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Cyclopentadienyl Ruthenium(II) Complex-Mediated Oxidation of Benzylic and Allylic Alcohols to Corresponding Aldehydes

Cyclopentadienyl Ruthenium(II) Complex-Mediated Oxidation of Benzylic and Allylic Alcohols to... Hindawi Heteroatom Chemistry Volume 2019, Article ID 5053702, 8 pages https://doi.org/10.1155/2019/5053702 Research Article Cyclopentadienyl Ruthenium(II) Complex-Mediated Oxidation of Benzylic and Allylic Alcohols to Corresponding Aldehydes 1 2 3 2,3 Ching-Yuh Chern, Ching-Chun Tseng, Rong-Hong Hsiao, Fung Fuh Wong , 4,5,6 and Yueh-Hsiung Kuo Department of Applied Chemistry, National Chia-Yi University, Chia-Yi 60004, Taiwan �e Ph.D. Program for Biotech Pharmaceutical Industry, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan School of Pharmacy, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan Department of Chinese Pharmaceutical Sciences and Chinese Medicine Resources, China Medical University, Taichung 40402, Taiwan Department of Biotechnology, Asia University, Taichung 41354, Taiwan Chinese Medicine Research Center, China Medical University, Taichung 40402, Taiwan Correspondence should be addressed to Fung Fuh Wong; wongfungfuh@yahoo.com.tw and Yueh-Hsiung Kuo; kuoyh@ mail.cmu.edu.tw Received 2 May 2019; Revised 9 July 2019; Accepted 17 July 2019; Published 18 August 2019 Academic Editor: Guillaume Berionni Copyright © 2019 Ching-Yuh Chern et al. ƒis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ƒis work reports an e‡cient method for the oxidation reaction of aliphatic, aromatic allylic, and benzylic alcohols into aldehydes catalyzed by the cyclopentadienyl ruthenium(II) complex (RuCpCl(PPh ) ) with bubbled O . ƒrough further optimizing 3 2 2 controlled studies, the tendency order of oxidation reactivity was determined as follows: benzylic alcohols > aromatic allylic alcohols >> aliphatic alcohols. In addition, this method has several advantages, including a small amount of catalyst (0.5 mol%) and selective application of high discrimination activity of aliphatic, aromatic allylic, and benzylic alcohols. adjunction metals such as Co or Cd, or presence of oxygen or 1. Introduction air under higher pressure and/or temperature, and/or Oxidation reactions are very useful functional trans- stronger basic conditions to obtain the desired products. formations in organic synthesis [1, 2]. Many of the metal- Furthermore, the rare earth elements have occupied an based oxidizing reagents have been developed to achieve the especially important place in the past two decades because of e‡cient oxidation of alcohols such as PI Au [3], ARP-Pt [4], their high reactivity in various catalytic processes [33–39]. Ru/Al O [5, 6], Pd/HAP [7, 8], Au-Pd/TiO [9], and HB Ru One of these rare elements is ruthenium. Some species of 2 3 2 [10]. However, these catalysts are generally di‡cult to obtain ruthenium have been widely developed and used as e‡cient because of their expensive cost and harsh production. In catalysts for oxidation reactions [40, 41]. addition, other oxidation methods for alcohols using the Among a variety of catalysts reported in the literature for the redox process of carbonyl compounds [42], the use of readily available carbon-supported metal catalysts [11–14] including many famous Pd/C [15–17], Pt/C [18–23], or Au/ ruthenium complexes has garnished signi¥cant attention. For C [24–30] catalysts [31, 32] were also enthusiastically in- example, several ruthenium complexes have been employed as vestigated. Unfortunately, these catalysts needed addition of catalysts for the hydrogenation of carbonyl compounds, 2 Heteroatom Chemistry Table 1: Study of oxidation conditions of benzyl alcohol 1a with RuCpCl(PPh ) . 3 2 OH RuCpCl(PPh ) 3 2 O , solvent, at reflux 1a 2a Entry Substrate Solvent Reaction condition Product Yield (%) 1 1a CH Cl Reflux for 24 h 2a Trace 2 2 2 1a CH Cl Reflux for 48 h 2a <10 2 2 3 1a Benzene Reflux for 24 h 2a Trace 4 1a Benzene Reflux for 48 h 2a <6 5 1a Acetone Reflux for 24 h 2a Nondetectable 6 1a Acetone Reflux for 48 h 2a Nondetectable 7 1a THF Reflux for 24 h 2a 54 8 1a THF Reflux for 48 h 2a 71 providing an efficient access to their corresponding alcohols. ACS grade of solvents (i.e., benzene, acetone, and THF) and For example, several ruthenium complexes [43] including the reaction time at room temperature or reflux. Based on RuCpCl(PPh ) , Ru(indenyl)Cl(PPh ) , [RuCp(MeCN) (PR )] the experimental results in Table 1, we observed that the 3 2 3 2 2 3 PF [44], [RuCp(MeCN) ]PF [45], and RuCpCl(diphosphine) ideal conditions for this reaction were to use THF as the 6 3 6 have been employed as catalysts for the isomerization of both reaction solvent and to reflux for 48 h. /e corresponding aliphatic [46] and aromatic [47–50] allylic alcohols into ketones oxidation product 2a can be afforded in 71% isolated yield or aldehydes. (entry 8 in Table 1). Consequently, optimization of the amount of the catalyst from 0.5, 1.0, 2.0, to 5 mol% was Literature has reported oxidation of allylic and benzylic alcohols for functional transformation such as that reported performed. However, this did not lead to any further im- by Prof. Pearson using trimethylamine-N-oxide in the provement. /e structure of product 2a was completely presence of iron carbonyl as a significant catalyst [51]. characterized by spectroscopic methods and consistent with However, several issues limit these catalysts’ applications an Aldrich-authentic sample. Following the results in Ta- such as the need for a toxic solvent, tedious complex re- ble 1, we found that RuCpCl(PPh ) possessed the oxidation 3 2 agents, and troublesome procedures. /erefore, we reported reactivity necessary for transferring benzyl alcohols to the a novel and interesting cyclopentadienyl ruthenium(II) corresponding benzaldehydes. complex (RuCpCl(PPh ) ), which oxidizes aromatic allylic In order to explore the substrate scope of the new oxidation 3 2 and benzylic alcohols into carbonyl compounds. Based on reaction, we first examined the reactions of substituted benzylic the further controlled experimental studies, we found the alcohols 1b–f containing either electron-donating or electron- tendency order of oxidative reactivity as benzylic alcohol- withdrawing groups 1b–d and disubstituted benzylic alcohols s> aromatic allylic alcohols>> aliphatic alcohols. 1e-f. Fortunately, most of the substituted benzylic alcohols 1b–f were successfully converted to the corresponding alde- hyde products 2b–f in moderate yields (>68%; Table 2). On the 2. Results and Discussion contrary, benzylic alcohols with para-Me (1b) and para-OMe Benzyl alcohol 1a is an important precursor for organic (1c) electron-donating groups were oxidized to give the cor- synthesis and a useful solvent because of its polarity, low responding aldehyde products 2b-c in 73% and 80% yields, toxicity, mildly pleasant aromatic odor, and low vapor respectively (entries 1 and 2 in Table 2). In addition, the pressure [52–54]. /e chemoselective oxidation property of conversion of para-CN-benzylic alcohol 1d to aldehyde 2d in compound 1a was also very useful in functional trans- 68% yield seemed to have lower reactivity compared to benzylic formations for the preparation of aldehydes [55, 56] and alcohols with electron-donating groups 1b-c (entries 1 and 2 in their dicarboxyl analogues [57, 58]. For these reasons, we Table 2). For disubstituted benzylic alcohols 1e and 1f, the most carried out a plausible oxidation of benzyl alcohol 1a. We effective results were achieved in 82% and 86% yields, re- spectively, demonstrating a strong and significant electron- preliminarily investigated a versatile oxidation method for compound 1a with 0.5 mol% amount of the cyclo- assisted effect (entries 4 and 5 in Table 2). Following the above pentadienyl ruthenium(II) complex (RuCpCl(PPh ) ) cat- study, it was found the cyclopentadienyl ruthenium(II) com- 3 2 alyst with bubbled O in CH Cl solution at reflux for 24 h. plex (RuCpCl(PPh ) ) possessed significant oxidizing activity 2 2 2 3 2 However, only trace amounts of benzaldehyde 2a were for discrimination of benzylic alcohols. achieved (entry 1 in Table 1). We then increased the reaction To further expand our study, we investigated simplified time to 48 h, which resulted in the desired oxidation product allylic systems such as (E)-3-arylprop-2-en-1-ols (1g) and 2a, but at a low yield (<10%; entry 2 in Table 1). To identify cyclohex-2-enol (1h). /e RuCpCl(PPh ) catalyst (0.5 mol%) 3 2 the optimal reaction conditions, we attempted to screen the successfully reacted towards (E)-3-arylprop-2-en-1-ols (1g) Heteroatom Chemistry 3 Table 2: Oxidation results of alcohols with RuCpCl(PPh ) at reflux in anhydrous THF solution. 3 2 Entry Alcohols 1b–k Reaction time (h) Yields of products 2b–k Yields (%) OH 1 1b ∼48 h 2b 73 Me Me OH 2 1c ∼48 h 2c 80 MeO MeO OH 3 1d ∼48 h 2d 68 NC NC MeO OH MeO 4 1e ∼48 h 2e 82 MeO MeO MeO OH MeO 5 1f ∼48 h 2f 86 Bno Bno OH 6 1g ∼48 h 2g 68 OH O 7 1h ∼48 h 2h 69 OH 8 1i ∼48 h 2i 17 9 1j CH (CH ) CH OH ∼48 h 2j CH (CH ) CHO 35 3 2 5 2 3 2 5 OH 10 1k ∼48 h 2k — — Me /e starting material was recovered. 4 Heteroatom Chemistry OH OH HO 10 9876543 ppm Figure 1: H NMR characteristic identification of compounds 3 (□), 4 (○), and 5 (∆). OH OH H RuCpCl(PPh ) 3 2 O , THF, at reflux HO O H O H 3 5 Reaction time 4h 21% <5% 24 h 48% 12% 48 h 54% 15% Scheme 1: Chemoreactivity oxidation results of 4-(hydroxymethyl)benzenepropanol 3. and cyclohex-2-enol (1h) in the presence of THF at reflux for alcohols. In addition, the oxidation reaction of benzylic ∼48 h to give the desired oxidation products 2g and 2h in 68% alcohol 1a with the RuCpCl(PPh ) catalyst (0.5 mol%) 3 2 and 69% yields, respectively (entries 6 and 7 in Table 2). /e was found to produce better isolated yields compared to yielding results of the allylic system 1g-h were noticeably both allylic alcohol 1g and aliphatic alcohol 1i. lower than the conversion of benzylic alcohols 1e-f. 4-(Hydroxymethyl)benzenepropanol 3 was synthesized To evaluate the substrate scope and limitation, this by the reported method as the important bifunctional sub- study has been extended to a variety of aliphatic or ali- strate for the chemoreactivity oxidation study [59]. We cyclic alcohols such as 3-arylpropan-1-ols (1i), heptan-1- initially carried out a careful study of possible oxidations of ol (1j), and 2-isopropyl-5-methylcyclohexanol ( 1k). In compound 3 with 1.25 mol% amount of the cyclopentadienyl general, a longer reaction time (∼60 h) was required ruthenium(II) complex (RuCpCl(PPh ) ) catalyst with 3 2 compared to benzyl and allylic alcohols 1a–h for oxidation bubbled O in CH Cl solution at reflux for 48 h. /e versatile 2 2 2 reaction (entries 8–10 in Table 2). Under the same con- oxidation was monitored by TLC and sampled for H NMR dition, aliphatic or alicyclic alcohols 1i–k presented poor characteristic identification (see Figure 1). When the reaction oxidizing reactivity, resulting in trace to 35% yields. For was performed for 4 h, the crude solution was sampled, compound 1k, no trace of the oxidation product was worked up, and eluted from the column. Most of starting detected in the H NMR spectrum of the crude reaction material 3 was recovered, and the corresponding mixture mixture (entry 10 in Table 2). Following the above study, it products 4-(3-hydroxypropyl)benzaldehyde 4 and the small was found the cyclopentadienyl ruthenium(II) complex amount of 4-(3-oxopropyl)benzaldehyde 5 were given out in (RuCpCl(PPh ) ) possessed significant oxidizing activity 21% and <5% yields, respectively (see Scheme 1 and Fig- 3 2 for discrimination of aromatic allylic and benzylic ure 1). When the reaction time was prolonged from 4 h to Heteroatom Chemistry 5 24 h or 48 h under the same condition, we observed that the atmospheric pressure. When the oxidization reaction was expected oxidation product 4 was significantly promoted completed, the solution was filtered through Celite, and the from 21% to 48% or 54% yields, respectively [60]. Celite bed was washed with hot THF. /e filtrate was Comparatively, only a small amount of oxidation product concentrated to remove THF under reduced pressure. /e 4-(3-oxopropyl)benzaldehyde 5 was achieved (∼5% to residue was added (CH Cl , 15 mL), washed with saturated 2 2 12% and 15%; Scheme 1) [60]. Based on the above ex- aqueous NaHCO (15 mL), and extracted with CH Cl 3 2 2 perimental results, we proved again that the benzylic (10 mL × 2). /e combined organic layers were washed with alcohol possessed more efficient oxidative reactivity than brine (15 mL), dried over MgSO , filtered, and concentrated the aliphatic alcohol. under reduced pressure. /e residue was purified by column chromatography (n-hexane/EtOAc � 4/1) on silica gel to give the corresponding aldehyde products 2a–j in 17–86% 3. Conclusions yields. /e physical properties and spectroscopic charac- We have successfully developed the oxidation reaction for teristics of the isolated aliphatic, aromatic allylic, and aliphatic, aromatic allylic, and benzylic alcohols with benzylic alcohols, including 2a–j, were consistent with those 0.5 mol% of the cyclopentadienyl ruthenium(II) complex of the authentic sample [61]. (RuCpCl(PPh ) ). Based on the further controlled studies, 3 2 Benzaldehyde (2a) [61]: light yellow liquid; H NMR the reactive tendency was determined as follows: benzylic (CDCl , 400 MHz): δ 7.48–7.52 (m, 2H, ArH), 7.58– alcohols> aromatic allylic alcohols>> aliphatic alcohols. 7.63 (m, 1H, ArH), 7.84–7.87 (m, 2H, ArH), and 9.99 (s, On the contrary, mono- and disubstituted benzylic alcohols 1H, CHO); C NMR (CDCl , 100 MHz): δ 129.94 with electron-donating groups can provide the best oxi- (2 × CH), 129.69 (2 × CH), 134.41, 136.34, and 192.37; dation results. In addition, this new method has several IR (KBr): 3071, 2832, 2674, 2560, 1686, 1424, 1325, advantages including a small amount of catalyst (0.5 mol%) − 1 1291, 934, and 709 cm ; MS (EI): 106 (78), 105 (100), and high discrimination activity of aliphatic, aromatic al- 78 (15), 77 (83), 51 (37), and 50 (13). lylic, and benzylic alcohols. 4-Tolualdehyde (2b) [61]: colorless liquid; H NMR 4. Experimental Section (CDCl , 400 MHz): δ 2.42 (s, 3H, Me), 7.31 (d, J � 7.9 Hz, 2H, ArH), 7.75 (dd, J � 6.6 and 1.6 Hz, 2H, All reagents were used as obtained commercially. All re- ArH), and 9.94 (s, 1H, CHO); C NMR (CDCl , actions were carried out under argon or nitrogen atmo- 100 MHz): δ 21.85, 129.68 (2 × CH), 129.83 (2 × CH), sphere and monitored by TLC. Flash column 134.16, 145.54, and 192.01; IR (KBr): 3043, 2945, 1672, − 1 chromatography was carried out on silica gel (230– 1574, 1282, 959, 947, 838, and 752 cm ; MS (EI): 120 400 mesh). An analytical thin-layer chromatography (TLC) (34), 119 (100), 92 (11), 91 (97), and 65 (18). was performed using precoated plates (silica gel 60 F-254) 4-Methoxybenzaldehyde (2c) [61]: light yellow liquid; purchased from Merck Inc. Flash column chromatography H NMR (CDCl , 400 MHz): δ 3.79 (s, 3H, OMe), purification was carried out by gradient elution using n- 6.90–6.93 (m, 2H, ArH), 7.73–7.76 (m, 2H, ArH), and hexane in ethyl acetate (EtOAc) unless otherwise stated. 9.79 (s, 1H, CHO); C NMR (CDCl , 100 MHz): δ Infrared (IR) spectra were measured with a Bomem 54.74, 113.61 (2 × CH), 129.24, 131.16 (2 × CH), 163.89, Michelson Series FT-IR spectrometer. /e wavenumbers and 189.98; IR (KBr): 3520, 3356, 2969, 2839, 2741, reported are referenced to the polystyrene absorption at − 1 1682, 1601, 1577, 1512, 1315, 1260, 1158, and 1601 cm . Absorption intensities are recorded by the fol- − 1 1026 cm ; MS (EI): 136 (72), 135 (100), 92 (14), lowing abbreviations: s, strong; m, medium; and w, weak. All 77.0(24), and 65 (10). proton and carbon-13 NMR spectra were obtained by Bruker instruments (400 MHz and100 MHz, respectively). 4-Cyanobenzaldehyde (2d) [61]: white crystal, m.p. Proton and carbon-13 NMR spectra were acquired using 99–102 C; H NMR (CDCl , 400 MHz): δ 7.83 (d, deuterochloroform (CDCl ) solvent. Multiplicities are J � 8.2 Hz, 2H, ArH), 7.98 (d, J � 8.2 Hz, 2H, ArH), and recorded by the following abbreviations: s, singlet; d, dou- 10.07 (s, 1H, CHO); C NMR (CDCl , 100 MHz): δ blet; t, triplet; q, quartet; m, multiplet; and J, coupling 117.57, 117.74, 129.90 (2 × CH), 132.92 (2 × CH), constant (Hz). ESI-MS analyses were performed on an 138.74, and 190.70; IR (KBr): 2850, 2231, 1707, 1608, − 1 Applied Biosystems API 300 mass spectrometer. High- 1571, 1387, 1295, 1203, 1172, 832, 737, and 546 cm ; resolution mass spectra were obtained from a JEOL JMS- MS (EI): 131 (84), 130 (100), 105 (28), 103 (23), HX110 mass spectrometer. 102.0(57), 91 (12), 77 (16), 76 (18), 75 (12), and 51 (12). 3,4-Dimethoxybenzaldehyde (2e) [61]: light yellow liquid; H NMR (CDCl , 400 MHz): δ 3.93 (s, 3H,- 4.1. Standard Procedure for the Oxidation of Aliphatic, Aro- OMe), 3.96 (s, 3H,-OMe), 6.98 (d, J � 8.2 Hz, 1H, ArH), matic Allylic, and Benzylic Alcohols 1a–j to Corresponding 7.40 (d, J � 7.6 Hz, 1H, ArH), 7.45 (dd, J � 8.2 and Aldehydes 2a–j with Cyclopentadienyl Ruthenium(II) 1.9 Hz, 1H, ArH), and 9.84 (s, 1H, CHO); C NMR Complex. Aliphatic, aromatic allylic, or benzylic alcohols (CDCl , 100 MHz): δ 55.60, 55.80, 108.63, 110.11, (1a–j, ∼1.0 mmol, 1.0 equiv) and RuCpCl(PPh ) (∼0.5 mol 3 2 126.42, 129.79, 149.26, 154.13, and 190.47; IR (KBr): %) with bubbled O were stirred in anhydrous THF (2.0 mL) 2938, 2840, 1683, 1588, 1513, 1462, 1421, 1268, 1243, and heated at reflux for 24–48 h under the argon 6 Heteroatom Chemistry − 1 1135, 1021, 811, and 731 cm ; MS (EI): 167 (18), 166 Disclosure (100), 165 (71), 159 (22), 95 (33), 91.0(40), 79 (14), 77 /is manuscript was also presented at National Annual (22), 73 (26), and 60 (15). Meeting of Chinese Chemical Society, National Sun Yat-sen 4-Benzyloxy-3-methoxybenzaldehyde (2f) [61]: orange 1 University (Kaohsiung), Taiwan, Nov 8∼9, 2018. liquid; H NMR (CDCl , 400 MHz): δ 3.89 (s, 3H, OMe), 5.19 (s, 2H, OCH ), 6.95 (d, J � 8.2 Hz, 1H, ArH), 7.30 (d, J � 7.3 Hz, 1H, ArH), 7.33–7.37 (m, 3H, Conflicts of Interest ArH), 7.39–7.42 (m, 3H, ArH), and 9.79 (s, 1H, CHO); /e authors declare that they have no conflicts of interest. C NMR (CDCl , 100 MHz): δ 55.85, 70.67, 109.22, 112.24, 126.39, 127.08 (2 × CH), 128.05, 128.56 (2 × CH), 130.14, 135.87, 149.90, 153.44, and 190.72; IR Authors’ Contributions (KBr): 2938, 2833, 2731, 1683, 1588, 1509, 1462, 1424, − 1 1264, 1237, 1135, 1026, and 734 cm ; MS (EI): 242 Ching-Yuh Chern and Ching-Chun Tseng contributed (45), 92 (43), 91 (100), and 65 (41). equally to this work. Cinnamaldehyde (2g) [61]: light yellow liquid; H NMR (CDCl , 400 MHz): δ 6.71 (qd, J � 1.82 and 7.70 Hz, 1H, Acknowledgments Me), 7.41–7.43 (m, 3H, ArH), 7.44 (d, J � 1.88 Hz, 1H, Me), 7.54–7.56 (m, 2H, ArH), and 9.69 (dd, J � 7.70 and /e authors are grateful to the Tsuzuki Institute for Tra- 2.42 Hz, CHO); C NMR (CDCl , 100 MHz): δ 128.42 ditional Medicine and the Ministry of Science and Tech- (3 × CH), 129.01 (2 × CH), 131.21, 133.88, 152.81, and nology of the Republic of China (MOST 107-2113-M-039- 193.73; IR (KBr): 3024, 2844, 1681, 1634, 1449, 1311, 006) for financial support. /is work was also financially − 1 and 1286 cm ; MS (EI): 132 (38), 131 (69), 130 (36), supported by Taiwan Ministry of Health and Welfare 103 (98), 102 (63), 91 (59), 78 (41), 77 (94), 76 (40), and Clinical Trial Center (MOHW107-TDU-B-212-123004) and 51 (65). Chinese Medicine Research Center, China Medical Uni- versity, through /e Featured Areas Research Center Pro- 2-Cyclohexen-1-one (2h) [61]: colorless liquid; H gram within the framework of the Higher Education Sprout NMR (CDCl , 400 MHz): δ 1.90–1.96 (m, 2H), 2.24– Project by the Ministry of Education (MOE) in Taiwan 2.29 (m, 2H), 2.33 (t, J � 13.48 Hz, 2H), 5.92 (d, (CMRC-CHM-4). J � 10.12 Hz, 1H), and 6.89–6.94 (m, 1H); C NMR (CDCl , 100 MHz): δ 22.56, 25.49, 37.92, 129.63, 150.61, and 199.40; IR (KBr) 3299, 2918, 2847, 1513, 1707, Supplementary Materials − 1 1380, 1241, 1210, and 832 cm ; MS (EI): 96 (36) and 68 (100). /e supplementary materials contain experimental details, characterization data of all compounds, and copies of H and 3-Phenylpropionaldehyde (2i) [61]: colorless liquid; H C NMR spectra and low mass. 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Cyclopentadienyl Ruthenium(II) Complex-Mediated Oxidation of Benzylic and Allylic Alcohols to Corresponding Aldehydes

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Copyright © 2019 Ching-Yuh Chern et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Heteroatom Chemistry Volume 2019, Article ID 5053702, 8 pages https://doi.org/10.1155/2019/5053702 Research Article Cyclopentadienyl Ruthenium(II) Complex-Mediated Oxidation of Benzylic and Allylic Alcohols to Corresponding Aldehydes 1 2 3 2,3 Ching-Yuh Chern, Ching-Chun Tseng, Rong-Hong Hsiao, Fung Fuh Wong , 4,5,6 and Yueh-Hsiung Kuo Department of Applied Chemistry, National Chia-Yi University, Chia-Yi 60004, Taiwan �e Ph.D. Program for Biotech Pharmaceutical Industry, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan School of Pharmacy, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan Department of Chinese Pharmaceutical Sciences and Chinese Medicine Resources, China Medical University, Taichung 40402, Taiwan Department of Biotechnology, Asia University, Taichung 41354, Taiwan Chinese Medicine Research Center, China Medical University, Taichung 40402, Taiwan Correspondence should be addressed to Fung Fuh Wong; wongfungfuh@yahoo.com.tw and Yueh-Hsiung Kuo; kuoyh@ mail.cmu.edu.tw Received 2 May 2019; Revised 9 July 2019; Accepted 17 July 2019; Published 18 August 2019 Academic Editor: Guillaume Berionni Copyright © 2019 Ching-Yuh Chern et al. ƒis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ƒis work reports an e‡cient method for the oxidation reaction of aliphatic, aromatic allylic, and benzylic alcohols into aldehydes catalyzed by the cyclopentadienyl ruthenium(II) complex (RuCpCl(PPh ) ) with bubbled O . ƒrough further optimizing 3 2 2 controlled studies, the tendency order of oxidation reactivity was determined as follows: benzylic alcohols > aromatic allylic alcohols >> aliphatic alcohols. In addition, this method has several advantages, including a small amount of catalyst (0.5 mol%) and selective application of high discrimination activity of aliphatic, aromatic allylic, and benzylic alcohols. adjunction metals such as Co or Cd, or presence of oxygen or 1. Introduction air under higher pressure and/or temperature, and/or Oxidation reactions are very useful functional trans- stronger basic conditions to obtain the desired products. formations in organic synthesis [1, 2]. Many of the metal- Furthermore, the rare earth elements have occupied an based oxidizing reagents have been developed to achieve the especially important place in the past two decades because of e‡cient oxidation of alcohols such as PI Au [3], ARP-Pt [4], their high reactivity in various catalytic processes [33–39]. Ru/Al O [5, 6], Pd/HAP [7, 8], Au-Pd/TiO [9], and HB Ru One of these rare elements is ruthenium. Some species of 2 3 2 [10]. However, these catalysts are generally di‡cult to obtain ruthenium have been widely developed and used as e‡cient because of their expensive cost and harsh production. In catalysts for oxidation reactions [40, 41]. addition, other oxidation methods for alcohols using the Among a variety of catalysts reported in the literature for the redox process of carbonyl compounds [42], the use of readily available carbon-supported metal catalysts [11–14] including many famous Pd/C [15–17], Pt/C [18–23], or Au/ ruthenium complexes has garnished signi¥cant attention. For C [24–30] catalysts [31, 32] were also enthusiastically in- example, several ruthenium complexes have been employed as vestigated. Unfortunately, these catalysts needed addition of catalysts for the hydrogenation of carbonyl compounds, 2 Heteroatom Chemistry Table 1: Study of oxidation conditions of benzyl alcohol 1a with RuCpCl(PPh ) . 3 2 OH RuCpCl(PPh ) 3 2 O , solvent, at reflux 1a 2a Entry Substrate Solvent Reaction condition Product Yield (%) 1 1a CH Cl Reflux for 24 h 2a Trace 2 2 2 1a CH Cl Reflux for 48 h 2a <10 2 2 3 1a Benzene Reflux for 24 h 2a Trace 4 1a Benzene Reflux for 48 h 2a <6 5 1a Acetone Reflux for 24 h 2a Nondetectable 6 1a Acetone Reflux for 48 h 2a Nondetectable 7 1a THF Reflux for 24 h 2a 54 8 1a THF Reflux for 48 h 2a 71 providing an efficient access to their corresponding alcohols. ACS grade of solvents (i.e., benzene, acetone, and THF) and For example, several ruthenium complexes [43] including the reaction time at room temperature or reflux. Based on RuCpCl(PPh ) , Ru(indenyl)Cl(PPh ) , [RuCp(MeCN) (PR )] the experimental results in Table 1, we observed that the 3 2 3 2 2 3 PF [44], [RuCp(MeCN) ]PF [45], and RuCpCl(diphosphine) ideal conditions for this reaction were to use THF as the 6 3 6 have been employed as catalysts for the isomerization of both reaction solvent and to reflux for 48 h. /e corresponding aliphatic [46] and aromatic [47–50] allylic alcohols into ketones oxidation product 2a can be afforded in 71% isolated yield or aldehydes. (entry 8 in Table 1). Consequently, optimization of the amount of the catalyst from 0.5, 1.0, 2.0, to 5 mol% was Literature has reported oxidation of allylic and benzylic alcohols for functional transformation such as that reported performed. However, this did not lead to any further im- by Prof. Pearson using trimethylamine-N-oxide in the provement. /e structure of product 2a was completely presence of iron carbonyl as a significant catalyst [51]. characterized by spectroscopic methods and consistent with However, several issues limit these catalysts’ applications an Aldrich-authentic sample. Following the results in Ta- such as the need for a toxic solvent, tedious complex re- ble 1, we found that RuCpCl(PPh ) possessed the oxidation 3 2 agents, and troublesome procedures. /erefore, we reported reactivity necessary for transferring benzyl alcohols to the a novel and interesting cyclopentadienyl ruthenium(II) corresponding benzaldehydes. complex (RuCpCl(PPh ) ), which oxidizes aromatic allylic In order to explore the substrate scope of the new oxidation 3 2 and benzylic alcohols into carbonyl compounds. Based on reaction, we first examined the reactions of substituted benzylic the further controlled experimental studies, we found the alcohols 1b–f containing either electron-donating or electron- tendency order of oxidative reactivity as benzylic alcohol- withdrawing groups 1b–d and disubstituted benzylic alcohols s> aromatic allylic alcohols>> aliphatic alcohols. 1e-f. Fortunately, most of the substituted benzylic alcohols 1b–f were successfully converted to the corresponding alde- hyde products 2b–f in moderate yields (>68%; Table 2). On the 2. Results and Discussion contrary, benzylic alcohols with para-Me (1b) and para-OMe Benzyl alcohol 1a is an important precursor for organic (1c) electron-donating groups were oxidized to give the cor- synthesis and a useful solvent because of its polarity, low responding aldehyde products 2b-c in 73% and 80% yields, toxicity, mildly pleasant aromatic odor, and low vapor respectively (entries 1 and 2 in Table 2). In addition, the pressure [52–54]. /e chemoselective oxidation property of conversion of para-CN-benzylic alcohol 1d to aldehyde 2d in compound 1a was also very useful in functional trans- 68% yield seemed to have lower reactivity compared to benzylic formations for the preparation of aldehydes [55, 56] and alcohols with electron-donating groups 1b-c (entries 1 and 2 in their dicarboxyl analogues [57, 58]. For these reasons, we Table 2). For disubstituted benzylic alcohols 1e and 1f, the most carried out a plausible oxidation of benzyl alcohol 1a. We effective results were achieved in 82% and 86% yields, re- spectively, demonstrating a strong and significant electron- preliminarily investigated a versatile oxidation method for compound 1a with 0.5 mol% amount of the cyclo- assisted effect (entries 4 and 5 in Table 2). Following the above pentadienyl ruthenium(II) complex (RuCpCl(PPh ) ) cat- study, it was found the cyclopentadienyl ruthenium(II) com- 3 2 alyst with bubbled O in CH Cl solution at reflux for 24 h. plex (RuCpCl(PPh ) ) possessed significant oxidizing activity 2 2 2 3 2 However, only trace amounts of benzaldehyde 2a were for discrimination of benzylic alcohols. achieved (entry 1 in Table 1). We then increased the reaction To further expand our study, we investigated simplified time to 48 h, which resulted in the desired oxidation product allylic systems such as (E)-3-arylprop-2-en-1-ols (1g) and 2a, but at a low yield (<10%; entry 2 in Table 1). To identify cyclohex-2-enol (1h). /e RuCpCl(PPh ) catalyst (0.5 mol%) 3 2 the optimal reaction conditions, we attempted to screen the successfully reacted towards (E)-3-arylprop-2-en-1-ols (1g) Heteroatom Chemistry 3 Table 2: Oxidation results of alcohols with RuCpCl(PPh ) at reflux in anhydrous THF solution. 3 2 Entry Alcohols 1b–k Reaction time (h) Yields of products 2b–k Yields (%) OH 1 1b ∼48 h 2b 73 Me Me OH 2 1c ∼48 h 2c 80 MeO MeO OH 3 1d ∼48 h 2d 68 NC NC MeO OH MeO 4 1e ∼48 h 2e 82 MeO MeO MeO OH MeO 5 1f ∼48 h 2f 86 Bno Bno OH 6 1g ∼48 h 2g 68 OH O 7 1h ∼48 h 2h 69 OH 8 1i ∼48 h 2i 17 9 1j CH (CH ) CH OH ∼48 h 2j CH (CH ) CHO 35 3 2 5 2 3 2 5 OH 10 1k ∼48 h 2k — — Me /e starting material was recovered. 4 Heteroatom Chemistry OH OH HO 10 9876543 ppm Figure 1: H NMR characteristic identification of compounds 3 (□), 4 (○), and 5 (∆). OH OH H RuCpCl(PPh ) 3 2 O , THF, at reflux HO O H O H 3 5 Reaction time 4h 21% <5% 24 h 48% 12% 48 h 54% 15% Scheme 1: Chemoreactivity oxidation results of 4-(hydroxymethyl)benzenepropanol 3. and cyclohex-2-enol (1h) in the presence of THF at reflux for alcohols. In addition, the oxidation reaction of benzylic ∼48 h to give the desired oxidation products 2g and 2h in 68% alcohol 1a with the RuCpCl(PPh ) catalyst (0.5 mol%) 3 2 and 69% yields, respectively (entries 6 and 7 in Table 2). /e was found to produce better isolated yields compared to yielding results of the allylic system 1g-h were noticeably both allylic alcohol 1g and aliphatic alcohol 1i. lower than the conversion of benzylic alcohols 1e-f. 4-(Hydroxymethyl)benzenepropanol 3 was synthesized To evaluate the substrate scope and limitation, this by the reported method as the important bifunctional sub- study has been extended to a variety of aliphatic or ali- strate for the chemoreactivity oxidation study [59]. We cyclic alcohols such as 3-arylpropan-1-ols (1i), heptan-1- initially carried out a careful study of possible oxidations of ol (1j), and 2-isopropyl-5-methylcyclohexanol ( 1k). In compound 3 with 1.25 mol% amount of the cyclopentadienyl general, a longer reaction time (∼60 h) was required ruthenium(II) complex (RuCpCl(PPh ) ) catalyst with 3 2 compared to benzyl and allylic alcohols 1a–h for oxidation bubbled O in CH Cl solution at reflux for 48 h. /e versatile 2 2 2 reaction (entries 8–10 in Table 2). Under the same con- oxidation was monitored by TLC and sampled for H NMR dition, aliphatic or alicyclic alcohols 1i–k presented poor characteristic identification (see Figure 1). When the reaction oxidizing reactivity, resulting in trace to 35% yields. For was performed for 4 h, the crude solution was sampled, compound 1k, no trace of the oxidation product was worked up, and eluted from the column. Most of starting detected in the H NMR spectrum of the crude reaction material 3 was recovered, and the corresponding mixture mixture (entry 10 in Table 2). Following the above study, it products 4-(3-hydroxypropyl)benzaldehyde 4 and the small was found the cyclopentadienyl ruthenium(II) complex amount of 4-(3-oxopropyl)benzaldehyde 5 were given out in (RuCpCl(PPh ) ) possessed significant oxidizing activity 21% and <5% yields, respectively (see Scheme 1 and Fig- 3 2 for discrimination of aromatic allylic and benzylic ure 1). When the reaction time was prolonged from 4 h to Heteroatom Chemistry 5 24 h or 48 h under the same condition, we observed that the atmospheric pressure. When the oxidization reaction was expected oxidation product 4 was significantly promoted completed, the solution was filtered through Celite, and the from 21% to 48% or 54% yields, respectively [60]. Celite bed was washed with hot THF. /e filtrate was Comparatively, only a small amount of oxidation product concentrated to remove THF under reduced pressure. /e 4-(3-oxopropyl)benzaldehyde 5 was achieved (∼5% to residue was added (CH Cl , 15 mL), washed with saturated 2 2 12% and 15%; Scheme 1) [60]. Based on the above ex- aqueous NaHCO (15 mL), and extracted with CH Cl 3 2 2 perimental results, we proved again that the benzylic (10 mL × 2). /e combined organic layers were washed with alcohol possessed more efficient oxidative reactivity than brine (15 mL), dried over MgSO , filtered, and concentrated the aliphatic alcohol. under reduced pressure. /e residue was purified by column chromatography (n-hexane/EtOAc � 4/1) on silica gel to give the corresponding aldehyde products 2a–j in 17–86% 3. Conclusions yields. /e physical properties and spectroscopic charac- We have successfully developed the oxidation reaction for teristics of the isolated aliphatic, aromatic allylic, and aliphatic, aromatic allylic, and benzylic alcohols with benzylic alcohols, including 2a–j, were consistent with those 0.5 mol% of the cyclopentadienyl ruthenium(II) complex of the authentic sample [61]. (RuCpCl(PPh ) ). Based on the further controlled studies, 3 2 Benzaldehyde (2a) [61]: light yellow liquid; H NMR the reactive tendency was determined as follows: benzylic (CDCl , 400 MHz): δ 7.48–7.52 (m, 2H, ArH), 7.58– alcohols> aromatic allylic alcohols>> aliphatic alcohols. 7.63 (m, 1H, ArH), 7.84–7.87 (m, 2H, ArH), and 9.99 (s, On the contrary, mono- and disubstituted benzylic alcohols 1H, CHO); C NMR (CDCl , 100 MHz): δ 129.94 with electron-donating groups can provide the best oxi- (2 × CH), 129.69 (2 × CH), 134.41, 136.34, and 192.37; dation results. In addition, this new method has several IR (KBr): 3071, 2832, 2674, 2560, 1686, 1424, 1325, advantages including a small amount of catalyst (0.5 mol%) − 1 1291, 934, and 709 cm ; MS (EI): 106 (78), 105 (100), and high discrimination activity of aliphatic, aromatic al- 78 (15), 77 (83), 51 (37), and 50 (13). lylic, and benzylic alcohols. 4-Tolualdehyde (2b) [61]: colorless liquid; H NMR 4. Experimental Section (CDCl , 400 MHz): δ 2.42 (s, 3H, Me), 7.31 (d, J � 7.9 Hz, 2H, ArH), 7.75 (dd, J � 6.6 and 1.6 Hz, 2H, All reagents were used as obtained commercially. All re- ArH), and 9.94 (s, 1H, CHO); C NMR (CDCl , actions were carried out under argon or nitrogen atmo- 100 MHz): δ 21.85, 129.68 (2 × CH), 129.83 (2 × CH), sphere and monitored by TLC. Flash column 134.16, 145.54, and 192.01; IR (KBr): 3043, 2945, 1672, − 1 chromatography was carried out on silica gel (230– 1574, 1282, 959, 947, 838, and 752 cm ; MS (EI): 120 400 mesh). An analytical thin-layer chromatography (TLC) (34), 119 (100), 92 (11), 91 (97), and 65 (18). was performed using precoated plates (silica gel 60 F-254) 4-Methoxybenzaldehyde (2c) [61]: light yellow liquid; purchased from Merck Inc. Flash column chromatography H NMR (CDCl , 400 MHz): δ 3.79 (s, 3H, OMe), purification was carried out by gradient elution using n- 6.90–6.93 (m, 2H, ArH), 7.73–7.76 (m, 2H, ArH), and hexane in ethyl acetate (EtOAc) unless otherwise stated. 9.79 (s, 1H, CHO); C NMR (CDCl , 100 MHz): δ Infrared (IR) spectra were measured with a Bomem 54.74, 113.61 (2 × CH), 129.24, 131.16 (2 × CH), 163.89, Michelson Series FT-IR spectrometer. /e wavenumbers and 189.98; IR (KBr): 3520, 3356, 2969, 2839, 2741, reported are referenced to the polystyrene absorption at − 1 1682, 1601, 1577, 1512, 1315, 1260, 1158, and 1601 cm . Absorption intensities are recorded by the fol- − 1 1026 cm ; MS (EI): 136 (72), 135 (100), 92 (14), lowing abbreviations: s, strong; m, medium; and w, weak. All 77.0(24), and 65 (10). proton and carbon-13 NMR spectra were obtained by Bruker instruments (400 MHz and100 MHz, respectively). 4-Cyanobenzaldehyde (2d) [61]: white crystal, m.p. Proton and carbon-13 NMR spectra were acquired using 99–102 C; H NMR (CDCl , 400 MHz): δ 7.83 (d, deuterochloroform (CDCl ) solvent. Multiplicities are J � 8.2 Hz, 2H, ArH), 7.98 (d, J � 8.2 Hz, 2H, ArH), and recorded by the following abbreviations: s, singlet; d, dou- 10.07 (s, 1H, CHO); C NMR (CDCl , 100 MHz): δ blet; t, triplet; q, quartet; m, multiplet; and J, coupling 117.57, 117.74, 129.90 (2 × CH), 132.92 (2 × CH), constant (Hz). ESI-MS analyses were performed on an 138.74, and 190.70; IR (KBr): 2850, 2231, 1707, 1608, − 1 Applied Biosystems API 300 mass spectrometer. High- 1571, 1387, 1295, 1203, 1172, 832, 737, and 546 cm ; resolution mass spectra were obtained from a JEOL JMS- MS (EI): 131 (84), 130 (100), 105 (28), 103 (23), HX110 mass spectrometer. 102.0(57), 91 (12), 77 (16), 76 (18), 75 (12), and 51 (12). 3,4-Dimethoxybenzaldehyde (2e) [61]: light yellow liquid; H NMR (CDCl , 400 MHz): δ 3.93 (s, 3H,- 4.1. Standard Procedure for the Oxidation of Aliphatic, Aro- OMe), 3.96 (s, 3H,-OMe), 6.98 (d, J � 8.2 Hz, 1H, ArH), matic Allylic, and Benzylic Alcohols 1a–j to Corresponding 7.40 (d, J � 7.6 Hz, 1H, ArH), 7.45 (dd, J � 8.2 and Aldehydes 2a–j with Cyclopentadienyl Ruthenium(II) 1.9 Hz, 1H, ArH), and 9.84 (s, 1H, CHO); C NMR Complex. Aliphatic, aromatic allylic, or benzylic alcohols (CDCl , 100 MHz): δ 55.60, 55.80, 108.63, 110.11, (1a–j, ∼1.0 mmol, 1.0 equiv) and RuCpCl(PPh ) (∼0.5 mol 3 2 126.42, 129.79, 149.26, 154.13, and 190.47; IR (KBr): %) with bubbled O were stirred in anhydrous THF (2.0 mL) 2938, 2840, 1683, 1588, 1513, 1462, 1421, 1268, 1243, and heated at reflux for 24–48 h under the argon 6 Heteroatom Chemistry − 1 1135, 1021, 811, and 731 cm ; MS (EI): 167 (18), 166 Disclosure (100), 165 (71), 159 (22), 95 (33), 91.0(40), 79 (14), 77 /is manuscript was also presented at National Annual (22), 73 (26), and 60 (15). Meeting of Chinese Chemical Society, National Sun Yat-sen 4-Benzyloxy-3-methoxybenzaldehyde (2f) [61]: orange 1 University (Kaohsiung), Taiwan, Nov 8∼9, 2018. liquid; H NMR (CDCl , 400 MHz): δ 3.89 (s, 3H, OMe), 5.19 (s, 2H, OCH ), 6.95 (d, J � 8.2 Hz, 1H, ArH), 7.30 (d, J � 7.3 Hz, 1H, ArH), 7.33–7.37 (m, 3H, Conflicts of Interest ArH), 7.39–7.42 (m, 3H, ArH), and 9.79 (s, 1H, CHO); /e authors declare that they have no conflicts of interest. C NMR (CDCl , 100 MHz): δ 55.85, 70.67, 109.22, 112.24, 126.39, 127.08 (2 × CH), 128.05, 128.56 (2 × CH), 130.14, 135.87, 149.90, 153.44, and 190.72; IR Authors’ Contributions (KBr): 2938, 2833, 2731, 1683, 1588, 1509, 1462, 1424, − 1 1264, 1237, 1135, 1026, and 734 cm ; MS (EI): 242 Ching-Yuh Chern and Ching-Chun Tseng contributed (45), 92 (43), 91 (100), and 65 (41). equally to this work. Cinnamaldehyde (2g) [61]: light yellow liquid; H NMR (CDCl , 400 MHz): δ 6.71 (qd, J � 1.82 and 7.70 Hz, 1H, Acknowledgments Me), 7.41–7.43 (m, 3H, ArH), 7.44 (d, J � 1.88 Hz, 1H, Me), 7.54–7.56 (m, 2H, ArH), and 9.69 (dd, J � 7.70 and /e authors are grateful to the Tsuzuki Institute for Tra- 2.42 Hz, CHO); C NMR (CDCl , 100 MHz): δ 128.42 ditional Medicine and the Ministry of Science and Tech- (3 × CH), 129.01 (2 × CH), 131.21, 133.88, 152.81, and nology of the Republic of China (MOST 107-2113-M-039- 193.73; IR (KBr): 3024, 2844, 1681, 1634, 1449, 1311, 006) for financial support. /is work was also financially − 1 and 1286 cm ; MS (EI): 132 (38), 131 (69), 130 (36), supported by Taiwan Ministry of Health and Welfare 103 (98), 102 (63), 91 (59), 78 (41), 77 (94), 76 (40), and Clinical Trial Center (MOHW107-TDU-B-212-123004) and 51 (65). Chinese Medicine Research Center, China Medical Uni- versity, through /e Featured Areas Research Center Pro- 2-Cyclohexen-1-one (2h) [61]: colorless liquid; H gram within the framework of the Higher Education Sprout NMR (CDCl , 400 MHz): δ 1.90–1.96 (m, 2H), 2.24– Project by the Ministry of Education (MOE) in Taiwan 2.29 (m, 2H), 2.33 (t, J � 13.48 Hz, 2H), 5.92 (d, (CMRC-CHM-4). J � 10.12 Hz, 1H), and 6.89–6.94 (m, 1H); C NMR (CDCl , 100 MHz): δ 22.56, 25.49, 37.92, 129.63, 150.61, and 199.40; IR (KBr) 3299, 2918, 2847, 1513, 1707, Supplementary Materials − 1 1380, 1241, 1210, and 832 cm ; MS (EI): 96 (36) and 68 (100). /e supplementary materials contain experimental details, characterization data of all compounds, and copies of H and 3-Phenylpropionaldehyde (2i) [61]: colorless liquid; H C NMR spectra and low mass. (Supplementary Materials) NMR (CDCl , 400 MHz): δ 2.77 (td, J � 7.28 and 0.87 Hz, 2H), 2.96 (t, J � 7.56 Hz, 2H), 7.20 (t, J � 7.38 Hz, 3H), 7.30 (t, J � 7.30 Hz, 2H), and 9.81 (t, References J � 1.34 Hz, CHO); C NMR (CDCl , 100 MHz): δ [1] T. Kondo, Y. U. Kimura, H. Yamada, and A. Toshimitsu, 28.08, 45.18, 126.28 (2 × CH), 128.35, 128.60 (2 × CH), “Ruthenium-based catalysts for aerobic oxidation of alcohols: 140.57, and 201.46; IR (KBr): 3029, 2929, 1709, 1604, − 1 transition metal catalysis in aerobic alcohol oxidation,” in 1496, 1456, and 1298 cm ; MS (EI):135 (22), 134 (100), Green Chemistry Series, F. Cardona and C. Parmeggiani, Eds., 133 (10), 118 (12), 117 (20), 105 (27), 92 (58), 91 (96), 78 pp. 70–91, Royal Society of Chemistry, London, U.K, 2014. (23), and 77 (11). [2] G. N. I. 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Heteroatom Chemistry – Hindawi Publishing Corporation

Published: Aug 18, 2019

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