Merging Photoredox and Organometallic Catalysts in

Angewandte

Zuschriften Heterogeneous Catalysis Very Important Paper

Chemie

Deutsche Ausgabe: DOI: 10.1002/ange.201809493 Internationale Ausgabe: DOI: 10.1002/anie.201809493

Merging Photoredox and Organometallic Catalysts in a Metal–Organic Framework Significantly Boosts Photocatalytic Activities Yuan-Yuan Zhu+, Guangxu Lan+, Yingjie Fan, Samuel S. Veroneau, Yang Song, Daniel Micheroni, and Wenbin Lin* Abstract: Metal–organic frameworks (MOFs) have been extensively used for single-site catalysis and light harvesting, but their application in multicomponent photocatalysis is unexplored. We report here the successful incorporation of an IrIII photoredox catalyst and a NiII cross-coupling catalyst into a stable Zr12 MOF, Zr12-Ir-Ni, to efficiently catalyze C@S bond formation between various aryl iodides and thiols. The proximity of the IrIII and NiII catalytic components to each other (ca. 0.6 nm) in Zr12-Ir-Ni greatly facilitates electron and thiol radical transfers from Ir to Ni centers to reach a turnover number of 38 500, an order of magnitude higher than that of its homogeneous counterpart. This work highlights the opportunity in merging photoredox and organometallic catalysts in MOFs to effect challenging organic transformations.

transition-metal catalysts in MOFs and their periodic arrangements facilitate the transfer of electrons and radicals to accelerate the catalytic cycle; 2) site isolation of transitionmetal catalysts prevent bimolecular deactivation to stabilize the photocatalytic systems; 3) pore confinement stabilizes photoredox catalysts in MOFs by a rebound mechanism; 4) MOF photocatalysts can be readily recycled and reused. Herein we report the first attempt at merging photoredox and organometallic dual catalysts in a MOF and its application in visible light driven cross-coupling between aryl iodides and thiols (Figure 1).

Metal–organic frameworks (MOFs) are crystalline porous

materials built from metal ions or clusters bridged by organic linkers.[1] MOFs have been employed to study light harvesting[2] and single-site catalysis.[3] Progress has also been made in incorporating photosensitizing components and catalytically active functionalities into MOFs to afford photocatalytic systems for H2 and O2 evolution and CO2 reduction, key half reactions for artificial photosynthesis.[4] However, few MOFs have been developed to catalyze organic photoreactions,[5] and all of them are based on single components that serve the role of photosensitization. Considering the success of MOFs in artificial photosynthesis,[2, 3] we posited that MOFs might provide a highly tunable platform to combine photoredox and transition-metal catalysts to drive multicomponent organic photoreactions.[6] Such MOF photocatalysts should provide several benefits: 1) high local concentrations (0.1–1m) of photoredox and [+]

[+]

[*] Prof. Y.-Y. Zhu, G. Lan, Y. Fan, S. S. Veroneau, Y. Song, D. Micheroni, Prof. W. Lin Department of Chemistry, the University of Chicago 929 E 57th Street, Chicago, IL 60637 (USA) E-mail: [email protected] Prof. Y.-Y. Zhu[+] School of Chemistry and Chemical Engineering Hefei University of Technology Hefei 230009 (China) Y. Fan College of Chemistry and Molecular Engineering Peking University Beijing 100871 (China) [+] These authors contributed equally to this work. Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201809493.

14286

Figure 1. Merging the photoredox Ir catalyst and the cross-coupling Ni catalyst in a Zr12 MOF enhances photocatalytic activities.

The dual catalytic MOF Zr12-Ir-Ni, containing the [IrIII(dF(CF3)ppy)2(DBB)]Cl photosensitizer [H2L; where dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine and DBB = 4,4’-di(4-benzoato)-2,2’-bipyridine] and (DBB)NiCl2 precatalyst, was synthesized by a combination of pre-functionalization and postsynthetic modification. First, treatment of [IrIII(dF(CF3)ppy)2]2Cl2 with Me2DBB afforded ([IrIII(dF(CF3)ppy)2(Me2DBB)]Cl) (Me2L), which was hydrolyzed under mild basic conditions to yield the ligand H2L (see the Supporting Information for synthetic details). A Zr12-type MOF, Zr12-Ir, was subsequently synthesized by a solvothermal reaction between ZrCl4 and mixed ligands of H2DBB and H2L in N,N-dimethylformamide (DMF) at 80 8C using acetic acid

T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. 2018, 130, 14286 –14290

Angewandte

Zuschriften as the modulator. 1H NMR analysis of the digested Zr12-Ir gave a DBB to L ratio of approximately 1:1 (see Figure S1 in the Supporting Information), affording a framework formula of Zr12(m3-O)8(m3-OH)8(m2-OH)6(DBB)4.5[IrIII(dF(CF3)ppy)2(DBB)]4.5. Transmission electron microscopy (TEM) imaging showed that Zr12-Ir adopted hexagonal plate morphology (Figure 2 a). High-resolution TEM (HRTEM) imaging and fast Fourier transform (FFT) of Zr12-Ir nanoplates revealed a sixfold symmetry (Figure 2 b), consistent with the projection of the Zr12-Ir structure in the (001) direction. Powder X-ray diffraction (PXRD) studies indicated that Zr12-Ir is isostructural with recently reported Hf12-DBP (Figure 2 d).[7] Postsynthetic metalation of Zr12-Ir with NiCl2·6 H2O afforded Zr12-Ir-Ni whose PXRD pattern and TEM image were unchanged from Zr12-Ir (Figure 2 c,d). The Ni content in Zr12-Ir-Ni MOF was determined to be 4.43 Ni per Zr12 SBU by inductively coupled plasma-mass spectrometry (ICP-MS), indicating complete metalation of DBB ligands. Thermogravimetric analysis also supported the complete coordination of DBB ligands with NiCl2 in Zr12-Ir-Ni (see Figure S2). The porous structures of Zr12-Ir and Zr12-Ir-Ni were evaluated by

Chemie

nitrogen sorption isotherms at 77 K (Figure 2 f) with Brunauer-Emmett-Teller (BET) surface areas of 580.6 and 472.3 m2 g@1, respectively. The extended X-ray absorption fine structure (EXAFS) spectrum of Zr12-Ir-Ni was well fitted with the model complex (bpy)NiCl2, indicating a tetrahedral coordination environment for the NiII centers in Zr12-Ir-Ni (Figure 2 e; see Figure S7). The prevalence of C@S bonds in organic chemicals and pharmaceuticals has spurred continuous interest in developing efficient methods for C@S bond formation.[8] In 2016, Oderinde et al. and Jouffroy et al. reported photoredox and Ni dual catalysts for cross-couplings of aryl iodides and thiophenols.[9] We decided to test the applicability of Zr12-IrNi as a photoredox and Ni dual catalyst in this type of crosscoupling reaction and to gain fundamental insights into designing multifunctional MOFs for photocatalytic reactions. Upon irradiation with blue LEDs (5 mW cm@2, 410 nm), stirring a 1.0 m acetonitrile solution of 4-iodobenzotrifluoride (1 a) and 4-methoxylthiophenol (2 a) in the presence of 0.02 mol % Zr12-Ir-Ni and 2,6-lutidine (2 equiv) at room temperature for 48 hours afforded (4-methoxyphenyl)(4-(trifluoromethyl)phenyl)sulfane (3 a) in 91 % yield (Table 1, Table 1: Cross-coupling of aryl iodide and thiophenol using Zr12-Ni-Ir as a photoredox/Ni dual catalyst.

Entry 1 2 3 4 5

Conv. [%][e]

Reaction conditions [a]

Zr12-Ir-Ni (0.02 mol %) Zr12-Ir-Ni (0.002 mol %)[a] C1 (0.002 mol %)[b] C2 (0.002 mol %)[c] C3 (0.002 mol %)[d]

91 45 5.7 2.2 < 0.1

TON 4550 22 500 2850 1100 < 50

[a] Standard MOF conditions: aryl iodide (0.5 mmol), thiophenol (0.75 mmol), 2,6-lutidine (1.0 mmol), Zr12-Ir-Ni (0.25 mg, 0.02 mol % loading or 0.025 mg, 0.002 mol % loading) in MeCN (0.5 mL, 1.0 m aryl iodide concentration). [b] Homogeneous control 1: C1 = (dtbppy)NiCl2 and Me2L instead of Zr12-Ir-Ni. [c] Homogeneous control 2: C2 = (dtbppy)NiCl2 and Zr12-Ir-QPDC instead of Zr12-Ir-Ni. [d] Heterogeneous control: C3 = Zr6-Ni-QPDC-NO2 and Zr12-Ir-QPDC instead of Zr12-Ir-Ni. [e] Conversions were determined by GC-MS.

Figure 2. a) TEM image and b) HRTEM image and FFT pattern (inset) of Zr12-Ir. c) TEM image of Zr12-Ir-Ni. d) PXRD patterns of Hf12-DBP, Zr12-Ir, Zr12-Ir-Ni, and Zr12-Ir-Ni after photocatalysis. e) EXAFS spectra and the fits in R-space at the Ni K-edge of Zr12-Ir-Ni showing the magnitude (solid lines) and real component (hollow squares) of the Fourier transform. f) Nitrogen sorption isotherms of Zr12-Ir and Zr12-Ir-Ni at 77 K. Angew. Chem. 2018, 130, 14286 –14290

entry 1), which corresponds to a turnover number (TON) of 4550. By lowering the Zr12-Ir-Ni loading to 0.002 mol %, a more impressive TON of 22 500 was obtained (entry 2). Control experiments verified that the nickel complex, photosensitizer, and light are all indispensable for this reaction (see Table S2). In addition, 2,6-lutidine and pyridine gave the best results among a series of organic bases examined (see Table S2). Acetonitrile was the best solvent among several polar solvents tested (see Table S2), whereas nonpolar solvents afforded only trace amounts of 3 a. We performed two homogeneous control reactions under identical reaction conditions (1.0 m aryl iodide and 0.002 mol % catalyst loading) to probe the advantages of this MOF-based dual catalyst. First, 3 a was obtained in only 5.7 % conversion when (dtbbpy)NiCl2 (dtbbpy = 4,4’-di-tert-


www.angewandte.de

14287

Zuschriften butyl-2,2’-bipyridine) and Me2L (C1) were used in place of Zr12-Ir-Ni (Table 1, entry 3). The corresponding TON of 2850 is almost eight times lower than that of Zr12-Ir-Ni. Second, when (dtbbpy)NiCl2 and Zr12-Ir-QPDC were used as the catalysts (C2), the conversion was only 2.2 % (TON = 1100), about 20 times lower than that of Zr12-Ir-Ni. Additionally, a heterogeneous control reaction catalyzed by a physical mixture of Zr6-Ni-QPDC-NO2 and Zr12-Ir-QPDC was conducted and only a trace amount of the product was detected with a TON of less than 50. These results provide preliminary evidence for the synergy between IrIII and NiII centers in Zr12Ir-Ni, likely by enhanced electron and radical transfers as well as increased stability of Ir and Ni catalysts in the MOF. Timedependent conversions confirmed that Zr12-Ir-Ni was about eight times more active than C1 at 0.02 mol % catalyst loading throughout the 16 hour reaction (Figure 3 a). These results clearly demonstrated the advantage of Zr12-Ir-Ni over its homogeneous counterparts in photocatalytic cross-coupling reactions.

Angewandte

Chemie

Table 2: Substrate scope of Zr12-Ni-Ir catalyzed cross-coupling between aryl iodides and thiols.

[a] 2,6-Lutidine, RT. [b] Pyridine, 55 8C. [c] Yield of isolated product. [d] TON.

Figure 3. a) Plots of conversion vs. time at 0.02 mol % catalyst loading of Zr12-Ir-Ni and C1. The conversions were determined by GC-MS. b) Plots of yield of 3 f obtained by using recovered Zr12-Ir-Ni (0.02 mol % catalyst loading) for six consecutive runs. c) Emission spectra of Zr12-Ir (2 W 10@5 m, MeCN) after the addition of different amounts of 2 a with 365 nm excitation. d) CVs of Me2L in 0.1 m TBAH/ MeCN and [Ni(Me2DBB)]Cl2 in 0.1 m TBAH/DMF on a glassy carbon disk working electrode with a Pt counter electrode and Ag wire quasireference. Potential sweep rate was 100 mVs@1. e) Proposed mechanism for the C@S cross-coupling reaction catalyzed by Zr12-Ir-Ni.

14288

www.angewandte.de

We then investigated the substrate scope of this MOFcatalyzed cross-coupling reaction (Table 2). In the presence of 0.02 mol % Zr12-Ir-Ni, reactions of diverse aryl iodides containing various functional groups, including trifluoromethyl, cyano, formyl, acetyl, and carbomethoxy groups, and three different aryl thiols bearing hydrogen, methyl, and methoxy groups afforded the corresponding coupling products in excellent yields (86–96 %) and with high TONs (4300–4800). Upon gentle heating at 55 8C, aryl iodides containing a methyl group, thiols bearing benzyl and cyclohexyl groups, and a cysteine derivative all underwent cross-coupling reactions at a 0.02 mol % Zr12-Ir-Ni loading to afford the desired products (3 p–ac) in good to excellent yields (60–99 %) and TONs (3000–4950). Impressively, for the most reactive substrates 1 b (R1 = CN) and 2 c (R2 = phenyl), the cross-coupled product 3 f was obtained in 77 % yield at 0.002 mol % loading of Zr12-IrNi, corresponding to an impressive TON of 38 500. Under the same reaction conditions, C1 afforded 3 f in 7.4 % yield with a TON of 3700. Zr12-Ni-Ir could be recovered and reused for


Angew. Chem. 2018, 130, 14286 –14290

Angewandte

Zuschriften at least five cycles for the synthesis of 3 f without loss of catalytic activity (Figure 3 b). The MOF recovered from the photoreaction remained crystalline as shown by PXRD (Figure 2 d) with less than 0.5 % leaching of Ir and nondetectable leaching of Ni. Simple mixing Zr12-Ir and Ni(NO3)2 in solution generated the dual catalyst in situ which afforded the desired cross-coupling product in an excellent yield. Finally, we probed the reaction mechanism of Zr12-Ir-Ni catalyzed C@S bond formation by photophysical and electrochemical measurements. We first determined if electron transfer between excited IrIII complex ([IrIII]*) in Zr12-Ir and either the thiophenol or aryl iodide could occur by performing luminescence quenching and cyclic voltammetric (CV) experiments. [IrIII]* Luminescence of Zr12-Ir was quenched by both 1 a and 2 a, which was fit to the Stern-Vçlmer equation to afford a KSV(1 a) of 0.101 : 0.003 and a KSV(2 a) of 0.365 : 0.012 (see Figure S56; Figure 3 c). 2 a is thus almost four times as effective as 1 a in quenching the [IrIII]* luminescence. The combination of luminescence and CV data indicated that the [IrIII]* species with a redox potential E0’ of + 1.27 V vs. SCE (Table S5) could oxidize 2 a (EP = 0.50 V vs. SCE with lutidine) but not 1 a (EP > 1.8 V vs. SCE; see Figures S59 and S60). These results suggest that the initial step of the photocatalysis involves the reductive quenching of [IrIII]* by 2 a to generate the thiophenol radical. 1 a likely quenched [IrIII]* luminescence by energy transfer. Luminescence of the [IrIII]* species in Me2L was also effectively quenched by (Me2DBB)NiCl2, affording a KSV(1 a) value of 0.743 : 0.008 (see Figure S58). CVs of (Me2DBB)NiCl2 showed four reduction peaks at @0.93, @1.28, @1.66, and @1.78 V vs. SCE in DMF (RNi,1, RNi,2, RNi,3 and RNi,4), corresponding to the NiII/I, NiI/0, Me2DBB0/C@ , and Ni0/0C@ couples, respectively (Figure 3 d).[10] Me2L showed three pairs of peaks at + 1.78, @1.03, and @1.40 V (vs. SCE in MeCN), corresponding to the IrIII/IV, DBB0/@, and dF(CF3)ppy@/@2 redox couples, respectively.[11] The irreversible broad peak at + 1.12 V likely originated from the oxidation of Me2DBB.[10b] The reversible reduction peak RIr,3 corresponding to DBB0/@ at @1.10 V is thus located between RNi,1 (@0.93 V) and RNi,2 (@1.28 V), indicating IrIII(DBB)@ can reduce NiII to NiI but cannot reduce NiI further to Ni0. These results show that the reduction of NiII to NiI by IrIII(DBB)@ is thermodynamically and kinetically feasible in the catalytic cycle. Based on the photophysical and electrochemical data, we propose the reaction mechanism in Figure 3 e for the dual catalysis by Zr12-Ir-Ni. This mechanism is similar to that proposed by Oderinde et al. for the homogeneous system.[9a] In this catalytic process, the Ir cycle and Ni cycle are dependent on each other through both electron and radical transfers. In Zr12-Ir-Ni, the Ir and Ni catalytic centers are in close proximity, about 0.6 nm from each other, which facilitates both electron and thiophenol radical transfers from Ir to Ni centers to significantly boost its photocatalytic activity. Given recent exciting developments on catalyzing C@C, C@O, and C@N cross-coupling reactions using homogeneous photoredox and transition-metal dual catalysts,[12] we expect that MOFs can provide a tunable platform to enhance the efficiency of these catalytic systems. Angew. Chem. 2018, 130, 14286 –14290

Chemie

In summary, we have developed a facile method to incorporate Ir and Ni dual catalysts into the framework of a robust and porous Zr12 MOF. This MOF-based dual photocatalyst exhibited an order of magnitude higher activities than its homogenous counterparts for C@S crosscoupling between aryl iodides and thiols, reaching a TON of 38500. Preorganization of single-site Ir photoredox and Ni cross-coupling catalysts in close proximity facilities electron and thiophenol radical transfers from Ir to NiII centers to speed up the catalytic cycle whereas site isolation of Ni catalysts and cavity confinement of Ir photoredox catalysts in the MOF likely increase the stability of this dual catalyst system. This work demonstrates for the first time the ability to merge photoredox and organometallic catalysts in MOFs to boost catalytic activities and highlights the opportunity in designing multifunctional MOF photocatalysts for challenging organic transformations.

Acknowledgements We acknowledge funding from the U.S. National Science Foundation (CHE-1464941). Y.Y.Z. acknowledges the financial support from National Natural Science Foundation (21771049) and China Scholarship Council. XAS analysis was performed at Beamline 10-BM, Advanced Photon Source (APS), Argonne National Laboratory (ANL). Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. We acknowledge Mr. Wenbo Han and Mr. Zhe Li for experimental assistance.

Conflict of interest The authors declare no conflict of interest. Keywords: heterogeneous catalysis · metal– organic frameworks · nickel · photochemistry · sulfides How to cite: Angew. Chem. Int. Ed. 2018, 57, 14090 – 14094 Angew. Chem. 2018, 130, 14286 – 14290 [1] a) H. Furukawa, K. E. Cordova, M. OQKeeffe, O. M. Yaghi, Science 2013, 341, 1230444; b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T. H. Bae, J. R. Long, Chem. Rev. 2012, 112, 724 – 781; c) J. P. Zhang, Y. B. Zhang, J. B. Lin, X. M. Chen, Chem. Rev. 2012, 112, 1001 – 1033; d) S. Kitagawa, R. Matsuda, Coord. Chem. Rev. 2007, 251, 2490 – 2509; e) L. Q. Ma, C. Abney, W. B. Lin, Chem. Soc. Rev. 2009, 38, 1248 – 1256; f) Y. J. Cui, B. Li, H. J. He, W. Zhou, B. L. Chen, G. D. Qian, Acc. Chem. Res. 2016, 49, 483 – 493; g) P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Ferey, R. E. Morris, C. Serre, Chem. Rev. 2012, 112, 1232 – 1268; h) D. Bradshaw, J. B. Claridge, E. J. Cussen, T. J. Prior, M. J. Rosseinsky, Acc. Chem. Res. 2005, 38, 273 – 282. [2] a) C. A. Kent, D. Liu, L. Ma, J. M. Papanikolas, T. J. Meyer, W. Lin, J. Am. Chem. Soc. 2011, 133, 12940 – 12943; b) C. A. Kent, D. Liu, T. J. Meyer, W. Lin, J. Am. Chem. Soc. 2012, 134, 3991 –


www.angewandte.de

14289

Angewandte

Zuschriften [3]

[4]

[5] [6]

3994; c) M. C. So, G. P. Wiederrecht, J. E. Mondloch, J. T. Hupp, O. K. Farha, Chem. Commun. 2015, 51, 3501 – 3510. a) M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 2012, 112, 1196 – 1231; b) M. Zhao, S. Ou, C.-D. Wu, Acc. Chem. Res. 2014, 47, 1199 – 1207; c) J. A. Johnson, B. M. Petersen, A. Kormos, E. Echeverr&a, Y.-S. Chen, J. Zhang, J. Am. Chem. Soc. 2016, 138, 10293 – 10298; d) T. Zhang, K. Manna, W. Lin, J. Am. Chem. Soc. 2016, 138, 3241 – 3249; e) K. Manna, P. Ji, F. X. Greene, W. Lin, J. Am. Chem. Soc. 2016, 138, 7488 – 7491. a) A. Fateeva, P. A. Chater, C. P. Ireland, A. A. Tahir, Y. Z. Khimyak, P. V. Wiper, J. R. Darwent, M. J. Rosseinsky, Angew. Chem. Int. Ed. 2012, 51, 7440 – 7444; Angew. Chem. 2012, 124, 7558 – 7562; b) Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu, Z. Li, Angew. Chem. Int. Ed. 2012, 51, 3364 – 3367; Angew. Chem. 2012, 124, 3420 – 3423; c) C. Wang, K. E. deKrafft, W. Lin, J. Am. Chem. Soc. 2012, 134, 7211 – 7214; d) C. Wang, J.-L. Wang, W. Lin, J. Am. Chem. Soc. 2012, 134, 19895 – 19908; e) X. J. Kong, Z. Lin, Z. M. Zhang, T. Zhang, W. Lin, Angew. Chem. Int. Ed. 2016, 55, 6411 – 6416; Angew. Chem. 2016, 128, 6521 – 6526. a) C. Wang, Z. Xie, K. E. deKrafft, W. Lin, J. Am. Chem. Soc. 2011, 133, 13445 – 13454; b) X. Yu, S. M. Cohen, J. Am. Chem. Soc. 2016, 138, 12320 – 12323. a) J. Xuan, W.-J. Xiao, Angew. Chem. Int. Ed. 2012, 51, 6828 – 6838; Angew. Chem. 2012, 124, 6934 – 6944; b) Y. Zheng, L. Lin, B. Wang, X. Wang, Angew. Chem. Int. Ed. 2015, 54, 12868 – 12884; Angew. Chem. 2015, 127, 13060 – 13077; c) J. Xie, H. Jin, A. S. K. Hashmi, Chem. Soc. Rev. 2017, 46, 5193 – 5203.

14290 www.angewandte.de

Chemie

[7] K. Lu, C. He, N. Guo, C. Chan, K. Ni, G. Lan, H. Tang, C. Pelizzari, Y.-X. Fu, M. T. Spiotto, Nat. Biomed. Eng. 2018, 2, 600 – 610. [8] a) I. P. Beletskaya, V. P. Ananikov, Chem. Rev. 2011, 111, 1596 – 1636; b) B. Liu, C.-H. Lim, G. M. Miyake, J. Am. Chem. Soc. 2017, 139, 13616 – 13619; c) M. Jiang, H. Li, H. Yang, H. Fu, Angew. Chem. Int. Ed. 2017, 56, 874 – 879; Angew. Chem. 2017, 129, 892 – 897. [9] a) M. S. Oderinde, M. Frenette, D. W. Robbins, B. Aquila, J. W. Johannes, J. Am. Chem. Soc. 2016, 138, 1760 – 1763; b) M. Jouffroy, C. B. Kelly, G. A. Molander, Org. Lett. 2016, 18, 876 – 879. [10] a) C. Amatore, A. Jutand, Organometallics 1988, 7, 2203 – 2214; b) H. G. Roth, N. A. Romero, D. A. Nicewicz, Synlett 2016, 27, 714 – 723. [11] M. S. Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, G. G. Malliaras, S. Bernhard, Chem. Mater. 2005, 17, 5712 – 5719. [12] a) J. C. Tellis, D. N. Primer, G. A. Molander, Science 2014, 345, 433 – 436; b) Z. Zuo, D. T. Ahneman, L. Chu, J. A. Terrett, A. G. Doyle, D. W. MacMillan, Science 2014, 345, 437 – 440; c) K. L. Skubi, T. R. Blum, T. P. Yoon, Chem. Rev. 2016, 116, 10035 – 10074; d) J. Twilton, C. Le, P. Zhang, M. H. Shaw, R. W. Evans, D. W. C. MacMillan, Nat. Rev. Chem. 2017, 1, 0052. Manuscript received: August 16, 2018 Accepted manuscript online: August 20, 2018 Version of record online: October 8, 2018


Angew. Chem. 2018, 130, 14286 –14290