Epoxidation with molecular oxygen

The search for new environmentally-friendly epoxidation methods using O2 as a sole oxidant has attracted much interest Although there has been some success with O2 and homogeneous catalyst systems in the liquid phase without the use of reducing reagents, there have been few reports concerning heterogeneous epoxidation of olefins [45, 46]. 

Co +-exchanged faujasite zeolite is a unique heterogeneous catalyst for liquid-phase epoxidation using O2 [45]. This catalyst is active only for styrene, and the conversion and yield of styrene oxide were 65 and 45%, respectively. The TON, based on Co ions, reached 12. The Co + ions, located in supercages, are thought to cause activation of O2 for epoxidation. 

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Hart, H. Lavrik, P. B. Synthetically useful epoxidations with molecular oxygen. /. Org. Chem. 1974, 39, 1793-1794. 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

Unsaturated fatty compounds are the preferred educts in industrial epoxidation. Numerous methods are available to transform then to the corresponding epoxides. Epoxidation with molecular oxygen [3], dioxiranes [4], hydrogen peroxide with methyltrioxorhenium as catalyst [5, 6], the Halcon process [7], or enzymatic reactions [8] are the most important industrial processes (cf. Section 2.4.3). 

W. Chen, J. Yamada, K. Matsumoto, Catalytic olefin epoxidation with molecular oxygen over supported amidate-bridged platinum blue complexes, Synth. Commun. 32 (2002) 17. 

Scheme 4. Epoxidation with molecular oxygen and alcohol catalyzed by bis(l,3-diketonato)oxovanadium(IV)...

MeO OMe Scheme 5. Nickel(II) complex catalyzed epoxidation with molecular oxygen and primary alcohol... 

Epoxidation is carried out by four different procedures (i) epoxidation with peracids such as peracetic acid or perbenzoic acid in the presence of an acid catalyst (ii) epoxidation with organic and inorganic peroxides, including transition metal catalysts (iii) epoxidation with halohydrins using hypoha-lous acids and their salts and (iv) epoxidation with molecular oxygen. 

On top of the oxidation reactions, the same team demonstrated that the silver-containing MOF-5 turned out to be active in propene epoxidation with molecular oxygen [126]. 

A complementary approach has been reported very recently [43]. hi this case negative charges were introduced into the salen ligand Iq (Fig. 14) with the aim of exchanging it on cationic supports, such as a layered double (Zn, Al) hydroxide. The expansion in the basal spacing indicated intercalation, at least partially, of the Ig-Mn complex between the layers of [Zn2,i5Alo,86(OH)6,o2]- The complex was used in the epoxidation of (i )-limonene with molecular oxygen and pivalaldehyde. The use of N-... 

Shiraishi, Y., Morishita, M., and Hirai, T. (2005) Acetonitrile-assisted highly selective photocatalytic epoxidation of olefins on Ti-containing silica with molecular oxygen. Chemical Communications (48), 5977—5979. 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

The catalytic properties Co-POM/Si02 were assessed in oxidation of IBA to isobutyric acid [96] and a-pinene to verbenol/verbenone [97] with molecular oxygen as well as in a-pinene/lBA co-oxidation to produce the corresponding epoxide and carboxylic acid [97]. In turn, the catalytic performance of Ti-POM/Si02 was examined in a-pinene oxidation to... 

Oxo-metal species participate in a wide range of biological and chemical oxidation reactions. Representative oxidizing enzyme, cytochrome P-450, which carries iron(III)-porphyrin complex as its active site, catalyzes various O-atom transfer reactions such as epoxidation, hydroxy-lation of C-H bond, and oxidation of sulfides. These reactions have been proven to proceed through cationic oxoiron(IV)-porphyrin species, which are generated by the oxidation of Fe(III) complex with molecular oxygen. This conversion from Fe(III) to 0=Fe(IV) species is a... 

Caps and coworkers studied the solvent effect in the epoxidation of stilbene by varying solvents and the supports [200], In methylcyclohexane (MCH), the activated radical species proposed were MCH peroxy radicals, which were formed by the radical transfer from TBHP and reaction with molecular oxygen. Except for MCH, the solvent effect is not fully understood however, the choice of solvent and supports that can trap or stabilize the radical species affected the catalytic performance of Au. 

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