Oxygenation of alkenes

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Most of the catalytic studies on the oxygenation of alkenes were carried out in the 1970s and 1980s in which typical rhodium complexes [9,10] such as [RhCKPPhals] [11], or even Rh Cls [12-14] with or without co-catalysts (Cu [14,15] or Bi [16]), and some iridium compoimds such as [ Ir (p.-Cl)HCl(cod) 2] [17] were used as catalyst precursors. 

The low effectiveness of the [Rh2"( Jt-OAc)4] system [31] in the oxygenation of alkenes has been attributed to its high oxidation potential to form the mixed-valence complex [Rh2" " ( Ji-OAc)4] on the basis of a relationship between the abihty of the complexes to transfer one electron and the effectiveness of the catalysts in allyhc oxidation, suggested by Kochi [33]. 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

While the oxygenation of alkenes represents the lion s share of methodology development, new and improved protocols for the [C=0 + C] approach to epoxides have also been reported. For example, the phase-transfer-catalyzed asymmetric Daizens reaction using the chiral Cinchona derivative 41 afforded spirocyclic epoxide 52 in good to excellent yields and fair to good ee s <99T6375>. 

Although the oxidation of ethylene to acetaldehyde was known for a number of years,506 its utility depended on the catalytic regeneration of Pd(0) in situ with cop-per(II) chloride discovered by Smidt and coworkers.507 508 Air oxidation of Cu(I) to Cu(n) makes a complete catalytic cycle. This coupled three-step transformation is known as the Wacker process [Eqs. (9.97)-(9.99)]. The overall reaction [Eq. (9.100)] is the indirect oxidation with oxygen of alkenes to carbonyl compounds ... 

In fact, the oxidation with singlet oxygen of alkenes not possessing allylic hydrogens is known to involve a 1,2-dioxetane intermediate15 16 (see Section 9.2.2). 

Photolysis, in the presence of oxygen, of alkenes containing an ally lie hydrogen atom leads to the formation of hydroperoxides. The sensitized process is more efficient, and often yields photoproducts different in structure from those obtained by nonsensitized photooxidation. Cyclohexadiene and related dienes on photolysis in the presence of oxygen yield the transannular peroxides. Thus, on photosensitized oxidation, a-terpinene (410) is converted into ascaridole (411).435 The equivalent process is not, in general, observed in acyclic dienes. Certain polynuclear aromatic hydrocarbons, such as anthracene and naphthacene and including the heterocycles 5,10-diphenyl-1-... 

Recent developments in the stereoselective singlet oxygen allylic photo-oxygenations of alkenes have been reviewed. A number of factors, such as solvent, electronic effects, and non-bonded interactions that dictate the ene product selectivity, and also the various mechanisms of this reaction, have been highlighted.239... 

The purpose of this chapter is to summarize the recent stereochemical and mechanistic features of the ene photo-oxygenation of alkenes and related substrates by singlet oxygen. 

Scheme 4 Proposed mechanism of the sensitized photo-oxygenation of alkene 1.

Scheme 13 Intermolecular isotope effects in the photo-oxygenation of alkenes 24 versus 26.

Table 5 Geminal Selectivity in the Photo-Oxygenation of Alkenes Bearing an Allylic Functionality3...

Scheme 17 Mechanism of the geminal selectivity on the photo-oxygenation of alkenes bearing a large alkyl substituent in the allylic position.

Scheme 10.2 Efficient use of 102 (0), generated near La or Ca cations in the mesopores of zeolite USY (a) or at the external surface of small zeolite Beta crystals (b), in the oxygenation of alkenes (CO-...

More than 40 years later did two different groups [2,3] isolate [2+2]-cycloadducts (1,2-dioxetanes) by photo-oxygenation of alkenes shortly after Kopecky had synthetized the first stable dioxetanes by base-catalyzed cyclization of a-bromohydroperoxides (Schs. 2,3) [4],... 

Photo sensitized oxygenation of alkenes with singlet oxygen can, in principle, proceed via three competitive reaction pathways [4 + 2]cycloaddition to e do-peroxides, ene reaction of allylic hydroperoxides, and [2 + 2]cycloaddition to 1,2-dioxetanes (see reference [681] for a review). With suitable olefinic substrates, the chemical outcome of such photo-oxygenation reactions can be strongly influenced by the solvent. This is shown in the somewhat simplified Eq. (5-145). 

W. Ando and Y. Moro-oka, Role of Oxygen in Chemistry and Biochemistry , Elsevier, Amsterdam, 1988. Includes chapters on (a) Cation-Radical Chain Catalyzed Oxygenation of Alkenes, by S. F. Nelsen, et al. (b) Photooxygenation of Organic Compounds via Thieir Radical Ions, by K. Mizono and Y. Otsuji (c) Triphenylpyrylium Sensitized Oxygenations, by K. Tokumaru a al. (d) Catalytic Oxidation of Metiiylarenes to Benzaldehydes, by R. A. Shddon and N. de Heij (e) Oxidation of Arylamines with Horseradish Peroxidase, 1 Fujimori et al. (f) Arene Hydroxylation by Cytochrome P-4S0, by M. Tsuda et al., and marry others. 

Direct oxygenation of alkenes other than ethene and propene is normally achieved in the liquid phase and in the presence of a catalyst or under u.v. irradiation. Thus aryl-oxirans (1 = 1 or 4) were obtained in 37-71% yields by autoxidation of the corresponding 1-phenyl-cycloalkenes in the presence of cobalt naphthenate at 50 °C. ... 

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