Transition metal-catalyzed aerobic oxidations

This chapter provides some highhghts in the innovative field of aerobic oxidation reactions in continuous flow. Topics include transition metal-catalyzed aerobic oxidations in continuous flow, photosensitized singlet oxygen oxidation in continuous flow, metal-free aerobic oxidations in continuous flow, aerobic cou-phng chemistry in continuous flow, and general prospects for scale-up. Ozonolysis is not covered in this chapter hereto, we refer to the literature [25—27]. 

Transition Metal-Catalyzed Aerobic Oxidations in Continuous Flow... 

Schultz MJ, Sigman MS (2006) Recent advances in homogeneous transition metal-catalyzed aerobic alcohol oxidations. Tetrahedron 62(35) 8227-8241... 

Recent Advances In Homogeneous Transition Metal-Catalyzed Aerobic Alcohol Oxidations Schultz, M.J. Sigman, M.S. Tetrahedron 2006, 62, 8227. 

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]. 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

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... 

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

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

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... 

Organic derivatives of tetrazane ( buzane ) H2N—NH—NH—NH2 have been known since 1893 (227), but to date no transition metal tetrazane complexes have been fully characterized. However, iron tetrazane species have been proposed as intermediates in the iron(II)-catalyzed aerobic oxidation of hydrazines (104,230) and are thought to account for the red coloration observed in some of these reactions (230). 

Efficient and regioselective iron-catalyzed aerobic oxidative reactions afforded 3,5-disubstituted isoxazoles 5 from homopropargylic alcohols 4, r-BuONO as the nitrogen source, and H2O under mild conditions (140L6298).A transition metal-free one-pot synthesis of 3,5-disubstituted isoxazoles used terminal alkynes by treatment with -BuLi, then aldehydes and iodine to afford intermediate a-alkynyl ketones 6 converted into isoxazoles 7 with hydroxylamine (14JOC2049). 

The development of transition-metal-catalyzed oxidative addition of heteroatoms to alkenes is closely linked to the discovery of the industrial Wacker process, which consists of an aerobic oxidative transformation of ethylene to acetaldehyde in the presence of water and a catalytic amount of palladium and copper salts [1-3]. This reaction is based on the pioneering observation by Phihpps on the stoichiometric oxidation of ethylene by palladium(II) salts in aqueous medium [4]. 

In biatyl synthesis, although transition metal catalyzed aryl-aryl crosscoupling reactions have been widely exemplified using Suzuki-type reactions, CDC reactions between sp C-H bonds leading to sp C-C bonds are scarce. In 2010, Katsuki reported an enantioselective CDC reaction between two naphthol moieties catalyzed by a chiral iron(salan) complex under aerobic oxidative conditions (Scheme 4.23). Using 4 mol% of the iron(salan) complex 23-A as the catalyst in toluene at 60 °C for 48 h, two different 2-naphthols were coupled and the cross-coupled derivatives were isolated with moderate yields (44-70%) and good ee (87-95%). It must be pointed out that the homocoupling derivatives are also obtained in low yields. 

In this multi-authored book selected authors in the field of oxidation provide the reader with an up to date of a number of important fields of modern oxidation methodology. Chapter 1 summarizes recent advances on the use of green oxidants such as H2O2 and O2 in the osmium-catalyzed dihydroxylation of olefins. Immobilization of osmium is also discussed and with these recent achievements industrial applications seem to be near. Another important transformation of olefins is epoxidation. In Chapter 2 transition metal-catalyzed epoxidations are reviewed and in Chapter 3 recent advances in organocatalytic ketone-catalyzed epoxidations are covered. Catalytic oxidations of alcohols with the use of environmentally benign oxidants have developed tremendously during the last decade and in Chapter 4 this area is reviewed. Aerobic oxidations catalyzed by N-hydroxyphtahmides (NHPI) are reviewed in Chapter 5. In particular oxidation of hydrocarbons via C-H activation are treated but also oxidations of aUcenes and alcohols are covered. 

Chart 1 Transition metal cocatalysts used in multicomponent aerobic palladium-catalyzed oxidation reactions... 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

Several efficient oxidation reactions with molecular oxygen were developed using transition-metal complexes coordinated by variuos ligands in combination with apprOTriate reductants. Recently, it was found that cyclic ketones such as 2-methylcyclohexanone and acetals of aldehyde such as propionaldehyde diethyl acetal were effectively employed in aerobic epoxidation of olefins catalyzed by cobalt(II) complexes. In the latter case, ethyl propionate and ethanol were just detected in nearly stoichiometric manner as coproducts (Scheme 12), therefore the reaction system is kept under neutral conditions during the epoxidation. 

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