Palladium catalysis aerobic oxidation

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

Wu, G. Wang, X. Guan, N. Li, L., Palladium on Graphene as Efficient Catalyst for Solvent-Free Aerobic Oxidation of Aromatic Alcohols Role of Graphene Support. Applied Catalysis B Environmental 2013,136-137,177-185. 

Following the development of aerobic conditions, N-heterocychc carbenes (NHCs) were found to be equally efficient under conditions of homogeneous palladium catalysis [33]. Although initially their apphcation had focused on aerobic alcohol oxidation, later examples include Wacker oxidation of styrenes and 2-aUyl phenol cych2ation.[34]... 

The catalytic aerobic oxidation of alcohols has a long history dating back to Dbbereiner s observation, in 1820, that ethanol is oxidized to acetic acid over platinum black. Indeed, this preceded the coining of the term catalysis, by Berzelius in 1835 The catalytic effect of platinum on the aerobic oxidation of cinnamyl alcohol was described by Strecker in 1855" and in the period 1912-1921 Wieland showed that finely divided palladium catalyzes the aerobic oxidation of primary alcohols to aldehydes in aqueous... 

Scheme 3 Triple-catalysis strategy for aerobic palladium-catalyzed oxidation reactions...

Goldberg H, Kaminker I, Goldfarb D et al (2009) Oxidation of carbon monoxide cocatalyzed by palladium(O) and the H5PV2M010O40 polyoxometalate probed by electron paramagnetic resonance and aerobic catalysis. Inorg Chem 48 7947-7952... 

Thus the first part of the book (Chapters 1 to 9) deals with NP catalysis, emphasizing the key role of NP supports the second part (Chapters 10 to 12) concerns specific metals (namely Pd, Ru, Ir and Au), and the last part (Chapters 13 to 18) focuses on specific substrates of particular interest for organic chemistry, hydrocarbon reforming and environmental aspects. Among the metals, Pd and Au are the most effective catalysts. Palladium is the most efficient catalyst for carbon-carbon bond formation thus, besides Chapter 10 that is devoted to PdNPs, their catalytic properties also spread over the first part of the book. Gold is the most efficient NP catalyst for a variety of aerobic (thus low-cost) oxidation reactions, and AuNP catalysis is covered in four chapters at the end of the second part and beginning of the third part. Each chapter is introduced in more detail in Chapter 1. 

Hydroxyapatite (CajQ(P04)g(0H)2) has also attracted considerable interest as a catalyst support. In these materials, wherein Ca sites are surrounded by P04 tetrahedra, the introduction of transition metal cations such as Pd into the apatite framework can generate stable monomeric phosphate complexes that are efficient for aerobic selox catalysis [99]. Carbon-derived supports have also been utihzed for this chemistry, and are particularly interesting because of the ease of precious metal recovery from spent catalysts simply by combustion of the support. Carbon nanotubes (CNTs) have received considerable attention in this latter regard because of their superior gas adsorption capacity. Palladium nanoparticles anchored on multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) show better selectivity and activity for aerobic selox of benzyl and cinnamyl alcohols [100, 101] compared to activated carbon. Interestingly, Pd supported on MWCNTs showed higher selectivity toward benzaldehyde, whereas activated carbon was found to be a better support in cinnamyl alcohol oxidation. Functionalized polyethylene glycol (PEG) has also been employed successfully as a water-soluble, low-cost, recoverable, non-toxic, and non-volatile support with which to anchor nanoparticulate Pd for selox catalysis of benzyl/cinnamyl alcohols and 2-octanol [102-104]. 

These ligand effects were the key in the development of suitable palladium oxidation catalysis under aerobic conditions [31, 6h]. These reactions can now proceed with molecular dioxygen as the sole reoxidant under homogeneous conditions (Figure 16.3). Catalytic reactions of this type have been characterized as... 

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