Supported Ruthenium Catalyst for the Heterogeneous Oxidation of Alcohols with Molecular Oxygen

Abstract

Round and round it goes! A supported ruthenium catalyst, easily prepared by treatment of RuCl3 with γ-Al2O3, is an efficient heterogeneous catalyst for the oxidations of alcohols with 1 atm of molecular oxygen or air without any additives (see scheme). The spent catalyst was recyclable without an appreciable loss of the catalytic activity and selectivity for the oxidation. The Ru/Al2O3 system had high catalytic activities for the oxidation of activated and non-activated alcohols with only 1 atm of O2 as shown in Table 1. Reaction selectivity was over 97 % in all cases and all primary and secondary benzylic alcohols were converted quantitatively into the corresponding benzaldehydes and ketones, respectively. Primary and secondary allylic alcohols afforded the corresponding enals or enones without intramolecular hydrogen transfer or geometrical isomerization of the double bonds. Additionally, the catalytic system efficiently oxidized non-activated alcohols to the corresponding carbonyl compounds: for the oxidation of 2-octanol, 2-octanone was produced in a 91 % yield (entry 14). Similarly, alicyclic alcohols such as cyclopentanol and cyclooctanol were selectively oxidized to the corresponding cyclic ketones (entries 15 and 16). Less reactive aliphatic primary alcohols, 1-octanol and 1-decanol, were also oxidized. However, increasing the reaction time did not improve the yields of the aldehydes because of successive oxidation to the corresponding carboxylic acids. The addition of a small amount of hydroquinone (1 equiv based on Ru) completely suppressed the over-oxidation, which afforded only aliphatic aldehydes in fairly good yields. Ru/Al2O3 catalyzed the oxidation of alcohols also containing nitrogen or sulfur atoms to the corresponding aldehydes in high yields (entries 17 and 18), while monomeric Ru complexes cannot affect catalytic oxidation of these alcohols because of the strong coordination to the metal center.10 Entry Substrate Product t [h] Conv. [%] Selectivity [%] 1 1 1 1 >99 >99 2 1 1 1 >99 >99 3 1 1 1 >99 >99 4 1 1 1 >99 >99 5 1 1 3 97 >99 6 1 1 1 >99 >99 7 1 1 1 >99 >99 8 1 1 1.5 >99 98 9[b] 1 1 6 84 >99 10 1 1 6 89 97 11[b,c] C7H15CH2OH C7H15CHO 4 87 98 12[b,c] C9H19CH2OH C9H19CHO 4 71 99 13 1 1 5 90 >99 14 1 1 2 91 >99 15[b] 1 1 8 92 >99 16[b] 1 1 6 81 >99 17 1 1 1.5 >99 >99 18[b] 1 1 2 93 >99 Among various Ru catalysts tested, Ru/Al2O3 showed the highest catalytic activity for the oxidation of benzyl alcohol at 356 K. Ru(OH)3⋅n H2O gave benzaldehyde in a 29 % yield (TON=12) after 20 h and other Ru compounds of RuCl3⋅n H2O, [RuCl2(PPh3)3]2, [RuCl2(p-cymene)], and RuO2 showed no catalytic activity. It was confirmed by induced coupled plasma techniques (ICP) that the Ru content of the used Ru/Al2O3 catalyst was the same as that of the fresh catalyst and that no Ru was in the filtrate. In addition, the oxidation of benzyl alcohol was completely stopped by the removal of Ru/Al2O3 from the reaction solution. These results indicate that any Ru species that leached into the reaction solution is not an active homogeneous catalyst and that the observed catalysis is truly heterogeneous in nature.22 The Ru/Al2O3 catalyst system is reusable (Figure 1). When the oxidation of benzyl alcohol was repeated seven times with the same sample of catalyst, benzaldehyde was quantitatively produced at the same rate as that of the first run. Recycling of Ru/Al2O3 catalyst for the oxidation of benzyl alcohol. Reaction conditions were as follows: benzyl alcohol (1 mmol), Ru/Al2O3 (Ru: 2.5 mol %), PhCF3 (1.5 mL), 356 K, under 1 atm of O2, 1 h. The conversion and selectivity were not changed by the addition of a free-radical trap, for example, 2,6-di-tert-butyl-4-methylphenol, and hydroquinone. The skeletal isomerization of a cyclopropyl ring used as a radical clock did not proceed at all. This evidence shows that a radical is not involved in the oxidation reaction. Primary alcohols were selectively oxidized even in the presence of secondary ones, an equimolar mixture of benzyl alcohol and 1-phenylethanol gave a mixture of benzaldehyde and acetophenone in 90 % and 24 % yields, respectively. Similarly, competitive oxidations of 1-octanol and 4-octanol showed 84 and 8 % conversions, respectively. These faster oxidations of primary alcohols show that Ru–alcoholate II (see scheme 1) is formed as an intermediate, since the formation of metal–alcoholate species is well known for such selective oxidation of primary hydroxy groups.23, 24 Competitive oxidations of para-substituted benzyl alcohols gave a Hammett ρ value of −0.461 (r2=0.99; Figure 2), which suggests that hydride abstraction from II to form the carbocation-type intermediate is involved in the oxidation path. Hammett plots for the aerobic oxidation of p-substituted benzyl alcohols by using Ru/Al2O3 catalyst. Reaction conditions were as follows: alcohol (1 mmol), Ru/Al2O3 (1 mol%), PhCF3 (1.5 mL), 333K, under 1 atm O2.1. The rate did not change with a decrease in oxygen partial pressure (P) from 1.0 to 0.2 atm (air). Kinetic data could well be expressed by the Michaelis-Menten type equation.27 The kinetic isotope effects (kH/kD) were 5.0 and 2.4 for the intramolecular competitive oxidation of α-deuterio-p-methylbenzyl alcohol and for the intermolecular competitive oxidation of benzyl alcohol and benzyl [D7]alcohol (C6D5CD2OH) at 333K, respectively. The apparent activation energies for steps 1 (alcoholate formation) and 2 (β elimination) were 40.0 and 54.9 kJ mol−1, respectively. On the basis of these facts, step 2 is rate-determining. In summary, Ru/Al2O3 can act as an efficient heterogeneous catalyst for the oxidation of alcohols with 1 atm of molecular oxygen or air, without additives. The high activity of, and ease with which, the catalyst can be recovered and reused raise the prospect of using this type of simple supported catalyst for organic syntheses and the industrial oxidation of alcohols. The catalyst would be considerably cheaper than organometallic and inorganic compounds. The Ru/Al2O3 catalyst was prepared by the modification of a literature preparation of Ru(OH)3⋅n H2O:28 γ-Al2O3 (2.0 g, JRC-ALO-4, BET surface area: 174 m2 g−1) was vigorously stirred with an aqueous solution of RuCl3 (60 mL, 8.3×10−3 M) for 15 min at room temperature. The initially brown aqueous phase became lighter, and the alumina powder turned dark gray. The solid was separated by filtration and washed with a large amount of water, then dried in vacuo. This solid was added to deionized water (30 mL), the pH of the solution was adjusted to 13.2 by the addition of an aqueous solution of NaOH (1.0 M) and the resulting slurry was stirred for 24 h. During this period, the powder changed from dark gray to dark green. The solid was separated by filtration and copiously washed with water, then dried in vacuo to afford 2.1 g of Ru/Al2O3 as a dark green powder. The contents of ruthenium and chloride was 1.4 and 0 wt %, respectively, and the BET surface area was 182 m2 g−1. The ESR spectrum had a signal at g=2.11, which is assigned to Ru3+ with low-spin d5 electron configuration.29 The XRD pattern was the same as that of the γ-Al2O3 support and no signals arising from ruthenium metal or dioxide were observed. Particles of ruthenium metal or dioxide were not detected by TEM. The IR spectrum showed a very broad ν(OH) band in the range of 2900–3700 cm−1. These facts suggest that ruthenium(III) hydroxide is highly dispersed on γ-Al2O3. Thus, the Ru/Al2O3 catalyst can be easily prepared in a quantitative yield and the content of Ru can be controlled. Oxidations of alcohols were typically carried out as follows. A suspension of Ru/Al2O3 (0.11 g, Ru: 2.5 mol %) in trifluorotoluene (1.5 mL) was stirred for 5 min. Benzyl alcohol (0.108 g, 1 mmol) was added to the reaction mixture and the suspension was purged with molecular oxygen. The resulting mixture was heated at 356 K for 1 h, and benzaldehyde was produced in >99 % yield determined by GC. After the reaction, the catalyst was separated by filtration (or centrifugation) and was further washed with acetone. The filtrate was evaporated in vacuo, and the crude product was purified by column chlomatography on silica gel, which gave 0.101 g of pure benzaldehyde (95 % yield). The separated Ru/Al2O3 was washed with an aqueous solution of NaOH (1.0 M) and water (30 mL), and dried in vacuo before recycling.