Aerobic oxidation of aldehydes

Co-containing POMs have been found to be among the most efficient catalysts for homogeneous aerobic oxidation and co-oxidation processes [91-93]. This prompted many researchers to design solid Co-POM-containing materials [78,94-100]. Thus, various Co-POMs have been deposited on cotton cloth [94] and silica [100], datively [95] or electrostatically [96,97] bonded to NH2-modified silica surfaces (vide infra) as well as intercalated in LDHs [78,98,99]. The resulting materials were successfully used for aerobic oxidation of aldehydes, alkenes, alkanes, alcohols and some other organic substrates. 

These results suggest the possibility of using the aerobic oxidation of aldehydes, catalyzed by NHPI, for the epoxidations of alkenes by peracids generated "in situ" under mild conditions [Eq. (6.11)]. 

F. Minisci, F. Recupero, A. Cecchetto, C. Punta, C. Gambarotti, F. Fontana, G. F. Pedulli, Polar effects in free-radical reactions. A novel homolytic acylation of heteroaromatic bases by aerobic oxidation of aldehydes, catalysed by N-hydroxyphthalimide and Co salts, /. Heter. Chem. 40 (2003) 325. 

The present oxidation reaction catalyzed by nickel(II) complex with the combined use of molecular oxygen and aldehyde was applied to the aerobic oxidation of aldehyde into the corresponding carboxylic acid, and the aerobic Baeyer-Villiger reaction (Scheme 10), respectively. 

Marsden, C., Taaming, E., Hansen, D., et al. (2008). Aerobic Oxidation of Aldehydes under Ambient Conditions Using Supported Gold Nanoparticle Catalysts, Green Chem., 10, pp. 168-170. 

Aerobic oxidation of aldehydes under ambient conditions using supported gold nanoparticle catalysts. Green Chemistry, 10(2), 168-70. 

Epoxidation reactions have been widely utilized for over 100 years with peradds, peroxides and, more recently, metal catalysts [7]. However, direct metal-catalyzed aerobic epoxidations are rare and generally require an aldehyde coreductant. In this case, the metal is proposed to catalyze radical formation (A-C, Scheme 5.2) followed by O2 insertion to form acyl peroxide D. Metal-catalyzed aerobic oxidation of aldehydes to peradds has previously been observed [8]. With the formation of species D, either an outer-sphere path similar to a peracid-type oxidation occurs (Path 1) or an inner-sphere metal-catalyzed oxidation in which the metal-based oxidant and substrate interact during oxygen transfer (Path 2 or 3). Mu-kaiyama and coworkers were the first to report an aerobic epoxidation of olefins catalyzed by transition metals using either a primary alcohol or an aldehyde as coreductants [9]. The role of the metal was probed through parallel studies of peracid and metal-catalyzed epoxidations of 2 which yielded different stereochemical outcomes. Therefore, a metal-centered mechanism for olefin epoxidation was proposed which implicates an oxygenase system. Path 2 or 3 (Table 5.1) [10]. 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

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... 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

Recently, two reports (218, 219) appeared showing that (iminosemiqui-nonato)copper(II) complexes also catalyze the aerobic oxidation of primary alcohols (ethanol, benzyl alcohol) to the corresponding aldehydes and H202. Complexes J and K shown in Fig. 33 have been isolated as active catalysts and the former has been characterized by X-ray crystallography. Detailed mechanistic studies have been performed that again show the close resemblance to GO. 

The supported Co2+-substituted Wells-Dawson POM, Cs6H2[P2W17061Co(OH2)], on silica was stable up to 773 K and catalyzed the heterogeneous oxidation of various aldehydes to the corresponding carboxylic acids with 02 as a sole oxidant [116], The H5PV2Mo10O40 POM, impregnated onto meso-porous MCM-41, catalyzed the aerobic oxidation of alkanes and alkenes using isobutyraldehyde as a... 

In the aerobic oxidation of the non-activated aliphatic primary and secondary alcohols to the corresponding aldehydes and ketones, co-catalysts or other additives are normally required 223-226). The catalytic aerobic oxidation of aromatic aldehydes to the corresponding carboxylic acids with Ni(acac)2 in ionic liquids was the first example of an aerobic oxidation in ionic liquids 227). 

Aerobic oxidation of primary alcohols to aldehydes and secondary alcohols to ketones was accomplished in ionic liquids (bmim, l-butyl-3-methyl-imidazolium cation) as RuCl2(PPh3)j/(bmim)V80°C RuClj or [RuCl Cp-cymene)] were also used... 

A copper-centered mechanism for the Cu-TEMPO-catalyzed aerobic oxidation of alcohols was proposed by Sheldon and co-workers, wherein the active catalytic Cu" species is generated by oxidation of a Cu species with TEMPO, in the presence of alcohol, with formation of TEMPOH (Scheme 3) [146]. The resulting Cu" species is then capable of oxidizing the alcoholate to the aldehyde or ketone species. Regeneration of the TEMPO radical species was achieved by rapid oxidation of TEMPOH with O2. 

The application of ionic liquids as a reaction medium for the copper-catalyzed aerobic oxidation of primary alcohols was reported recently by various groups, in attempts to recycle the relatively expensive oxidant TEMPO [150,151]. A TEMPO/CuCl-based system was employed using [bmim]PF6 (bmim = l-butyl-3-methylimodazolium) as the ionic liquid. At 65 °C a variety of allylic, benzylic, aliphatic primary and secondary alcohols were converted to the respective aldehydes or ketones, with good selectiv-ities [150]. A three-component catalytic system comprised of Cu(C104)2, dimethylaminopyridine (DMAP) and acetamido-TEMPO in the ionic liquid [bmpy]Pp6 (bmpy = l-butyl-4-methylpyridinium) was also applied for the oxidation of benzylic and allylic alcohols as well as selected primary alcohols. Possible recycling of the catalyst system for up to five runs was demonstrated, albeit with significant loss of activity and yields. No reactivity was observed with 1-phenylethanol and cyclohexanol [151]. 

The aerobic oxidation of alcohol under neutral or acidic conditions to produce the corresponding adds, which can avoid the neutralization of the carboxylate salts, is also an important R D issue. In Au-catalyzed alcohol oxidation in methanol, the corresponding methyl esters are obtained with high seledivity instead of carboxylic acids by using metal oxide supported Au NPs [157, 160], In this case, base is not necessary, or only a catalytic amount of base is required to promote the readion. However, in water, it was demonstrated that alcohols were not oxidized under acidic conditions [161] and only aldehydes were oxidized to carboxylic adds [162]. Even under solvent-free conditions or in organic solvents, alcohols were converted into aldehydes without base however, the alcohols were not fully oxidized to carboxylic acid under acidic conditions [163-166]. 

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