Palladium-Catalyzed C(sp2)–H Acetoxylation via Electrochemical

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance. Ming Yan , Yu Kawamata , and Phil S. Baran. Chemical Reviews 2017...
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Palladium-Catalyzed C(sp2)−H Acetoxylation via Electrochemical Oxidation Yi-Qian Li,†,‡ Qi-Liang Yang,‡ Ping Fang,‡ Tian-Sheng Mei,*,‡ and Dayong Zhang*,† †

Institute of Pharmaceutical Science, China Pharmaceutical University, Nanjing, P. R. China State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China

‡

S Supporting Information *

ABSTRACT: Palladium-catalyzed arene C(sp2)−H acetoxylation has emerged as a powerful tool to construct a carbon−oxygen (C−O) bond. However, the requirement of stoichiometric chemical oxidants for this transformation possesses a significant disadvantage. To solve this fundamental problem, we now report an anodic oxidation strategy to achieve arene C(sp2)−H acetoxylation.

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potentially toxic, have poor atom economy, and produce byproducts. The utilization of electric current as an oxidant for Pd(II)catalyzed C−H functionalization is an attractive alternative to stoichiometric reagents.13−16 We recently demonstrated Pdcatalyzed C(sp3)−H oxygenation via electrochemical oxidation (Scheme 1b).17 We questioned whether this anodic oxidation system would be compatible with aromatic C(sp2)−H acetxoylation. We report herein the first Pd-catalyzed oximedirected C(sp2)−H acetoxylation via anodic oxidation, in which electric current is used in place of stoichiometric oxidants (Scheme 1c). To investigate the formation of C(sp2)−O bonds, we chose oxime 1a as the substrate for the reaction under electrochemical conditions because the oxime moiety is easily manipulated and has extensive derivation potential (Table 1). Gratifyingly, the reaction of 1a with 10 mol % Pd(OAc)2 in the presence of tetrabutylammonium acetate (1 equiv) under constant-current electrolysis conditions at 1.0 mA (J = 0.75 mA·cm−2) gave a 75% isolated (78% by 1H NMR) yield of monoacetoxylated product 2a (entry 1). Among Pd catalysts that were tested, Pd(OAc)2 was found to be optimal (entries 1−4). The 1H NMR yield decreased to 23% when the amount of palladium catalyst was decreased to 5 mol % (entry 5). Increasing and decreasing the reaction temperature did not improve the acetoxylation (entries 6 and 7). Among the electrolytes that were tested, tetrabutylammonium acetate was found to be optimal (entries 8−12). Increasing or decreasing the electric current from 1.0 mA to 0.8 mA or 1.5 mA respectively gave lower yields (entries 13 and 14). Control experiments revealed that no reaction was observed in the absence of electric current or Pd(OAc)2 (entries 15 and 16). Having optimized the reaction conditions, we next examined the scope of the Pd(II)-catalyzed C−H acetoxylation reaction

uring the past decade, palladium-catalyzed C−H functionalization has emerged as a useful tool to construct carbon−carbon (C−C) and carbon−heteroatom (C−Y) bonds.1 Pd(II)-catalyzed aryl C(sp2)−H oxygenation, in particular, has received significant attention.2 Toward that end, in 2004, Sanford and co-workers reported the first example of Pd(II)-catalyzed pyridine-directed C(sp2)−H acetoxylation using PhI(OAc)2 as a stochiometric oxidant (Scheme 1a).3 Scheme 1. Approaches to Pd(II)-Catalyzed C−H Acetoxylation

Following this seminal report, a number of examples of Pdcatalyzed aryl C(sp2)−H oxygenation have been reported.4−10 However, in order to accomplish these transformations, in all cases a stoichiometric amount of exogenous chemical oxidants including PhI(OAc)2,6 K2S2O8,7 oxone,8 peroxide,9 and others10 is needed to convert organopalladium(II) intermediates into the PdIII or PdIV oxidation state to promote otherwise challenging C−O reductive elimination.11,12 These stoichiometric oxidants have drawbacks in that they are expensive and © 2017 American Chemical Society

Received: April 14, 2017 Published: May 24, 2017 2905

DOI: 10.1021/acs.orglett.7b01138 Org. Lett. 2017, 19, 2905−2908

Letter

Organic Letters Table 1. Reaction Optimizationa

Scheme 2. Substrate Scope of Oximesa,b

entry

variation from standard conditions above

yield (%)b

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

none 10 mol % Pd(TFA)2 instead of 10 mol % Pd(OAc)2 10 mol % Pd(dba)2 instead of 10 mol % Pd(OAc)2 10 mol % PdCl2 instead of 10 mol % Pd(OAc)2 5 mol % Pd(OAc)2 instead of 10 mol % Pd(OAc)2 60 °C instead of 40 °C 25 °C instead of 40 °C LiOAc instead of n-Bu4NOAc NaOAc instead of n-Bu4NOAc KOAc instead of n-Bu4NOAc n-Bu4NBF4 instead of n-Bu4NOAc n-Bu4NCl instead of n-Bu4NOAc 0.8 mA instead of 1 mA (25 h) 1.5 mA instead of 1 mA (19 h) no electric current no Pd(OAc)2

78 (75)c 74 66 30 23 69 54 51 76 59 14 9 55 58 NR NR

a

Reaction conditions: 1a (0.3 mmol), Pd(OAc)2 (10 mol %), nBu4OAc (1 equiv), acetic acid (3 mL) [anode], and n-Bu4OAc (1 equiv), acetic acid (3 mL) [cathode] in an H-type divided cell with two platinum electrodes and anion exchange membrane, 40 °C for 20 h. bThe yield was determined by 1H NMR with CH2Br2 as the internal standard. cIsolated yield.

a Reaction conditions: 1 (0.3 mmol), Pd(OAc)2 (10 mol %), nBu4NOAc (1 equiv), acetic acid (3 mL) [anode], and n-Bu4NOAc (1 equiv), acetic acid (3 mL) [cathode] in an H-type divided cell with two platinum electrodes and anion exchange membrane, 40 °C. Isolated yields are reported. bTemperature was 60 °C.

Scheme 3. Kinetic Isotopic Effect Experiments

by testing a series of oxime substrates containing different substitution patterns and various functional groups (Scheme 2). To our satisfaction, a variety of functional groups, including electron-donating groups (Me, OMe, t-Bu), halogens (F, Cl, Br and I), and some electron-withdrawing groups (CF3, CO2Me) successfully afforded the corresponding products (2b−2j). In contrast, substrates with cyano and nitro substituents were less reactive (2k and 2l). Interestingly, meta-substituted substrates were selectively acetoxylated at the less sterically crowded position (2m−2p). This regioselectivity could result from a steric effect upon the formation of the putative palladacyclic intermediate (see below), leading to cleavage of the less sterically hindered ortho-C−H bond. However, ortho-substituted substrate obtained the product with low yields due to the low conversion (2q). Additionally, when alkylphenone and diarylketone O-methyl oximes were examined, the acetoxylation still proceeded efficiently (2r and 2s). To our delight, 1cyclohexenylethanone O-methyl oxime could be converted into the corresponding acetoxylation product (2t). It is worth noting that this is the first example of palladium catalyzed oxime-assisted alkenyl C(sp2)−H bonds acetoxylation.18 To gain mechanistic insight into the reaction, we carried out kinetic isotope effect (KIE) experiments (Scheme 3). A slight isotope effect was detected in the parallel experiments (kH/kD = 1.5). Furthermore, the intermolecular kH/kD was determined to be 2.5. This indicates that the C−H cleavage occurs during the rate-determining step.19 Based on the above results and previous reports, a possible mechanism was proposed below (Scheme 4). Initially, the N atom of the oxime coordinates with Pd(II), followed by subsequent rate-determining C−H activation to generate the

Scheme 4. Proposed Catalytic Cycle

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DOI: 10.1021/acs.orglett.7b01138 Org. Lett. 2017, 19, 2905−2908

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Organic Letters

2012, 134, 7313. (d) Neufeldt, S. R.; Sanford, M. S. Org. Lett. 2010, 12, 532. (e) Reddy, B. V. S.; Reddy, L. R.; Corey, E. J. Org. Lett. 2006, 8, 3391. (f) Wang, D.-H.; Wu, D.-F.; Yu, J.-Q. Org. Lett. 2006, 8, 3387. (g) Giri, R.; Liang, J.; Lei, J.-G.; Li, J.-J.; Wang, D.-H.; Chen, X.; Naggar, I. C.; Guo, C.; Foxman, B. M.; Yu, J.-Q. Angew. Chem., Int. Ed. 2005, 44, 7420. (h) Dick, A. R.; Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2004, 126, 2300. (6) (a) Korwar, S.; Brinkley, K.; Siamaki, A. R.; Gupton, B. F.; Ellis, K. C. Org. Lett. 2015, 17, 1782. (b) Zhang, H.; Hu, R.-B.; Zhang, X.-Y.; Li, S.-X.; Yang, S.-D. Chem. Commun. 2014, 50, 4686. (c) Cheng, X.F.; Li, Y.; Su, Y.-M.; Yin, F.; Wang, J.-Y.; Sheng, J.; Vora, H. U.; Wang, X.-S.; Yu, J.-Q. J. Am. Chem. Soc. 2013, 135, 1236. (d) Cook, A. K.; Emmert, M. H.; Sanford, M. S. Org. Lett. 2013, 15, 5428. (e) Choy, P. Y.; Kwong, F. Y. Org. Lett. 2013, 15, 270. (f) Choy, P. Y.; Lau, C. P.; Kwong, F. Y. J. Org. Chem. 2011, 76, 80. (g) Wang, L.; Xia, X.-D.; Guo, W.; Chen, J.-R.; Xiao, W.-J. Org. Biomol. Chem. 2011, 9, 6895. (h) Chernyak, N.; Dudnik, A. S.; Huang, C.; Gevorgyan, V. J. Am. Chem. Soc. 2010, 132, 8270. (i) Stowers, K. J.; Sanford, M. S. Org. Lett. 2009, 11, 4584. (j) Desai, L. V.; Stowers, K. J.; Sanford, M. S. J. Am. Chem. Soc. 2008, 130, 13285. (k) Kalberer, E. W.; Whitfield, S. R.; Sanford, M. S. J. Mol. Catal. A: Chem. 2006, 251, 108. (l) Kalyani, D.; Sanford, M. S. Org. Lett. 2005, 7, 4149. (7) (a) Rit, R. K.; Yadav, R.; Sahoo, A. K. Org. Lett. 2014, 16, 968. (b) Gary, J. B.; Cook, A. K.; Sanford, M. S. ACS Catal. 2013, 3, 700. (c) Shan, G.; Yang, X.; Ma, L.; Rao, Y. Angew. Chem., Int. Ed. 2012, 51, 13070. (d) Mo, F.; Trzepkowski, L. J.; Dong, G. Angew. Chem., Int. Ed. 2012, 51, 13075. (e) Wang, G.-W.; Yuan, T.-T.; Wu, X.-L. J. Org. Chem. 2008, 73, 4717. (8) (a) Liang, Y.-F.; Wang, X.; Yuan, Y.; Liang, Y.; Li, X.; Jiao, N. ACS Catal. 2015, 5, 6148. (b) Kim, S. H.; Lee, H. S.; Kim, S. H.; Kim, J. N. Tetrahedron Lett. 2008, 49, 5863. (c) Desai, L. V.; Malik, H. A.; Sanford, M. S. Org. Lett. 2006, 8, 1141. (9) (a) Vickers, C.; Mei, T.-S.; Yu, J.-Q. Org. Lett. 2010, 12, 2511. (b) Giri, R.; Liang, J.; Lei, J.-G.; Li, J.-J.; Wang, D.-H.; Chen, X.; Naggar, I. C.; Guo, C.; Foxman, B. M.; Yu, J.-Q. Angew. Chem., Int. Ed. 2005, 44, 7420. (10) (a) Zhang, Y.-H.; Yu, J.-Q. J. Am. Chem. Soc. 2009, 131, 14654. (b) Zhang, J.; Khaskin, E.; Anderson, N. P.; Zavalij, P. Y.; Vedernikov, A. N. Chem. Commun. 2008, 3625. (11) For selected reviews on PdIV chemistry, see: (a) Hickman, A. J.; Sanford, M. S. Nature 2012, 484, 177. (b) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A. H. J. Chem. Soc., Chem. Commun. 1986, 0, 1722. For the selected examples on PdIII chemistry, see: (c) Powers, D. C.; Ritter, T. Acc. Chem. Res. 2012, 45, 840. (d) Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 17050. (e) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302. (f) Deprez, N. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 11234. (g) Cotton, F. A.; Koshevoy, I. O.; Lahuerta, P.; Murillo, C. A.; Sanaú, M.; Ubeda, M. A.; Zhao, Q. J. Am. Chem. Soc. 2006, 128, 13674. (12) For elegant mechanistic studies on C−O bond formation via reductive elimination from a high valent Pd center, see: (a) Camasso, N. M.; Pérez-Temprano, M. H.; Sanford, M. S. J. Am. Chem. Soc. 2014, 136, 12771. (b) Powers, D. C.; Lee, E.; Ariafard, A.; Sanford, M. S.; Yates, B. F.; Canty, A. J.; Ritter, T. J. Am. Chem. Soc. 2012, 134, 12002. (c) Lanci, M. P.; Remy, M. S.; Kaminsky, W.; Mayer, J. M.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 15618. (d) Racowski, J. M.; Dick, A. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 10974. (e) Dick, A. R.; Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc. 2005, 127, 12790. (13) For selected reviews on electroorganic synthesis, see: (a) Horn, E. J.; Rosen, B. R.; Baran, P. S. ACS Cent. Sci. 2016, 2, 302. (b) Waldvogel, S. R.; Selt, M. Angew. Chem., Int. Ed. 2016, 55, 12578. (c) Francke, R.; Quell, T.; Wiebe, A.; Waldvogel, S. R. In Organic Electrochemistry, 5th ed.; Hammerich, O., Speiser, B., Eds.; Taylor & Francis: Boca Raton, FL, 2015. (d) Francke, R.; Little, R. D. Chem. Soc. Rev. 2014, 43, 2492. (e) Yoshida, J.-i.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev. 2008, 108, 2265. (f) Sperry, J. B.; Wright, D. L. Chem. Soc. Rev. 2006, 35, 605. (g) Moller, K. D. Tetrahedron 2000, 56, 9527.

intermediate B. This Pd(II) complex is directly oxidized at the anode to form Pd(IV) complex C. The intermediate C then undergoes reductive elimination to afford the desired product, thereby closing the catalytic cycle. In conclusion, we have developed an efficient electrochemical method for the palladium-catalyzed C(sp2)−H bond acetoxylation. This reaction proceeds in the absence of toxic oxidants at a mild temperature and overcomes some drawbacks of traditional acetoxylation methodologies that employ stoichiometric oxidants.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01138. Detailed experimental procedures, compound characterization data, and copies of NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yi-Qian Li: 0000-0002-3794-2355 Ping Fang: 0000-0002-3421-2613 Tian-Sheng Mei: 0000-0002-4985-1071 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000), “1000-Youth Talents Plan”, NSF of China (Grant 21421091, 21572245), and S&TCSM of Shanghai (Grant 15PJ1410200).

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REFERENCES

(1) For selected reviews on Pd-catalyzed C−H functionalization, see: (a) Engle, K. M.; Mei, T.-S.; Wasa, M.; Yu, J.-Q. Acc. Chem. Res. 2012, 45, 788. (b) Wencel-Delord, J.; Dröge, T.; Liu, F.; Glorius, F. Chem. Soc. Rev. 2011, 40, 4740. (c) McMurray, L.; O'Hara, F.; Gaunt, M. J. Chem. Soc. Rev. 2011, 40, 1885. (d) Li, H.; Li, B.-J.; Shi, Z.-J. Catal. Sci. Technol. 2011, 1, 191. (e) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147. (f) Daugulis, O.; Do, H.-Q.; Shabashov, D. Acc. Chem. Res. 2009, 42, 1074. (2) For selected reviews on transition-metal-catalyzed C−H oxygenation, see: (a) Liu, B.; Shi, B.-F. Tetrahedron Lett. 2015, 56, 15. (b) Thirunavukkarasu, V. S.; Kozhushkov, S. I.; Ackermann, L. Chem. Commun. 2014, 50, 29. (c) Racowski, J. M.; Sanford, M. S. Top. Organomet. Chem. 2011, 35, 61. (3) Dick, A. R.; Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2004, 126, 2300. (4) For early examples of Pd-catalyzed C−H oxygenation of benzene, see: (a) Yoneyama, T.; Crabtree, R. H. J. Mol. Catal. A: Chem. 1996, 108, 35. (b) Eberson, L.; Jonsson, L. J. Chem. Soc., Chem. Commun. 1974, 885. (c) Henry, P. M. J. Org. Chem. 1971, 36, 1886. (5) For selected examples of Pd-catalyzed C(sp3)−H oxygenation, see: (a) Huang, Z.; Wang, C.; Dong, G. Angew. Chem., Int. Ed. 2016, 55, 5299. (b) Chen, F.-J.; Zhao, S.; Hu, F.; Chen, K.; Zhang, Q.; Zhang, S.-Q.; Shi, B.-F. Chem. Sci. 2013, 4, 4187. (c) Zhang, S.-Y.; He, G.; Zhao, Y.; Wright, K.; Nack, W. A.; Chen, G. J. Am. Chem. Soc. 2907

DOI: 10.1021/acs.orglett.7b01138 Org. Lett. 2017, 19, 2905−2908

Letter

Organic Letters (14) For selected recent examples on electroorganic synthesis, see: (a) Xiong, P.; Xu, H.-H.; Xu, H.-C. J. Am. Chem. Soc. 2017, 139, 2956. (b) Zhu, L.; Xiong, P.; Mao, Z.-Y.; Wang, Y.-H.; Yan, X.; Lu, X.; Xu, H.-C. Angew. Chem., Int. Ed. 2016, 55, 2226. (c) Badalyan, A.; Stahl, S. S. Nature 2016, 535, 406. (d) Horn, E. J.; Rosen, B. R.; Chen, Y.; Tang, J.; Chen, K.; Eastgate, M. D.; Baran, P. S. Nature 2016, 533, 77. (e) Wiebe, A.; Schollmeyer, D.; Dyballa, K. M.; Franke, R.; Waldvogel, S. R. Angew. Chem., Int. Ed. 2016, 55, 11801. (f) Lips, S.; Wiebe, A.; Elsler, B.; Schollmeyer, D.; Dyballa, K. M.; Franke, R.; Waldvogel, S. R. Angew. Chem., Int. Ed. 2016, 55, 10872. (g) Hayashi, R.; Shimizu, A.; Yoshida, J.-i. J. Am. Chem. Soc. 2016, 138, 8400. (h) Chen, J.; Yan, W. Q.; Lam, C. M.; Zeng, C. C.; Hu, L. M.; Little, R. D. Org. Lett. 2015, 17, 986. (i) Francke, R.; Little, R. D. J. Am. Chem. Soc. 2014, 136, 427. (j) Rosen, B. R.; Werner, E. W.; O’Brien, A. G.; Baran, P. S. J. Am. Chem. Soc. 2014, 136, 5571. (k) O’Brien, A. G.; Maruyama, A.; Inokuma, Y.; Fujita, M.; Baran, P. S.; Blackmond, D. G. Angew. Chem., Int. Ed. 2014, 53, 11868. (l) Redden, A.; Perkins, R. J.; Moeller, K. D. Angew. Chem., Int. Ed. 2013, 52, 12865. (15) For selected reviews on C−H functionalization via electrochemical oxidation, see: (a) Jiao, K.-J.; Zhao, C.-Q.; Fang, P.; Mei, T.S. Tetrahedron Lett. 2017, 58, 797. (b) Dudkina, Y. B.; Gryaznova, T. V.; Sinyashin, O. G.; Budnikova, Y. H. Russ. Chem. Bull. 2015, 64, 1713. (16) For selected examples on Pd-catalyzed C−H functionalization via electrochemical oxidation, see: (a) Grayaznova, T. V.; Dudkina, Y. B.; Islamov, D. R.; Kataeva, O. N.; Sinyashin, O. G.; Vicic, D. A.; Budnikova, Y. H. J. Organomet. Chem. 2015, 785, 68. (b) Gryaznova, T.; Dudkina, Y.; Khrizanforov, M.; Sinyashin, O.; Kataeva, O.; Budnikova, Y. J. Solid State Electrochem. 2015, 19, 2665. (c) Dudkina, Y. B.; Mikhaylov, D. Y.; Gryaznova, T. V.; Tufatullin, A. I.; Kataeva, O. N.; Vicic, D. A.; Budnikova, Y. H. Organometallics 2013, 32, 4785. (d) Dudkina, Y. B.; Mikhaylov, D. Y.; Gryaznova, T. V.; Sinyashin, O. G.; Vicic, D. A.; Budnikova, Y. H. Eur. J. Org. Chem. 2012, 2012, 2114. (e) Saito, F.; Aiso, H.; Kochi, T.; Kakiuchi, F. Organometallics 2014, 33, 6704. (f) Aiso, H.; Kochi, T.; Mutsutani, H.; Tanabe, T.; Nishiyama, S.; Kakiuchi, F. J. Org. Chem. 2012, 77, 7718. (g) Kakiuchi, F.; Kochi, T.; Mutsutani, H.; Kobayashi, N.; Urano, S.; Sato, M.; Nishiyama, S.; Tanabe, T. J. Am. Chem. Soc. 2009, 131, 11310. (h) Amatore, C.; Cammoun, C.; Jutand, A. Adv. Synth. Catal. 2007, 349, 292. (17) Yang, Q.-L.; Li, Y.-Q.; Ma, C.; Fang, P.; Zhang, X.-J.; Mei, T.-S. J. Am. Chem. Soc. 2017, 139, 3293. (18) For a Rh-catalyzed example, see: Yu, W.; Chen, J.; Gao, K.; Liu, Z.; Zhang, Y. Org. Lett. 2014, 16, 4870. (19) Simmons, E. M.; Hartwig, J. F. Angew. Chem., Int. Ed. 2012, 51, 3066.

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