Room Temperature Chemoselective Deoxygenation of Aromatic

Aug 20, 2018 - A rapid and practical protocol for the chemoselective deoxygenation of various aromatic ketones and aldehydes was described, which used...
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Room Temperature Chemoselective Deoxygenation of Aromatic Ketones and Aldehydes Promoted by a Tandem Pd/TiO2 + FeCl3 Catalyst Zhenhua Dong,† Jinwei Yuan,† Yongmei Xiao,† Pu Mao,† and Wentao Wang*,‡ †

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College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Lianhua Street 100, Zhengzhou 450001, China ‡ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China S Supporting Information *

ABSTRACT: A rapid and practical protocol for the chemoselective deoxygenation of various aromatic ketones and aldehydes was described, which used a tandem catalyst composed of heterogeneous Pd/TiO2 + homogeneous FeCl3 with the green hydrogen source, polymethylhydrosiloxane (PMHS). The developed catalytic system was robust and scalable, as exemplified by the deoxygenation of acetophenone, which was performed on a gram scale in an atmospheric environment utilizing only 0.4 mol % Pd/TiO2 + 10 mol % FeCl3 catalyst to give the corresponding ethylbenzene in 96% yield within 10 min at room temperature. Furthermore, the Pd/TiO2 catalyst was shown to be recyclable up to three times without an observable decrease in efficiency and it exhibited low metal leaching under the reaction conditions. Insights toward the reaction mechanism of Pdcatalyzed reductive deoxygenation for aromatic ketones and aldehydes were investigated through operando IR, NMR, and GCMS techniques.

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INTRODUCTION Chemoselective deoxygenation reactions continued to attract wide interest in modern organic synthesis because of the fundamental importance of the reaction in the synthesis of biologically active molecules as well as in industrial-scale procedures for the transformation of both biomass feedstocks and petroleum chemicals.1 There was a great demand for the development of new protocols, especially utilizing cheap and environmentally friendly catalysts. To date, many useful manners for the chemoselective deoxygenation of oxygencontaining organic molecules, such as alcohols, aldehydes, ketones, amides, epoxides, and nitro compounds, have been developed.2 Conventional methods for the reduction of carbonyl compounds to the corresponding alkanes included the Barton−McCombie (R3SnH),3 NaBH4, borane, Clemmensen (Zn/Hg, HCl), and Wolff−Kishner−Huang (N2H4, KOH) reductions with rigorous reaction conditions and stoichiometric amounts of unfriendly reagents. Subsequent developments led to mild stoichiometric protocols for the deoxygenation of carbonyl compounds with poor atom economy and undesirable byproduct, in which various metal hydride reagents were employed.4 To address these issues, hydrogen gas was developed for the deoxygenation of carbonyl compounds promoted by transition metal catalysts with H2O being the only byproduct.5 Although H2 was an atom efficient and nontoxic reducing agent, the reaction usually performed at high temperature and pressure with the explosion risk. In contrast, silanes can be considered as one of the most inexpensive and environmentally friendly hydride donors. Of the many available silanes, polymethylhydrosiloxane (PMHS), © 2018 American Chemical Society

a plentiful and green byproduct of the silicone industry, is a very attractive option.6 In this context, although homogeneous catalytic systems for the deoxygenation of aromatic alcohols, aldehydes, and ketones with PMHS have been developed,7 heterogeneous catalytic systems remain limited.8 Heterogeneous catalysts have remarkable advantages, such as simplified reusability and purification, the potential for incorporation in continuous reactors and microreactors over homogeneous systems, and promising positive environmental consequences.9 Therefore, the development of cheaper, simpler, and more efficient heterogeneous catalysts for rapid and chemoselective deoxygenation of aromatic ketones and aldehydes is highly desirable and challenging. Herein, we report a rapid, practical, and efficient method for the chemoselective deoxygenation of various aromatic ketones and aldehydes, using heterogeneous Pd/TiO2 + homogeneous FeCl3 as tandem catalysts together with the green hydride source PMHS.

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RESULTS AND DISCUSSION In initial experiments, we attempted the deoxygenation reaction of 4-methoxyacetophenone (1a) into 4-ethylanisole (2a) with various TiO2 supported metal nanoparticle catalysts. The supported metal catalysts were prepared by a routine deposition precipitation method. During the optimization of the reaction conditions, we found that Pd/TiO2 afforded a 5% yield of 4-ethylanisole (2a) in the presence of PMHS (Table 1, entry 6). Other metals such as Au, Ru, Ir, Rh, and Pt Received: July 3, 2018 Published: August 20, 2018 11067

DOI: 10.1021/acs.joc.8b01667 J. Org. Chem. 2018, 83, 11067−11073

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The Journal of Organic Chemistry Table 1. Optimization of Reaction Conditionsa

catalytic system had a remarkable synergistic effect on promoting this reaction (Table 1, entries 6 and 17). To show the general applicability of the catalytic system, various substrates were tested under the optimized reaction conditions. And the results were summarized in Table 2. In general, the catalytic experiments were conducted in the presence of 0.4 mol % Pd/TiO2 and 10 mol % FeCl3 at room temperature in 2−10 min. A variety of aromatic ketones and aldehydes performed well under these conditions. For example, Me, MeO, and F substituted acetophenone derivatives reacted smoothly in the presence of PMHS to give the corresponding alkanes in 85−98% yield (Table 2, entries 1−6). Similar good results were observed on applying long chain aromatic ketones and cyclic ketones (Table 2, entries 7−11), thus demonstrating the versatility of the catalytic system. It is worth noting that heterocyclic ketones such as chroman-4-one (1l) could also be used in this transformation and gave chromane (2l) in 92% yield in 10 min (Table 2, entry 12). Furthermore, various benzaldehydes were also tolerated well, thus giving the desired products in excellent yields (Table 2, entries 13−15). To evaluate the scalability of the present protocol, the deoxygenation of acetophenone (1f) was performed on a gram scale in an open flask setup using 0.4 mol % Pd/TiO2 and 10 mol % FeCl3. Gratifyingly, quantitative deoxygenation of the starting compound was observed after 10 min and the product ethylbenzene (2f) could be isolated in 96% yield (Table 2, entry 16). The deoxygenation of benzophenone by the current catalytic system occurred to give diphenylmethane in 52% yield, presumably because it presents too much steric hindrance for the deoxygenation reaction. However, aliphatic ketones exhibit more difficulty in being deoxygenated, as exemplified by the reaction of cyclohexanone which resulted in an 8% yield of the corresponding alcohol after 2 h and the deoxygenative product cyclohexane was not observed. To gain insight into the reaction mechanism, several experiments were conducted. Initially, the reactions were monitored using an operando IR spectroscopy. As shown in Figure 1, the kinetic profile clearly shows the peak of the substrate acetophenone at 1679 cm−1 was consumed, and a peak of the product ethylbenzene at 2960 cm−1 was accumulated. However, the peaks of the possible intermediates 1-phenylethan-1-ol were not observed obviously. These results indicated that acetophenones were rapidly deoxygenated to the corresponding ethylbenzenes under the standard reaction conditions promoted by Pd nanoparticles with FeCl3. GCMS and NMR techniques can provide some important details about the intermediates of the reaction. To our delight, a clear spectrum was obtained when the reaction mixture was investigated by GC-MS and NMR analysis. The intermediate 1-phenylethan-1-ol was detected at m/z values of 122.07 (Supporting Information pp S18−S21). Furthermore, 1phenylethan-1-ol was also detected by NMR spectroscopy (Supporting Information pp S17−S18). These results proved that intermediate 1-phenylethan-1-ol was involved in the catalytic cycle. A control experiment was also performed to prove the intermediate. As shown in Scheme 1, the substrates 1phenylethan-1-ol (1p) could be transformed to ethylbenzene (2f) in 5 min under optimized reaction conditions. The result further confirmed the reaction intermediate. The kinetic reaction results showed that acetophenone was reduced to 1-phenylethan-1-ol within 1 min, which was then further reduced to ethylbenzene in the following 10 min

yield (%)b entry

catalyst

1

Au/TiO2 (1 wt %) Ru/TiO2 (1 wt %) Pt/TiO2 (1 wt %) Ir/TiO2 (1 wt %) Rh/TiO2 (1 wt %) Pd/TiO2 (1 wt %) Pd/TiO2 (1 wt %) Pd/TiO2 (1 wt %) Pd/TiO2 (1 wt %) Pd/TiO2 (1 wt %) Pd/CeO2 (1 wt %) Pd/Al2O3 (1 wt %) Pd/FeOx (1 wt %) Pd/TiO2 (1 wt %) Pd/TiO2 (1 wt %) Pd/TiO2 (1 wt %) −

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

additive (mol %)

T (°C) t (min)

2a

3a

−

40

120

0

11

−

40

120

0

0

− − −

40 40 40

120 120 120

0 0 0

0 0 0

− HCl (10) AlCl3 (10) ZrCl4 (10) FeCl3 (10) FeCl3 (10)

40 40 40 rt rt rt

120 120 120 10 10 10

5 53 83 83 98 13

77 30 15 9 2 42

FeCl3 (10)

rt

10

73

13

FeCl3 (10)

rt

10

0

0

FeCl3 FeCl3 FeCl3 FeCl3

rt 0 50 rt

10 10 3 120

88 17 56 0

9 40 22 0

(5) (10) (10) (10)

a

The reaction was carried out with 0.4 mol % catalyst, 0.5 mmol of 4methoxyacetophenone 1a, 1.5 mL of CH3OH, 90 μL of PMHS, room temperature. bThe GC yield was obtained from the calibration curve using 1,3,5-trimethylbenzene as an internal standard.

nanoparticles displayed very low activity (Table 1, entries 1− 5). Previous exploratory/mechanistic studies showed that the reaction proceeded in two steps,7,8 where the ketone was first reduced to the alcohol, followed by hydrodeoxygenation to produce the saturated product. On the other hand, metal chloride complexes have been proven to be highly effective Lewis acid catalysts for deoxygenation of alcohols and carbonyl compounds, showing certain stability against moisture and air.10 These features aroused the intriguing question of whether metal chloride salts might be helpful catalysts in hydrogenolytic C−O bond cleavage of the alcohol. And then, various metal chlorides were screened (Table 1, entries 7−10). Among the Lewis acids tested as additives in the reaction, FeCl3 exhibited the highest catalytic activities (Table 1, entry 10). Note that HCl was less active (Table 1, entry 7), arguing that the transformation was primarily catalyzed by a Lewis acid (Fe(III)) rather than by a Brønsted acid (HCl) from metal chloride hydrolysis. Compared with TiO2, other supports for Pd nanoparticles such as CeO2, FeOx, and Al2O3 resulted in low yields of 4-ethylanisole (2a) (Table 1, entries 11−13). Further studies indicated that the efficiency of this transformation decreased when a lower catalyst loading was employed (Table 1, entry 14). Different reaction temperatures were investigated. The results showed room temperature provided a desirable yield (Table 1, entry 10 vs entries 15− 16). In addition, only a trace amount of the product was detected with FeCl3 or Pd/TiO2 alone which demonstrated the 11068

DOI: 10.1021/acs.joc.8b01667 J. Org. Chem. 2018, 83, 11067−11073

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The Journal of Organic Chemistry Table 2. Substrate Scope of the Chemoselective Deoxygenationa

a Unless otherwise noted, the reaction were carried out with 0.5 mmol substrate (1), 90 μL of PMHS, 0.4 mol % Pd/TiO2, 10 mol % FeCl3, 1.5 mL of CH3OH, room temperature for 2−10 min. bIsolated yields. cThe reaction was performed on a 10 mmol scale and resulted in a 96% yield of the isolated product.

Scheme 1. Deoxygenation of 1-Phenylethan-1-ol under Standard Conditions

A hot filtration experiment was carried out to rule out the possible contribution of homogeneous catalysis (Figure 3a). The Pd/TiO2 was separated from the reaction system by filtration at a conversion of 54% for substrate 1f, and the filtrate was examined by ICP-AES. The results showed that only a negligible amount of Pd (0.6 ppm corresponding to 0.55 wt % of initial charge) was detected in the solution and no further reaction occurred when a new batch of substrates was added into the filtrate. Evidently, the tandem reductive deoxygenation reaction indeed took place on the surface of the solid catalyst. The catalysts also exhibited good recyclability and stability. For example, the catalyst Pd/TiO2 was recovered by simple filtration and reused three times (Figure 3b) without significant loss of catalytic activity. The HRTEM images revealed highly dispersed Pd particles with a mean size of 4.02 nm, for which the narrowly distributed

Figure 1. IR profile of real-time in situ FT-IR spectroscopic analysis of the reductive deoxygenation of acetophenone. The peaks at 1679 and 2960 cm−1 correspond to the CO and benzyl C−H stretching vibrations of acetophenone and ethylbenzene, respectively.

(Figure 2). Clearly, 1-phenylethan-1-ol was an intermediate for the deoxygenatve product. This was consistent with the reported proposed ketone−alcohol−alkane reaction pathway.7,8 11069

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reaction time (2−10 min), a cheap and green reducing agent (PMHS), and a low loading of a readily available catalyst. Under the optimized reaction conditions, the Pd/TiO2 catalyst is recyclable up to three times without significant loss in activity. The synergistic interplay between the supported Pd catalyst and Lewis acid FeCl3 revealed in this work may offer a new guide to the design of other bifunctional catalysts in the green synthesis of fine chemicals.

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EXPERIMENTAL SECTION

General Experimental Information. Aldehydes, ketones, FeCl3, ZrCl4, AlCl3, and polymethylhydrosiloxanen were purchased from Aladdin Industrial Inc. Unless otherwise stated, all reagents were purchased from commercial suppliers and used without further purification. To facilitate the comparison, the metal contents in these catalysts were chosen as 1% by weight. NMR spectra were recorded at room temperature in CDCl3 on 400 MHz spectrometers. HRTEM images were recorded on a JEOL 2100F instrument operating at 120 kV. Prior to HRTEM characterization, the samples were dispersed in CH3CH2OH solution with ultrasonic treatment for 5 min and then dropped onto a carbon film on copper grid. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) was tested on an IRIS Intrepid II XSP instrument (Thermo Electron Corp.). Analytical thin-layer chromatography (TLC) was performed on silica gel, visualized by irradiation with UV light. For column chromatography, 200−300 mesh silica gel was used. Preparation of Catalysts. 1 wt % supported Pd catalysts were prepared by a routine deposition precipitation method. An aqueous solution of H2PdCl4 (0.047 mmol) in 20 mL of H2O was heated to 60 °C under vigorous stirring. The pH was adjusted to 8.0 by dropwise addition of NaOH (0.1 M), and then 500 mg of TiO2 were dispersed in the solution. The mixture was stirred for 3 h at 60 °C, after which the suspension was cooled to room temperature. Extensive washing with deionized water was then followed until it was free of chloride ions (determined by silver nitrate solution). Then, the solid was further dried at 80 °C for 12 h. Finally, the sample was reduced at 300 °C under the H2 flow (60 mL·min−1 g−1), followed by a passivation process with 1% (v/v) O2/N2 for 1 h at room temperature. The Pd loading on Pd/TiO2 was determined to be 1.09 wt % by ICP-AES analysis. General Experimental Procedure for the Pd/TiO2 Catalyzed Deoxygenation of Ketones and Aldehydes. The substrate aldehyde/ketone (0.5 mmol), 0.002 mmol of catalyst (based on metal palladium), 0.05 mmol of FeCl3, and 1.50 mL of CH3OH were sequentially added into a test tube with a magnetic stir bar. The mixture was stirred with an 800 rpm stirring speed for 3 min at room temperature. And then PMHS (90 μL, Si−H = 1.32 mmol) was added

Figure 2. Substrate and product concentration profiles as a function of the reaction time. Reaction conditions: 0.4 mol % Pd/TiO2 catalyst; 10 mol % FeCl3; 0.5 mmol of acetophenone 1f; 1.5 mL of CH3OH.

particle sizes range from 3.0 to 5.0 nm owing to the TiO2 stabilization (Figure 4). Based on the observations noted above, the following reaction mechanism can be proposed (Scheme 2). The reductive deoxygenation of carbonyl compounds can be divided into two major steps: (i) ketone is reduced to alcohol; (ii) alcohol is hydrodeoxygenated to produce the saturated product. The catalytic cycle begins with the dissociative adsorption of the PMHS onto the Pd nanoparticles surface. The Pd nanoparticles served as the main active sites to activate carbonyl group and adsorb H species in the first step (ketone → alcohol), presumably due to the strong affinity of Pd to the carbonyl group. The Lewis acidic FeCl3 would mainly activate the alcohol in the second step (alcohol → aromatic hydrocarbon). With the assistance of FeCl3, the resulting intermediate alcohol undergoes hydrodeoxygenation to give the observed product alkane.

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CONCLUSION In summary, a rapid and relatively environmentally benign method for reductive deoxygenation of aryl ketones and aldehydes has been uncovered using a tandem catalytic system comprised of Pd/TiO2 + FeCl3 under mild conditions in CH3OH. A variety of substrates with several functional groups performed well. The protocol has the advantages of short

Figure 3. Hot filtration (a) and reuse experiments (b) of the reaction: 0.5 mmol of acetophenone 1f, 0.4 mol % Pd/TiO2, 10 mol % FeCl3, 90 μL of PMHS, 1.5 mL of CH3OH. 11070

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Figure 4. HRTEM images and particle size distribution of Pd/TiO2.. Table 2, entry 5: 1-ethyl-4-fluorobenzene (2e),11 a colorless oil, 58.9 mg in 95% yield; calcd for C8H9F 124.069, found 123.948. Table 2, entry 6: ethylbenzene (2f),11 a colorless oil, 52.0 mg in 98% yield; calcd for C8H10 106.078, found 105.972. Table 2, entry 7: propylbenzene (2g),11 a colorless oil, 54.6 mg in 91% yield; calcd for C9H12 120.094, found 119.967. Table 2, entry 8: butylbenzene (2h),11 a colorless oil, 60.3 mg in 90% yield; calcd for C10H14 134.110, found 133.977. Table 2, entry 9: pentylbenzene (2i),11 a colorless oil, 57.0 mg in 77% yield; calcd for C11H16 148.125, found 148.016. Table 2, entry 10: 2,3-dihydro-1H-indene (2j),11 a colorless oil, 50.2 mg in 85% yield, calcd for C9H10; 118.078; found, 117.984. Table 2, entry 11:1,2,3,4-tetrahydronaphthalene (2k),11 a colorless oil, 59.4 mg in 90% yield; calcd for C10H12 132.094; found 131.943. Table 2, entry 12: chromane (2l),11 a white solid, 61.6 mg in 92% yield; calcd for C9H10O 134.073; found 133.924. Table 2, entry 13: toluene (2m),11 a colorless oil, 45.5 mg in 99% yield; calcd for C7H8 92.063; found 91.009. Table 2, entry 14: 1-methoxy-2-methylbenzene (2n),11 a colorless oil, 58.6 mg in 96% yield; calcd for C8H10O 122.073; found 121.880. Table 2, entry 15: 1-methoxy-3-methylbenzene (2o),11 a colorless oil, 58.0 mg in 95% yield; calcd for C8H10O 122.073; found 121.913.

Scheme 2. A Plausible Reaction Mechanism

under stirring. The resulting mixture was stirred at room temperature for 10 min. Upon completion of the reaction, the catalyst was filtrated and washed with CH3OH, dried under air, and then directly reused under the same conditions. The solvent was removed under reduced pressure, and the residue was purified (if necessary) by flash chromatography using pentane ether/ethyl acetate (10:1) as the eluent to give the corresponding deoxygenated products. All the products were known compounds. The products were confirmed by the comparison of their GC retention time, mass spectrum, and 1H NMR spectrum with the experimental values from National Institute of Standards and Technology (NIST). Table 2, entry 1: Following the general experimental procedure, the reaction mixture of 1a (0.5 mmol, 75.0 mg), Pd/TiO2 (0.4 mol %, 21.2 mg), FeCl3 (10 mol %, 8.0 mg), and PMHS (90 μL) was stirred in 1.5 mL of CH3OH at room temperature for 10 min to afford 2a (eluent: Pentane ether/ethyl acetate = 10:1) as a colorless oil, 64.6 mg in 95% yield. The structure of the product 2a was confirmed by gas chromatography−mass spectrometry,11 calcd for C9H12O, 136.089, found, 135.962. Table 2, entry 2: 1-ethyl-2-methylbenzene (2b),11 a colorless oil, 52.2 mg in 87% yield; calcd for C9H12 120.094, found 119.961. Table 2, entry 3: 1-ethyl-3-methylbenzene (2c),11 a colorless oil, 51.0 mg in 85% yield; calcd for C9H12 120.094, found 119.960. Table 2, entry 4: 1-ethyl-4-methylbenzene (2d),11 a colorless oil, 58.8 mg in 98% yield; calcd for C9H12 120.094, found 119.949.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01667.

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Characterization of the obtained deoxygenated products, 13 CNMR results of the reaction liquid mixture after acetophenone performed 1 and 4 min under the optimized conditions, GC-MS results of the reaction liquid mixture after the substrate acetophenone proceeded 1 min under the optimized conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 11071

DOI: 10.1021/acs.joc.8b01667 J. Org. Chem. 2018, 83, 11067−11073

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The Journal of Organic Chemistry ORCID

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Jinwei Yuan: 0000-0002-4226-1853 Wentao Wang: 0000-0002-3876-8627 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Project No. 21672210) for financial support. This work was generously supported by The Science and Technology Foundation of Henan Province (No. 172102310043), The Colleges and Universities Key Research Program Foundation of Henan Province (No. 17A150007), and Doctor Fund of Henan University of Technology (No. 2015BS013).

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DOI: 10.1021/acs.joc.8b01667 J. Org. Chem. 2018, 83, 11067−11073