Heterogeneous Transition-Metal-Free Alcohol Oxidation by Graphene Oxide Supported Iodoxybenzoic Acid in Water

Abstract

The metal-free oxidation of organic compounds is one of the most demanding reactions in chemical industries. As such, iodoxybenzoic acid (IBX) is an attractive reagent due to its metal-free oxidative activity. To apply IBX for the alcohol oxidation in water, IBX was immobilized on graphene oxide (GO). GO-supported IBX reagent exhibited excellent performance for alcohol oxidation reaction in water with greater than 90% selectivity. It could be reused without significant loss of oxidative activity.

The oxidation of organic compounds in water-based system has been demanding in fine-chemical industries due to its environmental and economic benefits.[2] Much research activity has focused on developing heterogeneous novel transition-metal catalysts (Pd,[3] Au,[4] Pt,[5] or bimetal alloy[6]) for alcohol oxidation reaction. However, they have several limitations: they require cocatalyst and lose activity as transition metals leach out. Accordingly, alcohol oxidation in water with transition-metal-free catalyst/reagent has been rarely reported. Developing transition-metal-free catalyst/reagent for alcohol oxidation became a challenging issue for green chemistry.[7]

Iodoxybenzoic acid (IBX) is a representative metal-free oxidizing reagent due to its advantages such as stability to moisture and oxygen, high selectivity, easy availability, and low toxicity.[8] Despite these, IBX is potentially explosive and poorly compatible with common solvents including water since strong hydrogen bonding exists among IBX monomers.[9] To overcome the problem, solid-supported IBX has been suggested to improve solvent compatibility.[10] [11] Over the past decade, the Zhdankin group has developed IBX amides for the homogeneous oxidation reaction,[12] and a few groups, including our group, reported heterogeneous IBX supported on polystyrene,[11,13] silica,[14] and macroporous bead.[10] The oxidizing ability of IBX could be improved by immobilization on solid supports in some solvents. Yet, it still remains as a challenge for IBX to be used in water system.

Graphene oxide (GO) is an emerging material that has ­excellent physical properties and water compatibility. It has been successfully applied as a solid support for heterogeneous catalysts, since it has many advantages such as ease of modification, high surface area, and few solvent limitations.[15] With these novel properties of GO, a new concept of ‘carbocatalysis’ was introduced as a mild and efficient carbon catalyst for organic synthesis.[16]

Here, we report the GO-supported IBX reagent (GO–IBX), which is highly compatible to water and has high loading of IBX as illustrated in Scheme [1]. GO–IBX showed good performance for alcohol oxidation reaction in water with >90% selectivity for various alcohol substrates. Moreover, it was recyclable up to at least five times without a significant loss of oxidizing activity.

GO–IBX was prepared as shown in Scheme [2]. The surface of GO, which contains caboxyl groups, was modified with ethylene diamine to introduce the 2-iodobenzoic acid (IBA) moiety on the GO surface. The coupling of ethylene diamine with carboxylic acid on GO was proceeded successfully by DIC/HOBt in DMF. The amino group loading was found to be 1.2 mmol/g of GO by elemental analysis. Then, IBA was coupled to the amino group on GO, affording GO–IBA (0.5 mmol/g of GO). It was activated to GO–IBX by an additional oxidation step in water. Similar to pristine GO, GO–IBX was compatible with hydrophilic solvents such as methanol, ethanol, and even water. This property is essential for developing a water-based IBA activation which could avoid toxic reagents or byproducts.[17] The coupling of GO with IBA and subsequent activation were confirmed by ATR FT-IR analysis. The peak intensity at 1663 cm^–1 which corresponds to the amide carbonyl group was shifted to 1649 cm^–1.

This indicates that IBA was covalently bonded to the amino group of GO which was also confirmed by the electron-dispersive spectrometry (EDS) analysis. When GO–IBA was activated to GO–IBX, the carbonyl peak was moved to 1628 cm^–1 caused by the formation of iodoso group (Supporting Information, Figure S1).

The morphological changes during each reaction step were investigated by the transmission electron microscopy (TEM) and X-ray diffractometry (XRD). TEM images of Figure [1(a)] show that GO sheets of less than 50 μm sizes were chemically exfoliated from natural graphite. After surface modification of GO [(Figure [1(b])] and IBA coupling step, the morphology of GO was unchanged [Figure [1(c])]. The XRD peak of natural graphite is 28° (2θ), and that of GO became 10° (2θ). The XRD patterns of GO were kept consistent even after IBA coupling on GO and subsequent activation of IBA to IBX (Supporting Information, Figure S2).

As shown in Table [1], alcohol oxidation reactions were performed successfully by GO–IBX in aqueous phase under mild conditions (50 °C). To evaluate the electronic effect of the substituent, three kinds of benzylic alcohols containing electron-donating as well as electron-withdrawing substituents were transformed into the corresponding aldehydes (Table [1], entries 1–3). Benzyl alcohol (Table [1], entry 1) and benzylic alcohol with an electron-donating methoxy group (Table [1], entry 2) was completely transformed into the corresponding aldehydes, while the alcohol with the electron-withdrawing nitro group (Table [1], entry 3) showed low conversion into the corresponding 4-nitrobenzaldehyde (31%). These results are in agreement with the previous report stating that disproportionation in the IBX-mediated oxidation mechanism is a rate-determining step and the oxidation of benzylic alcohol is influenced by the electron density of the benzylic group.[18]

Table 1 Oxidation Results of Various Alcohols Performed by GO–IBX under Aqueous Conditions^a

Entry	Substrate	Product	Conversion (%)	Selectivity (%)d
1			>99	>99
2			>99	>99
3b			31	98
4			>99	>99
5			>99	>99
6c			>99	>99
7c			>99	>99
8c			95	>99
9c			41	98

^a Reaction conditions: alcohol substrate (8.1 μmol), GO–IBX amide (2 equiv. of IBX), H₂O (1.6 mL), DMF (0.4 mL), 5% TFA, 50 °C, 12 h.

^b 24 h.

^c 10% TFA, 36 h.

^d Aldehydes or ketones percentage against overoxidized carboxylic acid.

In line with this theory, aromatic allylic alcohol (Table [1], entry 4), secondary alcohols (Table [1], entries 5 and 6) and aliphatic alcohols (Table [1], entries 8 and 9) were selectively converted into the corresponding carbonyl compounds by GO–IBX. All were selectively transformed into the corresponding carbonyl compunds with over 98% selectivity. The high selectivity of 1-octanol showed that Oxone was successfully washed out after the IBA activation step and did not exist on the surface of GO–IBX, since Oxone is more reactive toward aliphatic aldehyde than aromatic ones.[19] Furthermore, GO–IBX performed well for the oxidation of alcohols containing heteroatoms, such as sulfur, in high conversion and selectivity (Table [1], entry 7). Because GO could be an oxidant itself,[16a] we examined the influences of GO for the oxidation reaction under the same aqueous conditions and confirmed that the amount of oxidized products was negligible (<1%). GO–IBX could easily be reactivated by repetitive Oxone treatments after each oxidation reaction. As shown in Figure [2], the oxidizing ability of GO–IBX was preserved up to five times of reactivaction without any significant loss of activity (>90% conversion).

As expected, GO–IBX is found to be a suitable reagent for the recyclable heterogeneous oxidation reaction under aqueous conditions due to the chemical and physical stability and easiness of handling.

In conclusion, GO–IBX was prepared successfully with high loading (0.5 mmol/g) from exfoliated GO. It was compatible to water and showed excellent alcohol oxidation performance over 90% of conversion for most of alcohol substrates. After reactivation using Oxone in water, GO–IBX was reusable up to five times without significant loss of activity. We believe GO–IBX has a great potential to be a proper solid-supported oxidizing reagent for green chemical processes.

Preparation of Graphene Oxide (GO); General Procedure

GO was prepared by the modified Hummers’ method.[20]A 500 mL round-bottom flask, equipped with an egg-shaped magnetic stirring bar, was charged with natural graphite flakes [3.0 g; SP-1, Alfa Aesar (99%; 325 mesh)] and a mixture of concentrated H₂SO₄–H₃PO₄ (9:1, 400 mL) and KMnO₄ (18.0 g). The mixture was stirred for 1 h at r.t. and additional 12 h at 50 °C. The reaction mixture was cooled down to r.t. and cold H₂O (400 mL) and 30% H₂O₂ (3 mL) was poured into the reactor. The mixture was washed by centrifugation at 4000 rpm (30 min), and the supernatant was removed. The remaining solid was further washed with deionized H₂O (200 mL), 6 N HCl (200 mL), and EtOH (200 mL) sequentially. At each washing step, the mixture was centrifuged (4000 rpm for 30 min), and the supernatant was removed. The resulting GO was dried in vacuo.

Preparation of IBA-Coupled Graphene Oxide (GO–IBA); General Procedure

To introduce amino group on GO (GO–NH₂), ethylene diamine was grafted on the surface of GO. GO suspension in DMF (0.8 mg/mL, 250 mL), DIC (250 mg, 2 mmol), HOBt (270 mg, 2 mmol), and DIPEA (700 μL, 4 mmol) were mixed in a 500 mL of round-bottomed flask with an egg-shaped magnetic stirring bar. Thereafter, 120 mg (2 mmol) of ethylene diamine was added to the solution, and the mixture was stirred for 12 h at 30 °C. The resulting solution was washed with DMF (5×) by centrifugation (4000 rpm, 20 min). The loading level of the amino group on the surface of GO was 1.2 mmol/g of GO (N analysis, 3.4%), GO–NH₂ (200 mg), DIC (790 mg, 6 mmol), and DIPEA (2.1 mL, 12 mmol) were mixed in a 500 mL round-bottomed flask with an egg-shaped magnetic stirring bar. Thereafter, IBA (1.5 g, 6 mmol) was added to the solution, and the mixture was stirred for 8 h at 30 °C. The resulting GO–IBA was washed with DMF and H₂O by centrifugation (4000 rpm, 20 min) and dried in a freeze dryer.

Oxidation of GO–IBA to GO–IBX; General Procedure

The activation of GO–IBA to GO–IBX was performed according to previous reports.[15] After GO–IBA (125 mg) was suspended with H₂O (250 mL) in a round-bottomed flask, Oxone (950 mg, 1.5 mmol), and methane sulfonic acid (100 μL, 1.5 mmol) were added and stirred for 8 h at r.t. Thereafter, GO–IBA amide was separated by ultracentrifugation and washed with DMF (5×) and H₂O (3×). The resulting black solid was dried in a freeze dryer.

Alcohol Oxidation by GO–IBX; General Procedure

The alcohol oxidation reaction was carried out in a cylinder type glass reactor (Carousel 12 Plus Reaction Station, Radleys, U.K.) with a cross-shaped magnetic stirring bar. Primary or secondary alcohol substrates (1 μmol), GO–IBX amide (2 μmol of IBX), and H₂O (2 mL) were added to the reactor and stirred for 24 h at 80 °C. To analyze the oxidation product, the final solution was diluted 5× using acetone with internal standard (biphenyl) and analyzed by GC and GC–MS.

Acknowledgment

This work was supported by the WCU (World Class University) program (R32-2010-000-10213-0) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology and was also supported by a 2013 Research ­Grant from Kangwon National University.

• References and Notes

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• References and Notes

• 1 These authors contribute equally
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  Search in Google Scholar
• 3 Karimi B, Behzadnia H, Bostina M, Vali H. Chem. Eur. J. 2012; 18: 8634
  CrossrefSearch in Google Scholar
• 4 Shang C, Liu ZP. J. Am. Chem. Soc. 2011; 133: 9938
  CrossrefPubMedSearch in Google Scholar
• 5 Ng YH, Ikeda S, Harada T, Morita Y, Matsumura M. Chem. Commun. 2008; 3181
• 6 Soule JF, Miyamura H, Kobayashi S. J. Am. Chem. Soc. 2011; 133: 18550
  CrossrefPubMedSearch in Google Scholar
• 7a Marwah P, Marwah A, Lardy HA. Green Chem. 2004; 6: 570
  CrossrefSearch in Google Scholar
• 7b Zhang JT, Wang ZT, Wang Y, Wan CF, Zheng XQ, Wang ZY. Green Chem. 2009; 11: 1973
  CrossrefSearch in Google Scholar
• 8a Duschek A, Kirsch SF. Angew. Chem. Int. Ed. 2011; 50: 1524
  CrossrefPubMedSearch in Google Scholar
• 8b Yoon HJ, Choi JW, Jang HS, Cho JK, Byun JW, Chung WJ, Lee SM, Lee YS. Synlett 2011; 165
  Thieme Connect
• 8c Bernini R, Barontini M, Crisante F, Ginnasi MC, Saladino R. Tetrahedron Lett. 2009; 50: 6519
  CrossrefSearch in Google Scholar
• 9 Stevenson PJ, Treacy AB, Nieuwenhuyzen M. J. Chem. Soc., Perkin Trans. 2 1997; 589
• 10 Reed NN, Delgado M, Hereford K, Clapham B, Janda KD. Bioorg. Med. Chem. Lett. 2002; 12: 2047
  CrossrefSearch in Google Scholar
• 11 Sorg G, Mengel A, Jung G, Rademann J. Angew. Chem. Int. Ed. 2001; 40: 4395
  CrossrefSearch in Google Scholar
• 12 Zhdankin VV, Koposov AY, Netzel BC, Yashin NV, Rempel BP, Ferguson MJ, Tykwinski RR. Angew. Chem. Int. Ed. 2003; 42: 2194
  CrossrefSearch in Google Scholar
• 13 Lei Z, Denecker C, Jegasothy S, Sherrington DC, Slater NK. H, Sutherland AJ. Tetrahedron Lett. 2003; 44: 1635
  CrossrefSearch in Google Scholar
• 14 Mulbaier M, Giannis A. Angew. Chem. Int. Ed. 2001; 40: 4393
  CrossrefSearch in Google Scholar
• 15a Stankovich S, Dikin DA, Dommett GH. B, Kohlhaas KM, Zimney EJ, Stach EA, Piner RD, Nguyen ST, Ruoff RS. Nature 2006; 442: 282
  CrossrefSearch in Google Scholar
• 15b Liu FJ, Sun J, Zhu LF, Meng XJ, Qi CZ, Xiao FS. J. Mat. Chem. 2012; 22: 5495
  CrossrefSearch in Google Scholar
• 15c Scheuermann GM, Rumi L, Steurer P, Bannwarth W, Mulhaupt R. J. Am. Chem. Soc. 2009; 131: 8262
  CrossrefSearch in Google Scholar
• 16a Dreyer DR, Jia HP, Bielawski CW. Angew. Chem. Int. Ed. 2010; 49: 6813
  PubMedSearch in Google Scholar
• 16b Huang H, Huang J, Liu YM, He HY, Cao Y, Fan KN. Green Chem. 2012; 14: 930
  CrossrefSearch in Google Scholar
• 17 Frigerio M, Santagostino M, Sputore S. J. Org. Chem. 1999; 64: 4537
  CrossrefSearch in Google Scholar
• 18 DeMunari S, Frigerio M, Santagostino M. J. Org. Chem. 1996; 61: 9272
  CrossrefSearch in Google Scholar
• 19 Uyanik M, Akakura M, Ishihara K. J. Am. Chem. Soc. 2009; 131: 251
  CrossrefPubMedSearch in Google Scholar
• 20 Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun ZZ, Slesarev A, Alemany LB, Lu W, Tour JM. ACS Nano 2010; 4: 4806
  CrossrefPubMedSearch in Google Scholar

• Supplementary Material