SiO₂-supported RuCl₃/3-(dichloroiodo)benzoic acid: green catalytic system for the oxidation of alcohols and sulfides in water

Abstract

A green, recyclable and efficient catalytic oxidative system based on SiO₂-supported RuCl₃ and 3-(dichloroiodo)benzoic acid for the oxidation of alcohols and sulfides in water is developed. This catalytic oxidative system effects clean and efficient oxidation of a wide range of alcohols to the corresponding aldehydes and ketones, or sulfides to sulfoxides in high conversions with excellent chemoselectivity, under mild conditions and with an easy-work-up procedure. Furthermore, the SiO₂–RuCl₃ catalyst can be recovered by simple filtration and recycled in up to six consecutive runs without significant loss of activity. The reduced form of 3-(dichloroiodo)benzoic acid, 3-iodobenzoic acid, can be easily separated from reaction mixtures and converted back to 3-(dichloroiodo)benzoic acid by treatment with bleach and aqueous HCl in about 90% overall yield.

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The selective oxidation of alcohols and sulfides into the corresponding carbonyl compounds or sulfoxides is one of the most important processes for the production of fine and speciality chemicals and could be of interest to many scientists.^1,2 However, many traditional methods rely upon environmentally damaging oxidants, or harmful organic solvents,^3–5 or require vigorous reaction conditions.^6,7 This represents a major drawback from the green chemistry viewpoint.⁸ Thus, there is increasing demand for oxidation processes that are catalytic and use green, economic and efficient oxidants with special emphasis on benign solvents.^9–11

As highly efficient oxygen transfer agents, hypervalent iodine compounds have been widely used in organic synthesis for various synthetically useful oxidative transformations^12–28 with the advantages of easy handling, mild reaction conditions and low toxicity. However, their low atom economy remains a major drawback, as stoichiometric amounts of aryliodides or similar waste products are produced, which complicates isolation and purification of the products.^29–30 To overcome these disadvantages, recyclable and reusable hypervalent iodine reagents have been developed in recent years, involving both polymeric^31–42 and molecular^43–50 species.

The other approach is to combine a catalyst and a hypervalent iodine reagent as an efficient catalytic oxidation system. Homogeneous ruthenium catalysts are of particular interest for the oxidation of alcohols and other substrates using hypervalent iodine reagents as oxidants. Mueller and Godoy⁵¹ reported in 1981 that several ruthenium complexes can catalyze the oxidation of alcohols to carbonyl compounds and carboxylic acids using excess iodosylbenzene as the oxidant in dichloromethane solution at room temperature. Our group has also reported a highly efficient RuCl₃-catalyzed selective oxidation of alcohols to carbonyl compounds by (diacetoxyiodo)benzene (DIB)⁵² in aqueous acetonitrile. However, these reactions still occur in harmful organic solvents, while the use of water as a reaction solvent has attracted great attention and has become an active area of research in green chemistry.^53–55 Moreover, homogeneously catalyzed organic reactions commonly suffer from disadvantages, such as difficult separation of the product and recovery of the catalyst, low regioselectivity and long reaction times, etc. These problems can be addressed by the immobilization of homogeneous catalysts onto solid supports. Recently, silica-supported catalytic metallic species have been successfully employed in aqueous media,^56–58 since silica displays many advantageous properties—excellent stability (both chemical and thermal), high surface area, good accessibility, and organic groups can be robustly anchored to the surface to provide catalytic centers.^59–62 Furthermore, water has been found to be a promising medium for heterogeneous catalysis. Heterogeneous catalysts possess increased advantages over their homogeneous counterparts, however, a heterogeneous catalytic oxidative system, such as a silica-supported ruthenium catalyst, is still unknown.

Therefore, as a part of our studies^33–47 towards the development of practical and environmentally benign oxidation reactions, we herein report the design and synthesis of a recyclable SiO₂-supported RuCl₃ (SiO₂–RuCl₃) catalyst (Scheme 1) and develop a mild, simple, cost-effective and green procedure for the catalytic oxidation of alcohols and sulfides using the recyclable SiO₂–RuCl₃ catalyst and 3-(dichloroiodo)benzoic acid as a catalytic oxidative system in water under mild conditions (Scheme 2). 3-(Dichloroiodo)benzoic acid is a readily available and stable hypervalent iodine oxidant that can be easily regenerated and reused.^47,63

Our approach to the preparation of SiO₂-supported RuCl₃ (SiO₂–RuCl₃, 3) consists of building a suitable ligand structure 2 on the surface of commercial aminopropyl silica (AMPS, 1), followed by complexation with RuCl₃ (Scheme 1). The synthesized SiO₂–RuCl₃ was characterized by FT-IR and elemental analysis. The amount of ruthenium loaded on the surface of the silica gel was determined by elemental analysis (loading 0.047 mmol g⁻¹ based on Cl analysis).

Scheme 1 Synthesis of SiO₂-supported RuCl₃.

Various alcohols 4 were smoothly oxidized into corresponding carbonyl compounds 8 using 1.5 equiv. 3-(dichloroiodo)benzoic acid 6 as the oxidant in the presence of 0.23 mol% of SiO₂–RuCl₃3 in water at room temperature (Scheme 2). Conversions were measured by GC-MS with a prior column calibration using authentic samples of reactants and products. The reaction products were isolated by removal of the solid resin followed by aqueous work-up of the organic solution; products 8 were identified by direct comparison of the retention times and MS data with those obtained for authentic samples or by ¹H NMR. In the oxidation protocol (Scheme 2), the by-product, 3-iodobenzoic acid 7 was conveniently separated from the organic solution by treatment with aqueous NaHCO₃;⁴⁷ it can be recovered by filtration after acidification of the aqueous solution and converted to reagent 6 by reaction with commercial bleach and HCl.⁶³ The SiO₂-supported catalyst 3 was easily separated by filtration and directly reused several times without loss of its activity and selectivity.

Scheme 2 SiO₂–RuCl₃ catalyzed oxidation of organic substrates 4 and 5 using 3-(dichloroiodo)benzoic acid 6 as a recyclable oxidant.

The results of the SiO₂–RuCl₃ catalyzed oxidation of alcohols are summarized in Table 1. In most cases, the combination of SiO₂–RuCl₃ and 3-(dichloroiodo)benzoic acid resulted in the efficient oxidation and high conversion of alcohols. Primary benzylic alcohols were selectively oxidized to the corresponding aldehydes with nearly 100% conversion and in excellent yields (90–99%) after 2–3 h (Table 1, entries 1–3) or 12 h (entry 4). Secondary benzylic alcohols and alicyclic alcohols were converted to the respective ketones (entries 5–9). Simple primary alcohols were less reactive, e.g., phenyl acetaldehyde was obtained in 17% (entry 10), while the reaction of 1-octanol did not show any measurable conversion even after 24 h (entry 11). The formation of undesired side-products (e.g., carboxylic acids) was not observed. The oxidation of a glycol, 1-phenyl-1,2-ethanediol (entry 12), resulted in a partial C–C bond cleavage and afforded a mixture of benzaldehyde (74% yield) and 2-hydroxy-1-phenylethan-1-one (26% yield).

Table 1 SiO₂–RuCl₃ catalyzed oxidation of organic substrates using 3-(dichloroiodo)benzoic acid 6^a

Entry	Substrate	Product	Time/h	Conversion (%)^b	Yield (%)^c
a General oxidation procedure: to a suspension of alcohol 4 or sulfide 5 (0.2 mmol) and 3-(dichloroiodo)benzoic acid 6 (96 mg, 0.3 mmol) in 2 mL of water, SiO2–RuCl33 (10 mg, 0.23 mol%) was added whilst stirring at room temperature. The reaction mixture was stirred until the complete consumption of starting material 4 or 5 (monitored by TLC and GC-MS), then 2 mL of saturated aqueous NaHCO3 was added and the mixture stirred for 15–30 min; the solid (SiO2–RuCl3) was filtered, washed with EtOAc (1 mL × 3) and collected for the next run. The filtrate was extracted with EtOAc (3 mL × 3) and dried over Na2SO4. The aqueous layer was acidified with 10% HCl to pH = 3 and the 3-iodobenzoic acid precipitated, it was then filtered and collected for regenerating 3-(dichloroiodo)benzoic acid. The organic layer was concentrated to afford the respective product of oxidation 8 or 9. b Based on GC-MS analysis. c Yield of isolated product.
1			0.75	100	99
2			2.5	100	95
3			2.5	100	99
4			12	>99	93
5			2	100	99
6			3.5	>99	96
7			2.5	100	99
8			5	100	99
9			2.5	100	86
10			18	17	17
11	n-C7H15CH2OH	n-C7H15CHO	24	0	-
12			3	>99	74
					26
13			4.5	100	99
14			1.5	100	98
15			0.75	100	99

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The SiO₂–RuCl₃/3-(dichloroiodo)benzoic acid catalytic system can also be used for the oxidation of sulfides 5, as illustrated by the conversion of diaryl, and aryl alkyl sulfides to sulfoxides in excellent yields (entries 14 and 15). Interestingly, the SiO₂–RuCl₃/3-(dichloroiodo)benzoic acid system can oxidize indane to indanone in good yield after 4.5 h (entry 13).

A comparison of reactivity of the SiO₂–RuCl₃/3-(dichloroiodo)benzoic acid catalytic oxidative system with other oxidative systems based on RuCl₃ or oxidants in the absence of catalyst is summarized in Table 2. In particular, a comparison of the catalytic activity of the SiO₂-supported catalyst 3 with the common catalyst RuCl₃ showed a generally higher activity of catalyst 3. For example, the reaction of primary benzylic alcohols in the presence of 0.23 mol% RuCl₃ (Table 2, entry 2) required on average twice as long a time to reach 99% conversion of the alcohol to the respective aldehyde. In the absence of any ruthenium catalyst, the similar oxidation was extremely slow (Table 2, entry 1).

Table 2 Comparison of the reactivity of the SiO₂–RuCl₃/3-(dichloroiodo)benzoic acid catalytic oxidative system with other systems^a

Entry	Substrate	Catalyst	Oxidant (equiv.)	Product	Time/h^b	Conversion (%)^c	Yield (%)^d
a Reactions in entries 1–8 use 0.23 mol% catalyst or no catalyst unless otherwise noted. b All reactions of organic substrates (0.2 mmol) were performed at room temperature in H2O (2 mL). c Based on GC-MS analysis. d Isolated yield. e Isolated PhSO2Me f Using a SiO2–RuCl3/3-iodobenzoic acid tandem catalytic system composed of 0.1 equiv. 3-iodobenzoic acid and 0.23 mol% SiO2–RuCl3.
1		None	3-(Dichloroiodo)benzoic acid (1.5)		12	9	8
2		RuCl3	3-(Dichloroiodo)benzoic acid (1.5)		5	99	95
3		None	NaOCl (3)		24	< 2	0
4		SiO2–RuCl3	NaOCl (3)		0.75	100	84
5		None	NaOCl (3)		24	0	0
6		SiO2–RuCl3	NaOCl (3)		2	100	43
7		None	NaOCl (3)		17	100	90^e
8		SiO2–RuCl3	NaOCl (3)		1.5	100	94^e
9		SiO2–RuCl3/3-iodobenzoic acid	NaOCl (3)		0.75	100	39^f
10		SiO2–RuCl3/3-iodobenzoic acid	NaOCl (3)		2	39	27^f
11		SiO2–RuCl3/3-iodobenzoic acid	NaOCl (3)		1.5	100	80^e^,^f

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Similar to other oxidation procedures for alcohols and sulfides, organic solvents and/or bases,^64–69 even in the presence of a ruthenium complex as a catalyst,⁶⁹ are necessary when sodium hypochlorite (NaOCl) is employed as an oxidant. In the absence of any catalyst in water under mild conditions, the oxidative reactions of alcohols did not occur or did not show any measurable conversion after 24 h (Table 2, entries 3 and 5). Moreover, yields of target product in the presence of catalyst 3 and NaOCl (Table 2, entries 4 and 6) were much lower than those obtained using 3-(dichloroiodo)benzoic acid as an oxidant (Table 1, entries 1 and 5). Interestingly, thioanisole (Table 2, entries 7, 8 and 11) was converted to phenyl methyl sulfone using NaOCl as an oxidant with high yields in the absence of any catalyst or in the presence of catalyst 3 in water; however, catalyst 3 dramatically accelerated the reaction under milder conditions. In contrast, thioanisole gave exclusively phenyl methyl sulfoxide using 3-(dichloroiodo)benzoic acid as an oxidant (Table 1, entry 14), which indicated that 3-(dichloroiodo)benzoic acid showed excellent chemoselectivity.

A one-pot procedure for the oxidation of alcohols and sulfides based on the SiO₂–RuCl₃/3-iodobenzoic acid tandem catalytic system is especially attractive since catalyst 3 shows high catalytic activity and hypervalent iodine reagents in stoichiometric amounts are relatively expensive. We have carried out the oxidative reactions with catalytic amounts of both catalyst 3 and 3-iodobenzoic acid and using sodium hypochlorite as the stoichiometric oxidant (Table 2, entries 9–11). A one-pot procedure showed noticeably lower yields in the oxidation of alcohols and sulfide compared to the procedure employing the recyclable SiO₂–RuCl₃/3-(dichloroiodo)benzoic acid catalytic oxidative system in water (Table 1, entries 1, 5 and 14).

When using a heterogeneous catalyst, the important point is the deactivation and recyclability of the catalyst. To test this, a series of six consecutive runs in the case of entry 1 (Table 1) were carried out. After the first alcohol oxidation, the catalyst, SiO₂–RuCl₃, was separated from the reaction mixture by filtration, thoroughly washed with water, and then reused as the catalyst for the next run under the same conditions. As shown in Fig. 1, the benzaldehyde yields remained essentially constant for the six successive cycles, which demonstrates that there is no significant change in the activity of the catalyst, reflecting the high stability and reusability of the SiO₂–RuCl₃ catalyst. Moreover, it was experimentally confirmed that no Ru was present in the filtrate.

Fig. 1 Recyclability of the SiO₂–RuCl₃ catalyst for the oxidation of benzyl alcohol in water at room temperature (0.2 mmol substrate, 0.23 mol% SiO₂–RuCl₃ catalyst, 0.3 mmol 3-(dichloroiodo)benzoic acid, and 2 mL H₂O).

The reduced form of 3-(dichloroiodo)benzoic acid, 3-iodobenzoic acid, can be easily separated from the reaction mixtures and converted back to 3-(dichloroiodo)benzoic acid by treatment with bleach and aqueous HCl in about 90% overall yield. Thus, the recovery and recyclability of the catalyst, as well as the oxidant, make the process even more cost-effective.

In summary, we have developed a mild, simple, cost-effective and green procedure for the oxidation of alcohols and sulfides using a recyclable SiO₂–RuCl₃/3-(dichloroiodo)benzoic acid catalytic oxidative system in water under mild conditions. Moreover, the mild reaction conditions, high yields of products, ease of work-up, clean procedure, and excellent chemoselectivity make this method potentially attractive for the oxidation of organic compounds in the industrial setting.

Experimental

Preparation of SiO ₂ –RuCl₃: aminopropyl silica (AMPS, 5.0 g, 5 mmol) was added to a solution of dimethylacetamide (6.65 g, 50 mmol) in methanol (50 mL) and the reaction mixture was refluxed for 3 h. The mixture was cooled to room temperature, then RuCl₃ (0.41 g, 2 mmol) was added, and the resulting mixture was refluxed with stirring for 3 days. The solid product was filtered, washed with methanol, water and Et₂O until the washings became colorless. The catalyst, isolated as a dark yellow solid, was dried under vacuum for 5 h before use. Elemental analysis: C, 8.03%; H, 1.64%; N, 1.83%; Cl, 0.50%. The loading of Ru was 0.047 mmol g⁻¹ based on Cl analysis. FT-IR (KBr): 3432, 2950, 1648, 1400, 1098, 800, 564, and 470 cm⁻¹.

Acknowledgements

Jiang-Min Chen thanks the financial Foundation of Jiaxing University for supporting his visit to the University of Minnesota Duluth. This work was supported by a research grant from the National Science Foundation (CHE-1009038).

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