Palladium-Catalyzed Aerobic Alcohol Oxidation

A number of metal macrocycles were tested in the aerobic palladium-catalyzed oxidation of olefins and alcohols [3,4],... 

A remarkable number of palladium-catalyzed aerobic oxidation reactions of alcohols have been reported to date [15]. Unfortunately, although some progress has been made with heterogeneous Pd catalysts, such as Pd on activated carbon [16], Pd on pumice [17], Pd-hydrotalcite [18], Pd on Ti02 [19] and Pd/SBA-15 [20], most of these systems suffer from low catalytic activities and a limited substrate scope. 

Palladium-catalyzed aerobic oxidation of alcohols to aldehydes and ketones have been studied extensively in recent years, and a number of effective catalysts have been developed (Chart 2). This work has been the subject of several recent reviews [21,36-38,56,64-67] and will not be summarized in depth... 

Much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For a detailed overview the reader is referred to several excellent reviews [163]. The first synthetically useful system was reported in 1998, when Peterson and Larock showed that simple Pd(OAc)2 in combination with NaHC03 as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields [164, 165]. Recently, it was shown that replacing the non-green DMSO by an ionic liquid (imidazole-type) resulted in a three times higher activity of the Pd-catalyst [166]. 

Sigman MS, Jensen DR. Ligand-modulated palladium-catalyzed aerobic alcohol oxidations. Acc. Chem. Res. 2006 39 221-229. Schultz MJ, Sigman MS. Recent advances in homogeneous transition metal-catalyzed aerobic alcohol oxidations. Tetrahedron 2006 62 8227-8241. 

Ligand-Modulated Palladium-Catalyzed Aerobic Alcohol Oxidations Sigman, M.S. Jensen, D.R. Acc. Chem. Res. 2006, 39, 221. 

TABLE 2 Palladium-Catalyzed Aerobic Oxidation of Alcohols. 

Heterogeneous catalysts such as Ru-Al-Mg-hydrotaldte, Ru-Co-Al-hydrotalcite, Ru-hydroxyapatite (RuHAP) (Eq. (7.40)) [91], RU-AI2O3 [92a,b], and Ru/A10(OH) [92c] are highly efficient catalysts for aerobic oxidation of alcohols. In these oxidation reactions, the key step is postulated as the reaction of Ru-H with O2 to form Ru-OO H, in analogy to Pd-OOH, which has been shown to operate in the palladium-catalyzed Wacker-type asymmetric oxidation reaction [93]. It is noteworthy that ruthenium on carbon is simple and efficient for the oxidation of alcohols (Eq. (7.41)) [92d]. 

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... 

Palladium(II) is also capable of mediating the oxidation of alcohols via the hydridometal pathway shown in Figure 1. Blackburn and Schwarz first reported the Pd(OAc)2-NaOAc-catalyzed aerobic oxidation of alcohols in 1977. However, activities were very low, with turnover frequencies of the order of 1 h . Subsequently, much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For example, the giant palladium cluster, Pds6iphen6o(OAc)tgo , was shown to catalyze the aerobic oxidation of primary allylic alcohols to the corresponding a,0-unsaturated aldehydes (Reaction 14) . 

The paUadium(II) -catalyzed aerobic oxidation of alcohols is well documented. A general problem encountered in palladium-catalyzed aerobic oxidations is the sluggish reoxidation of Pd(0) to Pd(II), which results in the agglomeration of the Pd(0) particles to palladium black and accompanying deactivation of the catalyst. In the classical Wacker process for the oxidation of ethylene to acetaldehyde in an aqueous medium, this problem is circumvented... 

Figure 7.14 Palladium-catalyzed aerobic oxidation of alcohols in water.

Grunwaldt, J., Caravati, M. and Baiker, A. (2006). Oxidic or Metalhc Palladium Which is the Active Phase in Pd-Catalyzed Aerobic Alcohol Oxidation J. Phys. Chem. B, 110, pp. 25586-25589. 

Caspi, D., Ebner, D., Bagdanoff, J., et al. (2004). The Resolution of Important Pharmaceutical Building Blocks by Palladium-Catalyzed Aerobic Oxidation of Secondary Alcohols, A fv. Syrah. Catal., 346, pp. 185-189. 

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. A critical assessment of the use of palladium catalysts in the aerobic oxidation of alcohols has concluded that Pd(OAc)2-Et3N is the most versatile and convenient catalyst system and that this often functions under especially mild conditions.There have been many other recent advances in this field and such that there is now a wealth of methods available for effecting the palladium-catalyzed oxidation of alcohols. A procedure using pyridine under an oxygen atmosphere has been shown to convert benzylic and aliphatic alcohols into the corresponding aldehydes or ketones. The yields of product are frequently over 90%. Replacing pyridine with (—)-sparteine in such processes allows for the oxidative kinetic resolution of chiral secondary alcohols. 

Sigman MS, Jensen DR. Ligand-modulated palladium-catalyzed aerobic alcohol oxidations. Acc Chem Res. 2006 39 221-229. 

Chung K, Banik SM, De Crisci AG. Chemoselective Pd-catalyzed oxidation of polyols synthetic scope and mechanistic studies. J Am Chem Soc. 2013 135 7593-7602. Ebner DC, Bagdanoff JT, Ferreira EM. The palladium-catalyzed aerobic kinetic resolution of secondary alcohols reaction development, scope, and applications. Chem Eur... 

Parlett CMA, Bruce DW, Hondow NS, Newton MA, Lee AF, Wilson K. Mesoporous silicas as versatile supports to tune the palladium-catalyzed selective aerobic oxidation of aHylic alcohols. ChemCatChem. 2013 5 939-950. 

Caspi DD, Ebner DC, Bagdanoff JT, Stoltz BM. The resolution of important pharmaceutical building blocks by palladium-catalyzed aerobic oxidation of secondary alcohols. Adv. Synth. Catal. 2004 346 185-189. 

A recent example where amine ligands have made a significant impact is direct O2 palladium-catalyzed oxidations [2 c, 58, 59]. A key contribution which inspired much of the growth in the field was reported in 1998 by Uemura and coworkers where they discovered the combination of Pd(OAc)2 and pyridine as a system for aerobic alcohol oxidation (Scheme 5.18A) [60]. A greater than 2 1 ratio of pyridine to Pd(OAc)2 is required to avoid decomposition of the catalyst Mechanistic studies performed indicate pyridine not only acts to support Pd(0) during reoxidation with dioxygen, but also inhibits the rate of alcohol oxidation... 

Schultz MJ, Adler RS, Zitakiewicz W, Privalov T, Sigman MS (2005) Using mechanistic and computational studies to explain ligand effects in the palladium-catalyzed aerobic oxidation of alcohols. J Am Chem Soc 127 8499-8507... 

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

Similarly, a water-soluble palladium complex of a sulfonated phenanthroline ligand catalyzed the highly selective aerobic oxidation of primary and secondary alcohols in an aqueous biphasic system in the absence of any organic solvent (Figure 1.8) [40]. The liquid product could be recovered by simple phase separation, and the aqueous phase, containing the catalyst, used with a fresh batch of alcohol substrate, affording a truly green method for the oxidation of alcohols. 

Hou Z, Theyssen N, Brinkmann A et al (2005) Biphasic aerobic oxidation of alcohols catalyzed by polyethylene glycol)-stabilized palladium nanoparticles in supercritical carbon dioxide. Angew Chem Int Ed 44(9) 1346—1349... 

Chiral N-arylated imidazolinylidene ligands have been employed in the palladium(II) catalyzed aerobic oxidation of secondary alcohols to the corresponding ketones [55]. The chiral variant of this reaction, which does not generate a new element of chirality, is again based on the kinetic resolution of racemic mixtures. The active catalyst is formed in situ by a combination of two precursors, a dinuclear NHC-palladium(II) complex and an achiral (acetate) or chiral base ((-)-sparteine) (Scheme 18). 

A number of systems consist of a palladium salt, typically PdCb or Pd(OAc)2, with abase. For example, PdCb-NaOAc catalyzes the aerobic oxidation of secondary alcohols in ethylene carbonate under nuld conditions. Sheldon has carried out mechanistic investigations on a number of related Pd systems and shown that water-soluble complexes of Pd(II) with phenanthrohnes are stable, recyclable catalysts for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic liquid liquid system. The active catalyst is a dihydroxy-bridged palladium dimer. 

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