Photo-oxygenation mechanism

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

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... 

Singlet-Oxygen Ene-Sensitized Photo-Oxygenations Stereochemistry and Mechanisms... 

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

Scheme 8 Mechanism of the photo-oxygenation of allylic alcohol 14-E.

Scheme 14 Mechanism of photo-oxygenation of (, ( -dim ethyl styrene.

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

Many mechanisms had been proposed in the past to rationalize this selectivity (trioxanes, perepoxide, exciplex, dipolar, or biradical intermediates) however, it is now generally accepted [100] that the mechanism proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported (a) by the lack of stereoselectivity in the reactions with chiral oxazolines [101] and tiglic acid esters [102], (b) by the comparison of the diastereoselectivity of dialkyl-substituted acrylic esters [103] with structurally similar nonfunctionalized alkenes, (c) by intermolecular isotope effects [104] in the photo-oxygenation of methyl tiglate, and (d) by solvent effects on regioselec-tivity [105],... 

Scheme 26 Proposed mechanisms of the alkene photo-oxygenations in zeolites.

Oxidation of DMS to DMSO and DMSO. DMS is chemically and biochemically oxidized to dimethylsulfoxide (DMSO). Mechanisms for the in situ oxidation of DMS to DMSO in seawater have received little attention, even though this may be an important sink for DMS. Hydrogen peroxide occurs in surface oceanic waters (22) and is produced by marine algae (98). It may participate in a chemical oxidation of DMS, since peroxide oxidizes sulfides to sulfoxides (991. Photochemical oxidation of DMS to DMSO occurs in the atmosphere and DMSO is found in rain from marine regions (681. DMS is also photo-oxygenated in aqueous solution to DMSO if a photosensitizer is present natural compounds in coastal seawater catalyzed photo-oxidation at rates which may be similar to those at which DMS escapes from seawater into the atmosphere (1001. 

Kojima and co-workers also examined the photo-oxidation of stilbenes in zeolite Y [150]. The presence of O2 in the stilbene-NaY sample led to a red-shifting of the tail end of the diffuse reflectance spectrum and was assigned to the charge-transfer complex with O2. Upon excitation at 313/366 nm, both cis- and /ra .v-stilbene undergo photo-oxygenation to form benzaldehyde. Phenanthrene, the expected product from the ]2 -f 4] cycloaddition reaction of excited m-stilbene, was also formed. The details of the mechanism leading to benzaldehyde and phenanthrene... 

These reactive species will likely attack and decompose the drug or excipients in the formulation. This was demonstrated for primaquine (Kristensen et al., 1998), which can be photochemically stabilized by an inert atmosphere. Photo-oxidation reactions of type I (free radical) or type II (singlet oxygen) mechanisms can take place simultaneously in a competitive fashion. Oxygen concentration and the properties of the vehicle are factors influencing the distribution between the two processes. Free radical reactions are favored by polar vehicles such as water. 

The Type II photo-oxygenation reactions with which we are concerned occur by a completely different mechanism. The main feature of these reactions is that an activated oxygen is formed during the reaction, which can react stereoselectively with certain substrates to give the addition products. Today, we have reason to believe that the activated oxygen is the excited singlet oxygen, 02. 

Olefins containing at least one allylic hydrogen are suitable substrates and are of special importance and interest with regard to the intrinsic mechanism involved in their reactions with singlet oxygen. Allylic hydroperoxides are formed, but the mechanism of their formation is clearly distinct from that by which allylic hydroperoxides are produced in thermal or photochemically initiated (see example above for a Type I process) autoxidation reactions. This has unequivocally been shown with optically active limonene as a substrate, which gives rise to different products in free radical and Type II photo-oxygenation reactions (22, 57, 61). 

Figure 3, Mechanism for photo-oxygenation of a trisubstituted olefin...

We now discuss sensitized photo-oxygenation reactions and show how some of our theoretical results clarify some current controversies and give us new insight into the reaction mechanisms. 

Comparison of Terenin-Schenck Mechanism and the Singlet Oxygen Mechanism. Terenin (49, 50), and later Schenck (47), proposed the following mechanism for dye sensitized photo-oxygenation reactions ... 

Where the moloxide, [M. . . Oo], is the reactive intermediate which transfers oxygen to the acceptor. A, to form products AO2. This mechanism, which was given strong support by the extensive work of Schenck and co-workers (i5), was until recently the generally accepted mechanism for sensitized photo-oxygenation reactions. 

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