Combinatorial Nickel-Catalyzed Monofluoroalkylation of Aryl Boronic

Letter pubs.acs.org/OrgLett

Combinatorial Nickel-Catalyzed Monofluoroalkylation of Aryl Boronic Acids with Unactivated Fluoroalkyl Iodides Jie Sheng, Hui-Qi Ni, Ge Liu, Yan Li, and Xi-Sheng Wang* Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: A combinatorial nickel-catalyzed cross-coupling between arylboronic acids and unactived 1-fluoro-1-iodoalkanes has been developed, which demonstrated high efficiency, mild conditions, and excellent functional-group compatibility. Readily available nitrogen and phosphine ligands were combined with a nitrogen source, which in situ generated a variety of easily tunable catalysts to promote the fluoroalkylation for broad scopes of both coupling partners. This new strategy on combinatorial catalysis offers new solutions for nickel-catalyzed cross-coupling reactions.

T

alkyl) have emerged as interesting moieties, as the incorporation of a fluorine atom at the benzylic position could dramatically modify the biological activity of nonfluorinated compounds.7 For example, application of this strategy afforded monofluorinated mitochondrial complex 1 (Figure 1), an inhibitor for coronary

ransition-metal-catalyzed cross-coupling reactions have emerged as a powerful strategy for selective construction of carbon−carbon bonds in recent decades.1 Because of their key roles in the catalytic cycle to enhance both efficiency and selectivity of such reactions, the discovery of efficient ligands has come into major focus in transition-metal catalysis.2 While there is no universal ligand to accommodate all substrates and reactions, the design and synthesis of new skeletons or selective modification of the specific positions of known skeletons has been used as the most common tactics to develop novel ligands. However, these methods for tuning the steric and electronic effects of catalysts normally require multistep synthesis, which thus limits their utility in organic synthesis. Meanwhile, the selfassembly of a central metal with two different ligands would create various easily tunable catalysts,3 which offers an alternative solution to promote the target reaction via structural modification of catalysts. Such heteroleptic catalysts with two different ligands may be more active and selective than the homocombinational ones.4 In addition, this approach could easily generate greater catalyst diversity via a simple change of the combination of readily available ligands, which will make the finetuning of steric and electronic effects of catalysts more convenient. While combinatorial transition metal catalysis was normally considered only as a valuable path to discover optimal catalysts via high-throughput screening,3b,c,5 such easily tunable catalysts also demonstrate greater utility due to their in situ generation for a broader scope of substrates. Accordingly, the development of highly efficient catalysts via molecular recognition and self-assembly of different readily available ligands with metals offers an efficient solution to unresolved problems on transition-metal catalysis, especially for catalytic activity and reaction scope. The selective introduction of fluorine or fluorinated moieties into organic molecules has long been realized as a powerful strategy in drug design and development, as the fluorinated compounds can drastically enhance the metabolic stability, lipophilicity, and bioavailability of parent compounds.6 Among all fluorinated groups, monofluoroalkylated arenes (ArCHF© 2017 American Chemical Society

Figure 1. Representative bioactive molecules containing monlfluoroalkylated arenes.

artery disease with significantly improved IC50 bioactivity;7a a remarkable improvement in bioactivity was also detected when CH2 at the benzylic position was replaced with CHF in compound B, which dramatically changed the IC50 of the enzyme combination agent MMP-13.7b Despite its importance, the synthetic utility of ArCHF-alkyl is still very limited. Recently, a direct C−H fluorination at the benzylic position has been developed by the Groves, Lectka, Chen, and Tang groups, but still suffered from relatively poor functional-group compatibility.8 Meanwhile, the Gandelman group reported the only example of nickel-catalyzed cross-coupling of triphenylborane with 1-halo-1-fluoroalkanes to make monofluoroalkylated arene.9 However, the requirement of normally expensive and not readily available triarylboranes as the coupling partner will thus hamper its utility in organic synthesis. And most importantly, neither method is suitable for late-stage fluoroalkyReceived: July 2, 2017 Published: August 15, 2017 4480

DOI: 10.1021/acs.orglett.7b02012 Org. Lett. 2017, 19, 4480−4483

Letter

Organic Letters

(entry 7). To improve the yield further, other nickel sources were next examined under this combinational P/N ligand catalytic system, and Ni(OTf)2 was demonstrated as the optimal choice with a 91% yield (entry 14). Meanwhile, the amount of ligand was carefully studied, but both increasing and decreasing ligand loading only produced lower yields (entries 15−16; for details, see the Supporting Information (SI)). Lastly, control experiments confirmed that none of the desired products were detected in the absence of a nickel catalyst or ligands (entries 17−18). As nitrogen and phosphine ligand combinations demonstrated their decisive role in this catalytic monofluoroalkylation, a number of frequently used, commercially available ligands, including 12 nitrogen ligands (N1−N12) and 15 phosphine ligands (P1−P15), were chosen for combinatorial screening to discover the optimal catalysts. As shown in Figure 2, among all

lation of complex molecules because of the limited availability of the corresponding transformable precursors. Herein, we report the first example of combinatorial P/N ligands nickel-catalyzed monofluoroalkylation of arylboronic acids with unactivated fluoroalkyl iodides, in which high catalytic reactivity, mild conditions, and broad scope have been demonstrated.10 The key to success is the combination of different kinds of readily available nitrogen and phosphorus ligands with a nickel source,11 which in situ generated a variety of easily tunable catalysts to promote the fluoroalkylation between a broad scope of both coupling partners. This new strategy on combinatorial catalysis will thus offer new solutions for nickelcatalyzed cross-coupling reactions. We commenced our initial investigation with phenylboronic acid (1a) as the pilot substrate and 1-fluoro-1-iodo ethylbenzene (2a) as the coupling partner in the presence of a catalytic amount of Ni(NO3)2·6H2O (5 mol %) and K2CO3 in 1,4-dioxane at 80 °C. A careful investigation of ligands, including different kinds of nitrogen and phosphine ligands, has been carried out first. While phosphine ligands exhibited almost no catalytic reactivity, nitrogen ligands afforded the desired fluoroalkylated product 3a, but with low yields (Table 1, entries 1−6). For example, PPh3 (10 mol %) gave none of fluoroalkylated arene 3a and bpy (5 mol %) afforded 3a in only 16% yield. The mixture of PPh3 and bpy as a simple ligand combination greatly improved the yield to 57%, which clearly indicated that combinational P/N ligand catalysis could be the best choice to promote this fluoroalkylation reaction

Figure 2. Combinational screening for discovery of efficient catalysts.a Reaction conditions: 1a (2.0 equiv), 2a (0.2 mmol, 1.0 equiv), Ni(OTf)2 (5 mol %), ligand (5−10 mol %), K2CO3 (3.0 equiv), 1,4dioxane, 80 °C, 24 h. Yield was determined by 19F NMR spectroscopy using PhOCF3 as an internal standard. P1−P15: PPh3, PCy3, P(4OMeC6H4)3, P(4-Me C6H4)3, P(NEt2)3, PAd2Bn, PAd2(n-Bu), BINAP, dppe, dppp, dpph, dppf, Xantphos, X-phos, Dave-phos; N1−N12: bpy, dtbpy, dmbpy, dombpy, phen, Neocuproine, BPhen, DMEDA, TMEDA, L1, terpy, DMAP.

Table 1. Ni-Catalyzed Monofluoroalkylation: Optimization of Conditionsa

entry

Ni source

ligand (mol %)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(NO3)2·6H2O NiCl2 NiBr2 NiI2 Ni(acac)2 Ni(OAc)2 NiCl2·DME Ni(OTf)2 Ni(OTf)2 Ni(OTf)2

bpy (5) dtbpy (5) phen (5) L1 (5) DMEDA (5) PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (10) bpy (5)/PPh3 (15) bpy (10)/PPh3 (10) bpy (5)/PPh3 (10)

16 13 8 4 0 0 57 trace 80 77 trace 53 88 94(91) 89 86 0 0

Ni(OTf)2

a

these choices, bpy/PPh3, bpy/dppe, bpy/Xantphos, bpy/Xphos, phen/PCy3, dtbpy/dppe, and Bphen/dppe could furnish the desired product 3a with excellent yields (>95%), and approximately 10 more ligand combinations could afford good yields (about 85%). The diversity of ligand combinations provided extensive approaches to tune the properties of the nickel complex from both electronic and steric effects, which thus offered a novel solution to solve the problems of catalytic activity and substrate scope. With these optimal catalysts in hand, we next examined the scope by verifying the functional groups installed on the arylboronic acids. As shown in Figure 3, when bpy/PPh3 was selected as the ligand of choice, various electron-donating substituents, including Me (3b, 3i), n-C5H11 (3c), t-Bu (3d), Ph (3e, 3j), PhO (3f), MeS (3g), and TMS (3h) at the 4- or 3position on the phenyl ring of arylboronic acids were monofluoroalkylated smoothly in good to excellent yields (81−97%). However, electron-withdrawing groups were not be well tolerated under such conditions, giving only low to moderate yields using this bpy/Ni/PPh3 catalyst. Given that the alteration of ligand combinations definitely changed the steric and electronic effects of the catalyst system, other ligand combinations were screened to solve this problem (for details, see the SI). As expected, F (3l, 3q), Cl (3m, 3r, 3s), CF3 (3n), ester (3o), and ketone (3p) were completely compatible with

a Unless otherwise noted, the reaction conditions were as follows: 1a (0.4 mmol), 2a (0.2 mmol, 1.0 equiv), [Ni] (5 mol %), ligand (5−10 mol %), K2CO3 (0.6 mmol), 1,4-dioxane (2.0 mL), 80 °C, 24 h. bYield determined by 19F NMR spectroscopy using PhOCF3 as an internal standard; numbers in parentheses were yields of isolated products.

4481

DOI: 10.1021/acs.orglett.7b02012 Org. Lett. 2017, 19, 4480−4483

Letter

Organic Letters

employed accordingly (3ab, 3ac). Meanwhile, other polycyclic arenes were well tolerated in this reaction with different arylboronic acids (3ad). Additionally, the electronic properties of aromatic rings (3ae, 3af) in monofluoroalkyl iodides had no decisive influence on the reaction efficiency, and all these fluoroalkylating reagents could be coupled to arylboronic acids successfully with different ligand combinations changed in response. The 1-fluoro-1-iodoalkanes with the addition of other functional groups, such as ethers (3ag) and long-chain alkanes (3ah), were also well tolerated in this combinatorial catalytic system. Notably, this combinatorial nickel catalyst system showed high reactivity with ester- and sulfone-activated monofluoromethyl halides.10h Both BrCFHCO2Et (2i) and ICFHSO2Ph (2j) were smoothly coupled with arylboronic acids in excellent yields (3ai, 3aj), and electron-withdrawing substituents, which required a higher catalyst loading and ligand batch charging in our previous report,10h were well tolerated in this catalytic system with excellent yields (3yi, 3zj). To gain some insight into the mechanism of this transformation, a series of control experiments were next carried out. First, the reaction was completely quenched when 1.0 equiv of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added into the reaction system (eq 1, Figure S1, see the SI), which showed that this reaction may proceed via a radical path. To confirm this speculation, radical clock 4 was subjected to the standard conditions, giving the cycle-opening product 5 in 38% yield (eq 2, Figure S1, see the SI). This result demonstrated that a monofluoroalkyl radical did exist in the catalytic cycle. Next, the replacement of the Ni(II) species with Ni(0) afforded 3a in almost the same yield (90%, eq 3, Figure S1, see the SI), which indicated a possible Ni(0)/Ni(II) catalytic cycle.12 To determine how the Ni(0) species was generated in the catalytic system, 31P NMR was used to monitor the reaction. A 31P signal found at 22.7 ppm, the same as the signal detected in the bpy/Ni(COD)2/ PPh3 system, appeared in this bpy/Ni(OTf)2/PPh3 catalyst system in the presence of PhB(OH)2 and K2CO3. Taking also into account that little biphenyl could be detected in the reaction system by TLC and GC-MS, both results suggested that the catalytically active Ni(0) species was in situ generated via the Ni(II)-catalyzed homocoupling of arylboronic acid in the presence of base (for details, see the SI).13 However, at this stage, a Ni(I)/Ni(III) catalytic cycle could not be excluded. To demonstrate the synthetic potential and the functional group tolerance of this transformation, this catalytic system has been applied to the late-stage monofluoroalkylation of complex bioactive molecules. As shown in Figure 4, the ezetimibe-derived

Figure 3. Ni-catalyzed monofluoroalkylation: substrate scopea a Reaction conditions: 1 (2.0 equiv), 2 (0.2 mmol, 1.0 equiv), Ni(OTf)2 (5 mol %), bpy (5 mol %)/PPh3 (10 mol %), K2CO3 (3.0 equiv), 1,4-dioxane (2.0 mL), N2, 80 °C, 24 h. b Phen (5 mol %)/PCy3 (10 mol %). c bpy (5 mol %)/P(4-MeO C6H4)3 (10 mol %). d bpy (5 mol %), dppe (5 mol %). e bpy (5 mol %), X-phos (10 mol %). f dtbpy (5 mol %)/PPh3 (10 mol %). g Yield was determined by 19F NMR spectroscopy using PhOCF3 as an internal standard. The compound is unstable upon purification with silica gel chromatography. h 48 h.

this nickel-catalytic system when phen/PCy3 was selected as the ligand combination. This idea has also been used to realize the monofluoroalkylation of ortho-substituted arylboronic acids, which showed Me (3v), F (3w), and Cl (3x) on the 2-position of the phenyl ring afforded excellent yields with bpy/Ni(OTf)2/ P(4-OMePh)3 used as the catalyst. To test whether this method could be extended to different kinds of monofluoroalkylating reagents other than 2a, a great number of monofluoroalkyl iodides were next demonstrated in our reaction system. The length of the alkyl chain had no obvious influence on the efficiency of the reaction, while the combinatorial catalysts with different ligand combinations were

Figure 4. Fluoroalkylation of ezetimibe derivative 6.

boronic acid 6, made from ezetimibe in four steps, was fluoroalkylated successfully to 7 in excellent yield (89%). This late-stage modification of complex molecules clearly indicated the great potential of this method in drug discovery and development as an efficient and expedient tactic to make fluorine-containing analogues. In summary, we have developed a novel method for combinatorial nickel-catalyzed monofluoroalkylation of arylbor4482

DOI: 10.1021/acs.orglett.7b02012 Org. Lett. 2017, 19, 4480−4483

Letter

Organic Letters

(6) (a) Kirsch, P. Modern Fluoroorganic Chemistry: Synthesis Reactivity, Applications, 2nd ed.; Wiley-VCH: Weinheim, 2013. (b) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320. (c) Müller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881. (d) Hagmann, W. K. J. Med. Chem. 2008, 51, 4359. (7) (a) Purohit, A.; Benetti, B.; Hayes, M.; Guaraldi, M.; Kagan, M.; Yalamanchilli, P.; Su, F.; Azure, M.; Mistry, M.; Yu, M.; Robinson, S.; Dischino, D. D.; Casebier, D. Bioorg. Med. Chem. Lett. 2007, 17, 4882. (b) Booth, J. B.; Arbor, A.; O’Brien, P. M. PAT. Appl. Pub. US 2008/ 0161365A1, 2008. (c) Brickner, S. J.; Chen, J. M.; Li, Z. B.; Marfat, A.; Mitton-Fry, M. J.; Plotkin, M. A.; Reilly, U. S.; Subramanyam, C.; Zhang, Z.; Robinson, S. PAT. Appl. Pub. US 2012/0065188A1, 2012. (d) Liu, W.; Groves, J. T. Angew. Chem., Int. Ed. 2013, 52, 6024. (8) (a) Liu, W.; Groves, J. T. Angew. Chem., Int. Ed. 2013, 52, 6024. (b) Xia, J.-B.; Zhu, C.; Chen, C. J. Am. Chem. Soc. 2013, 135, 17494. (c) Bloom, S.; Pitts, C. R.; Woltornist, R.; Griswold, A.; Holl, M. G.; Lectka, T. Org. Lett. 2013, 15, 1722. (d) Bloom, S.; McCann, M.; Lectka, T. Org. Lett. 2014, 16, 6338. (e) Xu, P.; Guo, S.; Wang, L.; Tang, P. Angew. Chem., Int. Ed. 2014, 53, 5955. (9) (a) Jiang, X.; Sakthivel, S.; Kulbitski, K.; Nisnevich, G.; Gandelman, M. J. Am. Chem. Soc. 2014, 136, 9548. (b) Jiang, X.; Gandelman, M. J. Am. Chem. Soc. 2015, 137, 2542. (10) (a) Doi, H.; Ban, I.; Nonoyama, A.; Sumi, K.; Kuang, C.; Hosoya, T.; Tsukada, H.; Suzuki, M. Chem. - Eur. J. 2009, 15, 4165. (b) Guo, C.; Yue, X.; Qing, F.-L. Synthesis 2010, 2010, 1837. (c) Zhao, Y.; Gao, B.; Ni, C.; Hu, J. Org. Lett. 2012, 14, 6080. (d) Fujiwara, Y.; Dixon, J. A.; O’Hara, F.; Funder, E. D.; Dixon, D. D.; Rodriguez, R. A.; Baxter, R. D.; Herle, B.; Sach, N.; Collins, M. R.; Ishihara, Y.; Baran, P. S. Nature 2012, 492, 95. (e) Zhao, Y.; Ni, C.; Jiang, F.; Gao, B.; Shen, X.; Hu, J. ACS Catal. 2013, 3, 631. (f) Liang, Y.; Fu, G. C. J. Am. Chem. Soc. 2014, 136, 5520. (g) Hu, J.; Gao, B.; Li, L.; Ni, C.; Hu, J. Org. Lett. 2015, 17, 3086. (h) Su, Y.-M.; Feng, G.-S.; Wang, Z.-Y.; Lan, Q.; Wang, X.-S. Angew. Chem., Int. Ed. 2015, 54, 6003. (11) For examples of phosphines used as additives to enhance the catalytic reactivity, see: (a) Everson, D. A.; Shrestha, R.; Weix, D. J. J. Am. Chem. Soc. 2010, 132, 920. (b) Xiao, Y.-L.; Guo, W.-H.; He, G.-Z.; Pan, Q.; Zhang, X. Angew. Chem., Int. Ed. 2014, 53, 9909. For examples of nitriles used as additives to enhance the catalytic reactivity, see: (c) Ge, S.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 16330. (d) Ge, S.; Green, R. A.; Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 1617. (e) Yin, G.; Kalvet, I.; Englert, U.; Schoenebeck, F. J. Am. Chem. Soc. 2015, 137, 4164. For examples of pyridine or pyridine derivatives used as additives to enhance the catalytic reactivity, see: (f) Biswas, S.; Weix, D. J. J. Am. Chem. Soc. 2013, 135, 16192. (g) An, L.; Xiao, Y.-L.; Min, Q.-Q.; Zhang, X. Angew. Chem., Int. Ed. 2015, 54, 9079. (12) (a) González-Bobes, F.; Fu, G. C. J. Am. Chem. Soc. 2006, 128, 5360. (b) Zultanski, S. L.; Fu, G. C. J. Am. Chem. Soc. 2011, 133, 15362. (c) Everson, D. A.; Jones, B. A.; Weix, D. J. J. Am. Chem. Soc. 2012, 134, 6146. (d) Choi, J.; Fu, G. C. J. Am. Chem. Soc. 2012, 134, 9102. (e) Gallego, D.; Brück, A.; Irran, E.; Meier, F.; Kaupp, M.; Driess, M.; Hartwig, J. F. J. Am. Chem. Soc. 2013, 135, 15617. (f) Choi, J.; MartínGago, P.; Fu, G. C. J. Am. Chem. Soc. 2014, 136, 12161. (g) Lu, X.; Xiao, B.; Zhang, Z.; Gong, T.; Su, W.; Yi, J.; Fu, Y.; Liu, L. Nat. Commun. 2016, 7, 11129. (13) (a) Liu, C.; Shen, D.-M.; Chen, Q.-Y. J. Org. Chem. 2007, 72, 2732. (b) Li, Z.; Zhang, S.-L.; Fu, Y.; Guo, Q.-X.; Liu, L. J. Am. Chem. Soc. 2009, 131, 8815. (c) Quasdorf, K. W.; Antoft-Finch, A.; Liu, P.; Silberstein, A. L.; Komaromi, A.; Blackburn, T.; Ramgren, S. D.; Houk, K. N.; Snieckus, V.; Garg, N. K. J. Am. Chem. Soc. 2011, 133, 6352. (d) León, T.; Correa, A.; Martin, R. J. Am. Chem. Soc. 2013, 135, 1221. (e) Ren, Q.; Jiang, F.; Gong, H. J. Organomet. Chem. 2014, 770, 130. (f) Abe, T.; Mino, T.; Watanabe, K.; Sakamoto, M. Eur. J. Org. Chem. 2014, 2014, 6983. (g) Chen, Q.; Fan, X.-H.; Zhang, L.-P.; Yang, L.-M. RSC Adv. 2014, 4, 53885. (h) Liu, X.-W.; Echavarren, J.; Zarate, C.; Martin, R. J. Am. Chem. Soc. 2015, 137, 12470.

onic acids, in which the combination of nickel with phosphine and nitrogen ligands was crucial to increase the catalytic activity for monofluoromethylation. Both coupling partners were well tolerated in this catalytic system by tuning the electronic and steric properties of catalysts via combination of readily available ligands. Mechanistic investigations indicated a Ni0/NiII catalytic cycle involving a monofluoroalkyl radical were generated in situ. Further exploration of the mechanistic details of this catalytic cycle and application of this method to fluorine-containing modification of complex biologically active molecules are ongoing in our laboratory.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02012. Experimental procedure and characterization of all new compounds (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xi-Sheng Wang: 0000-0003-2398-5669 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), the National Basic Research Program of China (973 Program 2015CB856600), the National Science Foundation of China (21602213, 21522208, 21372209), and the Fundamental Research Funds for the Central Universities (WK2060190046) for financial support.

■

REFERENCES

(1) (a) Hartwig, J. F. Pure Appl. Chem. 1999, 71, 1417. (b) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461. (c) Rudolph, A.; Lautens, M. Angew. Chem., Int. Ed. 2009, 48, 2656. (d) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111, 1417. (e) Liu, C.; Zhang, H.; Shi, W.; Lei, A. Chem. Rev. 2011, 111, 1780. (f) Han, F.-S. Chem. Soc. Rev. 2013, 42, 5270. (g) Cherney, A. H.; Kadunce, N. T.; Reisman, S. E. Chem. Rev. 2015, 115, 9587. (2) (a) Gavrilova, A. L.; Bosnich, B. Chem. Rev. 2004, 104, 349. (b) Byrne, P. J.; Kitchen, J. A.; Gunnlaugsson, T. Chem. Soc. Rev. 2014, 43, 5302. (c) Trost, B. M.; Rao, M. Angew. Chem., Int. Ed. 2015, 54, 5026. (d) Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015, 54, 12236. (e) Pettinari, C.; Tǎbǎcaru, A.; Galli, S. Coord. Chem. Rev. 2016, 307, 1. (3) (a) Gennari, C.; Piarulli, U. Chem. Rev. 2003, 103, 3071. (b) Ding, K.; Du, H.; Yuan, Y.; Long, J. Chem. - Eur. J. 2004, 10, 2872. (c) Reetz, M. T. Angew. Chem., Int. Ed. 2008, 47, 2556. (d) Carboni, S.; Gennari, C.; Pignataro, L.; Piarulli, U. Dalton Trans. 2011, 40, 4355. (4) (a) Berrisford, D. J.; Bolm, C.; Sharpless, K. B. Angew. Chem., Int. Ed. Engl. 1995, 34, 1059. (b) Blaser, H.-U.; Studer, M. Acc. Chem. Res. 2007, 40, 1348. (c) Teichert, J. F.; Feringa, B. L. Angew. Chem., Int. Ed. 2010, 49, 2486. (5) (a) Francis, M. B.; Jamison, T. F.; Jacobsen, E. N. Curr. Opin. Chem. Biol. 1998, 2, 422. (b) Reetz, M. T. Angew. Chem., Int. Ed. 2001, 40, 284. (c) Satyanarayana, T.; Kagan, H. B. Adv. Synth. Catal. 2005, 347, 737. (d) Reyes, S. J.; Burgess, K. Chem. Soc. Rev. 2006, 35, 416. (e) Stanton, M. S.; Holcombe, J. A. J. Comb. Chem. 2007, 9, 359. 4483

DOI: 10.1021/acs.orglett.7b02012 Org. Lett. 2017, 19, 4480−4483