystallographic data for compound 7 (CIF)Experimental procedures, compound characterizationdata, NMR spectra, chiral HPLC traces, and crystallo-graphic data (PDF)
■ ACKNOWLEDGMENTSWe gratefully acknowledge funding from the National NaturalScience Foundation of China (grant nos. 21572184,21472154), the Natural Science Foundation of Fujian Provinceof China (grant no. 2017J06006), and the FundamentalResearch Funds for the Central Universities (grant no.20720160027). We thank Ms. Dandan Chen at XiamenUniversity for her assistance in geometry optimization of thetransition state.
■ REFERENCES(1) For selected reviews on visible-light photoredox catalysis, see:(a) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011,40, 102−113. (b) Xuan, J.; Xiao, W.-J. Angew. Chem., Int. Ed. 2012,51, 6828−6838. (c) Shi, L.; Xia, W. Chem. Soc. Rev. 2012, 41, 7687−
Scheme 7. Evidence for Light-Induced Oxidization of AlkylTrifluoroborate by CuII
Scheme 8. A Proposed Reaction Mechanism
Figure 4. A proposed transition state simulated by CYLview 1.0.
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