a ligand free pd oac 2 catalyst for the wacker oxidation of styrene derivatives using hydrogen peroxide as the oxidant

Accepted Manuscript Original article A ligand-free Pd(OAc)2 catalyst for the Wacker oxidation of styrene derivatives using hydrogen peroxide as the oxidant Xiaomeng Xia, Xi Gao, Junhui Xu, Chuanfeng Hu, Xinhua Peng PII: DOI: Reference: S1319-6103(16)30101-6 http://dx.doi.org/10.1016/j.jscs.2016.10.004 JSCS 841 To appear in: Journal of Saudi Chemical Society Received Date: Revised Date: Accepted Date: 30 August 2016 11 October 2016 23 October 2016 Please cite this article as: X Xia, X Gao, J Xu, C Hu, X Peng, A ligand-free Pd(OAc)2 catalyst for the Wacker oxidation of styrene derivatives using hydrogen peroxide as the oxidant, Journal of Saudi Chemical Society (2016), doi: http://dx.doi.org/10.1016/j.jscs.2016.10.004 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain A ligand-free Pd(OAc)2 catalyst for the Wacker oxidation of styrene derivatives using hydrogen peroxide as the oxidant Xiaomeng Xia a, Xi Gao a, Junhui Xu a, Chuanfeng Hu a and Xinhua Peng a,b,* a School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b Lianyungang Institute, Nanjing University of Science and Technology, Lianyungang 222006, China *Corresponding author Tel.: +86-25-8431-5520; fax: +86-25-8431-7639; e-mail: xhpeng@mail.njust.edu.cn Graphical abstract Abstract: A feasible palladium-catalyzed Wacker oxidation system has been developed, which is characterized by not adding ligand and using hydrogen peroxide as the sole oxidant Compared with the traditional Wacker system, the newly developed method offers a cost-efficient and environment-friendly choice without using copper species, and can be applied to more complex substrates Keywords: Wacker oxidation; Hydrogen peroxide; Palladium acetate Introduction The Wacker oxidation, a method of great importance for the synthesis of acetaldehyde in industry, was initially reported in 1959 [1-2] To effectively oxidize more valuable substrates, the Tsuji-Wacker oxidation has been developed and widely used in the synthesis of natural products, pharmaceuticals, and commodity chemicals [3-6] The traditional Tsuji-Wacker oxidation generally involves PdCl2, CuCl and O2 in a DMF–H2O solvent system (Fig 1A) In this classical system, the key step, reoxidization of Pd(0) to active Pd (II) species was realized by using copper salts as the oxidant However, the inevitable contamination caused by copper ions during this process has been recognized as a considerable limitation In order to replace the copper species, various new oxidation systems, such as Sc(OTf)3 [7], organic peroxides [8-12], 1,4-benzoquinone [13], CrO3 [14] and oxygen [15-16], have been reported Although these modifications offer a copper-free Wacker-type oxidation, there is still an urgent need to explore a facile, cost-effective and environmentally friendly method Based on the principles of green chemistry, low concentration of aqueous hydrogen peroxide is an ideal oxidant for Wacker oxidation Initially, Mimoun’s group developed a new H2O2 oxidation system with palladium acetate [17] A disadvantage of this method is that it is not effective for the oxidation of aromatic olefins (Fig 1B) To enhance the applicability of this new oxidation system, many palladium complexes have been chosen and made excellent performance [18-24] Furthermore, some polymers of aniline and their derivatives could also form complexes with Pd(II) and the catalytic effect is worth to study [25-27] Recently, Wang and coworkers have reported an efficient and economical oxidation system, using Pd(OAc)2, TFA and O2 in a DMSO-H2O solvent system [28] This method could realize the Tsuji−Wacker oxidation of terminal olefins and especially styrenes to methyl ketones (Fig 1C) Inspired by their work, we explored the possibility of realizing the Pd(II)-catalyzed ligand-free Wacker-type oxidation of styrene derivatives using H2O2 as the sole oxidant Experimental 2.1 General information All chemical reagents and solvents (analytical grade) were purchased from Aladdin Industrial Inc and used without further purification All experiments were monitored by thin-layer chromatography (TLC) 1H and 13C spectra were recorded on a Bruker Avance III 500 MHz Digital NMR spectrometer or 300 MHz NMR spectrometer, using CDCl3 as a solvent The yield was determined by Shimadzu GC-2014C gas chromatography based on the external standard method Column chromatography was performed on silica gel (300-400 mesh) using petroleum ether (PE)/ ethyl acetate (EA) 2.2 Typical procedures for oxidation of olefins to the corresponding carbonyl compounds To the solution of styrene (1.0 mmol) in CH3CN (10 mL), Pd(OAc)2 (0.05 mmol) and H2SO4 (70 wt%, 10 µL) were added The mixture was stirred for 15 at room temperature Then H2O2 (30 wt% 6.0 mmol) was added in a dropwise manner and the mixture was heated to 65 ˚C until the reaction was fully completed (monitored by TLC) The organic extracts were concentrated under reduced pressure and purified by column chromatography Results and discussion 3.1 Optimization of reaction conditions In preliminary experiments, styrene was treated with Pd(OAc)2 (5 mol%), H2O2 (30 wt%, equiv) and H2SO (70 wt%, 10 µL) as co-catalyst in CH3CN (10 mL) at room temperature The yield of desired acetophenone was only 12% (Table 1, entry 1) The reaction rate could be substantially enhanced by appropriately increasing the temperature, whereas higher temperature led to a yield decline (entries 2-4) By further optimizing the amount of H2SO4 and H2O2, 10 µL of H2SO4 and equiv of H2O2 were found to be the optimal conditions, providing 80% yield of desired acetophenone (Table 1, entry 3) Subsequently, the effects of solvent were evaluated: Based on entries 3, 10 and 11, the optimal amount of acetonitrile was found to be 10 mL Moreover, using other solvents like ethanol, acetone, ethyl acetate, tetrahydrofuran, 1,4-dioxane and ethylene carbonate, the desired product could also be obtained, but the yields were unsatisfactory (entry 12) Then we replaced H2SO with other acids such as HCl, HNO3, H3PO4, HCOOH, CH3COOH or H2C2O4, which resulted in lower yields (entry 13) These results implied that the acetonitrile and H2SO4 played important roles in this reaction To determine the effect of transformation from Pd(OAc)2 to the other palladium catalyst, we tested palladium chloride, as shown in entry 14 The latter catalyst did not show a better catalytic effect than Pd(OAc)2 In summary, optimization is realized as the following conditions: Pd(OAc)2 (5 mol%), sulfuric acid (10 µL), hydrogen peroxide (6 equiv) in acetonitrile (10 mL) at 65˚C 3.2 Substrate scope of the Wacker oxidant reaction With this optimized system in hand, various styrene derivatives were selected for further research and the results were summarized in Table As same as aniline compounds [29-30], styrenes are problematic substrates for the reaction due to that they are prone to polymerization To our delight, these substrates could obtain good to excellent yields and no polymeric products were detected in all cases Meanwhile, compared with entries and 2, the yield of desired product with a substituent in the 2-position declined (entry 3) The results implied that the position of the substituent could influence the reactivity of the double bond Moreover, following the same procedure, we could see that several electron-withdrawing groups on the phenyl ring tend to result in lower yields except the fluoro-substituent (entries 4-10) Then trans-stilbene was utilized as a substrate to explore the effect of internal olefins under identical conditions Unsurprisingly, trans-stilbene was also oxidized to the corresponding ketone in good yield, which implied that this method was also suitable for the internal olefins (entry 11) Due to the special role of methyl ketone compounds in organic synthesis [31], at the end of the experiment, styrene was further reacted in a 10 mmol scale and the desired product was separated in excellent yield (entry 12) 3.3 Typical NMR spectra analyses of products As shown in Figure 2, the structures of the synthesized compounds were confirmed by 1H NMR spectra Singlet for ─CO─CH3 protons was observed at 2.58 ppm in all the three compounds A group of peaks situated at 7.13-8.01 ppm were ascribed to the aromatic protons of the phenyl ring In compound B, one additional singlet located at 2.41 ppm for methyl protons was observed Figure thereafter illustrates the 13C NMR spectra of these as-prepared compounds It is obvious that singlets appearing at 196.33 ppm, 197.72 ppm and 197.98 ppm could be ascribed to the carbonyl carbon and the rest corresponding peaks were found at their expected positions Different from the other two compounds, the peaks situated at 165.65 ppm, 130.82 ppm and 115.51 ppm were individually split due to the 13C-19F coupling in compound A 3.4 Systematic comparison with other methods of synthesizing acetophenone The results of our method have been compared with representative reports about oxidation of styrene with respect to their catalytic system, oxidant, solvent and yield (Table 3) The application of cheap and clean oxidant O2, recyclable heterogeneous catalysts are advantages of previous methods Such kind of modifications have offered a wider scope to Wacker oxidation, whereas the ligand-free catalytic system we apply here has made the oxidation process lower in cost, higher in reactivity and easier to operate 3.5 Plausible mechanism of the reaction On the basis of the former efforts [17, 35-36], a plausible mechanism was shown in Scheme The palladium acetate formed a palladium hydroperoxidic species by the addition of H2O2 and H2SO4 (step 1) This species transferred oxygen to the olefin through a pseudocyclic hydroperoxypalladation mechanism of the coordinated olefin, obtaining the corresponding ketone and palladium hydroxyl species (step 2-3) In the presence of excess H2O2, this palladium hydroxyl species regenerated the palladium hydroperoxidic species, completing the catalytic cycle (step 4) Conclusions In this study, we have developed a green and efficient method by which terminal and internal olefins could be transformed into the corresponding ketones using ligand-free Pd(OAc)2 as the catalyst We believe that the simple Pd(OAc)2 catalyst is of considerable potential in Wacker oxidation using hydrogen peroxide as the green oxidant Furthermore, the newly developed system may be used as a practical method for the synthesis of ketones Notes and references School of Chemical Engineering, Nanjing 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for styrenes O 10 mol% Pd(OAc)2, equiv TFA R atm O2, DMSO:H2O, 70℃ R Fig Literature examples of Wacker oxidation Fig Typical H NMR spectra of products Fig Typical 13C NMR spectra of products Pd O O S Pd(OAc)2 + H2SO4 O O H2O2 STEP H2SO4 Pd H2O OOH STEP4 H2O2 Pd HOO Ph STEP2 OH Ph OH O Pd STEP2&3 OOH OOH Ph Pd OOH OOH OH Pd CH2 STEP3 O O H O Ph H Ph Scheme Plausible Mechanism of the Reaction Table Optimization of reaction conditions a Entry Cat H2SO4 H 2O CH3CN Temp Time Yield b (µL) (eq) (mL) (˚C) (h) (%) Pd(OAc)2 10 10 rt 24 12 Pd(OAc)2 10 10 55 72 Pd(OAc)2 10 10 65 80(76) Pd(OAc)2 10 10 75 73 Pd(OAc)2 25 10 65 82 Pd(OAc)2 10 65 63 Pd(OAc)2 - 10 65 trace Pd(OAc)2 10 12 10 65 64 Pd(OAc)2 10 10 65 57 10 Pd(OAc)2 10 20 65 78 11 Pd(OAc)2 10 65 72 12 c Pd(OAc)2 10 10 65