Catalyzed Aerobic Oxidation

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... 

Late Transition Metal-Oxo Compounds and Open-Framework Materials that Catalyze Aerobic Oxidations Rui Cao, Jong Woo Han, Travis M. Anderson, Daniel A. Hillesheim, Kenneth I. Hardcastle, Elena Slonkina, Britt Hedman, Keith O. Hodgson, Martin L. Kirk, Djamaladdin G. Musaev, Keiji Morokuma, Yurii V. Geletii and Craig L. Hill... 

A palladium-catalyzed aerobic oxidative annulation of indoles, in the presence of ethyl nicotinate, has been disclosed.137,13711 The stereochemical outcome of this reaction indicates that an initial C-H functionalization at C(2) of the indole, followed by yvv/-carbopa 11 adation and ry -/ -H-elimination, operates (Equation (163)).137 This process has also been employed for the synthesis of benzofuran analogs.1373... 

LATE TRANSITION METAL-OXO COMPOUNDS AND OPEN-FRAMEWORK MATERIALS THAT CATALYZE AEROBIC OXIDATIONS... 

III. Open-Framework Materials that Catalyze Aerobic Oxidations Acknowledgments References... 

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]. 

Scheme 2 Simplified catalytic cycle for palladium-catalyzed aerobic oxidation ( oxidase ) reactions...

Interest in peroxopalladium(II) complexes has returned in recent years. Research efforts have shifted away from the oxygen-atom-transfer reactivity of these complexes toward their role in oxidase -like reactions depicted in Scheme 2. The success of recent Pd-catalyzed aerobic oxidation reactions is linked to the identification of oxidatively robust ancillary hgands that stabilize Pd to prevent catalyst decomposition and promote efficient oxygenation of Pd. ... 

In a second study, the protonolysis of (IMes)2Pd(02), 17, was investigated [114]. Addition of one equivalent of acetic acid generates the hydroperoxo-Pd complex, 32, which has imdergone cis-trans isomerization in the protonolysis step (Scheme 9). The ability to isolate and characterize this complex reveals that protonolysis of the second Pd - O bond is much slower than the first. Addition of a second equivalent of acetic acid forms the diacetate complex, 33, but only after 3 days at room temperature. The systematic studies summarized in Eqs. 17 and 18 and Schemes 8 and 9 reveal the strong influence of ancillary ligands on fundamental rate constants associated with aerobic oxidation of Pd to Pd . Similar effects undoubtedly will impact the success of Pd-catalyzed aerobic oxidation reactions. 

A general simplified mechanism for palladium-catalyzed aerobic oxidation reactions and the different intermediates is given in Scheme 13. 

Late Transition Metal- Compounds and Open-Framework Materials that Catalyze Aerobic Oxidations Rui Coo, Jong Woo Han, Travis M. Anderson, Daniel A. Hilleskeim, Kenneth... 

Pd-catalyzed aerobic oxidations with hidroquinone-Cu(OAc)2701 and hydroqui-none-iron phthalocyanine529 were also achieved. On the basis of the transformation of 1,2-dideuterocyclohexene, conclusive evidence for the involvement of the ir-allyl intermediate in the quinone-based system has been reported 702... 

Novel practical methods using various reagents, such as [Co(OAc)Br],1355 sulfur trioxide,1356 or ds-dioxoruthenium complexes,1357 were developed to transform alkynes to 1,2-diketones. Radical-catalyzed aerobic oxidation using A-hydro-xyphthalimide combined with a transition metal (Co, Cu, or Mn) affords a,P-acetylenic ketones in good yields.1358 Oxidation by the HOF. acetonitrile complex yields diketones, ketoepoxides, or cleavage products.1359 Ozonolysis of acetylenes combined with trapping techniques affords to isolate various derivatives.1360,1361 New information for the ozonolysis of acetylene was acquired by quantum-chemical investigatons.1362... 

PV2M010O40 has usually been used in the acid form. H5PV2M010O40 catalyzes aerobic oxidative cleavage of cycloalkanes, 1-phenylalkanes, and ketones. For example, the oxidation of 2,4-dimethyl cyclopentanone and 2-methylcyclo-hexanone gives 5-oxo-3-methylhexanoic acid and 6-oxoheptanoic acid, respectively, in yields higher than 90% [285, 286). Bromination of arenes with HBr [287), oxidative dehydrogenation of cyclohexadiene [288, 289) and a-terpinene [290), oxidation of 2,4-dimethylphenol [291) and sulfides [292) are other examples. 

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. 

Scheme 3.5 PdCl2-catalyzed aerobic oxidation of styrene in SCCO2/PEG (Reproduced from Ref. [36] with permission from The Royal Society of Chemistry)...

Haimov A, Neumann R (2002) Polyethylene glycol as a non-ionic liquid solvent for polyoxometalate catalyzed aerobic oxidation. Chem Commun 8 876-877... 

Jiang N, Ragauskas AJ (2005) Copper(II)-catalyzed aerobic oxidation of primary alcohols to aldehydes in ionic liquid [bmpy]PF6. Org Lett 7(17) 3689-3692... 

II. First Generation Copper-Catalyzed Aerobic Oxidation Protocol... 

Copper-Catalyzed Aerobic Oxidation of Alcohols Using DBAD-H2... 

Scheme 1 General mechanism for Pd-catalyzed aerobic oxidation reaction...

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