Efficient, versatile and practical palladium-catalyzed highly regioselective ortho-halogenation of azoxybenzenes

Abstract

A highly efficient and practical strategy for regio-selective ortho-halogenation (I, Br, Cl) of azoxybenzenes with NXS in the presence of palladium catalysts has been developed in good to excellent yields. The reaction proceeds smoothly and can tolerate a variety of functional groups. Moreover, this chemistry can be applied to substrates in at least a gram scale.

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Introduction

Over the past decade, aryl halides have emerged as extremely valuable starting materials for constructing complex skeletons in synthetic elaboration due to their role as precursors for the synthesis of organometallic reagents,¹ nucleophilic aromatic substitution² as well as transition-metal catalyzed cross-coupling reactions.³ Generally, the most common methods for the synthesis of halogenated arenes involve electrophilic aromatic substitution⁴ and ortho-lithiation followed by a halogen quench,⁵ which shows many limitations and suffers from several notable disadvantages.^6,7b With regard to the development of selective halogenation reactions, the emergence of direct C–H functionalization/halogenation provides an atom-economic strategy,^8,9 which produces halogenated aromatic products with high selectivity that would not ordinarily be obtained under traditional conditions. Since the pioneering work of Sanford,⁷ Shi¹⁰ and Yu,¹¹ the groups of Bedford¹² and Xu¹³ illustrated the major importance of these ortho-selective transformations with the assistance of diverse directing groups. However, although significant progress in C–H bond functionalization/halogenation has been achieved, application of this strategy to create carbon–halogen bonds for more diverse structures is still surprisingly underdeveloped (Scheme 1).

Scheme 1 Pd-catalyzed ortho-halogenation of azoxybenzenes with NXS.

Owing to their unique structure and liquid crystalline properties,¹⁴ azoxy compounds are able to serve as polymer inhibitors, stabilizers, dyes and key materials in electronic devices.¹⁵ Furthermore, it is well known that regioselective control of C–H bond activation in azoxy compounds is an extreme challenge, due to the two different coordinating sites provided by the directing groups¹⁶ (N or O). In spite of the development of numerous methods for the synthesis of azoxy compounds,^9l,17 exploration in the sterically hindered ortho-functionalization of azoxybenzenes is very limited.¹⁸ Recently, Wang^18a and ourselves^18c,d reported the ortho-acylation of azoxybenzenes, and subsequently a method for the alkenylation of azoxy compounds was established.^18b As far as we know, there are no reports in the curet literature on the ortho-halogenation of azoxybenzenes, which represent a profoundly more desirable target as they are synthetically highly important yet this goal is still unmet. Encouraged by our previous research¹⁹ and as part of our ongoing interest in C–H activation, herein we describe a novel and efficient approach to the synthesis of a variety of azoxy halides by Pd-catalyzed ortho-C–H bond halogenation employing commercially available electrophilic halogenating reagents—particularly N-bromo-, N-chloro-, and N-iodosuccinimide.

Results and discussion

Our initial investigation was carried out by examining azoxybenzene (1a, 1.0 equiv.) and NBS (1.1 equiv.) in the presence of 10 mol% Pd(OAc)₂ in AcOH at 100 °C for 12 h (Table 1, entry 1). To our delight, the ortho-bromo-product was isolated in 87% yield, and further screening of solvents showed no difference in this reaction (Table 1, entries 2–5). Inspired by the previous discovery of the large beneficial effects of Brønsted acids in promoting the electrophilicity of palladium(II) catalysts²⁰ and rendering NBS a more effective source of Br⁺,²¹ we attempted to improve the desired product yield. Much to our surprise, this reaction was dramatically affected by the addition of Brønsted acids, and the corresponding yield increased to 85% in DCE with p-toluenesulfonic acid (PTSA) as an additive in contrast to 18% without the acid (Table 1, entry 9). Other Brønsted acids, including acetic acid and trifluoroacetic acid, also proved to be effective. It was found that the reaction temperature was crucial to this transformation, and the best result (97%) was observed at 40 °C (Table 1, entry 13), while increasing or lowering the reaction temperature lowered the efficiency. Subsequently, different Pd catalysts were also investigated. Only Pd(OAc)₂ facilitated this procedure with any appreciable conversion. Finally, the optimized reaction conditions for the ortho-C–H halogenation of azoxy compounds were determined to be NBS (1.1 equiv.) with 10 mol% Pd(OAc)₂ as the catalyst, 0.5 equiv. of PTSA as additive, and DCE as solvent, at 40 °C under air for 12 h.

Table 1 Optimization of the reaction condition^a,b

Entry	Catalyst	Additive	Solvent	Yield^b [%]
a All the reactions were carried out in the presence of 0.2 mmol of 1a, 0.22 mmol of NBS and 0.1 mmol of acid (if any) in 1.0 mL of solvents at 100 °C under air condition. b Isolated yields. c At 80 °C. d At 60 °C. e At 40 °C. f At room temperature.
1	Pd(OAc)2		AcOH	87%
2	Pd(OAc)2		DCE	18%
3	Pd(OAc)2		DME	n.r.
4	Pd(OAc)2		Toluene	Trace
5	Pd(OAc)2		MeCN	55%
6	Pd(OAc)2	AcOH	MeCN	56%
7	Pd(OAc)2	TsOH·H2O	MeCN	58%
8	Pd(OAc)2	AcOH	DCE	30%
9	Pd(OAc)2	TsOH·H2O	DCE	85%
10	Pd(OAc)2	TFA	DCE	69%
11	Pd(OAc)2	TsOH·H2O	DCE	88%^c
12	Pd(OAc)2	TsOH·H2O	DCE	92%^d
13	Pd(OAc)2	TsOH·H2O	DCE	97%^e
14	Pd(OAc)2	TsOH·H2O	DCE	80%^f
15	PdCl2	TsOH·H2O	DCE	60%
16	Pd(TFA)2	TsOH·H2O	DCE	62%
17	Pd(PPh3)4	TsOH·H2O	DCE	57%

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With a reliable protocol in hand, the scope of the ortho-bromination of azoxy compounds was investigated. The results, summarized in Table 2, revealed that the reaction had good compatibility with several functional groups, and good to excellent yields were obtained for most cases. For example, nearly quantitative yields were obtained with methyl group substituted substrates, and an interesting regioselectivity was found with meta-substituted aromatic rings due to the steric hindrance (Table 2, 2ba–2da). It is noteworthy that the reaction can tolerate various halogen groups such as fluoro, chloro and bromo, which afford the desired products in good yields under a slightly higher temperature and could be used for further transformations into other important skeletons (Table 2, 2fa–2ha). However, in the presence of methoxyl, a strongly electron-donating group, the corresponding product was obtained in a slightly lower efficiency along with unidentified by-products (Table 2, 2ea). Moreover, when a trifluoromethyl-substituted azoxy compound was exposed to the reaction conditions, the desired bromination product was observed in moderate yield even at high temperature (Table 2, 2ia). Much to our pleasure, this transformation could also be successfully extended to unsymmetrical azoxybenzenes with excellent yields (Table 2, 2ka–2la), and the desired regioselectivity was obtained thanks to the assistance of the N-atom in the azoxy group (vide infra).

Table 2 Scope of the ortho-bromination of azoxybenzene with NBS^a,b

a All the reactions were carried out in the presence of 0.2 mmol of 1, 0.22 mmol of NBS and 0.1 mmol of TsOH·H₂O in 1.0 mL DCE at 40 °C under air condition. b Isolated yields. c At 80 °C. d At 100 °C.

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To further establish the power of this strategy, we next examined the chlorination and iodination of azoxy compounds under the same reaction conditions. Although moderate yields were obtained with simple replacement of NBS by NCS or NIS under otherwise identical conditions,²² we were pleased to observe that a subtle change in the solvent exerts significant effect on the corresponding ortho-chlorination and iodination reactions. Much to our pleasure, azoxybenzene was found to generate the ortho-chlorinated and ortho-iodinated products in 96% and 95% yields in AcOH, respectively (see ESI† for more details). More importantly, for most of the substrates we employed, moderate to good yields were observed in this reaction and good tolerance to the chemically active functional groups was also revealed (Tables 3 and 4). Interestingly, high selectivity was obtained for the chlorination and iodination reactions when meta-substituted azoxybenzene was involved, which led to the formation of the single regioselective products in excellent yields. It is important to note that this chemistry was successfully extended to polyhalogenated aromatic azoxy compounds in spite of relatively low efficiencies, which are hard to prepare by traditional methods.

Table 3 Scope of the ortho-chlorination of azoxybenzene with NCS^a,b

a All the reactions were carried out in the presence of 0.2 mmol of 1 and 0.22 mmol of NCS in 1.0 mL AcOH at 100 °C under air condition. b Isolated yields. c At 120 °C.

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Table 4 Scope of the ortho-iodination of azoxybenzene with NIS^a,b

a All the reactions were carried out in the presence of 0.2 mmol of 1 and 0.22 mmol of NIS in 1.0 mL AcOH at 60 °C under air condition. b Isolated yields. c At 80 °C. d At 120 °C.

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A distinctive feature of transition-metal catalyzed C–H functionalization is its limited reacting scale, which is a key restraining factor in the use of many strategies in organic synthesis. For this purpose, the reaction was attempted on a gram scale under the established conditions. We were delighted to observe that 2.48 g (yield 90%) of 2-bromo-azoxybenzene, 1.97 g (yield 85%) of 2-chloro-azoxybenzene and 2.66 g (yield 82%) of 2-iodo-azoxybenzene were obtained with 10 mmol (1.98 g) of azoxybenzene as the reactant and 5% Pd(OAc)₂ as the catalyst, which demonstrates the possibility to use this transformation in general organic synthesis.

Based on our experimental results and previous literature,^7,9f a possible mechanism was proposed to account for the ortho-halogenation of azoxy compounds, which is depicted in Scheme 2. Initially, a five-membered palladacycle intermediate I is generated with the assistance of the N atom by chelation-directed C–H activation, which is generally considered to be a better coordinating atom than O. Oxidative addition of NXS to the palladacycle leads to the formation of Pd(IV) complex II,^7,23 followed by reductive elimination to afford ortho-halogenated azoxy compounds and regenerate active Pd(II) to continue the catalytic cycle.

Scheme 2 Proposed reaction mechanism.

Conclusions

In summary, we have developed an efficient, versatile and practical catalytic system for the synthesis of ortho-halogenated azoxybenzenes via a regio-selective directing-group-assisted strategy. The potential for gram scale functionalization has been demonstrated and different NXS can be employed as effective halogenating reagents in this transformation. Further investigations on the application of this chemistry and other functionalizations of azoxy compounds are ongoing in our laboratory.

Experimental

General experimental information

All reactions involving air- and moisture-sensitive reagents were carried out under a nitrogen atmosphere. Toluene, DMF, 1,2-dichloroethane, DMSO, 1,4-dioxane and CH₃CN were distilled from appropriate drying agents prior to use. All chemicals were purchased from Aldrich and used without further purification. Thin-layer chromatography (TLC) was performed using 60 mesh silica gel plates visualized with short-wavelength UV light (254 nm). Silica gel 60 (230–400 mesh) was used for column chromatography. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker INOVA-400. NMR spectra were recorded on a 400 instrument (400 MHz for ¹H and 100 MHz for ¹³C). Chemical shifts (δ) were measured in ppm relative to TMS δ = 0 for ¹H, or to chloroform δ = 77.0 for ¹³C as internal standard. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constants, J, reported in hertz. Mass data were measured with Thermo Scientific DSQ II mass spectrometer. Azoxybenzenes were prepared from arylamines, according to the literature.¹

General catalytic procedure for ortho-bromination of azoxybenzenes with NBS

A mixture of azoxybenzene (39.6 mg, 0.2 mmol, 1.0 equiv.), TsOH·H₂O (19.0 mg, 0.1 mmol, 0.5 equiv.), NBS (39.2 mg, 0.22 mmol, 1.1 equiv.) and Pd(OAc)₂ (4.5 mg, 0.02 mmol, 10 mol%) in DCE (1 mL) was stirred under air at 40 °C for 12 h followed by cooling. The volatiles were removed under reduced pressure. The contents were subjected to flash chromatography to give the corresponding product (97%) as a pale yellow oil. The purified material was dried under an oil-pump vacuum.

2-(2-Bromophenyl)-2-oxo-1-phenylhydrazin-2-ium-1-ide (2aa). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.17 (d, J = 12.0 Hz, 2 H), 7.71–7.63 (m, 2 H), 7.52–7.42 (m, 4 H), 7.34 (t, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 149.33, 143.66, 134.03, 130.86, 130.26, 128.77, 128.22, 125.40, 124.81, 115.11. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₉BrN₂ONa⁺: 298.9796, Found [M + Na]⁺: 298.9744.

2-(2-Bromo-4-methylphenyl)-2-oxo-1-(p-tolyl)hydrazin-2-ium-1-ide (2ba). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.10 (d, J = 8.0 Hz, 2 H), 7.55–7.43 (m, 2 H), 7.29 (d, J = 4 Hz, 2 H), 7.22 (d, J = 8.0 Hz, 1 H), 2.41 (d, J = 8.0 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃) δ: 147.10, 141.52, 141.45, 140.86, 134.21, 129.32, 128.73, 125.52, 124.55, 114.75, 21.61, 20.93. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃BrN₂ONa⁺: 327.0103, Found [M + Na]⁺: 326.9997.

2-(2-Bromo-5-methylphenyl)-2-oxo-1-(m-tolyl)hydrazin-2-ium-1-ide (2ca). Deep red oil. ¹H NMR (400 MHz, CDCl₃) δ: 7.95 (d, J = 8.0 Hz,2 H), 7.55 (d, J = 8.0 Hz, 1 H), 7.46 (d, J = 8.0 Hz, 1 H), 7.40–7.36 (m, 1 H), 7.24 (d, J = 8.0 Hz, 1 H), 7.15–7.13 (m, 1 H), 2.42 (s, 3 H), 2.38 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 149.11, 143.71, 138.79, 138.57, 133.61, 131.62, 130.98, 128.55, 125.80, 125.29, 122.43, 111.57, 21.43, 20.79. HRMS (ESI) ([M + H]⁺) Calcd for C₁₄H₁₄BrN₂O⁺: 305.0290, Found [M + H]⁺: 305.0208.

2-(2-Bromo-6-methylphenyl)-2-oxo-1-(o-tolyl)hydrazin-2-ium-1-ide (2da). Deep yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.18–8.15 (m, 1 H), 7.53 (d, J = 4.0 Hz, 1 H), 7.34–7.19 (m, 5 H), 2.44 (s, 3 H), 2.42 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 148.80, 141.96, 134.90, 132.67, 130.96, 130.87, 130.15, 129.95, 129.12, 125.90, 121.60, 115.26, 18.48, 17.19. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃BrN₂ONa⁺: 327.0103, Found [M + Na]⁺: 327.0035.

2-(2-Bromo-4-methoxyphenyl)-1-(4-methoxyphenyl)-2-oxohydrazin-2-ium-1-ide (2ea). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.28 (d, J = 8.0 Hz, 2 H), 7.62 (d, J = 12.0 Hz, 1 H), 7.19 (d, J = 4.0 Hz, 1 H), 6.98 (d, J = 12.0 Hz, 2 H), 6.94–6.92 (m, 1 H), 3.88 (s, 3 H), 3.85 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 160.71, 160.30, 142.97, 137.76, 127.82, 125.91, 118.67, 115.97, 113.70, 113.65, 55.92, 55.51.HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃BrN₂O₃Na⁺: 359.0002, Found [M + Na]⁺: 358.9878.

2-(2-Bromo-4-fluorophenyl)-1-(4-fluorophenyl)-2-oxohydrazin-2-ium-1-ide (2fa). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.25 (dd, J = 8.0 Hz, J = 12.0 Hz, 2 H), 7.68 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H), 7.45 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H), 7.17 (t, J = 8.0 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 162.92 (d, J = 252.0 Hz), 162.16 (d, J = 253.0 Hz), 145.82, 140.10 (d, J = 3.0 Hz), 128.01(d, J = 10.0 Hz), 126.36(d, J = 9.0 Hz), 121.25(d, J = 26.0 Hz), 116.23 (d, J = 10.0 Hz), 115.76 (d, J = 23.0 Hz), 115.37 (d, J = 23.0 Hz). HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇BrF₂N₂ONa⁺: 334.9602, Found [M + Na]⁺: 334.9737.

2-(2-Bromo-4-chlorophenyl)-1-(4-chlorophenyl)-2-oxohydrazin-2-ium-1-ide (2ga). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.14 (d, J = 8.0 Hz, 2 H), 7.72 (s, 1 H), 7.62 (d, J = 8.0 Hz, 1 H), 7.47–7.42 (m, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 147.68, 141.96, 136.40, 135.90, 133.73, 129.02, 128.42, 126.94, 125.79, 115.97. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇BrCl₂N₂ONa⁺: 366.9017, Found [M + Na]⁺: 366.9335.

1-(4-Bromophenyl)-2-(2,4-dibromophenyl)-2-oxohydrazin-2-ium-1-ide (2ha). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.06 (d, J = 8.0 Hz, 2 H), 7.88 (d, J = 4.0 Hz, 1 H), 7.63–7.58 (m, 3 H), 7.54 (d, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 148.09, 142.28, 136.46, 132.02, 131.37, 127.07, 125.98, 124.29, 124.25, 116.13. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇Br₃N₂ONa⁺: 454.8001, Found [M + Na]⁺: 454.7833.

2-(2-Bromo-4-(trifluoromethyl)phenyl)-2-oxo-1-(4-(trifluoromethyl)phenyl)hydrazin-2-ium-1-ide (2ia). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.22 (d, J = 12.0 Hz, 2 H), 8.01 (s, 1 H), 7.79–7.74 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃) δ: 151.06, 145.59, 133.23 (q, J_C–F = 33.0 Hz), 131.62 (q, J_C–F = 33.0 Hz), 131.49 (d, J_C–F = 4.0 Hz), 126.00 (q, J_C–F = 3.0 Hz), 125.57, 125.40, 122.96 (q, J_C–F = 118.0 Hz), 122.95 (q, J_C–F = 118.0 Hz), 115.86. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₇BrF₆N₂ONa⁺: 434.9538, Found [M + Na]⁺: 434.9381.

2-(2-Bromo-6-methoxyphenyl)-2-oxo-1-phenylhydrazin-2-ium-1-ide (2ka). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.15 (d, J = 8.0 Hz, 2 H), 7.51–7.42 (m, 3 H), 7.26–2.25 (m, 2 H), 7.00 (d, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 152.88, 143.73, 130.59, 130.14, 128.70, 125.48, 125.38, 124.63, 116.46, 111.49, 56.52. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₃H₁₁BrN₂O₂Na⁺: 328.9902, Found [M + Na]⁺:328.9902.

2-(2,6-Dibromophenyl)-2-oxo-1-phenylhydrazin-2-ium-1-ide (2la). White solid. ¹H NMR (400 MHz, CDCl₃) δ: 8.15 (d, J = 8.0 Hz, 2 H), 7.65 (d, J = 8.0 Hz, 2 H), 7.54–7.46 (m, 3 H), 7.20 (t, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 143.40, 132.59, 130.92, 130.57, 128.84, 125.42, 116.65. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₈Br₂N₂ONa⁺: 376.8901, Found [M + Na]⁺: 376.8903.

General catalytic procedure for ortho-chlorination/iodination of azoxybenzenes with NCS/NIS

A mixture of azoxybenzene (39.6 mg, 0.2 mmol, 1.0 equiv.), NCS/NIS (0.22 mmol, 1.1 equiv.) and Pd(OAc)₂ (4.5 mg, 0.02 mmol, 10 mol%) in AcOH (1 mL) was stirred under air at 60 °C or 100 °C for 12 h as specified in Tables 2 and 3 followed by cooling. The volatiles were removed under reduced pressure. The contents were subjected to flash chromatography to give the corresponding products (96% and 95%) pale yellow oils. The purified materials were dried under an oil-pump vacuum.

2-(2-Chlorophenyl)-2-oxo-1-phenylhydrazin-2-ium-1-ide (3aa). Deep red oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.15 (d, J = 8.0 Hz, 2 H), 7.68–7.65 (m, 1 H), 7.53–7.47 (m, 3 H), 7.43–7.38 (m, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 147.68, 143.70, 130.89, 130.67, 130.22, 128.74, 127.50, 126.74, 125.38, 124.79. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₉ClN₂ONa⁺: 255.0301, Found [M + Na]⁺: 255.0212.

2-(2-Chloro-4-methylphenyl)-2-oxo-1-(p-tolyl)hydrazin-2-ium-1-ide (3ba). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.10 (d, J = 8.0 Hz, 2 H), 7.56 (d, J = 8.0 Hz, 1 H), 7.32–7.25 (m, 3 H), 7. 17 (d, J = 8.0 Hz, 1 H), 2.41 (d, J = 4.0 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃) δ: 145.54, 141.66, 141.31, 140.79, 131.15, 129.31, 128.04, 126.40, 125.53, 124.63, 21.57, 21.00. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃ClN₂ONa⁺: 283.0614, Found [M + Na]⁺: 283.0519.

2-(2-Chloro-5-methylphenyl)-2-oxo-1-(m-tolyl)hydrazin-2-ium-1-ide (3ca). Deep red oil. ¹H NMR (400 MHz, CDCl₃) δ: 7.97 (d, J = 8.0 Hz, 2 H), 7.48 (d, J = 4.0 Hz, 1 H), 7.40–7.37 (m, 2 H),7.26–7.21 (m, 2 H), 2.43 (s, 3 H), 2.40 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 147.34, 143.65, 138.59, 138.07, 131.41, 131.05, 130.51, 128.56, 125.83, 125.15, 123.47, 122.46, 21.45, 20.78. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃ClN₂ONa⁺: 283.0614, Found [M + Na]⁺: 283.0512.

2-(2-Chloro-6-methylphenyl)-2-oxo-1-(o-tolyl)hydrazin-2-ium-1-ide (3da). Deep red oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.12–8.10 (d, J = 8.0 Hz, 1 H), 7.36–7.27 (m, 5 H), 7.23 (t, J = 8.0 Hz, 1 H), 2.43 (s, 3 H), 2.40 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 147.27, 142.10, 134.69, 132.64, 130.86, 129.63, 129.46, 129.03, 127.83, 126.67, 125.94, 121.58, 18.40, 16.96. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃ClN₂ONa⁺: 283.0614, Found [M + Na]⁺: 283.0611.

2-(2-Chloro-4-methoxyphenyl)-1-(4-methoxyphenyl)-2-oxohydrazin-2-ium-1-ide (3ea). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.29 (s, 1 H), 8.27 (s, 1 H), 7.64 (d, J = 12.0 Hz, 1 H), 7.00 (d, J = 4.0 Hz, 1 H), 6.99 (d, J = 0.0 Hz, 1 H), 6.97 (t, J = 4.0 Hz, 1 H), 6.89 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H), 3.88 (s, 3 H), 3.85 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 160.70, 160.38, 141.31, 137.81, 127.83, 126.00, 115.63, 113.69, 113.04, 55.90, 55.50. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃ClN₂O₃Na⁺: 315.0512, Found [M + Na]⁺: 315.0404.

2-(2-Chloro-4-fluorophenyl)-1-(4-fluorophenyl)-2-oxohydrazin-2-ium-1-ide (3fa). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.27–8.24 (m, 2 H), 7.72–7.69 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H), 7.29–7.26 (m, 1 H), 7.20–7.15 (m, 2 H), 7.15–7.10 (m, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 162.90 (d, J = 252.0 Hz), 162.20 (d, J = 253.0 Hz), 144.14, 140.10 (d, J = 3.0 Hz), 128.40 (d, J = 11.0 Hz), 128.03 (d, J = 8.0 Hz), 126.44 (d, J = 10.0 Hz), 118.26 (d, J = 25.0 Hz), 115.75 (d, J = 22.0 Hz), 114.80 (d, J = 22.0 Hz). HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇ClF₂N₂ONa⁺: 291.0113, Found [M + Na]⁺: 291.0110.

1-(4-Chlorophenyl)-2-(2,4-dichlorophenyl)-2-oxohydrazin-2-ium-1-ide (3ga). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.14 (d, J = 12.0 Hz, 2 H), 7.65 (d, J = 8.0 Hz, 1 H), 7.56 (d, J = 4.0 Hz, 1 H), 7.46 (d, J = 8.0 Hz, 2 H), 7.40 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 146.00, 141.97, 136.36, 135.91, 130.80, 129.01, 127.92, 127.82, 126.96, 125.86. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇Cl₃N₂ONa⁺: 322.9522, Found [M + Na]⁺: 322.9409.

2-(4-Bromo-2-chlorophenyl)-1-(4-bromophenyl)-2-oxohydrazin-2-ium-1-ide (3ha). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.07 (s, 1 H), 8.05 (s, 1H), 7.71 (s, 1 H), 7.63–7.62 (m, 1 H), 7.61–7.59 (m, 1 H), 7.57–7.55 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃) δ: ¹³C NMR (101 MHz, CDCl₃) δ: 146.46, 142.33, 133.61, 132.02, 130.76, 127.99, 127.10, 126.03, 124.30, 124.14. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇Br₂N₂ONa⁺: 410.8511, Found [M + Na]⁺: 410.8362.

2-(2-Chloro-4-(trifluoromethyl)phenyl)-2-oxo-1-(4-(trifluoromethyl)phenyl)hydrazin-2-ium-1-ide (3ia). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.22 (d, J = 8.0 Hz, 2 H), 7.84 (d, J = 8.0 Hz, 2 H), 7.77 (d, J = 8.0 Hz, 2 H), 7.72 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 149.38, 145.61, 133.24 (q, J_C–F = 33.0 Hz), 131.67 (q, J_C–F = 33.0 Hz), 128.47 (q, J_C–F = 4.0 Hz), 127.87, 126.02 (q, J_C–F = 4.0 Hz), 125.61, 125.53, 124.84 (q, J_C–F = 3.0 Hz), 123.02 (q, J_C–F = 105.0 Hz), 123.01 (q, J_C–F = 106.0 Hz). HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₇ClF₆N₂ONa⁺: 391.0049, Found [M + Na]⁺: 390.9896.

2-(2-Iodophenyl)-2-oxo-1-phenylhydrazin-2-ium-1-ide (4aa). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.17 (t, J = 8.0 Hz, 2 H), 7.94 (d, J = 8.0 Hz, 1 H), 7.65–7.63 (m, 1 H), 7.53–7.42 (m, J = 8.0 Hz, 4 H), 7.21–7.17 (m, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 152.60, 143.58, 140.42, 130.99, 130.26, 129.07, 128.77, 125.39, 124.32, 87.97. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₉IN₂ONa⁺: 346.9657, Found [M + Na]⁺: 346.9650.

2-(2-Iodo-4-methylphenyl)-2-oxo-1-(p-tolyl)hydrazin-2-ium-1-ide (4ba). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (d, J = 8.0 Hz, 2 H), 7.77 (s, 1 H), 7.52 (d, J = 8.0 Hz, 1 H), 7.30 (d, J = 8.0 Hz, 2 H), 7.26 (d, J = 4.0 Hz, 1 H), 2.43 (s, 3 H), 2.38 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 150.44, 141.45, 140.79, 140.64, 140.09, 129.56, 129.30, 125.50, 124.02, 87.84, 21.60, 20.67. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃IN₂ONa⁺: 374.9970, Found [M + Na]⁺: 374.9964.

2-(2-Iodo-5-methylphenyl)-2-oxo-1-(m-tolyl)hydrazin-2-ium-1-ide (4ca). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 7.97 (d, J = 4.0 Hz, 2 H), 7.78 (d, J = 8.0 Hz, 1 H), 7.44 (d, J = 0.0 Hz, 1 H), 7.39 (t, J = 8.0 Hz, 1 H), 7.25 (d, J = 4.0 Hz, 1 H), 7.00–6.98 (m, 1H), 2.43 (s, 3 H), 2.38 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 152.47, 143.58, 139.97, 139.71, 138.55, 131.91, 130.99, 128.53, 125.77, 124.93, 122.43, 83.74, 21.46, 20.82. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃IN₂ONa⁺: 374.9970, Found [M + Na]⁺: 374.9963.

2-(2-Iodo-6-methylphenyl)-2-oxo-1-(o-tolyl)hydrazin-2-ium-1-ide (4da). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.28 (dd, J = 4.0 Hz, J = 8.0 Hz, 1 H), 7.72 (d, J = 8.0 Hz, 1 H), 7.31–7.23 (m, 4 H), 7.06–7.01 (m, 1 H), 2.43 (d, J = 4.0 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃) δ: 152.23, 141.82, 137.32, 135.18, 132.02, 131.05, 130.84, 130.20, 129.20, 125.81, 121.67, 88.49, 18.60, 17.54. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃IN₂ONa⁺: 374.9970, Found [M + Na]⁺: 374.9963.

2-(2-Iodo-4-methoxyphenyl)-1-(4-methoxyphenyl)-2-oxohydrazin-2-ium-1-ide (4ea). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.30 (t, J = 4.0 Hz, 1 H), 8.27 (t, J = 4.0 Hz, 1 H), 7.59 (d, J = 8.0 Hz, 1 H), 7.43 (d, J = 8.0 Hz, 1 H), 7.01–6.99 (m, 1 H), 6.98–6.94 (m, 2 H), 3.86 (s, 3 H), 3.84 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ: 160.67, 160.07, 146.25, 137.67, 127.78, 125.27, 125.09, 114.35, 113.69, 88.60, 55.86, 55.49. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₁₃IN₂O₃Na⁺: 406.9869, Found [M + Na]⁺: 406.9860.

2-(4-Fluoro-2-iodophenyl)-1-(4-fluorophenyl)-2-oxohydrazin-2-ium-1-ide (4fa). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.27–8.23 (m, 2 H), 7.68–7.63 (m, 2 H), 7.22–7.16 (m, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 162.82 (d, J = 252.0 Hz), 161.79 (d, J = 257.0 Hz), 149.00, 139.96 (d, J = 3.0 Hz), 127.95 (d, J = 8.0 Hz), 127.32 (d, J = 25.0 Hz), 125.68 (d, J = 10.0 Hz), 116.04 (d, J = 23.0 Hz), 115.72 (d, J = 23.0 Hz), 88.35 (d, J = 9.0 Hz). HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇F₂IN₂ONa⁺: 382.9464, Found [M + Na]⁺: 382.9326.

2-(4-Chloro-2-iodophenyl)-1-(4-chlorophenyl)-2-oxohydrazin-2-ium-1-ide (4ga). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.15 (d, J = 8.0 Hz, 2 H), 7.96 (d, J = 2.0 Hz, 1 H), 7.59 (d, J = 8.0 Hz, 1 H), 7.47 (d, J = 8.0 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃) δ: 151.01, 141.94, 139.93, 136.33, 135.89, 129.19, 129.03, 126.94, 125.19, 88.46. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇Cl₂IN₂ONa⁺: 414.8878, Found [M + Na]⁺: 414.8710.

2-(4-Bromo-2-iodophenyl)-1-(4-bromophenyl)-2-oxohydrazin-2-ium-1-ide (4ha). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (s, 1 H), 8.06 (d, J = 8.0 Hz, 2 H), 7.63–7.60 (m, 3 H), 7.52 (d, J = 8.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃) δ: 151.40, 142.56, 142.24, 132.14, 132.02, 127.07, 125.43, 124.38, 124.27, 88.87. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₂H₇Br₂IN₂ONa⁺: 502.7867, Found [M + Na]⁺: 502.7859.

2-(2-Iodo-4-(trifluoromethyl)phenyl)-2-oxo-1-(4-(trifluoromethyl)phenyl)hydrazin-2-ium-1-ide (4ia). Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.22 (d, J = 8.0 Hz, 3 H), 7.80–7.75 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃) δ: 154.44, 145.55, 137.78 (d, J_C–F = 4.0 Hz), 133.12 (q, J_C–F = 34.0 Hz), 131.62 (q, J_C–F = 32.0 Hz), 126.42 (q, J_C–F = 3.0 Hz), 126.03 (q, J_C–F = 4.0 Hz), 125.58, 124.72, 122.84 (q, J_C–F = 140.0 Hz), 122.83 (q, J_C–F = 142.0 Hz), 88.20. HRMS (ESI) ([M + Na]⁺) Calcd for C₁₄H₇F₆IN₂ONa⁺: 482.9405, Found [M + Na]⁺: 482.9388.

Acknowledgements

We are grateful to the Northwest University (NF14020), Education Department of Shaanxi Provincial Government (14JK1735), the National Natural Science Foundation of China (21402071) and National Found for Fostering Talents of Basic Science (NFFTBS-J1103311, J1210057) for financial support.

Notes and references

1. (a) N. Sotomayor and E. Lete, Curr. Org. Chem., 2003, 7, 275; (b) Handbook of Grignard Reagents, ed. G. S. Silverman and P. E. Rakita, Dekker, New York, 1996.
2. Organic Reaction Mechanism, ed. M. R. Crampton and A. J. Knipe, Wiley, New York, 2007.
3. (a) Metal Catalyzed Cross Coupling Reactions, ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998; (b) A. de Meijere and F. Diederich, Metal-Catalyzed Cross-Coupling Reactions, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2004; (c) K. C. Nicolaou, P. G. Bulger and D. Sarlah, Angew. Chem., Int. Ed., 2005, 44, 4442; (d) J. P. Corbet and G. Mignani, Chem. Rev., 2006, 106, 2651; (e) L. C. Campeau and K. Fagnou, Chem. Commun., 2006, 1253; (f) L. X. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133; (g) G. P. McGlacken and I. J. S. Fairlamb, Eur. J. Org. Chem., 2009, 4011; (h) I. P. Beletskaya and A. V. Cheprakov, Coord. Chem. Rev., 2004, 248, 2337; (i) S. V. Ley and A. W. Thomas, Angew. Chem., Int. Ed., 2003, 42, 5400.
4. (a) R. Taylor, Electrophilic Aromatic Substitution, John Wiley, New York, 1990; (b) P. B. De la Mare, Electrophilic Halogenation, Cambridge University Press, Cambridge, 1976, ch. 5; (c) E. B. Merkushev, Synthesis, 1988, 923.
5. V. Snieckus, Chem. Rev., 1990, 90, 879.
6. (a) Y. Zhang, K. Shibatomi and H. Yamamoto, Synlett, 2005, 2837 and references therein; (b) G. K. S. Prakash, T. Mathew, D. Hoole, P. M. Esteves, Q. Wang, G. Rasul and G. A. Olah, J. Am. Chem. Soc., 2004, 126, 15770; (c) K. Tanemura, T. Suzuki, Y. Nishida, K. Satsumabayashi and T. Horaguchi, Chem. Lett., 2003, 32, 932; (d) H. Firouzabadi, N. Iranpoor and M. Shiri, Tetrahedron Lett., 2003, 44, 8781; (e) P. V. Vyas, A. K. Bhatt, G. Ramachandraiah and A. V. Bedekar, Tetrahedron Lett., 2003, 44, 4085; (f) K. Lee, H. K. Cho and C. Song, Bull. Korean Chem. Soc., 2002, 23, 773; (g) J. Iskra, S. Stavber and M. Zupan, Synthesis, 2004, 1869; (h) G. K. Dewkar, S. V. Narina and A. Sudalai, Org. Lett., 2003, 5, 4501; (i) B. V. Tamhankar, U. V. Desai, R. B. Mane, P. P. Wadgaonkar and A. V. Bedekar, Synth. Commun., 2001, 31, 2021; (j) D. Roche, K. Prasad, O. Repic and T. J. Blacklock, Tetrahedron Lett., 2000, 41, 2083; (k) D. C. Braddock, G. Cansell and S. A. Hermitage, Synlett, 2004, 461; (l) P. A. Evans and T. A. Brandt, J. Org. Chem., 1997, 62, 5321.
7. (a) D. Kalyani, A. R. Dick, W. Q. Anani and M. S. Sanford, Tetrahedron, 2006, 62, 11483; (b) D. Kalyani, A. R. Dick, W. Q. Anani and M. S. Sanford, Org. Lett., 2006, 8, 2523; (c) A. R. Dick, K. L. Hull and M. S. Sanford, J. Am. Chem. Soc., 2004, 126, 2300; (d) S. R. Whitfield and M. S. Sanford, J. Am. Chem. Soc., 2007, 129, 15142; (e) S. R. Whitfield and M. S. Sanford, Organometallics, 2008, 27, 1683.
8. For selected reviews, see: (a) G. Dyker, Handbook of C-H Transformations: Applications in Organic Synthesis, Wiley-VCH, Weinheim, 2005; (b) Activation and Functionalization of C-H Bond, ed. K. I. Goldberg and A. S. Goldman, ACS Symposium Series 885, American Chemical Society, Washington, DC, 2004; (c) R. Giri, B.-F. Shi, K. M. Engle, N. Maugel and J.-Q. Yu, Chem. Soc. Rev., 2009, 38, 3242; (d) J.-Q. Yu and Z.-J. Shi, C-H Activation, Springer, Berlin, Germany, 2010.
9. Selected recent examples: (a) R. Giri, X. Chen, X.-S. Hao, J.-J. Li, J. Liang, Z.-P. Fan and J.-Q. Yu, Tetrahedron: Asymmetry, 2005, 16, 3502; (b) K. L. Hull, W. Q. Anani and M. S. Sanford, J. Am. Chem. Soc., 2006, 128, 7134; (c) J.-J. Li, R. Giri and J.-Q. Yu, Tetrahedron, 2008, 64, 6979; (d) X. Zhao, E. Dimitrijevic and V. M. Dong, J. Am. Chem. Soc., 2009, 131, 3466; (e) F. Kakiuchi, T. Kochi, H. Mutsutani, N. Kobayashi, S. Urano, M. Sato, S. Nishiyama and T. Tanabe, J. Am. Chem. Soc., 2009, 131, 11310; (f) E. Dubost, C. Fossey, T. Cailly, S. Rault and F. Fabis, J. Org. Chem., 2011, 76, 6414; (g) N. Schröder, J. Wencel-Delord and F. Glorius, J. Am. Chem. Soc., 2012, 134, 8298; (h) A. John and K. M. Nicholas, J. Org. Chem., 2012, 77, 5600; (i) D. R. Fahey, J. Organomet. Chem., 1971, 27, 283; (j) Y. Lian, R. G. Bergman, L. D. Lavis and J. A. Ellman, J. Am. Chem. Soc., 2013, 135, 7122; (k) H. Li, P. Li and L. Wang, Org. Lett., 2013, 15, 620; (l) X.-T. Ma and S.-K. Tian, Adv. Synth. Catal., 2013, 355, 337.
10. X. Wan, Z. Ma, B. Li, K. Zhang, S. Cao, S. Zhang and Z. Shi, J. Am. Chem. Soc., 2006, 128, 7416.
11. (a) T.-S. Mei, D.-H. Wang and J.-Q. Yu, Org. Lett., 2010, 12, 3140; (b) J.-J. Li, T.-S. Mei and J.-Q. Yu, Angew. Chem., Int. Ed., 2008, 47, 6452; (c) T.-S. Mei, R. Giri, N. Maugel and J.-Q. Yu, Angew. Chem., Int. Ed., 2008, 47, 5215; (d) R. Giri, X. Chen and J.-Q. Yu, Angew. Chem., Int. Ed., 2005, 44, 2112.
12. (a) R. B. Bedford, J. U. Engelhart, M. F. Haddow, C. J. Mitchell and R. L. Webster, Dalton Trans., 2010, 39, 10464; (b) R. B. Bedford, M. F. Haddow, C. J. Mitchell and R. L. Webster, Angew. Chem., Int. Ed., 2011, 50, 5524.
13. B. Song, X. Zheng, J. Mo and B. Xu, Adv. Synth. Catal., 2010, 352, 329.
14. T. Ikeda and O. Tsu, Science, 1995, 268, 1873.
15. (a) J. M. Huang, J. F. Kuo and C. Y. Chen, J. Appl. Polym. Sci., 1995, 55, 1217; (b) D. Campbell, L. R. Dix and P. Rostron, Dyes Pigm., 1995, 29, 77.
16. (a) S. D. Kahn, W. D. Hehre and J. A. Pople, J. Am. Chem. Soc., 1987, 109, 1871; (b) H. Basch and T. Hoz, Mol. Phys., 1997, 91, 789.
17. For selected examples, see: (a) Z. Hou, Y. Fujiware and H. Taniguchi, J. Org. Chem., 1988, 53, 3118; (b) N. Sakai, K. Fuji, S. Nabeshima, R. Ikeda and T. Konakahara, Chem. Commun., 2010, 46, 3173; (c) Although regioselective halogenation of azobenzenes has been developed (9l), the azoxy compounds could not be afforded and/or isolated purely by oxidation of corresponding aromatic azo compounds.
18. (a) H. J. Li, P. H. Li, Q. Zhao and L. Wang, Chem. Commun., 2013, 49, 9170; (b) H. J. Li, X. Y. Xie and L. Wang, Chem. Commun., 2014, 50, 4218; (c) M. Sun, L.-K. Hou, X.-X. Chen, X.-J. Yang, W. Sun and Y.-S. Zang, Adv. Synth. Catal., 2014, 356, 3789; (d) L. Hou, X. Chen, S. Li, S. Cai, Y. Zhao, M. Sun and X.-J. Yang, Org. Biomol. Chem., 2015, 13, 4160; (e) M. Yi, X. Cui, C. Zhu, C. Pi, W. Zhu and Y. Wu, Asian J. Org. Chem., 2015, 4, 38.
19. (a) M. Sun, H. Y. Zhang, Q. Han, K. Yang and S. D. Yang, Chem. – Eur. J., 2011, 17, 9566; (b) M. Sun, Y. N. Ma, Y. M. Li, Q. P. Tian and S. D. Yang, Tetrahedron Lett., 2013, 54, 5091; (c) Y. M. Li, M. Sun, H. L. Wang, Q. P. Tian and S. D. Yang, Angew. Chem., Int. Ed., 2013, 52, 3972.
20. For a review, see: J. Wencel-Delord, T. Dröge, F. Liu and F. Glorius, Chem. Soc. Rev., 2011, 40, 4740.
21. T. Oberhauser, J. Org. Chem., 1997, 62, 4504.
22. The corresponding chlorinated/iodinated products were isolated in 44% and 31% yields, respectively. See ESI† for more details.
23. For a review on organopalladium(IV) chemistry, see: L.-M. Xu, B.-J. Li, Z. Yang and Z.-J. Shi, Chem. Soc. Rev., 2010, 39, 712.

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Footnote

† Electronic supplementary information (ESI) available: Details experimental procedures, complete characterization data, copies of NMR spectra for all crucial compounds. CCDC 1054759. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ob02153g

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