Aerobic oxidation of benzyl alcohol

Figure 5.5 Example of drastic activity enhancements. Aerobic oxidation of benzyl alcohol to benzaldehyde in scC02 over TPAP entrapped in aged ( , ) and fresh ( ) 75% methyl-modified silica matrix. (Reproduced from Adv. Fund. Mater., with permission.)...

Figure 32. Proposed mechanism for the aerobic oxidation of benzyl alcohol by Complex E. [Adapted from (212).]...

The use of a novel N3O2 ligand set provided an unsaturated five-coordinated Cu complex (19), capable of performing the aerobic oxidation of benzyl alcohol to benzaldehyde in the presence of Cu(CF3S03)2 as initial oxidant, with 44 turnovers in 24 h [163]. The initial step produces the active bis-phenoxy radical species 20 (Eq. 13). Under exclusion of air, greatly reduced catalytic activity was observed, indicative of reoxidation of the active catalyst by O2, although the possibihty for H2O2 (which is a likely side-product) to act as oxidant could not be ruled out. 

Our own work in the area of aerobic oxidations was inspired by the exquisite research performed on the structure and reactivity of the binuclear copper proteins (7), hemocyanin and tyrosinase, and by the seminal contribution of Riviere and Jallabert (8). These two authors have shown that the simple copper complex CuCl - Phen (Phen = 1,10-phenanthroline) promoted the aerobic oxidation of benzylic alcohols to the corresponding aromatic aldehydes and ketones (Fig. 2). 

TABLE 8 Results of the aerobic oxidation of benzyl alcohol in supercritical C02 at 150 bar and 353 K (Grunwaldt and Baiker, 2005). 

In summary, we have developed a recyclable heterogeneous catalyst for the bleach oxidation of alcohols and polyols. In contrast to previously reported systems, neither a chlorinated hydrocarbon solvent nor a bromide cocatalyst is necessary to achieve good activity. Besides bleach-oxidation, PIPO is also effective in the CuCl/nitroxyl catalysed aerobic oxidation of benzyl alcohol. A further advantage of our system is that PIPO is readily prepared from inexpensive and commercially available raw materials. We believe that it will find wide application in organic synthesis. 

For example, PEG-200 and PEG-400 (the number refers to the average molecular weight) were used as solvents for the aerobic oxidation of benzylic alcohols catalyzed by the polyoxometalate, H5PV2Mo10O40 [8]. Combination of the same polyoxometalate with Pd(II) was used to catalyze the Wacker oxidation of propyl-... 

Metal nanoparticles embedded in thermosensitive core-shell microgel particles can also work efficiently as catalyst for this reaction. Figure 13 shows the oxidation reaction of benzyl alcohol to benzaldehyde in aqueous media by using microgel-metal nanocomposite particles as catalyst. All reactions were carried out at room temperature using aerobic conditions. It is worth noting that the reaction conditions are very mild and no phase transfer catalyst is needed. It has been found that microgel-metal nanocomposites efficiently catalyze the aerobic oxidation of benzyl alcohol at room temperature. No byproducts have been detected by GC after the reaction, and water is the only product formed besides the aldehyde. 

Scheme 9.3 Recycling and reuse of copper catalyst and solvent in the aerobic oxidation of benzyl alcohol.

Industrially performed catalytic oxidation reactions often suffer from drawbacks such as poor conversion and selectivity due to overoxidation, corrosive reaction media, lack of solvent and catalyst recycling, and negative environmental impact due to evaporation of the solvents. In order to provide a methodology that addresses these problems, ionic liquids have been investigated as reaction media. For example, the aerobic oxidation of benzyl alcohol and alkylbenzene to benzaldehyde and benzoic acids was performed in l-butyl-2,3-dimethylimidazolium tetrafluoroborate ([C4dmim][BF ]) using palladium and cobalt complexes respectively [34, 35]. 

They used this catalyst for the aerobic oxidation of benzyl alcohol to benzalde-hyde in scCO as solvent. This conversion is time dependent, and most of the alcohol was converted to aldehyde after 24 h. The pressure also influences the reaction. The increase in the pressure of CO was found to accelerate the reaction rate as well as the yield. The catalyst could be reused directly after in situ extraction of the products using scCO, and it remains active after being reused for four times. 

Xie Y, Zhang Z, Hu S, Song J, Li W, Han B (2008) Aerobic oxidation of benzyl alcohol in supercritical CO catalyzed by perruthenate immobilized on polymer supported ionic liquid. Green Chem 10 278-282... 

In a subsequent comparison of various mediators, in the laccase-catalyzed aerobic oxidation of benzylic alcohols (Table II), TEMPO proved to be the most effective 47,50). 

There have been several reports on the use of catalytic NO sources for aerobic alcohol oxidation in the absence of other redox mediators. In 1981, Tovrog disclosed that catalytic quantities of Lewis acids and Co(III)—nitro complexes could be used for the aerobic oxidation of benzyl alcohol and cyclo-heptanol in moderate and low yields, respectively (Scheme 15.1, top) [10]. Under anaerobic conditions with stoichiometric pyCo(TPP)N02 (py= pyridine,... 

In another approach, Beletskaya reported the aerobic oxidation of benzyl alcohol to benzaldehyde using catalytic NH NOj in trifluoroacetic acid (TFA) (Scheme 15.2a) [11]. This was based on previous reports on the stoichiometric NO -mediated oxidation of benzyl alcohol in strongly acidic solutions [12]. In 1994, Levina disclosed a system for aerobic aliphatic alcohol oxidation using catalytic NaNOj in perchloric acid (Scheme 15.2b) [13]. 

Scheme 15.1 (Top) LCo(lll)N02- and Lewis acid-catalyzed aerobic oxidation of benzyl alcohol and cycloheptanol. (Bottom) Proposed mechanism for substrate oxidation.

Unsupported Au nanoclusters (or those contacting an inert support material such as BN) exhibit strong size-dependent reactivity, with optimal oxidation performance typically reached < 5 nm diameter [59], For example, colloidal gold stabilized by polyvinylpyrrolidone (PVP) shows pronounced size effects in the aerobic oxidation of benzylic alcohols in water under ambient conditions [60]. Figure 2.1 illustrates this phenomenon for p-hydroxybenzyl alcohol oxidation, wherein 1.3 nm Au clusters achieve 80 % conversion, whereas 9.5 nm clusters are catalytically dead. Differential oxygen adsorption onto these gold clusters is believed to play a crucial role in regulating reactivity. 

Table 7.2 Catalytic performance in aerobic oxidation of benzyl alcohol on Pd catalysts at 383 K [35].

Elucidating the Reaction Mechanism of Aerobic Oxidation of Benzyl Alcohol 381... 

With this brief introduction to this technique, we now look into some examples where these in situ spectroscopic methods have been used effectively to elucidate the mechanism of heterogeneously catalyzed selective aerobic oxidation of benzyl alcohol to benzaldehyde, and in the identification of the active sites of supported metal catalysts. 

This substantiates the positive role of O2 in promoting benzaldehye and benzoic acid formation. From the above-mentioned two examples, it is apparent that there are many parallel reactions in the aerobic oxidation of benzyl alcohol, besides the direct dehydrogenation of benzyl alcohol to benzaldehyde, which result in the undesired by-products. On the basis of these studies and other literature evidence, a network of reactions occurring during the aerobic oxidation of benzyl alcohol over Pd/AljOj catalyst is presented in Scheme 12.1. After elucidating the mechanisms of reactions that lead to by-products, the next step in catalyst development is the complete elimination or at least suppression of these byproducts. It is logical to assume that all these reactions may not have the same active sites, and it is important to identify the different active sites for different reactions. 

Determination of the Active Sites in Aerobic Oxidation of Benzyl Alcohol I 385... 

The active site responsible for the aerobic oxidation of alcohols over Pd/AljO, catalysts has long been debated [96-lOOj. Many reports claim that the active site for this catalyst material is the metallic palladium based on electrochemical studies of these catalysts [100, 101]. On the contrary, there are reports that claim that palladium oxide is the active site for the oxidation reaction and the metalhc palladium has a lesser catalytic activity [96,97). In this section, we present examples on how in situ XAS combined with other analytical techniques such as ATR-IR, DRIFTS, and mass spectroscopic methods have been used to study the nature of the actual active site for the supported palladium catalysts for the selective aerobic oxidation of benzylic alcohols. Initially, we present examples that claim that palladium in its metallic state is the active site for this selective aerobic oxidation, followed by some recent examples where researchers have reported that ojddic palladium is the active site for this reaction. Examples where in situ spectroscopic methods have been utilized to arrive at the conclusion are presented here. For this purpose, a spectroscopic reaction cell, acting as a continuous flow reactor, has been equipped with X-ray transparent windows and then charged with the catalyst material. A liquid pump is used to feed the reactants and solvent mixture into the reaction cell, which can be heated by an oven. The reaction was monitored by a transmission flow-through IR cell. A detailed description of the experimental setup and procedure can be found elsewhere [100]. Figure 12.10 shows the obtained XAS results as well as the online product analysis by FTIR for a Pd/AljOj catalyst during the aerobic oxidation of benzyl alcohol. 

From the earlier discussions on the mechanism of benzyl alcohol oxidation, it is clear that there are many different reactions that are active in parallel when using supported palladium catalysts [94]. Examples from the previous sections suggested that metallic palladium is the active site for the aerobic oxidation of benzyl alcohol, but generally the surface of these powder catalysts comprises... 

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