oxidative acyloxylation of 2-aryl-benzo[d]thiazoles

Journal of Organometallic Chemistry 739 (2013) 33e39

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Palladium-catalyzed CeH bond functionalization/oxidative acyloxylation of 2-aryl-benzo[d]thiazoles Qiuping Ding*, Huafang Ji, Ziyi Nie, Qin Yang, Yiyuan Peng* Key Laboratory of Functional Small Organic Molecules, Ministry of Education and College of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 January 2013 Received in revised form 4 April 2013 Accepted 10 April 2013

A chelation-assisted Pd-catalyzed ortho-acyloxylation reaction of the 2-arylbenzo[d]thiazole is described via sp2 CeH bond activation. A wide substrate scope with good functional group tolerance has been demonstrated, affording mono- or diacyloxylation products in moderate to good yields. This method is an alternative route for the preparation of 2-arylbenzo[d]thiazole derivatives via a CeH activation mechanism. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Arylbenzothiazole CeH activation Acyloxylation

1. Introduction The developing of transition metal-catalyzed carbonehydrogen (CeH) bond activation is a big leap in knowledge of how to cleave and functionalize inert CeH bonds effectively. During the past several decades, great progress has been made in the direct formation of carbonecarbon [1e7] and carboneheteroatom (CeN [8e 10], CeS [11e13], CeX [14e22], and CeO [14,23e37]) bonds from both of unactivated sp2 and sp3 CeH bonds via CeH activation. However, these transformations suffer from two fundamental challenges: (i) reactivity of unactivated CeH bonds and (ii) regioselectivity. Much endeavor for the challenges has been done from three aspects. The first effective measure is choosing transition metals (Pd, Ru, Rh, and Cu) to make CeH bonds activation and increase the rates of reactions. A second method is the use of a variety of different oxidants [such as iodine(III) oxidant, iodine(I) oxidant, peroxide oxidant, and dioxygen] to effect the transformation via producing high oxidation state metal intermediate which was involved in mechanism. The third one is the use of substrates with directing group which can coordinate with transition metal center via favorable five- or six-membered transition states. Among these, significant progress has been made in directing-group-assisted transition metal-catalyzed CeH acetoxylation. The substrates containing directing-group include benzoquinoline [14], azobenzene

* Corresponding authors. Tel./fax: þ86 791 88120380. E-mail addresses: [email protected] (Q. Ding), [email protected] (Y. Peng). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.04.026

[14], pyrazole [14,32], imine [14], pyridine [14,27,29,33e35], 2pyrimidyldiisopropylsilyl (PyrDipSi) [31] oxime ether [23,24,36], Boc-protected N-methylamine [25], anilide [26], sulfoximine [30], and benzoxazole [36]. For example, Sanford reported the elegant work of Pd-catalyzed chelate-directed oxidative acetoxylation of sp2 and sp3 CeH bonds [14]. Yu developed the pyridyl groupdirected Cu(OAc)2-catalyzed oxidative acyloxylation of arene CeH bonds in HOAc/Ac2O using oxygen as terminal oxidant [17]. Benzothiazole is a common structural motif in many natural products and pharmaceuticals that exhibit remarkable antitumor activities [38e41]. Therefore, many efforts have been given for the development of new approaches for the construction of benzothiazole derivatives. Recently, we developed an efficient method for the arylation of 2-arylbenzothiazoles via CeH activation [42]. Our interest in heterocycle [43e46] and CeH functionalization [42,47] led us to explore the possibility of acyloxylation using 2-arylbenzo[d]thiazole as the reaction substrate. Herein, we report a Pdcatalyzed oxidative acyloxylation of arene CeH bonds using benzo [d]thiazole as directing group. 2. Experimental 2.1. General remarks All reactions were performed in reaction tubes under nitrogen atmosphere. Flash column chromatography was performed using silica gel (60- A pore size, 32e63 mm, standard grade). Analytical thin-layer chromatography was performed using glass plates precoated with 0.25 mm 230e400 mesh silica gel impregnated with

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Q. Ding et al. / Journal of Organometallic Chemistry 739 (2013) 33e39

a fluorescent indicator (254 nm). Thin layer chromatography plates were visualized by exposure to ultraviolet light. Organic solutions were concentrated on rotary evaporators at w20 Torr (house vacuum) at 25e35  C. Commercial reagents and solvents were used as received. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded in CDCl3 using a Bruker Avance (AV400) spectrometer in parts per million from internal tetramethylsilane on the d scale. HRMS were obtained using a Nova NanoSEM 200 (FEI) instrument with ESI ionization. 2.2. General procedure for CeH bond activation/acyloxylation of 2arylbenzothiazoles 1 with iodine(III) oxidants 2 In a 20 mL reaction tube, a mixture of 2-arylbenzothiazole 1 (0.3 mmol, 1.0 equiv), iodine(III) oxidants 2 (0.9 mmol, 3.0 equiv), and Pd(OAc)2 (5 mol %) in dried CH3CN (1.0 mL) was stirred at 90  C for 6e12 h. After completion of the reaction as indicated by TLC, the mixture was cooled to room temperature, the resulting mixture was extracted with ethyl acetate (3  20 mL). The organic layer was evaporated under vacuum, and then the residue was purified by flash column chromatography on silica gel to provide the corresponding pure product 3 and (or) 4. 2.2.1. 2-(Benzo[d]thiazol-2-yl)-1,3-phenylene diacetate 4aa 1 H NMR (400 MHz, CDCl3) d 2.24 (s, 6H), 7.16 (d, J ¼ 8.0 Hz, 2H), 7.43 (d, J ¼ 7.6 Hz, 1H), 7.48e7.52 (m, 2H), 7.93 (d, J ¼ 8.0 Hz, 1H), 8.09 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.2, 120.7, 121.3, 121.4, 123.5, 125.7, 126.3, 131.0, 135.6, 149.4, 152.8, 158.4, 169.0; HRMS (ESI): m/z [M þ H]þ calcd for C17H14NO4S: 328.0644; found: 328.0640. 2.2.2. 2-(6-Methylbenzo[d]thiazol-2-yl)-1,3-phenylene diacetate 4ba 1 H NMR (400 MHz, CDCl3) d 2.23 (s, 6H), 2.50 (s, 3H), 7.15 (d, J ¼ 8.4 Hz, 2H), 7.31 (d, J ¼ 8.4 Hz, 1H), 7.48 (t, J ¼ 8.0 Hz, 1H), 7.72 (s, 1H), 7.96 (J ¼ 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.1, 21.6, 120.8, 121.0, 121.2, 123.0, 127.9, 130.8, 135.9, 149.4, 151.0, 157.3, 168.9; HRMS (ESI): m/z [M þ H]þ calcd for C18H16NO4S: 342.0800; found: 342.0808. 2.2.3. 2-(Benzo[d]thiazol-2-yl)-3-methylphenyl acetate 3ca 1 H NMR (400 MHz, CDCl3) d 2.06 (s, 3H), 2.34 (s, 3H), 7.05 (d, J ¼ 8.4 Hz, 1H), 7.22 (d, J ¼ 7.6 Hz, 1H), 7.38e7.46 (m, 2H), 7.70e 7.55 (m, 1H), 7.94 (d, J ¼ 8.0 Hz, 1H), 8.13 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.3, 20.9, 120.5, 121.5, 123.6, 125.4, 126.1, 126.9, 128.2, 130.6, 136.1, 139.7, 149.1, 153.2, 162.8, 169.4; HRMS (ESI): m/z [M þ H]þ calcd for C16H14NO2S: 284.0745; found: 284.0740. 2.2.4. 3-Methyl-2-(6-methylbenzo[d]thiazol-2-yl)phenyl acetate 3da 1 H NMR (400 MHz, CDCl3) d 2.05 (s, 3H), 2.33 (s, 3H), 2.52 (s, 3H), 7.04 (d, J ¼ 8.4 Hz, 1H), 7.20 (d, J ¼ 7.6 Hz, 1H), 7.33 (d, J ¼ 8.4 Hz, 1H), 7.38 (t, J ¼ 8.0 Hz, 1H), 7.72 (s, 1H), 8.01 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.3, 20.8, 21.6, 120.4, 121.2, 123.1, 126.9, 127.7, 128.2, 130.5, 131.8, 135.6, 136.3, 139.7, 149.1, 151.3, 161.7, 169.4; HRMS (ESI): m/z [M þ H]þ calcd for C17H16NO2S: 298.0902; found: 298.0906. 2.2.5. 2-(6-Chlorobenzo[d]thiazol-2-yl)-3-methylphenyl acetate 3ea 1 H NMR (400 MHz, CDCl3) d 2.06 (s, 3H), 2.34 (s, 3H), 7.06 (d, J ¼ 8.0 Hz, 1H), 7.22 (d, J ¼ 8.0 Hz, 1H), 7.40 (t, J ¼ 8.0 Hz, 1H), 7.49 (dd, J ¼ 2.0, 4.8 Hz, 1H), 7.91 (d, J ¼ 2.0 Hz, 1H), 8.02 (d, J ¼ 8.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.3, 20.8, 120.5, 121.1, 124.3, 126.4, 127.0, 128.3, 130.8, 131.4, 137.3, 139.6, 149.0, 151.7, 163.3, 169.2; HRMS (ESI): m/z [M þ H]þ calcd for C16H13ClNO2S: 318.0356; found: 318.0350.

2.2.6. 2-(6-Fluorobenzo[d]thiazol-2-yl)-3-methylphenyl acetate 3fa 1 H NMR (400 MHz, CDCl3) d 2.07 (s, 3H), 2.35 (s, 3H), 7.06 (d, J ¼ 8.0 Hz, 1H), 7.21e7.29 (m, 2H), 7.41 (t, J ¼ 8.0 Hz, 1H), 7.61 (dd, J ¼ 2.4, 8.0 Hz, 1H), 8.06 (dd, J ¼ 4.8, 9.2 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.3, 20.8, 107.6 (d, 2JCeF ¼ 26.0 Hz), 114.9 (d, 2JCeF ¼ 25.0 Hz), 120.5, 124.6 (d, 3JCeF ¼ 10.0 Hz), 126.5, 128.3, 130.7, 137.1 (d, 3JCeF ¼ 11.0 Hz), 139.6, 149.1, 149.8, 159.4 (d, 1JCeF ¼ 245.0 Hz), 162.4 (d, 4JCeF ¼ 4.0 Hz), 169.3; HRMS (ESI): m/z [M þ H]þ calcd for C16H13FNO2S: 302.0651; found: 302.0651. 2.2.7. 2-(Benzo[d]thiazol-2-yl)-3-fluorophenyl acetate 3ga 1 H NMR (400 MHz, CDCl3) d 2.31 (s, 3H), 7.07 (d, J ¼ 8.0 Hz, 1H), 7.17 (t, J ¼ 8.8 Hz, 1H), 7.41e7.53 (m, 3H), 7.94 (d, J ¼ 8.0 Hz, 1H), 8.10 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.2, 114.1 (d, 2JCe 3 F ¼ 22.0 Hz), 116.1 (d, JCeF ¼ 14.0 Hz), 119.8, 121.4, 123.4, 125.7, 126.3, 131.5 (d, 3JCeF ¼ 10.0 Hz), 135.6, 149.5, 152.8, 157.2, 160.8 (d, 1 JCeF ¼ 252.0 Hz), 169.4; HRMS (ESI): m/z [M þ H]þ calcd for C15H11FNO2S: 288.0495; found: 288.0499. 2.2.8. 3-Fluoro-2-(6-methylbenzo[d]thiazol-2-yl)phenyl acetate 3ha 1 H NMR (400 MHz, CDCl3) d 2.29 (s, 3H), 2.50 (s, 3H), 7.05 (d, J ¼ 8.0 Hz, 1H), 7.14 (t, J ¼ 9.2 Hz, 1H), 7.31 (d, J ¼ 8.0 Hz, 1H), 7.44 (d, J ¼ 6.8 Hz, 1H), 7.72 (s, 1H), 7.97 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.2, 21.6, 114.0 (d, 2JCeF ¼ 23.0 Hz), 116.3 (d, 3JCe 3 F ¼ 14.0 Hz), 119.8, 121.0, 123.0, 127.9, 131.3 (d, JCeF ¼ 10.0 Hz), 1 135.8, 135.9, 149.5, 150.9, 156.0, 160.8 (d, JCeF ¼ 252.0 Hz), 169.4; HRMS (ESI): m/z [M þ H]þ calcd for C16H13FNO2S: 302.0651; found: 302.0651. 2.2.9. 2-(6-Chlorobenzo[d]thiazol-2-yl)-3-fluorophenyl acetate 3ia 1 H NMR (400 MHz, CDCl3) d 2.32 (s, 3H), 7.07 (d, J ¼ 8.0 Hz, 1H), 7.18 (t, J ¼ 8.8 Hz, 1H), 7.46e7.50 (m, 2H), 7.92 (s, 1H), 7.99 (d, J ¼ 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.2, 114.1 (d, 2JCe 3 F ¼ 23.0 Hz), 115.7 (d, JCeF ¼ 14.0 Hz), 119.9, 121.0, 124.3, 127.2, 131.7, 131.8, 136.7, 149.4, 151.2, 157.7, 160.8 (d, 1JCeF ¼ 253.0 Hz), 169.3; HRMS (ESI): m/z [M þ K]þ calcd for C15H9ClFKNO2S: 359.9664; found: 359.9646. 2.2.10. 3-Fluoro-2-(6-fluorobenzo[d]thiazol-2-yl)phenyl acetate 3ja 1 H NMR (400 MHz, CDCl3) d 2.32 (s, 3H), 7.07 (d, J ¼ 8.4 Hz, 1H), 7.17 (t, J ¼ 9.2 Hz, 1H), 7.25 (t, J ¼ 8.8 Hz, 1H), 7.48 (t, J ¼ 6.8 Hz, 1H), 7.62 (d, J ¼ 8.0 Hz, 1H), 8.03 (q, J ¼ 4.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.1, 107.5 (d, 2JCeF ¼ 26.0 Hz), 114.1 (d, 2JCeF ¼ 22.0 Hz), 115.1 (d, 2JCeF ¼ 24.0 Hz), 115.8 (d, 2JCeF ¼ 25.0 Hz), 119.9, 124.5 (d, 3 JCeF ¼ 9.0 Hz), 131.6 (d, 3JCeF ¼ 11.0 Hz), 136.6, 136.7, 149.4, 156.9, 160.7 (d, 1JCeF ¼ 245.0 Hz), 160.8 (d, 1JCeF ¼ 253.0 Hz), 169.3; HRMS (ESI): m/z [M þ H]þ calcd for C15H10F2NO2S: 306.0400; found: 306.0389. 2.2.11. 2-(Benzo[d]thiazol-2-yl)-3-chlorophenyl acetate 3ka 1 H NMR (400 MHz, CDCl3) d 2.05 (s, 3H), 7.18 (q, J ¼ 3.2 Hz, 1H), 7.44e7.48 (m, 3H), 7.54 (t, J ¼ 7.2 Hz, 1H), 7.96 (d, J ¼ 8.0 Hz, 1H), 8.16 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.7, 121.6, 122.0, 123.8, 125.7, 126.3, 126.9127.7, 131.3, 134.9, 136.2, 150.0, 152.9, 160.2, 169.0; HRMS (ESI): m/z [M þ K]þ calcd for C15H10ClKNO2S: 341.9758; found: 341.9745. 2.2.12. 3-Chloro-2-(6-methylbenzo[d]thiazol-2-yl)phenyl acetate 3la 1 H NMR (400 MHz, CDCl3) d 2.05 (s, 3H), 2.52 (s, 3H), 7.15e7.18 (m, 1H), 7.35 (d, J ¼ 8.0 Hz, 1H), 7.42e7.45 (m, 2H), 7.74 (s, 1H), 8.03 (d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.7, 21.6, 121.3, 121.9, 123.3, 127.0, 127.7, 127.9, 131.2, 134.9, 136.0, 136.5, 150.1, 151.1, 159.1, 169.0; HRMS (ESI): m/z [M þ H]þ calcd for C16H13ClNO2S: 318.0356; found: 318.0349.

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2.2.13. 3-Chloro-2-(6-chlorobenzo[d]thiazol-2-yl)phenyl acetate 3ma 1 H NMR (400 MHz, CDCl3) d 2.06 (s, 3H), 7.18 (d, J ¼ 7.2 Hz 1H), 7.43e7.51 (m, 3H), 7.93 (s, 1H), 8.06 (d, J ¼ 8.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.7, 121.2, 122.0, 124.6, 126.4, 127.2, 127.8, 131.6, 131.8, 134.8, 137.4, 150.0, 151.5, 160.7, 168.9; HRMS (ESI): m/z [M þ K]þ calcd for C15H9Cl2KNO2S: 375.9368; found: 375.9362. 2.2.14. 3-Chloro-2-(6-fluorobenzo[d]thiazol-2-yl)phenyl acetate 3na 1 H NMR (400 MHz, CDCl3) d 2.06 (s, 3H), 7.17 (d, J ¼ 6.8 Hz, 1H), 7.27 (t, J ¼ 8.0 Hz, 1H), 7.42e7.47 (m, 2H), 7.63 (d, J ¼ 7.2 Hz, 1H), 8.09 (q, J ¼ 4.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.7, 107.7 (d, 2 JCeF ¼ 16.0 Hz), 115.1 (d, 2JCeF ¼ 24.0 Hz), 122.0, 124.8 (d, 3JCe 3 F ¼ 10.0 Hz), 126.5, 127.8, 131.5, 134.8, 137.3 (d, JCeF ¼ 12.0 Hz), 149.6, 150.0, 159.9, 160.8 (d, 1JCeF ¼ 246.0 Hz), 168.9; HRMS (ESI): m/z [M þ K]þ calcd for C15H9ClFKNO2S: 359.9664; found: 359.9649. 2.2.15. 2-(Benzo[d]thiazol-2-yl)-4-chlorophenyl acetate 3oa 1 H NMR (400 MHz, CDCl3) d 2.46 (s, 3H), 7.18 (d, J ¼ 8.8 Hz, 1H), 7.38e7.44 (m, 2H), 7.50 (d, J ¼ 7.6 Hz, 1H), 7.91 (d, J ¼ 8.0 Hz, 1H), 8.08 (d, J ¼ 8.0 Hz, 1H), 8.35 (d, J ¼ 2.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.7, 121.4, 123.6, 125.1, 125.7, 126.6, 127.5, 129.7, 131.1, 132.0, 135.5, 146.7, 152.7, 160.8, 168.8; HRMS (ESI): m/z [M þ H]þ calcd for C15H11ClNO2S: 304.0199; found: 304.0197. 2.2.16. 4-Chloro-2-(6-chlorobenzo[d]thiazol-2-yl)phenyl acetate 3pa 1 H NMR (400 MHz, CDCl3) d 2.46 (s, 3H), 7.18 (d, J ¼ 8.4 Hz, 1H), 7.42e7.46 (m, 2H), 7.86 (d, J ¼ 1.6 Hz, 1H), 7.96 (d, J ¼ 8.8 Hz, 1H), 8.31 (d, J ¼ 2.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 21.7, 121.0, 124.3, 125.0, 126.9, 127.4, 129.6, 131.4, 131.7, 132.1, 136.6, 146.7, 151.1, 161.1, 168.6; HRMS (ESI): m/z [M þ H]þ calcd for C15H10Cl2NO2S: 337.9809; found: 337.9812. 2.2.17. 2-(Benzo[d]thiazol-2-yl)-4-methylphenyl acetate 3qa 1 H NMR (400 MHz, CDCl3) d 2.43 (s, 3H), 2.44 (s, 3H), 7.11 (d, J ¼ 8.8 Hz, 1H), 7.29 (d, J ¼ 8.4 Hz, 1H), 7.38 (t, J ¼ 8.0 Hz, 1H), 7.49 (t, J ¼ 7.6 Hz, 1H), 7.90 (d, J ¼ 8.0 Hz, 1H), 8.08 (d, J ¼ 8.0 Hz, 1H), 8.12 (s, 1H); 13C NMR (100 MHz, CDCl3) d 20.9, 21.7, 121.3, 123.3, 123.5, 125.3, 125.6, 126.3, 130.4, 132.2, 135.4, 136.3, 146.2, 152.9, 162.7, 169.3; HRMS (ESI): m/z [M þ H]þ calcd for C16H14NO2S: 284.0745; found: 284.0741. 2-(benzo[d]thiazol-2-yl)-4-methyl-1,3-phenylene diacetate 4qa. 1 H NMR (400 MHz, CDCl3) d 2.22 (s, 3H), 2.23 (s, 3H), 2.25 (s, 3H), 7.08 (d, J ¼ 8.0 Hz, 1H), 7.38 (d, J ¼ 8.0 Hz, 1H), 7.43 (t, J ¼ 8.0 Hz, 1H), 7.51 (t, J ¼ 8.0 Hz, 1H), 7.93 (d, J ¼ 7.6 Hz, 1H), 8.09 (d, J ¼ 8.0 Hz, 1H); 13 C NMR (100 MHz, CDCl3) d 20.8, 21.1, 22.7, 120.7, 120.9, 121.4, 123.5, 125.5, 126.2, 129.7, 132.6, 135.7, 147.4, 147.9, 152.9, 158.9, 168.5, 169.0; HRMS (ESI): m/z [M þ H]þ calcd for C18H16NO4S: 342.0800; found: 342.0804. 2.2.18. 4-Methyl-2-(6-methylbenzo[d]thiazol-2-yl)phenyl acetate 3ra 1 H NMR (400 MHz, CDCl3) d 2.42 (s, 3H), 2.44 (s, 3H), 2.49 (s, 3H), 7.10 (d, J ¼ 8.0 Hz, 1H), 7.26e7.31 (m, 2H), 7.69 (s, 1H), 7.95 (d, J ¼ 8.0 Hz, 1H), 8.09 (s, 1H); 13C NMR (100 MHz, CDCl3) d 20.9, 21.6, 21.7, 121.0, 122.8, 123.4, 125.7, 128.0, 130.3, 132.0, 135.5, 135.6, 136.3, 146.0, 151.1, 161.6, 169.4; HRMS (ESI): m/z [M þ H]þ calcd for C17H16NO2S: 298.0902; found: 298.0905. 4-methyl-2-(6-methylbenzo[d]thiazol-2-yl)-1,3-phenylene diacetate 4ra. 1H NMR (400 MHz, CDCl3) d 2.20 (s, 3H), 2.21 (s, 3H), 2.23 (s, 3H), 2.50 (s, 3H), 7.06 (d, J ¼ 8.4 Hz, 1H), 7.30e7.36 (m, 2H), 7.71 (s, 1H), 7.96 (d, J ¼ 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 16.4, 20.8, 21.1, 21.6, 120.8, 120.9, 121.0, 123.0, 127.8, 129.6, 132.5, 135.8, 135.9, 147.3, 147.8, 151.0, 157.7, 168.6, 169.1; HRMS (ESI): m/z [M þ H]þ calcd for C19H18NO4S: 356.0957; found: 356.0952.

35

2.2.19. 2-(6-Chlorobenzo[d]thiazol-2-yl)-4-methylphenyl acetate 3sa 1 H NMR (400 MHz, CDCl3) d 2.42 (s, 3H), 2.44 (s, 3H), 7.11 (d, J ¼ 8.0 Hz, 1H), 7.29 (d, J ¼ 8.0 Hz, 1H), 7.43 (d, J ¼ 8.0 Hz, 1H), 7.86 (d, J ¼ 2.0 Hz, 1H), 7.95 (d, J ¼ 8.8 Hz, 1H), 8.09 (s, 1H); 13C NMR (100 MHz, CDCl3) d 20.9, 21.7, 120.9, 123.5, 124.0, 125.2, 127.2, 130.3, 131.2, 132.5, 136.4, 136.6, 146.1, 151.4, 163.1, 169.2; HRMS (ESI): m/z [M þ H]þ calcd for C16H13ClNO2S: 318.0356; found: 318.0350. 2-(6-chlorobenzo[d]thiazol-2-yl)-4-methyl-1,3-phenylene diacetate 4sa. 1H NMR (400 MHz, CDCl3) d 2.14 (s, 3H), 2.16 (s, 3H), 2.17 (s, 3H), 7.01 (d, J ¼ 8.4 Hz, 1H), 7.31 (d, J ¼ 8.4 Hz, 1H), 7.39 (dd, J ¼ 1.6, 8.4 Hz, 1H), 7.83 (d, J ¼ 1.6 Hz, 1H), 7.91 (d, J ¼ 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 16.6, 21.1, 21.4, 120.7, 121.3, 124.6, 127.4, 130.1, 132.0, 133.1, 133.2, 137.3, 147.7, 148.2, 151.8, 159.8, 168.6, 169.2; HRMS (ESI): m/z [M þ H]þ calcd for C18H15ClNO4S: 376.0410; found: 376.0401. 2.2.20. 2-(6-Fluorobenzo[d]thiazol-2-yl)-4-methylphenyl acetate 3ta 1 H NMR (400 MHz, CDCl3) d 2.42 (s, 3H), 2.44 (s, 3H), 7.11 (d, J ¼ 8.4 Hz, 1H), 7.22 (dt, J ¼ 2.8, 8.8 Hz, 1H), 7.28 (dd, J ¼ 1.6, 8.4 Hz, 1H), 7.57 (dd, J ¼ 2.4, 8.0 Hz, 1H), 8.00 (dd, J ¼ 4.8, 8.8 Hz, 1H), 8.07 (d, J ¼ 1.6 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.9, 21.7, 107.3 (d, 2 JCeF ¼ 26.0 Hz), 114.9 (d, 2JCeF ¼ 24.0 Hz), 123.4, 124.2 (d, 3JCe F ¼ 9.0 Hz), 125.3, 130.2, 132.3, 136.3, 136.5, 146.1, 149.6, 160.5 (d, 1 JCeF ¼ 244.0 Hz), 162.4 (d, 4JCeF ¼ 3.0 Hz), 169.2; HRMS (ESI): m/z [M þ H]þ calcd for C16H13FNO2S: 302.0651; found: 302.0651. 2.2.21. 2-(Benzo[d]thiazol-2-yl)-3-methylphenyl pivalate 3cb 1 H NMR (400 MHz, CDCl3) d 1.00 (s, 9H), 2.29 (s, 3H), 7.02 (d, J ¼ 8.0 Hz, 1H), 7.19 (d, J ¼ 7.6 Hz, 1H), 7.39 (d, J ¼ 8.0 Hz, 1H), 7.43 (d, J ¼ 8.0 Hz, 1H), 7.51 (t, J ¼ 8.0 Hz, 1H), 7.92 (d, J ¼ 8.0 Hz, 1H), 8.11 (d, J ¼ 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 19.9, 26.8, 38.9, 120.2, 121.4, 123.5, 125.3, 126.1, 127.1, 127.7, 130.5, 136.1, 139.4, 149.5, 153.2, 162.9, 176.6; HRMS (ESI): m/z [M þ H]þ calcd for C19H20NO2S: 326.1215; found: 326.1210. 2.2.22. 2-(6-Fluorobenzo[d]thiazol-2-yl)-3-methylphenyl pivalate 3fb 1 H NMR (400 MHz, CDCl3) d 1.01 (s, 9H), 2.29 (s, 3H), 7.02 (d, J ¼ 8.0 Hz, 1H), 7.19 (d, J ¼ 7.6 Hz, 1H), 7.24 (dt, J ¼ 2.4, 8.0 Hz, 1H), 7.40 (t, J ¼ 8.0 Hz, 1H), 7.60 (dd, J ¼ 2.4, 8.0 Hz, 2H), 8.06 (dd, J ¼ 4.4, 8.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 19.9, 26.7, 38.9, 107.6 (d, 2 JCeF ¼ 27.0 Hz), 114.9 (d, 2JCeF ¼ 25.0 Hz), 120.3, 124.4 (d, 3JCe 3 F ¼ 9.0 Hz), 126.7, 127.7, 130.7, 137.0 (d, JCeF ¼ 9.0 Hz), 139.3, 149.4, 149.8, 160.5 (d, 1JCeF ¼ 245.0 Hz), 162.7 (d, 3JCeF ¼ 4.0 Hz), 176.6; HRMS (ESI): m/z [M þ H]þ calcd for C19H19FNO2S: 344.1121; found: 344.1125. 2.2.23. 4-Methyl-2-(6-methylbenzo[d]thiazol-2-yl)phenyl pivalate 3rb 1 H NMR (400 MHz, CDCl3) d 1.40 (s, 9H), 2.41 (s, 3H), 2.47 (s, 3H), 6.99 (d, J ¼ 8.0 Hz, 1H), 7.25 (d, J ¼ 8.4 Hz, 1H), 7.27 (d, J ¼ 8.4 Hz, 1H), 7.68 (s, 1H), 7.93 (s, 1H), 7.94 (d, J ¼ 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.8, 21.6, 27.3, 39.2, 121.1, 122.8, 123.3, 126.3, 127.9, 130.8, 131.9, 135.4, 135.7, 135.9, 146.7, 151.4, 162.1, 177.1; HRMS (ESI): m/z [M þ H]þ calcd for C20H22NO2S: 340.1371; found: 340.1376. 2.2.24. 2-(6-Fluorobenzo[d]thiazol-2-yl)-4-methylphenyl pivalate 3tb 1 H NMR (400 MHz, CDCl3) d 1.40 (s, 9H), 2.41 (s, 3H), 6.99 (d, J ¼ 8.4 Hz, 1H), 7.21 (dt, J ¼ 2.4, 8.8 Hz, 1H), 7.27 (dd, J ¼ 1.6, 8.0 Hz, 1H), 7.57 (dd, J ¼ 1.6, 8.0 Hz, 1H), 7.91 (d, J ¼ 1.6 Hz, 1H), 7.99 (dd, J ¼ 4.8, 8.8 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 20.8, 27.3, 39.2, 107.5 (d, 2JCeF ¼ 26.0 Hz), 114.9 (d, 2JCeF ¼ 25.0 Hz), 123.3, 124.2 (d, 3 JCeF ¼ 9.0 Hz), 125.9, 130.7, 132.3, 136.1, 136.5 (d, 3JCeF ¼ 11.0 Hz), 146.7, 149.9, 160.5 (d, 1JCeF ¼ 245.0 Hz), 162.9 (d, 4JCeF ¼ 3.0 Hz), 177.1; HRMS (ESI): m/z [M þ H]þ calcd for C19H19FNO2S: 344.1121; found: 344.1118.

36

Q. Ding et al. / Journal of Organometallic Chemistry 739 (2013) 33e39

3. Results and discussion Initial investigations were performed using the reaction of 2phenylbenzo[d]thiazole 1a with PhI(OAc)2 in CH3CN as the model reaction and Pd(OAc)2 as the catalyst in sealed tube. The results showed that the effect of temperature was important for this transformation. No reaction takes place at room temperature (Table 1, entry 1). When temperature is changed from room temperature to 60  C, low yield 24% of mono-acetoxylation product was observed (Table 1, entry 2). If the temperature rises even further, the results appear obvious better. When the reaction was performed at 70  C, 2-phenylbenzo[d]thiazole was converted to diacetoxylated product 4aa in a yield of 66% together with a small amount (12%) of mono-acetoxylation product 3aa (Table 1, entry 3). Further studies indicate that better result 82% 4aa with 8% 3aa was isolated when temperature is 90  C (Table 1, entry 4). Several other palladium sources, such as Pd(PhCN)2Cl2, Pd(CH3CN)2Cl2, Pd(PPh3)2Cl2, PdCl2, and Pd3(dba)2 were screened. To our surprise, the yields were far from satisfactory in this transformation (Table 1, entries 5e9). Subsequently, we investigated the acetoxylation reaction in various solvents. The results suggested that the solvent was also crucial for this transformation. Other solvents, such as DMF, DMA, THF, dioxane, and toluene were not exhibit higher catalytic activity (Table 1, entries 10e14). Further examination showed that changing the amount of PhI(OAc)2 did not effect the yield distinctly, but affected the ratio of di- to mono-acetoxylated compound apparently. Reducing the oxidant PhI(OAc)2 loading decreased the ratio of 4aa to 3aa dramatically, while increasing the loading improved it visibly (Table 1, entry 4 vs entries 15e17). For example, diacetoxylated product 4aa was obtained in 84% yield with only trace of mono-acetoxylation product 3aa when 5 equiv of PhI(OAc)2 was used (Table 1, entry 17). With the optimized conditions in hand, we next explored the effect of the benzo[d]thiazole directing group on the ortho CeH

acetoxylation of arenes. The results showed that the benzo[d] thiazole group directed CeH bond functionalization protocol is broadly applicable to various substitutes affording corresponding acetoxylated products in moderate to good yields (Table 2). At first, the ortho CeH acetoxylation of electronically neutral compound 6methyl-2-phenylbenzo[d]thiazole 1b was investigated (Table 2, entry 2). The di-acetoxylated product 4ba was obtained as the major product in 63% yield with trace of mono-acetoxylated product 3ba, and the ratio of 4ba:3ba determined by 1H NMR is >30:1. Subsequently, all of the substrates bearing either an electron-donating or electron-withdrawing ortho-substituents at the phenyl could be applied to form the desired products efficiently (Table 2, entries 3e14). Furthermore, comparable yields were obtained regardless of the electronic nature of R2. Mono-acetoxylated products 3caena were isolated in moderate to good yields from ortho-substituted 2-arylbenzo[d]thiazoles 1cen. For example, the reaction of 2-(o-tolyl)benzo[d]thiazole 1c under the standard conditions gave 3ca in 71% yield. A similar yield 70% of the monoacetoxylated product 3ka was isolated from the reaction of 2-(2chlorophenyl)benzo[d]thiazole 1k. Then, the ortho acetoxylation of meta-substituted 2-arylbenzo[d]thiazoles was also investigated. For substrates containing two ortho CeH bonds, the reaction gave monoacetoxylated product 3 as a major product and the diacetoxylated product 4 were isolated in small amounts in most cases. The efficiency of this transformation was relatively high when an electron-donating substituent is present (Table 2, entries 15 and 16 vs entries 17 and 19). However, the selectivity of mono- to diacetoxylation was comparatively low when substrate with an electron-donating substituent (Table 2, entries 17 and 18). For example, the reaction of 2-(3-chlorophenyl)benzo[d]thiazole 1o containing an electron-withdrawing chloro substituent gave the mono- and di-acetoxylated products 3oa and 4oa in 60% (isolated) and 8% (determined by 1H NMR) yields respectively, with better regioselectivity (Table 2, entry 15). However, the reaction of 2-(m-

Table 1 Reaction optimization for the CeH acetoxylation of 1a.a

Entry

Pd (cat)

Solvent

T/ C

Yield (%)b 3aa

Yield (%)b 4aa

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

Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 PdCl2 Pd2(dba)3 Pd(PPh3)2Cl2 Pd(CH3CN)2Cl2 Pd(PhCN)2Cl2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN DMF THF Dioxane DMA Toluene CH3CN CH3CN CH3CN

r.t. 60  C 70  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C 90  C

NR 24 12 8 12 8 Trace Trace Trace 60 NR 16 10 30 31 4 Trace

e e 66 82 e e e e e Trace e Trace e 20 46 82 84

a b c d e

Reaction conditions: 2-phenylbenzo[d]thiazole 1a (0.3 mmol), PhI(OAc)2 2a (3 equiv, 0.9 mmol), Pd(OAc)2 (5 mol%), solvent (1 mL), 6 h. Isolated yield based on 1a. The ratio of 1a/2a ¼ 1/2. The ratio of 1a/2a ¼ 1/4. The ratio of 1a/2a ¼ 1/5.

Q. Ding et al. / Journal of Organometallic Chemistry 739 (2013) 33e39

37

Table 2 Ortho-Acetoxylation of 2-arylbenzo[d]thiazole 1a

Entry

1c

Starting material 1

Major product

1a

Yield (%)b

Ratio of 3:4

84 (4aa)

e

63 (4ba)

<1:30d

71 (3ca)

e

51 (3da)

e

70 (3ea)

e

50 (3fa)

e

65 (3ga)

e

53 (3ha)

e

48 (3ia)

e

56 (3ja)

e

70 (3ka)

e

55 (3la)

e

4aa

2c

1b

4ba

3

1c

3ca

4

1d

3da

5

1e

3ea

6

1f

3fa

7

1g

3ga

8

1h

3ha

9

1i

3ia

10

1j

3ja

11

1k

3ka

12

1l

3la

13

75 (3ma)

1m

3ma

e (continued on next page)

38

Q. Ding et al. / Journal of Organometallic Chemistry 739 (2013) 33e39

Table 2 (continued ) Entry

Starting material 1

Major product

14

1n

1p

1q

1r

1s

62 (3pa þ 4pa)

4:1e

86 (3qa þ 4qa)

1.8:1e

90 (3ra þ 4ra)

1:1e

87 (3sa þ 4sa)

2.3:1e

55 (3ta)

>20:1d

3sa

20

1t

e

5:1e

3ra

19

c

68 (3oa þ 4oa)

3qa

18

d

e

3pa

17

a

70 (3na)

3oa

16

b

Ratio of 3:4

3na

15

1o

Yield (%)b

3ta

Reaction conditions: 2-arylbenzo[d]thiazole 1 (0.3 mmol), PhI(OAc)2 2a (3 equiv, 0.9 mmol), Pd(OAc)2 (5 mol%), CH3CN (1 mL), 90  C, 6e12 h. Isolated yield based on 1. PhI(OAc)2 2a (5 equiv, 1.5 mmol). Determined by 1H NMR of crude product. Based on the isolated yield of 3 and 4.

tolyl)benzo[d]thiazole 1q containing an electron-donating methyl group under the catalytic conditions afforded 3qa and 4qa in 55% and 31% yields, respectively (Table 2, entry 17). The difference of the efficiency and the selectivity is obvious from the electronic effect of the substituent. Therefore, it is not surprising that 6-chloro-2-(3chlorophenyl)benzo[d]thiazole 1p was also acetoxylated in comparably low yield 62% and relatively high regioselectivity 4/1 (Table 2, entry 16), while 6-chloro-2-(m-tolyl)benzo[d]thiazole 1s with high yield 87% and low regioselectivity 2.3/1 (Table 2, entry 19). However, in case of 6-fluoro-2-(m-tolyl)benzo[d]thiazole 1t, afforded mainly the mono-acetoxylated product 3ta in a moderate

yield 53% (Table 2, entry 20). The results suggest that the efficiency and selectivity is governed by combination of steric and electronic effect of the substrate. Next, we explored the acyloxylation reaction of 6-methyl-2phenylbenzo[d]thiazole 1c with PhI(OPiv)2 2b, and found that good yield was obtained. Then, other acyloxylation reactions were also investigated as shown in Scheme 1. The results showed that substrates containing an ortho substituent on the aromatic ring, single product was obtained with good yield. While to those with a meta substituent on the aromatic ring, good regioselectivity was observed for acyloxylation of the less sterically hindered o-CeH bond.

Scheme 1. ortho Acyloxylation of 2-arylbenzo[d]thiazole 1 with PhI(OPiv)2 2b (Reaction conditions are the same as in Table 2).

Q. Ding et al. / Journal of Organometallic Chemistry 739 (2013) 33e39

39

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Scheme 2. Proposed catalytic cycle.

Based on well-documented literature on palladium-catalyzed CeH acyloxylation reaction of arenes [14,23e37], a possible pathway involving a PdII/IV species was proposed in Scheme 2. Firstly, the N of benzo[d]thiazole coordinated with Pd(OAc)2 to form a fivemembered cyclic Pd complex A, which might give rise to the PdIV intermediate B through the oxidation by PhI(OAc)2. Subsequently, reductive elimination would afford the desired product 3 and regenerate the active catalyst PdII species for the next catalytic cycle. Of course we still cannot exclude the mechanistic possibility through a Pd0/PdII or PdIII pathway in the present acyloxylation reaction [48]. 4. Conclusion In conclusion, we have developed a chelation-assisted Pdcatalyzed ortho-acyloxylation reaction of the 2-arylbenzo[d]thiazole sp2 CeH bond, affording mono- or di-acyloxylation products in moderate to good yields. A wide substrate scope with good functional group tolerance has been demonstrated. Although the results are not comparable with the similar approach developed by Yu and Sanford, a new and useful CeH functionalization/oxidative acyloxylation has been described. This method is an alternative route for the preparation of 2-arylbenzo[d]thiazole derivatives via a CeH activation mechanism.

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]

Acknowledgments Financial Supported from National Natural Science Foundation of China (21002042), Jiangxi Educational Committee (GJJ12169), and Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201217) is gratefully acknowledged.

[39]

[40] [41] [42]

Appendix A. Supplementary data

[43] [44] [45]

Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.jorganchem.2013.04.026.

[46] [47] [48]

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