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. 2023 Oct 23;8(44):41983–41990. doi: 10.1021/acsomega.3c07577

Palladium/Iron-Catalyzed Wacker-Type Oxidation of Aliphatic Terminal and Internal Alkenes Using O2

Mayu Miyazaki 1, Yasuyuki Ura 1,*
PMCID: PMC10634151  PMID: 37969998

Abstract

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The Wacker-type oxidation of aliphatic terminal alkenes proceeds using a Pd/Fe catalyst system under mild reaction conditions using 1 atm O2 without other additives. The use of 1,2-dimethoxyethane/H2O as a mixed solvent was effective. The slow addition of alkenes is also important for improving product yields. Fe(III) citrate was the most efficient cocatalyst among the iron complexes examined, whereas other complexes such as FeSO4, Fe2(SO4)3, Fe(NO3)3, and Fe2O3 were also operative. This method is also applicable to aliphatic internal alkenes, which are generally difficult to oxidize using conventional Pd/Cu catalyst systems. The gram-scale synthesis and reuse of the Pd catalysts were also demonstrated.

Introduction

Tsuji–Wacker oxidation is one of the most well-known homogeneous transition metal-catalyzed reactions.1−18 The Wacker-type oxidation furnishes methyl ketones from terminal alkenes, typically catalyzed by Pd and Cu salts under O2 (Scheme 1a). However, Cu salts are generally moderately toxic,19 and alternative methods that do not use Cu salts as cocatalysts have been investigated. Catalyst systems that involve the direct oxidation of Pd(0) by O2 in DMA, NMP,20 DMA/H2O,21,22 DMSO/H2O (with 1 equiv of CF3CO2H),23 MeOH/DMSO (with Pd NPs/ZrO2),24 MeOH/H2O (with cationic Pd complexes with NN bidentate ligands),25i-PrOH (with Pd(OAc)2/pyridine),26 and ethylene carbonate27 have been developed. These reactions require high temperatures (60–100 °C) and/or high O2 pressures (2–10 atm) to smoothly promote the oxidation step. Polyoxometalates,28−32 alkyl nitrites and nitrite salts,33−37 and Ir or BiVO4 photocatalysts38 have been reported as cocatalyst alternatives to Cu salts. The nonredox metal ion Sc(III) can also operate as an efficient cocatalyst in the Pd(OAc)2/Sc(OTf)3/O2 system.39

Scheme 1. Pd-Catalyzed Wacker-Type Oxidations Using a Cu or Fe Complex as a Cocatalyst or an Oxidant.

Scheme 1

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Fe complexes are generally less toxic than Cu complexes,19 and in addition, iron is more abundant and less expensive than copper. For Pd-catalyzed Wacker-type oxidations involving Fe complexes, Baeckvall et al. developed an elegant biomimetic catalytic system that uses an Fe complex and p-hydroquinone (HQ) as cocatalysts, that is, Pd(OAc)2/Fe(Pc) (Pc = phthalocyanine)/HQ/HClO4/O2 (Scheme 1b),40 in which the Pd(0/II)–HQ/p-benzoquinone (BQ)–Fe(red/ox) triple catalytic system is operative. The addition of HClO4 suppresses the precipitation of Pd(0).40 Grubbs et al. successfully applied a Pd(OAc)2/Fe(Pc)/BQ/HBF4/O2 catalyst system to internal alkenes.41 Fernandes et al. reported a Pd-catalyzed Wacker-type oxidation using a stoichiometric amount of Fe salts, such as Fe2(SO4)3, as oxidants for Pd (Scheme 1c).42 Because Fe complexes can be oxidized by O2,40 we hypothesized that a Pd/Fe/O2 catalyst system without other additives could be developed. Herein, we report a simple, Cu- and other additive-free, Pd/Fe-catalyzed Wacker-type oxidation using 1 atm O2 as the terminal oxidant that proceeds at room temperature (Scheme 1d). Aliphatic terminal and internal alkenes could be used as substrates. The gram-scale synthesis and reuse of the Pd catalyst were also demonstrated.

Results and Discussion

Initially, the effects of the reaction conditions were examined by using 1-octene (1a) as the substrate (Table 1). PdCl2 (5 mol %) and Fe(III) citrate·nH2O (5 mol %) were used as catalysts, and 1,2-dimethoxyethane (DME)/H2O (3:1) was used as a mixed solvent. O2 (1 atm) was used as the terminal oxidant. Substrate 1a was slowly added (over 5 h) to the reaction solution at room temperature using a syringe pump, and the mixture was stirred for an additional 1 h. Standard conditions afforded 2-octanone (2a) in 97% yield (entry 1). Product 2a was not obtained in the absence of PdCl2 (entry 2). Although the reaction proceeded in the absence of the Fe complex, the yield of 2a was moderate (entry 3). The slow addition of 1a was also important for increasing the product yield (entry 4 vs 1). The use of air (1 atm) instead of O2 decreased both the conversion of 1a and the yield of 2a, and increased the yield of internal alkenes via the isomerization of 1a (entry 5). The catalytic activities of PdCl2(MeCN)2 and PdCl2(PhCN)2 were comparable to those of PdCl2 (entries 6 and 7), whereas PdCl2(cod) (cod = 1,5-cyclooctadiene) and Pd(OAc)2 were inefficient (entries 8 and 9).

Table 1. Effect of Reaction Conditionsa.

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entry change from standard conditions Conv. of 1a (%)b yield of 2a (%)b yield of internal alkenes (%)b
1 none 100 97 0
2 no PdCl2 52 0 0
3 no Fe(III) citrate·nH2O 96 43 7
4 no slow addition of 1a 100 72 5
5 under air 56 29 24
6 PdCl2(MeCN)2 instead of PdCl2 100 95 0
7 PdCl2(PhCN)2 instead of PdCl2 100 95 0
8 PdCl2(cod) instead of PdCl2 89 26 28
9 Pd(OAc)2 instead of PdCl2 40 10 0
10c DME/H2O (1:1) 100 66 2
11c DME/H2O (5:1) 100 86 3
12 diglyme/H2O (3:1) 100 91 0
13 THF/H2O (3:1) 100 67 6
14 1,4-dioxane/H2O (3:1) 100 82 4
15 Et2O/H2O (3:1) 60 2 25
16 MeOH/H2O (3:1) 100 23 16
17 EtOH/H2O (3:1) 100 71 4
18 DMF/H2O (3:1) 86 33 1
19 MeCN/H2O (3:1) 75 44 18
20 FeSO4·7H2O (5 mol %) instead of Fe(III) citrate·nH2O 100 89 0
21 Fe2(SO4)3·nH2O (2.5 mol %) instead of Fe(III) citrate·nH2O 100 84 0
22 Fe(NO3)3·9H2O (5 mol %) instead of Fe(III) citrate·nH2O 100 91 0
23 Fe2O3 (2.5 mol %) instead of Fe(III) citrate·nH2O 100 83 0
24 FeCl2 (5 mol %) instead of Fe(III) citrate·nH2O 100 76 8
25 FeCl3 (5 mol %) instead of Fe(III) citrate·nH2O 90 49 20
26d PdCl2 (2 mol %), Fe(III) citrate·nH2O (4 mol %), 16 h 100 96 0
27e PdCl2 (1 mol %), Fe(III) citrate·nH2O (4 mol %), 16 h 100 92 0
28f PdCl2 (0.5 mol %), Fe(III) citrate·nH2O (10 mol %), 70 h 100 88 3
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a

Reaction conditions: 1a (0.50 mmol), PdCl2 (0.025 mmol), Fe(III) citrate·nH2O (0.025 mmol), DME (3.0 mL), H2O (1.0 mL), r.t., O2 (1 atm). 1a was added over 5, 15, or 69 h using a syringe pump, and the reaction mixture was stirred for an additional 1 h (6, 16, or 70 h in total).

b

Determined by 1H NMR.

c

Average of two runs.

d

PdCl2 (0.010 mmol), Fe(III) citrate·nH2O (0.020 mmol).

e

PdCl2 (0.005 mmol), Fe(III) citrate·nH2O (0.020 mmol).

f

1a (1.0 mmol), PdCl2 (0.005 mmol), Fe(III) citrate·nH2O (0.10 mmol).

The effect of the solvents was also critical (entries 10–19). Varying the DME/H2O ratio to 1:1 or 5:1 resulted in lower product yields (entries 10 and 11). The use of ethereal solvents miscible with H2O other than DME, such as, diglyme, THF, and 1,4-dioxane, afforded 2a in good to high yields (entries 12–14). In contrast, the use of Et2O significantly reduced the product yield, probably due to its poor miscibility with H2O (entry 15). Other polar solvents that are miscible with H2O, such as MeOH, DMF, and MeCN, instead of DME decreased the yield of 2a considerably, whereas EtOH produced a relatively good result (entries 16–19).

Fe complexes other than Fe(III) citrate·nH2O were also examined as cocatalysts. Several Fe(II) and Fe(III) complexes, such as FeSO4·7H2O, Fe2(SO4)3·nH2O, Fe(NO3)3·9H2O, and Fe2O3, were effective (entries 20–23). FeCl2 afforded 2a in good yield (entry 24), whereas FeCl3 was almost ineffective (entry 25 vs 3). The Pd catalyst loading was also investigated. The use of 2, 1, or 0.5 mol % of PdCl2 still afforded high product yields by increasing the Fe/Pd ratio and prolonging the reaction time (entries 26–28).

The optimized reaction conditions were then applied to various terminal alkenes (Scheme 2). In addition to linear alkenes 1a–1c, branched alkenes such as 4-methyl-1-pentene (1d) and 3-cyclohexyl-1-propene (1e) also afforded the corresponding methyl ketones 2d and 2e in good yields. 4-Phenyl-1-butene (1f) was also suitable. Notably, bulky alkenes vinylcyclohexane (1g) and 3,3-dimethyl-1-butene (1h) furnished ketones 2g and 2h in moderate yields, respectively. 5-Hexen-1-ol (1i) and 9-decen-1-ol (1j) afforded the corresponding products in good to high yields. In the case of allyl phenyl ether (1k), methyl ketone 2k and aldehyde 3k were formed in a 1.7:1 ratio. The formation of the aldehyde could be due to the oxygen atom, which functions as a directing group.7,18 Alkenes with a carboxy or benzyloxycarbonyl group 1l–1n are also applicable. Haloalkenes 6-chloro-1-hexene (1o) and 7-bromo-1-heptene (1p) afforded methyl ketones 2o and 2p in high yields, respectively, without damaging the halogen group. As for dienes, although 1,7-octadiene (1q) afforded the corresponding diketone 2q in good yield, 1,5-hexadiene (1r) did not furnish the corresponding diketone 2r at all. Styrene (1s), 4-acetoxystyrene (1t), and 3-trifluoromethylstyrene (1u) resulted in the corresponding mixtures of acetophenones 2s–2u, arylacetaldehydes 3s–3u,7,18,43−46 and arylaldehydes 4s–4u.45,46

Scheme 2. Scope of Terminal Alkenes.

Scheme 2

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Reaction conditions: 1 (0.50 mmol), PdCl2 (0.025 mmol), Fe(III) citrate·nH2O (0.025 mmol), DME (3.0 mL), H2O (1.0 mL), r.t., O2 (1 atm). 1 was added over 15 h using a syringe pump, and the reaction mixture was stirred for an additional 1 h (16 h in total). Isolated yields are shown.

Slow addition: 5 h + 1 h (6 h in total).

Isolated as 2,4-dinitrophenylhydrazone derivatives.

The formation ratios of the mixtures are indicated in parentheses.

NMR yields.

The same reaction conditions were applied to internal alkenes (Scheme 3). Because it is generally difficult for Pd/Cu catalyst systems to oxidize internal alkenes, Cu-free catalyst systems have predominantly been developed.37,41,47−56 3-Hexene (5a, cis and trans mixture) afforded 3-hexanone (6a), along with a small amount of 2-hexanone (2b). Trans-4-octene (trans-5b) selectively furnished 4-octanone (6b) in a moderate yield. 2-Octene (5c, cis and trans mixture) yielded a mixture of 2-octanone (2a) and 3-octanone (6c) in a 2.6:1 ratio. Trans-methyl crotonate (5d) afforded methyl acetoacetate (6d) exclusively. As cycloalkenes, cyclohexene (5e) and cyclooctene (5f) were examined; however, cyclohexanone (6e) was obtained in a low yield (17%) and cyclooctanone (6f) was not formed at all. Although methyl (2Z)-2-methyl-2-butenoate was used as a trisubstituted alkene, oxidation did not proceed.

Scheme 3. Scope of Internal Alkenes.

Scheme 3

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Reaction conditions: 5 (0.50 mmol), PdCl2 (0.050 mmol), Fe(III) citrate·nH2O (0.050 mmol), DME (3.0 mL), H2O (1.0 mL), r.t., O2 (1 atm). 5 was added over 23 h using a syringe pump, and the reaction mixture was stirred for an additional 1 h (24 h in total). Isolated yields are shown (the products were isolated as 2,4-dinitrophenylhydrazone derivatives). The formation ratios of the mixtures are indicated in parentheses.

Gram-scale synthesis was also attempted (Scheme 4). 1-Tetradecene (1c, 1.18 g, 6.00 mmol) was converted to 2-tetradecanone (2c, 1.07 g, 5.04 mmol) using 3 mol % of PdCl2 and 5 mol % of Fe(III) citrate·nH2O without decreasing the product yield.

Scheme 4. Gram-Scale Synthesis.

Scheme 4

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The reusability of the Pd catalyst was also examined (Scheme 5). After the first reaction using 1a as the substrate and PdCl2 (2 mol %) and Fe(III) citrate·nH2O (10 mol %) as the catalysts, the reaction mixture was extracted with n-hexane, and the aqueous layer containing the Pd catalyst was reused for the next reaction. Fe(III) citrate·nH2O and DME were added to the aqueous layer, and 1a was added over 15 h to the solution under O2. This procedure was repeated thrice. Although the NMR yield of 2a gradually decreased with each cycle, the overall recovery and reusability of the Pd catalysts were reasonable.

Scheme 5. Reuse of the Pd Catalyst.

Scheme 5

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To clarify whether Fe(III) complexes can oxidize Pd(0) species, a Wacker-type oxidation was performed using a stoichiometric amount of Pd2(dba)3·CHCl3 (dba = dibenzylideneacetone) in the absence or presence of Fe(III) citrate·nH2O, under an argon atmosphere (Scheme 6). The results show that Fe(III) citrate is essential for the reaction to proceed and that Fe(III) can oxidize Pd(0) to Pd(II).

Scheme 6. Stoichiometric Oxidation Using Pd2(dba)3.

Scheme 6

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An outline of the proposed mechanism is presented in Scheme 7. Alkenes are oxidized to ketones by Pd(II), and the formed Pd(0) species are reoxidized to Pd(II) by the higher oxidation state of the Fe complex (Feox). The reduced Fe complex (Fered) is reoxidized by O2. Ethereal solvents may stabilize Fe complexes via coordination. In situ formed Pd–H species isomerize terminal alkenes to internal alkenes. Higher concentrations of terminal alkenes in the reaction mixture tend to promote isomerization into internal alkenes; thus, the slow addition of substrates inhibits isomerization.

Scheme 7. Outline of the Proposed Mechanism.

Scheme 7

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Conclusions

We developed a Pd/Fe-catalyzed Wacker-type oxidation using O2 as the terminal oxidant that proceeds under mild reaction conditions in the absence of other additives. Ethereal solvent/H2O systems, particularly DME/H2O, are suitable reaction media. The addition of Fe cocatalysts, especially Fe(III) citrate, significantly promoted the reaction, and they functioned as reoxidants for the Pd(0) species. The slow addition of terminal alkenes is also a key factor in suppressing the isomerization to internal alkenes. Bulky terminal alkenes, such as 3,3-dimethyl-1-butene can also be oxidized. Aliphatic internal alkenes, which are generally difficult to oxidize by conventional Pd/Cu catalyst systems, were also converted to the corresponding ketones. The gram-scale synthesis proceeded successfully without a decrease in the product yield. The Pd catalyst was reused by collecting and reusing the aqueous layer after each reaction. We believe that this Pd/Fe/O2 catalyst system can be applied to other reactions related to the Wacker-type oxidation.

Experimental Section

General Information

All reactions were performed under an oxygen atmosphere unless otherwise noted using standard Schlenk techniques. PdCl2 and Fe complexes were commercially available and used as received. Pd2(dba)3·CHCl3 was prepared as described in the literature.57 Because the number of water molecules in Fe(III) citrate·nH2O is unknown, the mass was calculated as an anhydrous complex. DME (>99.0% purity), diglyme (>98.0% purity), and other organic solvents (>99.5% purities) were purchased from FUJIFILM Wako and used as received. THF was dried over Na/benzophenone and distilled. Ion-exchanged water was used for the reaction. Compound 1n was prepared as described in the literature.58 Alkenes other than 1n were commercially available and used as received. Flash column chromatography was performed using silica gel SILICYCLE SiliaFlash F60 (40–63 μm, 230–400 mesh). NMR spectra were recorded on a Bruker AV-300N (300 MHz (1H), 75 MHz (13C)) spectrometer or a JEOL JNM AL-400 (400 MHz (1H), 100 MHz (13C)) spectrometer. Chemical shift values (δ) were expressed relative to SiMe4 for 1H and 13C NMR. Elemental analysis was performed using a PerkinElmer 2400II analyzer. High-resolution mass spectra were recorded on a JEOL JMS-T100LC spectrometer (ESI-TOF MS) with positive ionization mode. For identification of known isolated products, the spectral data were compared to those reported in the literature.

Wacker-Type Oxidation of Terminal Alkenes 1

Method A: To a reaction vessel, PdCl2 (4.4 mg, 0.025 mmol) and Fe(III) citrate·nH2O (6.1 mg, 0.025 mmol) were added, and O2 was purged. To the mixture were added DME (3.0 mL) and H2O (1.0 mL), and the reaction mixture was stirred at room temperature. Immediately, 1 (0.50 mmol) was added slowly over 5 or 15 h by a syringe pump, and the reaction mixture was stirred for an additional 1 h (6 or 16 h in total). The product was derivatized to 2,4-dinitrophenylhydrazone using a literature procedure.44 The crude material was purified by silica gel column chromatography. Method B: After the reaction completed, to the mixture was added CHCl3. The aqueous layer was further extracted with CHCl3 (thrice). The combined organic layer was dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated under a vacuum. The crude material was purified by silica gel column chromatography.

2-Octanone (2a)

Method A was applied. Compound 2a was isolated as 2,4-dinitrophenylhydrazone derivatives 2a’ and 2a”.59 Purification by silica gel column chromatography (hexane/ethyl acetate = 60:1 to 7:1) afforded an orange solid (141 mg, 0.457 mmol, 91% yield).

2-Hexanone (2b)

Method A was applied. Compound 2b was isolated as 2,4-dinitrophenylhydrazone derivatives 2b’ and 2b”. Purification by silica gel column chromatography (hexane/ethyl acetate = 40:1 to 5:1) afforded an orange solid (117 mg, 0.417 mmol, 83% yield). 1H NMR for 2b’ (300 MHz, CDCl3) δ 10.97 (s, 1H), 9.03 (d, J = 2.5 Hz, 1H), 8.23 (dd, J = 9.6, 2.6 Hz, 1H), 7.90 (d, J = 9.6 Hz, 1H), 2.41 (t, J = 7.3 Hz, 2H), 2.04 (s, 3H), 1.65–1.55 (m, 2H), 1.44–1.32 (m, 2H), 0.94 (t, J = 7.3 Hz, 3H). 13C{1H} NMR for 2b’ (75 MHz, CDCl3) δ 157.6, 144.2, 136.4, 128.9, 127.8, 122.4, 115.4, 37.7, 27.3, 21.3, 14.8, 12.9. HRMS (ESI): m/z calcd for C12H16N4O4 [M + H]+ 281.1244, found 281.1254. Anal. Calcd for C12H15N4O4: C, 51.42; H, 5.75; N, 19.99. Found: C, 51.10; H, 5.75; N, 19.80.

2-Tetradecanone (2c)60

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 15:1) afforded a white solid (89 mg, 0.42 mmol, 84% yield).

4-Methyl-2-pentanone (2d)

Method A was applied. Compound 2d was isolated as 2,4-dinitrophenylhydrazone derivatives 2d’ and 2d”.59 Purification by silica gel column chromatography (hexane/ethyl acetate = 40:1 to 5:1) afforded an orange solid (99 mg, 0.35 mmol, 70% yield).

3-Cyclohexyl-2-propanone (2e)

Method A was applied. Compound 2e was isolated as 2,4-dinitrophenylhydrazone derivative 2e’.59 Purification by silica gel column chromatography (hexane/ethyl acetate = 60:1 to 7:1) afforded an orange solid (122 mg, 0.38 mmol, 76% yield).

4-Phenyl-2-butanone (2f)60

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 30:1 to 3:1) afforded a colorless oil (67 mg, 0.45 mmol, 90% yield).

1-Cyclohexylethanone (2g)

Method A was applied. Compound 2g was isolated as 2,4-dinitrophenylhydrazone derivative 2g’. Purification by silica gel column chromatography (hexane/ethyl acetate = 40:1 to 5:1) afforded an orange solid (78 mg, 0.25 mmol, 51% yield). 1H NMR for 2g’ (300 MHz, CDCl3) δ 11.02 (s, 1H), 9.12 (d, J = 2.6 Hz, 1H), 8.29 (dd, J = 9.6, 2.6 Hz, 1H), 7.96 (d, J = 9.6 Hz, 1H), 2.38–2.25 (m, 1H), 2.04 (s, 3H), 1.92–1.73 (m, 5H), 1.46–1.18 (m, 5H). 13C{1H} NMR for 2g’ (75 MHz, CDCl3) δ 160.7, 144.4, 136.5, 128.9, 127.9, 122.5, 115.5, 46.2, 29.1, 24.99, 24.97, 13.5. HRMS (ESI): m/z calcd for C14H18N4O4Na [M + Na]+ 329.1226, found 329.1240. Anal. Calcd for C14H19.2N4O4.6 (2g’·0.6 H2O): C, 53.02; H, 6.10; N, 17.67. Found: C, 53.16; H, 5.95; N, 17.31.

3,3-Dimethyl-2-butanone (2h)

Method A was applied. Compound 2h was isolated as 2,4-dinitrophenylhydrazone derivative 2h’.59 Purification by silica gel column chromatography (hexane/ethyl acetate = 60:1 to 7:1) afforded an orange solid (81 mg, 0.29 mmol, 57% yield). 1H NMR for 2h’ (300 MHz, CDCl3) δ 11.01 (s, 1H), 9.12 (d, J = 2.6 Hz, 1H), 8.28 (dd, J = 9.6, 2.4 Hz, 1H), 7.96 (d, J = 9.6 Hz, 1H), 2.04 (s, 3H), 1.24 (s, 9H). 13C{1H} NMR for 2h’ (75 MHz, CDCl3) δ 163.7, 145.5, 137.5, 129.9, 129.0, 123.5, 116.6, 39.3, 27.5, 12.1. HRMS (ESI): m/z calcd for C12H17N4O4 [M + H]+ 281.1244, found 281.1254. Anal. Calcd for C12H16N4O4: C, 51.42; H, 5.75; N, 19.99. Found: C, 51.27; H, 5.82; N, 19.74.

6-Hydroxy-2-hexanone (2i)

Method A was applied. Compound 2i was isolated as 2,4-dinitrophenylhydrazone derivatives 2i’ and 2i”. Purification by silica gel column chromatography (hexane/ethyl acetate = 8:1 to ethyl acetate only) afforded an orange solid (99 mg, 0.33 mmol, 67% yield). 1H NMR for 2i’ (300 MHz, CDCl3) δ 11.04 (s, 1H), 9.11 (d, J = 2.5 Hz, 1H), 8.28 (dd, J = 9.6, 2.4 Hz, 1H), 7.94 (d, J = 9.6 Hz, 1H), 3.72 (t, J = 6.1 Hz, 2H), 2.48 (t, J = 7.6 Hz, 2H), 2.07 (s, 3H), 1.81–1.60 (m, 4H), 1.47 (br s, 1H). 13C{1H} NMR for 2i’ (75 MHz, CDCl3) δ 157.9, 145.2, 137.6, 130.0, 128.9, 123.5, 116.4, 62.4, 38.7, 32.0, 22.3, 15.9. HRMS (ESI): m/z calcd for C12H16N4O5Na [M + Na]+ 319.1018, found 319.1021. Anal. Calcd for C12H16N4O5: C, 48.65; H, 5.45; N, 18.91. Found: C, 48.54; H, 5.32; N, 18.89.

10-Hydroxy-2-decanone (2j)61

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 2:1) afforded a white solid (79 mg, 0.46 mmol, 92% yield).

1-Phenoxy-2-propanone (2k)62 and 3-Phenoxypropanal (3k)63

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 20:1 to 2:1) afforded a pale yellow oil (46 mg, 0.31 mmol, 62% yield (2k/3k = 1.7:1)).

5-Oxohexanoic Acid (2l)64

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 1:1) afforded a colorless oil (57 mg, 0.44 mmol, 88% yield).

9-Oxodecanoic Acid (2m)65

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 1:1) afforded a white solid (89 mg, 0.48 mmol, 96% yield).

5-Oxohexanoic Acid Benzyl Ester (2n)66

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 20:1 to 3:1) afforded a pale yellow oil (94 mg, 0.43 mmol, 85% yield).

6-Chloro-2-hexanone (2o)

Method A was applied. Compound 2o was isolated as 2,4-dinitrophenylhydrazone derivatives 2o’ and 2o”. Purification by silica gel column chromatography (hexane/ethyl acetate = 20:1 to 2:1) afforded an orange solid (146 mg, 0.464 mmol, 93% yield). 1H NMR for 2o’ (300 MHz, CDCl3) δ 11.05 (s, 1H), 9.14 (d, J = 2.5 Hz, 1H), 8.31 (dd, J = 9.6, 2.6 Hz, 1H), 7.96 (d, J = 9.6 Hz, 1H), 3.61 (t, J = 6.0 Hz, 2H), 2.48 (t, J = 6.9 Hz, 2H), 2.08 (s, 3H), 1.91–1.80 (m, 4H). 13C{1H} NMR for 2o’ (75 MHz, CDCl3) δ 157.2, 145.1, 137.6, 130.0, 128.9, 123.5, 116.4, 44.6, 38.0, 31.8, 23.1, 16.0. HRMS (ESI): m/z calcd for C12H16ClN4O4[M + H]+ 315.0855, found 315.0845. Anal. Calcd for C12H15ClN4O4: C, 45.80; H, 4.80; N, 17.80. Found: C, 45.54; H, 4.57; N, 17.93.

7-Bromo-2-heptanone (2p)67

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 10:1) afforded a pale yellow oil (90 mg, 0.47 mmol, 93% yield).

2,7-Octanedione (2q)68

Method B was applied. Purification by silica gel column chromatography (hexane/ethyl acetate = 3:1) afforded a pale yellow oil (55 mg, 0.39 mmol, 77% yield).

Wacker-Type Oxidation of Internal Alkenes 5

To a reaction vessel, PdCl2 (8.9 mg, 0.050 mmol) and Fe(III) citrate·nH2O (12 mg, 0.050 mmol) were added, and O2 was purged. To the mixture, DME (3.0 mL) and H2O (1.0 mL) were added and the reaction mixture was stirred at room temperature. Immediately, 5 (0.50 mmol) was added slowly over 23 h by a syringe pump, and the reaction mixture was stirred for an additional 1 h (24 h in total). The product was derivatized to 2,4-dinitrophenylhydrazone using a literature procedure.44 The crude material was purified by silica gel column chromatography.

3-Hexanone (6a)

Compounds 6a and 2b were obtained as a mixture of corresponding 2,4-dinitrophenylhydrazone derivatives 6a’, 6a”, and 2b’ from 3-hexene (5a, cis and trans mixture). Purification by silica gel column chromatography (hexane/ethyl acetate = 40:1 to 5:1) afforded an orange solid (87 mg, 0.31 mmol, 62% yield ((6a’ + 6a”)/2b’ = 9.3:1)). 1H NMR for 6a’ and 6a” (300 MHz, CDCl3) δ 11.24 (s, 1H for 6a’ or 6a”), 11.21 (s, 1H for 6a” or 6a’), 9.11 (d, J = 2.6 Hz, 1H), 8.28 (dd, J = 9.6, 2.5 Hz, 1H), 7.96 (d, J = 9.6 Hz, 1H for 6a’ or 6a”), 7.95 (d, J = 9.6 Hz, 1H for 6a” or 6a’), 2.48–2.36 (m, 4H), 1.76–1.60 (m, 2H), 1.23 (t, J = 7.8 Hz, 3H for 6a’ or 6a”), 1.21 (t, J = 7.4 Hz, 3H for 6a” or 6a’), 1.07 (t, J = 7.3 Hz, 3H for 6a’ or 6a”), 1.00 (t, J = 7.4 Hz, 3H for 6a” or 6a’). 13C{1H} NMR for 6a’ and 6a” (75 MHz, CDCl3) δ 162.83, 162.80, 145.3, 137.4, 129.9, 128.8, 123.55, 123.53, 116.34, 116.31, 38.8, 32.2, 30.7, 23.2, 19.4, 18.8, 14.4, 13.8, 10.4, 9.4. HRMS (ESI): m/z calcd for C12H16N4O4Na [M + Na]+ 303.1069, found 303.1059. Anal. Calcd for C12H16.2N4O4.1 (6a’·0.1 H2O): C, 51.10; H, 5.79; N, 19.86. Found: C, 51.07; H, 5.95; N, 19.56.

4-Octanone (6b)

Compound 6b was obtained as 2,4-dinitrophenylhydrazone derivatives 6b’ and 6b” from trans-4-octene (trans-5b). Purification by silica gel column chromatography (hexane/ethyl acetate = 60:1 to 7:1) afforded an orange solid (85 mg, 0.28 mmol, 55% yield). 1H NMR for 6b’ and 6b” (300 MHz, CDCl3) δ 11.22 (s, 1H), 9.09 (d, J = 2.6 Hz, 1H), 8.29–8.24 (m, 1H), 7.93 (d, J = 9.6 Hz, 1H), 2.42–2.35 (m, 4H), 1.74–1.34 (m, 6H), 1.10–0.94 (m, 6H). 13C{1H} NMR for 6b’ and 6b” (75 MHz, CDCl3) δ 162.22, 162.16, 145.25, 145.23, 137.3, 129.87, 129.86, 128.7, 123.5, 116.30, 116.27, 39.4, 37.1, 32.2, 30.1, 28.2, 27.2, 23.0, 22.4, 19.5, 18.8, 14.4, 13.9, 13.8, 13.7. HRMS (ESI): m/z calcd for C14H20N4O4Na [M + Na]+ 331.1382, found 331.1381. Anal. Calcd for C14H20N4O4: C, 54.54; H, 6.54; N, 18.17. Found: C, 54.55; H, 6.70; N, 17.80.

3-Octanone (6c)

Compounds 2a and 6c were obtained as a mixture of corresponding 2,4-dinitrophenylhydrazone derivatives 2a’, 2a”, 6c’, and 6c” from 2-octene (5c, cis and trans mixture). Purification by silica gel column chromatography (hexane/ethyl acetate = 60:1 to 7:1) afforded an orange solid (61 mg, 0.20 mmol, 40% yield ((2a’ + 2a”)/(6c’ + 6c”) = 2.6:1)). 1H NMR for 6c’ and 6c” (300 MHz, CDCl3) δ 11.20 (s, 1H for 6c’ or 6c”), 11.02 (s, 1H for 6c” or 6c’), 9.10 (d, J = 2.6 Hz, 1H), 8.27 (dd, J = 9.6, 2.5 Hz, 1H), 7.96 (d, J = 9.6 Hz, 1H), 2.48–2.37 (m, 4H), 1.67–1.58 (m, 2H), 1.43–1.29 (m, 4H), 1.23 (t, J = 7.7 Hz, 3H for 6c’ or 6c”), 1.20 (t, J = 7.4 Hz, 3H for 6c” or 6c’), 0.94–0.88 (m, 3H). 13C{1H} NMR for 2a”, 6c’ and 6c” (75 MHz, CDCl3) δ 163.0, 159.5, 145.31, 145.27, 145.1, 137.40, 137.37, 129.9, 128.8, 128.7, 123.52, 123.51, 116.3, 116.2, 36.8, 32.0, 31.44, 31.36, 31.1, 30.7, 30.3, 29.4, 25.8, 25.1, 24.9, 23.7, 23.2, 22.45, 22.41, 22.3, 13.97, 13.94, 13.8, 10.4, 9.4. Some signals for 2a”, 6c’, and 6c” were overlapped with those for 2a’. HRMS (ESI): m/z calcd for C14H20N4O4Na [M + Na]+ 331.1382, found 331.1396. Anal. Calcd for C14H20N4O4: C, 54.54; H, 6.54; N, 18.17. Found: C,54.39; H, 6.74; N, 17.93.

Methyl Acetoacetate (6d)

Compound 6d was obtained as a mixture of corresponding 2,4-dinitrophenylhydrazone derivatives 6d’ and 6d”. Purification by silica gel column chromatography (hexane/ethyl acetate = 12:1 to 1:1) afforded an orange solid (97 mg, 0.33 mmol, 65% yield). 1H NMR for 6d’ (300 MHz, CDCl3) δ 11.04 (s, 1H), 9.05 (d, J = 2.6 Hz, 1H), 8.26 (dd, J = 9.6, 2.5 Hz, 1H), 7.90 (d, J = 9.6 Hz, 1H), 3.74 (s, 3H), 3.48 (s, 2H), 2.15 (s, 3H). 13C{1H} NMR for 6d’ (75 MHz, CDCl3) δ 169.6, 150.7, 144.8, 138.0, 129.9, 129.3, 123.2, 116.4, 52.3, 44.1, 16.1. HRMS (ESI): m/z calcd for C11H12N4O6Na [M + Na]+ 319.0655, found 319.0644. Anal. Calcd for C11H12.4N4O6.2 (6d’·0.2 H2O): C, 44.06; H, 4.17; N, 18.69. Found: C, 44.10; H, 4.02; N, 18.53.

Cyclohexanone (6e)(69)

Compound 6e was obtained as a corresponding 2,4-dinitrophenylhydrazone derivative 6e’. Purification by silica gel column chromatography (hexane/ethyl acetate = 30:1 to 7:1) afforded a yellow solid (23 mg, 0.083 mmol, 17% yield).

Gram-Scale Synthesis of 2c

To a reaction vessel, PdCl2 (32 mg, 0.18 mmol) and Fe(III) citrate·nH2O (73 mg, 0.30 mmol) were added, and O2 was purged. To the mixture, DME (36 mL) and H2O (12 mL) were added, and the reaction mixture was stirred at room temperature. Immediately, 1c (1.53 mL, 1.18 g, 6.00 mmol) was added slowly over 15 h by a syringe pump, and the reaction mixture was stirred for an additional 1 h (16 h in total). After the reaction was completed, the mixture was filtered by Celite. CHCl3 was added to the filtrate, and the aqueous layer was extracted with CHCl3 (five times). The combined organic layer was dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated under a vacuum. The crude material was purified by silica gel column chromatography (hexane/ethyl acetate = 50:1 to 7:1). Compound 2c was obtained as a white solid (1.07 g, 5.04 mmol, 84% yield).

Reuse of the Pd Catalyst

To a reaction vessel, PdCl2 (5.3 mg, 0.030 mmol) and Fe(III) citrate·nH2O (37 mg, 0.15 mmol) were added, and O2 was purged. To the mixture were added DME (9.0 mL) and H2O (3.0 mL), and the reaction mixture was stirred at room temperature. Immediately, 1a (235 μL, 1.50 mmol) was added slowly over 15 h by a syringe pump, and the reaction mixture was stirred for an additional 1 h (16 h in total). After the reaction was completed, to the mixture was added hexane. The aqueous layer was further extracted with hexane (thrice). After the organic layer was removed, the remaining organic solvents were evaporated under a vacuum. To the reaction vessel with only the aqueous layer remaining, Fe(III) citrate·nH2O (37 mg, 0.15 mmol) and DME (9.0 mL) were added and the reaction mixture was stirred at room temperature. Immediately, 1a (235 μL, 1.50 mmol) was added slowly over 15 h by a syringe pump, and the reaction mixture was stirred for an additional 1 h (16 h in total). Thereafter, these manipulations were repeated.

Stoichiometric Oxidation Using Pd2(dba)3

To a reaction vessel were added Pd2(dba)3·CHCl3 (26 mg, 0.025 mmol) and Fe(III) citrate·nH2O (none or 73 mg, 0.30 mmol), and Ar was purged. To the mixture, DME (1.5 mL), H2O (0.50 mL), HCl in 1,4-dioxane (50 μL, 4 M solution), and 1a (7.8 μL, 0.050 mmol) were added, and the reaction mixture was stirred at room temperature for 6 h. A portion of the reaction mixture (0.20 mL) and CDCl3 (0.40 mL) were mixed and the samples were filtered. The organic layers were analyzed by 1H NMR spectroscopy.

Acknowledgments

This study was supported by JSPS KAKENHI Grant Number JP18H03914 (Y.U.) and Micron Research Grant in the Field of Science and Technology at Nara Women's University (M.M.). We appreciate prof. Yasutaka Kataoka (Nara Women's University) for helpful discussion.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c07577.

  • Copies of 1H and 13C NMR spectral data (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c07577_si_001.pdf (2.4MB, pdf)

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Associated Data

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Supplementary Materials

ao3c07577_si_001.pdf (2.4MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

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