English

• 1.   Introduction

  Aryl ketones are important structural motifs in dyes, fragrances, fine chemicals, pharmaceuticals and agrochemicals.^[1] Conventionally, building such structure involves Friedel-Crafts acylation and oxidation of secondary alcohols by various oxidants.^[2] Although it is effective as documented in almost all organic chemistry textbooks, these processes suffer from some drawbacks, such as the formation of waste materials due to the use of excessive Lewis acids and oxidants (Scheme 1). The coupling of arenes with aliphatic olefins and carbon monoxide using ruthenium as a catalyst has provided an alternative pathway.^[3] The initial example developed by Moore and co-workers^[4] involves Ru₃(CO)₁₂-catalyzed acylation of pyridine with CO and olefins. Recently, palladium-catalyzed acylation of C—H bonds has been one of the most practical, atom- and step-economical methods in synthetic chemistry. In 2009, Cheng et al.^[5] first described palladium-catalyzed ortho-acylation reaction of arenes containing a pyridyl as a directing group with aryl aldehydes. Later, palladium-catalyzed C(sp²)—H bond acylation of 2-arylpyridines, ^{[5, 6]} oximes, ^[7] amides, ^[8] acetanilides, ^[9] indolizines, ^[10] azobenzenes^[11] and benzothiazoles^[12] using aldehydes, alcohols, α-oxocarboxylic acids, carboxylic acids, α-diketones, toluene derivatives and amines as acylation sources has been subsequently developed.

  Scheme 1

  

  Scheme 1.  Preparation of aryl ketones

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  Azoxy compounds are important materials and useful intermediates in electronic devices due to their liquid crystalline properties. They also have wide applications in the fields of pharmaceuticals, analytic chemistry, functional materials, organic synthesis, and the dye industry.^[13] In view of their importance, a number of methods have been established to build a structural skeleton.^[14] Recently, Wang, Sun and Cui group^[15] realized ortho-acylation of azoxybenzenes using aldehydes, α-oxocarboxylic acids and alcohols (Scheme 1). However, most of these processes are performed in organic solvents. From the perspective of green chemistry, water is an ideal chemical solvent, because it has natural, cheap, non-toxic, environmental characteristics.^[16] In the past few years, acylation reactions in water have occasionally been reported.^[17] Herein, we independently report a facile and efficient protocol to acylated azoxybenzenes via palladium-catalyzed C—H bond activation using alcohols as acylation reagents under aqueous conditions (Scheme 1).

  2.   Results and discussion

  Our initial investigation was focused on the acylation of azoxybenzene (1a) with benzylalcohol (2a). The results were summarized in Table 1. The desired product 3a was obtained in an isolated yield of 15% in water using 10 mol% PdCl₂ as catalyst and 70% tert-butyl hydroperoxide (TBHP) in water as oxidant (Entry 1, Table 1). To improve the solubility of the reactants in water, 5 mol% sodium dodecyl sulfate (SDS) was added to the system. Pd(OAc)₂ provided a higher yield compared to PdCl₂ and Pd(TFA)₂ (Entry 4 vs. Entries 2, 3, Table 1). Subsequently, various oxidants were tested in the presence of Pd(OAc)₂ under air atmosphere (Entries 5~11). The yields could not be improved by using di-tert-butylperoxide (DTBP) and AgOAc (Entries 5, 6). Only a trace amount of the desired product 3a were observed when K₂S₂O₈ and (NH₄)₂S₂O₈ as the oxidants (Entries 7, 8). benzoquinone (BQ), 2, 3-dichloro- 5, 6-dicyano-1, 4-benzoquinone (DDQ) and H₂O₂ were found to be ineffective for this process (Entries 9~11). Other phase-transfer catalysts (PTC) showed that SDS is superior to other phase-transfer catalysts such as tetrabutylammonium bromide (TBAB), 18-crown-6, or tween 80 (Entries 12~14). Meanwhile, decreasing the temperature resulted in a slightly decrease in the yield (Entry 15). When the reaction was carried out under nitrogen atmosphere, the desired product 3a was also obtained in 75% yield (Entry 16), which indicated that oxygen molecule was tolerated. After surveying a variety of catalysts, oxidants, PTC and reaction temperatures, the optimized reaction conditions were identified as follows: 10 mol% Pd(OAc)₂ as catalyst, TBHP as oxidant, SDS as the phase- transfer catalyst, and H₂O as solvent, at 100 ℃ under air atmosphere for 24 h.

  Table 1

  

  Table 1.  Optimization of the reaction conditions^a

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  Entrya	Catalyst	Oxidant	PTC	Yield b/%
  1	PdCl2	TBHP		15
  2	PdCl2	TBHP	SDS	35
  3	Pd(TFA)2	TBHP	SDS	68
  4	Pd(OAc)2	TBHP	SDS	80
  5	Pd(OAc)2	DBHP	SDS	20
  6	Pd(OAc)2	AgOAc	SDS	15
  7	Pd(OAc)2	K2S2O8	SDS	Trace
  8	Pd(OAc)2	(NH4)2S2O8	SDS	Trace
  9	Pd(OAc)2	BQ	SDS	NDe
  10	Pd(OAc)2	DDQ	SDS	NDe
  11	Pd(OAc)2	H2O2	SDS	NDe
  12	Pd(OAc)2	TBHP	TBAB	20
  13	Pd(OAc)2	TBHP	18-Crown-6	42
  14	Pd(OAc)2	TBHP	Tween 80	10
  15c	Pd(OAc)2	TBHP	SDS	65
  16d	Pd(OAc)2	TBHP	SDS	75
  a Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst (10 mol%), oxidant (4.5 equiv.), PTC (5 mol%) and water (2.0 mL), under air, 100 ℃, 24 h. b Isolated yield. c 80 ℃. d Nitrogen protection.e ND is not detected.

  With the optimal reaction conditions in hand, the generality and the scope of the substrates for this transformation were tested(Table 2, 3a~3m). A wide range of primary alcohols were firstly evaluated in reaction with azoxybenzene (1a). In general, the reactions of primary alcohols with electron-donating groups (such as CH₃, OCH₃) or weak electron-withdrawing groups (such as F, Cl, Br) on the aromatic ring afforded the desired products in 55%~78% yields (3b~3f). The presence of the strong electron-

  Table 2

  

  Table 2.  Scope of azoxybenzenes and alcohols ^a

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  withdrawing group, such as (4-nitrophenyl)methanol, restrained the reaction, providing the product in only 32% yield (3g). Steric hindrance on the aryl group of primary alcohols had no significant effect, furnishing the corresponding products in moderate yields (3h~3l). Notably, when azoxybenzene (1a) was coupled with (3-nitro-phenyl)methanol (2l) under the optimized reaction conditions, a satisfactory yield of the corresponding product 3l was achieved. Disubstituted benzyl alcohols 3i provided the desired products in 63% yield. Aliphatic alcohol exhibited low efficiency in this transformation. For example, the n-butyl alcohol delivered 34% yield of 3m.

  Heteroarylaldehydes, such as 2-pyridinecarboxaldehyde, 2-furanaldehyde and 2-thiophenecarboxaldehyde, were not suitable substrates. To expand the scope of this method, some typical disubstituted azoxybenzenes were examined. The results demonstrated that azoxy derivatives with electron-donating groups gave better yields than those with electron-withdrawing groups. For instance, the reactions of 4, 4'-dimethylazoxybenzene, 4, 4'-diisopropylazoxybenzene 4, 4'-dimethoxyazoxybenzene, 3, 3'-dimethylazoxybenzene, and 2, 2'-dimethylazoxybenzene provided the corresponding products in 68%~73% yields (3n~3r). The electron-withdrawing group in the substrate, such as F and Br, provided 3s and 3t in 32% and 33% yields.

  To clarify the reaction mechanism, some control experiments were carried out (Scheme 2). When the palladium-catalyzed acylation of 1a with 2a was carried out in the absence of TBHP (Scheme 2a, Condition A), no acylated product 3a was detected. Moreover, addition of a radical scavenger 2, 2, 6, 6-tetramethylpiperidyl-1-oxyl (TEMPO) to the reaction mixture under the standard reaction conditions made this acylation suppressed, suggesting the possibility of a radical process (Scheme 2a, Condition B). Furthermore, it was found that benzaldehyde was formed under the reaction conditions without the azoxybenzene (Scheme 2b). Then, the reaction between azoxybenzene (1a) and benzaldehyde were examined, and the product 3a was provided in 75% yield (Scheme 2c).

  Scheme 2

  

  Scheme 2.  Control experiments

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  Based on the results obtained and the literature, ^{[15, 17, 18]} a tentative reaction mechanism for the palladium-catalyzed ortho-C—H acylation of azoxybenzene (1a) was proposed and depicted in Scheme 3. The palladium catalyst reacted with azoxybenzene by chelation-directed C—H activation to generate complex A (Figure 1). Meanwhile, with the effect of TBHP, the benzaldehyde, which was produced from the oxidation of benzylalcohol (2a), generated an acyl radical. Then, the complex A would react with the acyl radical, affording the oxidative addition product as the dimeric Pd^III or reactive Pd^IV species B.^[19] Finally, the reductive elimination of intermediate B afforded coupling product 3a and regenerated Pd(II) for next cycle.

  Scheme 3

  

  Scheme 3.  Proposed reaction mechanism

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  Figure 1

  

  Figure 1.  X-Ray molecular structures of complex A

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  3.   Conclusions

  In summary, a simple, efficient and environmentally benign protocol for the synthesis of acylated azoxybenzenes via Pd-catalyzed C—H regioselective functionalization has been developed. The method is characterized by its wide substrate scope, high regioselectivity, mild reaction conditions, and readily available starting materials. Further investigations to expand the substrate scope and application of such chemistry in organic synthesis are underway.

  4.   Experimental section

  4.1   General information

  Melting points were measured on a microscopic apparatus and were uncorrected. ¹H NMR spectra were recorded on a 400 MHz spectrometer in deuterated chloroform. The chemical shifts (δ) are reported relative to tetramethylsilane. ¹³C NMR spectra were recorded using a 100 MHz spectrometer. The chemical shifts are reported relative to residual CHCl₃ (δ_C＝77.00). High-resolution mass spectrometry (HRMS) was performed on a Q-TOF spectrometer with micromass MS software using electrospray ionization (ESI). X-ray analysis was performed with a single-crystal X-ray diffractometer. Unless otherwise noted, all of the reagents were purchased from commercial suppliers and used without purification.

  4.2   General procedure for preparation of azoxy- benzenes

  All of the azoxybenzenes were prepared from arylamines, according to the literature.^[20] H₂O₂ (30%, 0.92 mL, 9.00 mmol) was added to a solution of arylamine (0.27 mL, 3.00 mmol) and SeO₂ (33.3 mg, 0.30 mmol) in MeOH (10 mL). The reaction mixture was stirred at room temperature (r.t.) until complete consumption of the starting material, which was followed by thin-layer chromatography (TLC) (20 h). The solvent was evaporated under vacuum. The residue was partitioned between CH₂Cl₂ (20 mL) and H₂O (20 mL). The organic layer was separated, and the aqueous phase was extracted with CH₂Cl₂ (20 mL×2). The combined organic phase were dried (MgSO₄) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (CH₂Cl₂-pentane, V:V＝1:10) to afford the desired azoxybenzene derivatives.

  4.3   Typical procedure for the acylation of azobenzenes

  Azoxybenzene (1a, 39.6 mg, 0.20 mmol), benzyl alcohol (2a, 63 μL, 0.40 mmol), Pd(OAc)₂ (4.48 mg, 0.02 mmol), TBHP (126 uL, 70%, 0.9 mmol), SDS (2.88 mg, 0.01 mmol) and H₂O (2.0 mL) were mixed in a dry reaction tube. The mixture was stirred at 100 ℃ under air for 24 h. When the reaction was completed, the crude mixture was cooled to room temperature. The mixture was extracted with ethyl acetate three times. The filtrate was concentrated in vacuum and the resulting residue was purified by preparative thin layer chromatography (silica gel, ethyl acetate/petroleum ether, V:V＝1:10) to give the desired product 1-(2-benzoylphenyl)-2-phenyldiazene 1-oxide (3a)^[15] as a yellow viscous liquid (48.3 mg, 80%). ¹H NMR (400 MHz, CDCl₃) δ: 8.24~8.22 (m, 1H), 7.76 (d, J＝7.4 Hz, 2H), 7.70~7.64 (m, 4H), 7.53~7.51 (m, 1H), 7.46 (t, J＝6.2 Hz, 1H), 7.36 (t, J＝6.1 Hz, 2H), 7.29 (d, J＝3.2 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.0, 147.2, 143.1, 136.9, 134.8, 133.1, 131.4, 130.5, 129.9, 128.9, 128.8, 128.5, 125.0, 123.4; HRMS (ESI) calcd for C₁₉H₁₅N₂O₂ [M+H]⁺ 303.1128, found 303.1133.

  1-(2-(4-Methylbenzoyl)phenyl)-2-phenyldiazene 1-oxide (3b):^[15] yellow liquid (37.9 mg, 60%). ¹H NMR (400 MHz, CDCl₃) δ: 8.22~8.21 (m, 1H), 7.73~7.71 (m, 2H), 7.65~7.63 (m, 4H), 7.51~7.48 (m, 1H), 7.32~7.27 (m, 3H), 7.16 7.16 (d, J＝7.9 Hz, 2H), 2.33 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.1, 144.3, 143.6, 135.4, 134.8, 131.6, 130.7, 130.2, 129.6, 129.4, 128.8, 125.4, 123.7, 22.0; HRMS (ESI) calcd for C₂₀H₁₇N₂O₂ [M+H]⁺ 317.1285, found 317.1289.

  1-(2-(4-Methoxybenzoyl)phenyl)-2-phenyldiazene 1-oxide (3c):^[15] yellow liquid (36.5 mg, 55%). ¹H NMR (400 MHz, CDCl₃) δ: 8.22~8.20 (m, 1H), 7.75 (d, J＝8.4 Hz, 3H), 7.63~7.61 (m, 2H), 7.49~7.47 (m, 1H), 7.31~7.27 (m, 3H), 6.84 (d, J＝8.8 Hz, 2H), 3.79 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.2, 163.9, 143.6, 135.5, 131.6, 131.6, 130.6, 130.4, 130.2, 129.1, 128.9, 125.4, 123.7, 114.20, 55.8; HRMS (ESI) calcd for C₂₀H₁₇N₂O₃ [M+H]⁺ 333.1234, found 333.1238.

  1-(2-(4-Fluorobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3d):^[15] yellow liquid (46.1 mg, 72%). ¹H NMR (400 MHz, CDCl₃) δ: 8.24~8.22 (m, 1H), 7.77~7.73 (m, 4H), 7.66 (t, J＝9.3 Hz, 2H), 7.50~7.48 (m, 1H), 7.31~7.29 (m, 3H), 7.03 (t, J＝10.3 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 192.9, 167.3, 164.7, 143.5, 134.8, 133.8, 131.8, 131.8, 131.7, 130.5, 129.1, 128.9, 125.4, 123.8, 116.2, 115.9; HRMS (ESI) calcd for C₁₉H₁₄FN₂O₂ [M+H]⁺ 321.1034, found 321.1037.

  1-(2-(4-Chlorobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3e):^[15] yellow liquid (52.4 mg, 78%), m.p. 64.6~65.2 ℃. ¹H NMR (400 MHz, CDCl₃) δ: 8.26~8.23 (m, 1H), 7.75 (d, J＝8.0 Hz, 2H), 7.70 (d, J＝8.4 Hz, 2H), 7.65 (t, J＝6.6 Hz, 3H), 7.51~7.49 (m, 1H), 7.34~7.31 (m, 5H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.1, 143.4, 139.8, 135.7, 134.6, 131.8, 131.1, 130.5, 130.5, 129.2, 129.1, 129.0, 125.4, 123.8; HRMS (ESI) calcd for C₁₉H₁₄ClN₂O₂ [M+H]⁺, 337.0738, found 337.0743.

  1-(2-(4-Bromobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3f):^[15] yellow liquid (57.0 mg, 75%). m.p. 62.5~63.4 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.25~8.23 (m, 1H), 7.74 (d, J＝7.4 Hz, 2H), 7.66 (t, J＝4.7 Hz, 2H), 7.61 (d, J＝8.3 Hz, 2H), 7.49 (d, J＝8.0 Hz, 3H), 7.35~7.30 (m, 3H). ¹³C NMR (100 MHz, CDCl₃) δ: 193.3, 143.4, 136.2, 134.6, 132.2, 131.8, 131.1, 130.6, 130.5, 129.1, 129.0, 128.6, 125.4, 123.8; HRMS (ESI) calcd for C₁₉H₁₄BrN₂O₂ [M+H]⁺ 381.0233, found 381.0236.

  1-(2-(4-Nitrobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3g):^[15] yellow liquid (22.2 mg, 32%). ¹H NMR (400 MHz, CDCl₃) δ: 8.30~8.28 (m, 4H), 8.18 (d, J＝8.6 Hz, 2H), 7.89 (d, J＝8.6 Hz, 2H), 7.77~7.70 (m, 4H), 7.56~7.54 (m, 1H), 7.33~7.31 (m, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 190.4, 150.4, 143.2, 142.0, 133.9, 132.1, 131.6, 130.9, 129.7, 129.2, 125.4, 124.1, 123.8; HRMS (ESI) calcd for C₁₉H₁₄N₃O₄ [M+H]⁺ 348.0979, found 348.0981.

  1-(2-(2-Methylbenzoyl)phenyl)-2-phenyldiazene 1-oxide (3h):^[15] yellow liquid (36.7 mg, 58%). ¹H NMR (400 MHz, CDCl₃) δ: 8.05~8.03 (m, 1H), 7.64~7.60 (m, 3H), 7.57~7.54 (m, 2H), 7.32~7.25 (m, 5H), 7.14~7.07 (m, 2H), 2.53 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 195.7, 148.3, 143.7, 140.2, 136.6, 136.1, 132.2, 131.1, 131.4, 131.3, 130.7, 130.0, 128.7, 125.7, 123.8, 21.5; HRMS (ESI) calcd for C₂₀H₁₇N₂O₂ [M+H]⁺ 317.1285; found 317.1289.

  1-(2-(2, 4-Dichlorobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3i): yellow liquid (46.6 mg, 63%). ¹H NMR (400 MHz, CDCl₃) δ: 8.06 (d, J＝7.4 Hz, 1H), 7.79~7.77 (m, 2H), 7.68~7.63 (m, 3H), 7.48 (d, J＝8.3 Hz, 1H), 7.37~7.31 (m, 4H), 7.16~7.14 (m, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 191.4, 143.6, 138.6, 134.8, 134.7, 134.6, 132.5, 131.9, 131.3, 130.6, 129.9, 128.9, 127.2, 125.5, 123.9; HRMS (ESI) calcd for C₁₉H₁₃Cl₂N₂O₂ [M+H]⁺ 371.0349, found 371.0352.

  1-(2-(3-Methoxybenzoyl)phenyl)-2-phenyldiazene 1-oxide (3j): yellow liquid (43.1 mg, 65%). ¹H NMR (400 MHz, CDCl₃) δ: 8.24~8.20 (m, 1H), 7.73~7.71 (m, 2H), 7.66~7.63 (m, 2H), 7.53~7.51 (m, 1H), 7.38 (s, 1H), 7.33~7.31 (m, 3H), 7.25~7.20 (m, 2H), 7.04~6.99 (m, 1H), 3.75 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.1, 160.1, 143.5, 138.7, 135.1, 131.7, 130.9, 129.8, 129.2, 128.8, 125.3, 123.7, 122.1, 120.2, 112.9, 55.8; HRMS (ESI) calcd for C₂₀H₁₇N₂O₃ [M+H]⁺ 333.1234, found 333.1238.

  1-(2-(3-Chlorobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3k):^[15] yellow liquid (39.6 mg, 59%). ¹H NMR (400 MHz, CDCl₃) δ: 8.26~8.24 (m, 1H), 7.77~7.74 (m, 3H), 7.68~7.66 (m, 2H), 7.58 (d, J＝7.8 Hz, 1H), 7.51~7.49 (m, 1H), 7.42 (d, J＝7.9 Hz, 1H), 7.35~7.25 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.0, 143.4, 139.0, 135.2, 134.5, 133.3, 131.8, 131.2, 130.5, 130.2, 129.1, 129.0, 129.0, 127.2, 125.4, 123.8; HRMS (ESI) calcd for C₁₉H₁₄ClN₂O₂ [M+H]⁺ 337.0738, found 337.0743.

  1-(2-(3-Nitrobenzoyl)phenyl)-2-phenyldiazene 1-oxide (3l): yellow liquid (38.8 mg, 56%). ¹H NMR (400 MHz, CDCl₃) δ: 8.57~8.56 (m, 1H), 8.30~8.27 (m, 2H), 8.03 (d, J＝7.7 Hz, 1H), 7.78~7.75 (m, 2H), 7.73~7.69 (m, 2H), 7.57~7.52 (m, 2H), 7.34~7.30 (m, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 192.0, 148.7, 147.5, 143.3, 138.9, 134.3, 133.7, 132.1, 131.6, 130.8, 130.1, 129.1, 129.0, 127.5, 125.5, 124.0, 123.7; HRMS (ESI) calcd for C₁₉H₁₄N₃O₄ [M+H]⁺ 348.0979, found 348.0982.

  1-(2-Butyrylphenyl)-2-phenyldiazene 1-oxide (3m):^[15] yellow liquid (18.1 mg, 34%). ¹H NMR (400 MHz, CDCl₃) δ: 8.10 (d, J＝8.1 Hz, 3H), 7.58~7.56 (m, 2H), 7.49~7.46 (m, 2H), 7.43~7.40 (m, 2H), 2.73 (t, J＝7.0 Hz, 2H), 1.71 (q, J＝10.4, 3.0 Hz, 2H), 0.92 (t, J＝7.4 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ: 203.4, 171.5, 143.9, 137.2, 131.6, 130.6, 130.6, 129.1, 127.8, 125.8, 123.9, 45.3, 18.1, 14.5; HRMS (ESI) calcd for C₁₆H₁₇N₂O₂ [M+H]⁺ 269.1285, found 269.1288.

  1-(2-Benzoyl-4-methylphenyl)-2-(p-tolyl)diazene 1-oxide (3n):^[15] yellow liquid (46.9 mg, 71%). ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝8.4 Hz, 1H), 7.76 (d, J＝7.4 Hz, 2H), 7.65 (d, J＝8.4 Hz, 2H), 7.47~7.41 (m, 2H), 7.34 (t, J＝7.7 Hz, 2H), 7.29 (s, 1H), 7.08 (d, J＝8.3 Hz, 2H), 2.47 (s, 3H), 2.30 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.7, 145.3, 142.4, 141.3, 137.4, 135.0, 133.3, 131.3, 129.5, 129.4, 129.1, 128.8, 125.5, 123.5, 21.9, 21.6; HRMS (ESI) calcd for C₂₁H₁₉N₂O₂ [M+H]⁺ 331.1441, found 331.1445.

  1-(2-Benzoyl-4-isopropylphenyl)-2-(4-isopropyl-phenyl)diazene 1-oxide (3o):^[15] yellow liquid (52.5 mg, 68%). ¹H NMR (400 MHz, CDCl₃) δ: 8.16 (d, J＝8.5 Hz, 1H), 7.77 (d, J＝7.5 Hz, 2H), 7.67 (d, J＝8.5 Hz, 2H), 7.50~7.44 (m, 2H), 7.38~7.33 (m, 3H), 7.13 (d, J＝8.5 Hz, 2H), 3.08~2.97 (m, 1H), 2.91~2.81 (m, 1H), 1.30 (d, J＝6.8 Hz, 6H), 1.19 (d, J＝6.7 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.8, 153.1, 151.5, 145.4, 141.6, 137.5, 135.1, 133.3, 129.2, 128.8, 128.8, 127.1, 126.8, 125.5, 123.6, 34.4, 34.4, 24.0; HRMS (ESI) calcd for C₂₅H₂₇N₂O₂ [M+H]⁺ 387.2067, found 387.2072.

  1-(2-Benzoyl-4-methoxyphenyl)-2-(4-methoxyphenyl)-diazene 1-oxide (3p):^[15] yellow liquid (50.7 mg, 70%). ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (d, J＝9.1 Hz, 1H), 7.75 (d, J＝9.0 Hz, 2H), 7.69~7.67 (m, 2H), 7.36 (t, J＝8.6 Hz, 1H), 7.27 (d, J＝2.7 Hz, 1H), 7.27~7.16 (m, 1H), 7.02~6.99 (m, 1H), 6.84 (d, J＝2.6 Hz, 1H), 6.68 (d, J＝9.0 Hz, 2H), 3.78 (s, 3H), 3.69 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.2, 161.9, 160.8, 137.5, 137.3, 133.3, 129.1, 128.8, 127.9, 127.7, 125.2, 116.0, 113.9, 56.3, 55.8; HRMS (ESI) calcd for C₂₁H₁₉N₂O₄ [M+H]⁺ 363.1339, found 363.1344.

  1-(2-Benzoyl-5-methylphenyl)-2-(m-tolyl)diazene 1-oxide (3q):^[15] yellow liquid (48.2 mg, 73%). ¹H NMR (400 MHz, CDCl₃) δ: 7.98 (s, 1H), 7.75 (d, J＝7.6 Hz, 2H), 7.47~7.40 (m, 5H), 7.34 (t, J＝7.6 Hz, 2H), 7.16 (t, J＝8.7 Hz, 1H), 7.07 (d, J＝7.5 Hz, 1H), 2.52 (s, 3H), 2.27 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.5, 147.8, 143.6, 141.7, 138.5, 137.6, 133.2, 132.2, 130.9, 129.3, 129.1, 128.8, 128.6, 125.6, 124.0, 122.3, 21.7, 21.6; HRMS (ESI) calcd for C₂₁H₁₉N₂O₂ [M+H]⁺ 331.1441, found 331.1449.

  1-(2-Benzoyl-6-methylphenyl)-2-(o-tolyl)diazene 1-oxide (3r):^[15] yellow liquid (46.1 mg, 70%). ¹H NMR (400 MHz, CDCl₃) δ: 7.79 (d, J＝7.5 Hz, 2H), 7.64 (d, J＝8.0 Hz, 1H), 7.53~7.44 (m, 3H), 7.42~7.33 (m, 3H), 7.20~7.13 (m, 2H), 7.07 (t, J＝4.6 Hz, 1H), 2.54 (s, 3H), 2.22 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.5, 148.2, 142.4, 136.9, 135.4, 134.4, 134.2, 133.7, 132.3, 130.9, 130.1, 129.5, 129.3, 128.7, 127.6, 126.0, 121.5, 18.7, 18.4; HRMS (ESI) calcd for C₂₁H₁₉N₂O₂ [M+H]⁺ 331.1441, found 331.1447.

  1-(2-Benzoyl-4-fluorophenyl)-2-(4-fluorophenyl)-diazene 1-oxide (3s):^[15] yellow liquid (21.6 mg, 32%). ¹H NMR (400 MHz, CDCl₃) δ: 8.27 (dd, J＝8.1, 3.4 Hz, 1H), 7.81~7.75 (m, 4H), 7.50 (d, J＝7.27 Hz, 1H), 7.40~7.31 (m, 3H), 7.20 (dd, J＝5.2, 2.5 Hz, 1H), 6.97 (t, J＝8.9 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 192.7, 164.1 (d, J_C-F＝254.3 Hz), 163.1 (d, J_C-F＝251.7 Hz), 143.4, 139.8 (d, J_C-F＝3.1 Hz), 137.4 (d, J_C-F＝3.5 Hz), 136.6, 133.8, 129.1 (d, J_C-F＝11.6 Hz), 127.9 (d, J_C-F＝8.5 Hz), 126.1 (d, J_C-F＝8.9 Hz), 117.7 (d, J_C-F＝23.0 Hz), 116.3 (d, J_C-F＝24.6 Hz), 115.9 (d, J_C-F＝22.3 Hz); HRMS (ESI) calcd for C₁₉H₁₃F₂N₂O₂ [M+H]⁺ 339.0940, found 339.0943.

  1-(2-Benzoyl-5-bromophenyl)-2-(3-bromophenyl)-diazene 1-oxide (3t): yellow liquid (30.2 mg, 33%). ¹H NMR (400 MHz, CDCl₃) δ: 8.37 (d, J＝1.69 Hz, 1H), 7.88 (d, J＝1.82 Hz, 1H), 7.82~7.80 (m, 1H), 7.75 (d, J＝7.8 Hz, 2H), 7.57~7.55 (m, 1H), 7.51 (d, J＝7.4 Hz, 1H), 7.43~7.40 (m, 4H), 7.19 (t, J＝7.3 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.3, 144.2, 136.8, 135.0, 133.9, 133.4, 130.7, 130.6, 130.2, 129.1, 129.1, 127.9, 127.0, 124.6, 124.3, 122.5; HRMS (ESI) calcd for C₁₉H₁₃Br₂N₂O₂ [M+H]⁺ 458.9338, found 458.9336.

  Supporting Information The spectroscopic characterization of the products 3a~3t, X-ray crystallographic data of dimer Pd complex A and GC-MS spectra of Scheme 2c. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

  
  
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• 

  Scheme 1  Preparation of aryl ketones

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  Scheme 2  Control experiments

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  Scheme 3  Proposed reaction mechanism

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  Figure 1  X-Ray molecular structures of complex A

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  Table 1.  Optimization of the reaction conditions^a

  Entrya	Catalyst	Oxidant	PTC	Yield b/%
  1	PdCl2	TBHP		15
  2	PdCl2	TBHP	SDS	35
  3	Pd(TFA)2	TBHP	SDS	68
  4	Pd(OAc)2	TBHP	SDS	80
  5	Pd(OAc)2	DBHP	SDS	20
  6	Pd(OAc)2	AgOAc	SDS	15
  7	Pd(OAc)2	K2S2O8	SDS	Trace
  8	Pd(OAc)2	(NH4)2S2O8	SDS	Trace
  9	Pd(OAc)2	BQ	SDS	NDe
  10	Pd(OAc)2	DDQ	SDS	NDe
  11	Pd(OAc)2	H2O2	SDS	NDe
  12	Pd(OAc)2	TBHP	TBAB	20
  13	Pd(OAc)2	TBHP	18-Crown-6	42
  14	Pd(OAc)2	TBHP	Tween 80	10
  15c	Pd(OAc)2	TBHP	SDS	65
  16d	Pd(OAc)2	TBHP	SDS	75
  a Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst (10 mol%), oxidant (4.5 equiv.), PTC (5 mol%) and water (2.0 mL), under air, 100 ℃, 24 h. b Isolated yield. c 80 ℃. d Nitrogen protection.e ND is not detected.

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  Table 2.  Scope of azoxybenzenes and alcohols ^a

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•