Alkenes epoxidation and hydroxylation

While it is well established that HO—ONO can be involved in such two-electron processes as alkene epoxidation and the oxidation of amines, sulfides and phosphines, the controversy remains concerning the mechanism of HO-ONO oxidation of saturated hydrocarbons. Rank and coworkers advanced the hypothesis that the reactive species in hydrocarbon oxidations by peroxynitrous acid, and in lipid peroxidation in the presence of air, is the discrete hydroxyl radical formed in the homolysis of HO—ONO. The HO—ONO oxidation of methane (equation 7) on the restricted surface with the B3LYP and QCISD methods gave about the same activation energy (31 3 kcalmol" ) irrespective of basis set size . ... 

The hydroperoxy functionality can be introduced into an alkene by a singlet oxygen ene reaction and subsequently reduced quantitatively to an allylic alcohol, by addition of reducing agents such as PPhs, Me2S or NaBHj ". In addition, the allylic hydroperoxides can be transformed stereospecilically in the presence of Ti(OPr-i)4 to an epoxy allylic alcohol, where epoxide and hydroxyl functionalities are cis to each other (e.g. substrate 160, Scheme 58) - . ... 

Whereas important progress has been made regarding the use of metalloporphyrins as catalysts for alkene epoxidations and alkane hydroxyla-tions, work concerning the mechanism of hydroxylation of aromatic hydrocarbons has received only limited attention. In fact, the main problem encountered with the design of systems capable of performing such oxidative reactions is in the preparation of superstructured porphyrins for the selective complexation of aromatic compounds. 

B. Bahramian, V. Mirkhani, M. Moghadam, S. Tangestaninejad, Selective alkene epoxidation and alkane hydroxylation with sodium periodate catalyzed by cationic Mn(lll)-salen supported on Dowex MSCl, Appl. Catal. A 301 (2006) 169. 

This chapter describes computational strategies for investigating the species in the catalytic cycle of the enzyme cjdochrome P450, and the mechanisms of its main processes alkane hydroxylation, alkene epoxidation, arene hydroxylation, and sulfoxidation. The methods reviewed are molecular mechanical (MM)-based approaches (used e.g., to study substrate docking), quantum mechanical (QM) and QM/MM calculations (used to study electronic structure and mechanism). 

Figure 2.14. Oxidation-states and orbital occupancy diagrams in various stages of alkene epoxidation and alkane hydroxylation. Occupations within parentheses show the alternative spin arrangement, of the odd electron, in the corresponding low-spin state.

Porphyrins were first introduced into clays in 1977 by the physical absorption of porphyrin molecules into montmor-illonite in aqueous solutions." The most common examples are the binding of tetracationic M(TMPyP) porphyrins, M = Co(II), Mn(III), Fe(III), into montmor-illonite clays. Co(TMPyP) was the first porphyrin to be intercalated into montmorillonite by ion exchange in acid solution. The interlayer distance expanded from 27 to 37 A upon intercalation. UV-visible studies revealed the retention of cobalt ions in the porphyrin molecules. Mansuy and coworkers have extended this approach and prepared the Mn-porphyrin intercalated materials. These solids are efficient alkene epoxidation and alkane hydroxylation catalysts." Additionally, the catalyst exhibited a marked shape selectivity in favor of small linear alkanes when compared to more bulky substrates. It was also shown that... 

Oxirans.— Epoxides are readily available from the catalytic oxidation of alkenes with 2-hydroperoxyhexafluoropropan-2-ol (HPHI) (1), which itself is easily prepared from hexafluoroacetone hydrate (2) and hydrogen peroxide (Scheme 1). HPHI is claimed to have similarities to the natural flavin hydroperoxides which are implicated in epoxidations and hydroxylations by external flavoprotein mono-oxygenases. Yields are typically in the range 77—92%. 

A tetracationic Mn porphyrin complex immobilized on monmorillonite was found to be efficient for alkene epoxidation and alkane hydroxylation by PhIO, with a higher ability than the corresponding homogeneous or silica-supported Mn porphyrin complexes... 

The reaction is carried out using a titanium silicalite-1 (TS-1) zeolite catalyst [30, 122]. This type of catalyst is known to accelerate the selective oxidation of alcohols, epoxidation of alkenes and hydroxylation of aromatics. These reactions have importance for fine-chemical production. 

Mechanistic studies103 revealed that chiral ketone-mediated asymmetric epoxidation of hydroxyl alkenes is highly pH dependent. Lower enantioselectivity is obtained at lower pH values at high pH, epoxidation mediated by chiral ketone out-competes the racemic epoxidation, leading to higher enantioselectivity. (For another mechanistic study on ketone-mediated epoxidation of C=C bonds, see Miaskiewicz and Smith.104)... 

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