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Green, Catalytic Oxidation of Alcohols in Water

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Science  03 Mar 2000:
Vol. 287, Issue 5458, pp. 1636-1639
DOI: 10.1126/science.287.5458.1636

Abstract

Alcohol oxidations are typically performed with stoichiometric reagents that generate heavy-metal waste and are usually run in chlorinated solvents. A water-soluble palladium(II) bathophenanthroline complex is a stable recyclable catalyst for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic water-alcohol system. The use of water as a solvent and air as the oxidant makes the reaction interesting from both an economic and environmental point of view.

Traditionally, oxidations of alcohols are performed with stoichiometric amounts of inorganic oxidants, notably chromium(VI) reagents (1). These oxidants are not only relatively expensive, but they also generate copious amounts of heavy-metal waste. Moreover, the reactions are often performed in environmentally undesirable solvents, typically chlorinated hydrocarbons. In a constant search for cleaner (“greener”) technologies, there is a definite need for catalytic oxidations that use dioxygen (O2) or hydrogen peroxide as the stoichiometric oxidant (2). These oxidants are atom efficient (3) and produce water as the only by-product.

Although the advantages of using oxygen in alcohol oxidation are evident, reports on this particular subject are still scarce. Most reports involve the oxidation of activated benzylic and allylic alcohols (4) or use the Mukaiyama co-oxidation method (5). Only recently have a few examples involving oxidations of nonactivated alcohols with O2 been reported. Many examples of homogeneous systems make use of palladium (6), copper (7), or ruthenium compounds (8), typically in toluene as solvent. However, if these oxidations could be performed in water, they would be considerably safer, cheaper, and more environmentally friendly than many of the processes in use today (9). Moreover, when a water-soluble catalyst is used in a biphasic system, most products can be separated by simple decantation, and the catalyst solution can be recycled. In contrast, the use of an organic solvent, such as toluene, necessitates a tedious distillation and cumbersome recovery of the catalyst. Furthermore, the method is not suitable for products that have boiling points close to that of the organic solvent used (10). Despite the advantage of using water as a solvent, none of the catalyst systems mentioned here have been shown to operate in water. In fact, most reactions are performed under anhydrous conditions, which seems unpractical because, by definition, water is formed during the reaction. Development of a catalyst system that not only is stable toward water but is also completely soluble in this solvent seems highly desirable.

Supported noble metals, such as palladium or platinum on carbon, are known to catalyze the aerobic oxidation of alcohols in an aqueous medium, but the method is generally limited to water-soluble substrates, such as carbohydrates (11). Moreover, primary alcohols are oxidized to carboxylic acids, and one equivalent (1 equiv) of base is required. Herein we report an extremely effective aerobic oxidation of alcohols, both of activated and nonactivated hydroxyl groups, performed in water with a soluble catalyst. The latter is a water-soluble palladium complex of bathophenanthroline disulfonate (PhenS*) (12) (Fig. 1).

Figure 1

Aerobic oxidation of alcohols catalyzed by water-soluble PhenS*Pd(OAc)2 (0.25 to 0.5 mole percent). R, alkyl, aryl.

Most alcohols (certainly those larger than C4–OH) are only partly soluble in water, which gives a constant alcohol concentration in the aqueous phase and thus a constant reaction rate. For the smaller, more soluble alcohols such as 2-pentanol, cyclopentanol, or cyclohexanol, the initial turnover frequency is ∼100 mmol/mmol per hour (13). This rate is about one order of magnitude faster than other aerobic oxidations of nonactivated secondary alcohols reported. Because the reaction rate is largely determined by the solubility of the alcohols, the reaction rate gradually decreases for higher alcohols, but even alcohols such as 2-octanol or 2-nonanol are oxidized at appreciable rates of 20 and 14 mmol/mmol per hour, respectively. Higher rates may be achieved for these alcohols through the addition of cosolvents or compounds such as alkanesulfonates or anthraquinone-2-sulfonate, which increase the alcohol concentration in the water phase. Nonactivated secondary alcohols show remarkably high reactivity; this is unexpected because others noted that some of the substrates given in Table 1 are notoriously difficult to oxidize (7).

Table 1

Conversion of primary and secondary alcohols. (All yields are for the pure, isolated products.) Conditions were as follows: 1° alcohol and 1-phenylethanol (10 mmol), 2° alcohol (20 mmol), PhenS*Pd(OAc)2 (0.05 mmol), substrate/catalyst ratio of 200 to 400, water (50 g), NaOAc (1 mmol), pH ∼11.5, 100°C, and 30-bar air pressure. Selectivity is based on the yield determined by gas chromatography with an external standard.

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The scope of the method is illustrated (Table 1) by the wide range of both primary and secondary allylic, benzylic, and aliphatic alcohols that can be oxidized in high conversions and selectivities. For example, with cyclopentanol, a turnover number of 400 was achieved to give cyclopentanone in 90% isolated yield (13). This reaction occurred without a substantial loss of activity or selectivity and with the option to reuse the catalyst (14). In the case of primary alcohol oxidation, a substrate/catalyst ratio of only 200 was used, because primary alcohols react more slowly than secondary alcohols for different reasons (15, 16).

The secondary alcohols were all oxidized selectively, although the “activated” 1-phenylethanol and 3-penten-2-ol react relatively slowly. The allylic and benzylic primary alcohols all reacted to form their corresponding aldehydes, whereas the nonactivated 1-hexanol reacted further to form hexanoic acid. Formation of this acid decreases the solution pH, and the reaction rate decreased. Adding TEMPO (the stable free radical 2,2,6,6-tetramethylpiperidinyl-1-oxyl) stops the reaction at the aldehyde. However, the reaction is slow, presumably due to coordination of TEMPO to the metal. Thus, the conversion of primary aliphatic alcohols may be directed to form either aldehydes or carboxylic acids. The latter product is especially interesting, because formation of a carboxylic acid from a primary alcohol with the oxidants known today is usually accompanied with the formation of 1 equiv of salt as a coproduct. The presence of an ether functionality (in butyl proxitol) does not affect the catalyst. Other functional groups, such as sulfides or amines, apparently coordinate strongly to palladium, and no reaction was observed with alcohols containing these functionalities. Similarly, the double bonds in 3-penten-2-ol and 3-methyl-2-buten-1-ol presumably coordinate to palladium as in Wacker-type reactions, but these only slow the reaction down. However, Wacker-type products were not detected in these cases.

Mechanistic studies of olefin oxidations with this catalyst showed that, under neutral conditions, the catalyst precursor is a dimeric palladium species with two bridging hydroxy ligands (17). Increasing the pH values up to pH ∼10 will yield a similar dimeric species with three bridging hydroxy ligands (18), apparently without changing the rate equation. The addition of sodium acetate (NaOAc) further increases the reaction rate. The reaction may also be carried out at neutral pH at only marginally lower rates. We propose that coordination of the alcohol to the metal center splits the dimeric precursor (Fig. 2). Next, a base abstracts a proton from the coordinated alcohol to form a palladium alcoholate species that subsequently undergoes β-hydride elimination to give the alkanone, water, and a zerovalent palladium species. Oxidation of the latter with O2 gives a palladium peroxide. Reaction of this peroxide with 1 equiv of zerovalent palladium yields the starting palladium dimer. Reoxidation of Pd(0) seems to be facilitated by NaOAc, which avoids the irreversible formation of a palladium mirror (19). A minimum of 1 mmol of NaOAc is recommended to avoid this palladium black formation.

Figure 2

The catalytic cycle proposed for alcohol oxidation with the aqueous-soluble PhenS*Pd(II) catalyst. Ar, aryl; B, base.

If the reactions are carried out under pressure in an autoclave, the oxygen content of the gas phase does not seem to have any influence on the reaction, because the oxygen concentration in the aqueous phase is maintained at a high level. Therefore, catalytic oxidations could be carried out under an atmosphere of pure oxygen, of air, or of 8% oxygen in nitrogen. The latter is generally used in practice, because mixtures of 8% oxygen in an inert gas with organic compounds fall outside the explosion limits. In our case, the use of water as solvent increases the amount of inert material in the gas phase and allows for the safe use of air as the oxidant. Another benefit of the use of an autoclave is that loss of volatile substrates and/or products is avoided, which would occur on bubbling air through the solution at atmospheric pressure at 100°C.

  • * To whom correspondence should be addressed. E-mail: R.A.Sheldon{at}tnw.tudelft.nl

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