β-Cyclodextrin promoted oxidation of aldehydes to carboxylic acids in water

Available online at www.sciencedirect.comCHINESEScienceDirectC HEMICALL .ETTERSELSEVIERChinese Chemical Letters 20 (2009) 139-142www.elsevier.com/locate/ccletβ-Cyclodextrin promoted oxidation of aldehydesto carboxylic acids in waterDong Po Shia, Hong Bing Ji b,*a College of Chemistry and Environmental Engineering, Yangize Universiry, Jingzhou 434023, Chinab School of Chemistry and Chemical Engineering of Sun Yat Sen Universiry, Guangzhou 510275, ChinaReceived 11 June 2008AbstractA facile, eficient and substrate-selective oxidation of aldehydes to carboxylic acids with NaClO catalyzed by β-cyclodextrin inwater has been developed. A series of aldehydes which could form inclusion complex with β-cyclodextrin (B-CD) were oxidizedselectively with excellent yields.C 2008 Hong Bing Ji. Published by Elsevier B. V. on behalf of Chinese Chemical Society. All rights reserved.Keywords: Substrate -ective; Oxidation; β-Cyclodexrin (β-CD); AldehydesOxidation of aldehydes to carboxylic acids is a pivotal reaction in organic synthesis [1- -2]. Lots of successful andsophisticated methods have been developed for these oxidations [3- 5]. However, there are various limitations such asthe use of metal-based catalysts, inorganic or organic promoters and organic solvents and reagents. The growingawareness of the need for green chemistry is required to investigate aldehydes oxidation under mild conditions [6].Sato et al. [7] reported a metal-free oxidation of aldehydes to carboxylic acids under aqueous and organic biphasicconditions; because of the absence of metallie catalysts, this environmentally friendly procedure was highly desirable.Previously, we have developed two effective catalysts including metalloporphyrins complexes [8] and Ru-Co(OH)2- CeO2 [9] for the oxidation of aldehydes. Inspired by those excellent results, most recently, we havedeveloped a biomimetic and metal-free approach for the aldehydes oxidation catalyzed by β-cyclodextrin in water.β-Cyclodextrin, which is cyclic oligosaccharide with hydrophobic cavity, can catalyze chemical reactions withhigh selectivity by supramolecular catalysis via non-covalent bonding of host-guest complexes [10] as seen inenzymes; and the corresponding reports [11-13] about oxidation and organic synthesis catalyzed by β-cyclodextrinand its derivatives in water involving the formation of host- guest complexes are increasing. In our efforts to developbiomimetic approaches for chemical reactions involving β-cyclodextrin in water, we focused on its substrate-selectivereactions [14-16]. In comparison with the general methods about the oxidation of various alcohols in NaClO-KBrsystem reported by Surendra et al. [11], only those substrates with suitable space configuration could be successfullyoxidized in our previous report [15]. It had been shown earlier that substrate-selective deprotection of various ethyleneacetals [16] and substrate- selective oxidation of sulfides [13] were achieved in the same way. In continuation of our* Corresponding author.E-mail address: jihb@ mailsysu.edu.cn (HB. Ji).中国煤化工1001-8417/$ - see front matter◎2008 Hong Bing Ji. Published by Elsevier B.V. on bJMYHCNMH GAll rights reserved.doi: 10.1016/.cclet.2008.10.037140D.P. Shi, H.B. Ji/Chinese Chemical Letters 20 (2009) 139 -142CHOB-cyclodextrin10% NaClO, 5% HCI, H2OScheme 1. Oxidation of benzaldehyde in the presence of β-cyclodextrin.efforts, we report herein an efficient, substrate- selective and practical method for the oxidation of aldehydes tocarboxylic acids with NaClO as an oxidant catalyzed by β-cyclodextrin in water.1. Experimentalβ-Cyclodextrin (0.5 mmol) was dissolved in water (25 mL) at 55 °C, and the substrate (0.5 mmol) was addedslowly with stirring. Then followed by addition of 5 mL of 10% NaClO and 5 mL of 5% HCl subsequent stirring wascontinued until the reaction had gone to completion. When the reaction was finished, the mixture was extracted byethyl acetate. The crude product was analyzed by GC-MS (Shimadzu GCMS QP2010) and compared with authenticsamples.The oxidation of benzaldehyde as a representative sample (Scheme 1) has been performed to optimize the reactionconditions with various oxidants and different temperatures, the results are shown in Table 1.2. Results and discussionThe results show that neither weak oxidant O2 nor moderate oxidant H2O2 could be used as an effective oxidanttowards benzaldehyde oxidation (entries 1-3). However, the strong oxidant NaClO greatly prompted the oxidation,and nearly complete conversion from benzaldehyde to benzoic acid could be achieved within 2 h at 55 °C (entry 10). It .should be noticed that HCI additive is crucial as the reaction rate was much lower in the absence of β-cyclodextrin(entry 12) and benzaldehyde could not be oxidized at all without hydrochloric acid (entry 11). The reactiontemperature affected the oxidation greatly; when the temperature was lower than 55 °。C, the conversion was notcomplete (entries 4 -10). Therefore, NaClO as an oxidant and 55 °C as the suitable reaction temperature were chosenas the optimum reaction conditions (entry 10).Using the optimal conditions reported in Table 1, the reaction was explored on various other aldehydes. The resultsare summarized in Table 2.It is known that β-cyclodextrin and substrates can form host -guest complex. This complexation depends on thesize, shape and hydrophobicity of the guest molecule. In the present research benzylic primary aldehydes underwentsmooth oxidation (entries 1- -8). Commonly, the existence of electron- donating group such as -CH3 group promotedTable 1Effect of the different oxidants and under different temperatures' '.EntryT(C)Time (h)Conv. (%)Yield (%)Air5530% H2O2 (5 mL, 44.1 mmol)10% NaCIO (5 mL, 6.7 mmol)r.210% NaClO (5 mL, 6.7 mmol)75745(6610% NaClo (5 mL, 6.7 mmol)5063171C)9)8115:1125S中国煤化工.40a Reaction condition: 0.5 mmol β-CD, 25 mL H2O, 5 mL 5% HCI.MYHCNMH GWithout HClWithout β-cyclodextrin.D.P. Shi, H.B. Ji/Chinese Chemical Letters 20 (2009) 139- -142141Table 2Oxidation of different aldehydes".EntrySubstrateProductTime (h)Conv. (%)Yield (%)2999PicolinaldehydePicolinic acid1002-Methylbenzaldehyde2-Methylbenzoic acid1.52-Methoxybenzaldehyde2-Methoxybenzoic acid<74-Methoxybenzaldehyde4-Methoxybenzoic acid10<2-Nitrobenzaldehyde2-Nitrobenzoic acid(4-Nitrobenzaldehyde4-Nitrolbenzoic acid1282-Chlorobenzaldehyde3-Phenylpropanal3-Phenylpropanoic acid60ButyraldehydeButyric acid71HeptanalHeptanoic acidOctanalOctanoic acid7(a Reaction condition: 0.5 mmol β-CD, 25 mL H2O, 5 mL 5% HCl, 5 mL 10% NaCIO, 55 °C.The yields of 2-methoxyphenol and 4 -methoxyphenol were 92%.the oxidation of aldehydes to carboxylic acids (entry 3). Unfortunately, 2- and 4-methoxybenzaldehydes afforded thecarboxylic acids with a yield of < <7% (entries 4- -5), showing that strong electron donating group does not favoroxidation of benzylic primary aldehydes to the corresponding carboxylic acids, but more likely to form thecorresponding phenols for the migration of the- Ar group [17]. Benzaldehyde derivatives with an electron-withdrawing group produced the benzoic acids in reasonable yields. However, the existence of electron-withdrawinggroup lowered the reaction rate and the oxidation of 2-nitrobenzaldehyde for 5 h hardly gave any products (entry 6),indicating that the space configuration of guest molcules is more important for smooth conversion than the influencefrom electron effect in the present catalytic system (entries 7-8). This could be further supported by the fact that theheteroaromatic primary aldehyde such as picolinaldehyde could be converted to picolinic acid with the similarreaction rate as benzaldehyde oxidation (entry 2).The length between benzene ring and -CHO group could significantly influence the selectivity, 3-phenylpropanalreacted for 2 h till the completion of conversion, but the selectivity is only 63%, indicating the β-cyclodextrin-substrate complex is important for the selective conversion (entry 9). This could be further evidenced by othersubstrates, with which the formed complex is less stable. For example, straight-chain aliphatic aldehydes such asbutyraldehyde exhibited some reactivity towards the oxidation (entries 10-12), however, the bulk of the butyl groupoccupying less space than an aromatic group, might explain the low selectivity towards oxidation (entry 10). Thatheptanal and octanal selectively gave the corresponding carboxylic acid with a yield of 71 and 70%, respectively, iscompatible to the view that the steady complexation between long chain aliphatic aldehydes and β-cyclodextrin mightbe hardly formed (entries 11- 12) [16].In conclusion, we have developed a facile, substrate- selective and transition metal-free oxidation of aldehydescatalyzed by B-cyclodextrin with cheap NaClO oxidant using water as the only solvent. The β-cyclodextrin could beeasily recycled by the addition of organic solvents such as ethyl acetate or acetone after the reaction.AcknowledgmentsThe authors thank the National Natural Science Foundation of China (No. 20776053) and the Program for NewCentury Excellent Talents in University (No. NCET-06-740) for providing financial support for this project.References[] M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph Ser. 186, American Chemical Society, Washingon, DC, 1990.[2] x. Huang, YG. Wang, z.c. Chen, New Organie Synthesis Chemistry, Chemical[3] R.C. Larock, Comprehensive Organic Transformations, VCH Publishers, New Y中国煤化工[4] S. Sagar, B. Nilotpal, B.B. Jubaraj, J. Mol. Catal. A: Chem. 229 (2005) 171.:TYHCNMHG[5] H. Joshua, Tetrahedron Lett. 41 (2000) 6627.[6] H.B. Ji, Y.B. She, Green Oxidation and Reduction, China Petrochemical Press, Beijing, 2005.142D.P. Shi, H.B. Jji/Chinese Chemical Letters 20 (2009) 139 -142[7] K. Sato, M. Hyodo, J. Takagi, M. Aoki, R. 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