Oxidation photochemical pathways

Although thermodynamically favorable, reductive dissolution of Fe(III)(hydr)oxides by some metastable ligands (even those, such as oxalate, that can form surface complexes) does not occur in the absence of light. The photochemical pathway is depicted in Fig. 9.3e. In the presence of light, surface complex formation is followed by electron transfer via an excited state (indicated by ) either of the iron oxide bulk phase or of the surface complex. (Light-induced reactions will be discussed in Chapter 10.)... 

Although ketone groups can produce free radicals in polymers also by other photochemical pathways, their role in initiation seems to be considerably less during initial stages of oxidation than that of hydroperoxides. 

Azinemonoxides are cleaved by two distinct photochemical pathways. One involves oxygen migration and the other involves a pericyclic ring closure. 4) Both processes are represented in the mass spectra of the compounds. Similar rearrangements occur in both the photochemistry and mass spectrometry of aryl nitrones, aryl N-oxides and aromatic azoxybenzenes. )... 

As mentioned previously, oxidative degradation pathways can be complex, and it is useful to consider the chemistry of oxidative degradation. The oxidative degradation of pharmaceuticals has been discussed in the literature,23-27 and we assert that there are three major pathways, (1) autoxidation or radical-mediated oxidation, (2) peroxide-mediated, and (3) photochemically induced (see Table 5). Traditionally, dilute aqueous peroxide solutions have been used for oxidative stress testing of pharmaceuticals. Landmark papers by Boccardi23 24 in 1992 and 1994 identified autoxidation as the major oxidative degradation pathway for pharmaceuticals. Boccardi showed that the use of radical initiators such as... 

E) Reactions [13.33]-[13.39] make up the photochemical pathways for the oxidation of SO2 to H2SO4 mediated by hydroxyl radical and ozone, which are generated from the interaction of the exposure light with moisture and oxygen, respectively, in the exposure chamber. 

Tervalent copper and nickel are involved in the autoxidation reactions of [Cu(H 3G4)] and [Ni(H 3G4)] respectively. In the case of nickel, decomposition of [Ni(H 3G4)] proceeds by decarboxylation of the terminal carboxy-group adjacent to the peptide nitrogen. - With copper, decomposition of [Cu-(H sG4)] proceeds through a carbon-centred free radical produced by abstraction of a hydrogen atom from the peptide backbone. Bulky carbon substituents assist the stabilization of the higher-oxidation state ions, and a study of the stabilities of leucyl tripeptide complexes with copper(ii) and nickel(u) has been reported. Copper(iii) and nickel(iii) tripeptide complexes of a-aminoisobutyric acid are thermally stable but are readily decomposed by photochemical pathways. Resonance Raman and other studies with copper(iii) peptide complexes have also been reported. ... 

In a few cases alkyl complexes can be synthesized by the photoaddition of dichloromethane or chloroform to a low-valent transition metal complex, but the usual products from these halogen abstraction reactions are the dichloro complexes. Photoinduced oxidative addition reactions are not always the result of the generation of an excited state that is more reactive than the ground state. An alternative photochemical pathway can involve the initial dissociation of a ligand to generate a coordinately unsaturated intermediate, which is then activated toward oxidative addition of the substrate. An example of such a reaction is... 

Reaction (12-9) shows the photochemical dissodation of NO2. Reaction (12-10) shows the formation of ozone from the combination of O and molecular O2 where M is any third-body molecule (principally N2 and O2 in the atmosphere). Reaction (12-11) shows the oxidation of NO by O3 to form NO2 and molecular oxygen. These three reactions represent a cyclic pathway (Fig. 12-4) driven by photons represented by hv. Throughout the daytime period, the flux of solar radiation changes with the movement of the sun. However, over short time periods (—10 min) the flux may be considered constant, in which case the rate of reaction (12-9) may be expressed as... 

Stability of a drug substance and product is monitored throughout the development and clinical phases. This monitoring requires stability-indicating assay methodology, and this is a subject that is separate from performulation per se. In most instances, the major, feasible decomposition products are identified early [51], and as such it is known if the pathways are hydrolytic, oxidative, or photochemical. 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

Figure 6. Proposed atom (Cl ) transfer pathway for the photochemical oxidation...

The Iron Cycle in the Photic Zone of Surface Waters In the photic zone the formation of iron(II) occurs as a photochemical process. The photochemical iron II) formation proceeds through different pathways 1) through the photochemical reductive dissolution of iron(III)(hydr)oxides, and 2) through photolysis of dissolved iron(lll) coordination compounds, Fig. 10.16. 

Arylindenes undergo sigmatropic 1,5-shifts, induced oxidatively [267], thermally [268], photochemically [269], and re-ductively [270]. The reductive rearrangement of 1,1,3-triphenylindene, yielding the dianion of l,2,3-triphenyl-2H-indene, has been the subject of voltammetric investigations in DMF-TBAP [271]. The reaction follows an EEC or EDisp C pathway. 

It is important to note that the reactions are fundamentally different from similar radical cation Diels-Alder reactions initiated with the use of a photochemical electron-transfer reaction [35, 36]. In photochemical reactions, a one-electron oxidation of the substrate leads to a cycloaddition that is then terminated by a back electron transfer . No net change is made in the oxidation state of the substrate. However, the reaction outlined in Scheme 13 involves a net two-electron oxidation of the substrate. Hence, the two pathways are complementary. 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

Iron oxide dissolution can proceed by a variety of pathways, viz. protonation, com-plexation and reduction, photochemical and biological. 

CgoO (1) can also be prepared by allowing toluene solutions of CgQ to react with dimethyldioxirane (Scheme 8.3) [28], The so-obtained product is identical to that prepared by photochemical epoxidation [15], Upon treatment of CgQ with dimethyldioxirane, a second product is formed simultaneously (Scheme 8.3), which was identified to be the 1,3-dioxolane 6. Upon heating 6 in toluene for 24 h at 110 °C, no decomposition could be observed by HPLC, implying that 1 and 6 are formed by different pathways. Replacement of dimethyldioxirane with the more reactive methyl(trifluoromethyl)dioxirane allows much milder reaction conditions [29]. At 0 °C and a reaction time of only some minutes this reaction renders a CgQ conversion rate of more than 90% and higher yields for CgoO as well as for the higher oxides. 

---