Photo-oxidation of alkenes

The 02 ion appears to play an important role in a number of photooxidation reactions (see Section VI,C) for example, the photo-oxidation of alkenes over TiOz. However, it seems likely that OJ is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=0 systems (e.g., V2Os supported on PVG) gives rise to an excited charge transfer state such as V4 + -0 . This excited state can react as O- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. /). In the presence of oxygen, 03 is formed which then reacts further with organic molecules. 

In 1999, Clennan and Sram reported a study of the photo-oxidations of a series of tetrasubstituted alkenes (Fig. 5) in methylene blue-doped zeolite Y [11], The ene regiochemistries are very sensitive to the size of the allylic substituent, R, in solution. The A/B ratio increases from 0.49 to 2.4 as the substituent, R, is changed from methyl to ferr-butyl. This phenomenon has been attributed [12] to a sterically induced lengthening of the carbon-2 oxygen bond in the perepoxide intermediate I and subsequent preferred opening of this long bond (Fig. 5). 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

In this section we will discuss photochemical synthesis of macrocycle, photoini-tiated oxidation of alkenes, photo-Fries rearrangement, photocycloaddition and chiral photochemical synthesis within a number of microreactors. 

Also, intermediate peroxides are formed in the oxidation of perfluorinated alkenes, e.g. in the photo-oxidation of perfluoroethene and perfluoropropene for the formation of Fomblin (Ausimont Co.) perfluoro-polyether fluids [198, 199]. 

The objective of present research was to provide a better understanding of the chemical processes involved in production and loss of ozone in the troposphere. This was achieved by providing kinetic and mechanistic data for several reactions of peroxy radicals involved in the photo-oxidation of volatile organic compounds (VOC). Additional aims were to determine the product quantum yields in the photolysis of carbonyl compounds, and to investigate the mechanism in the ozonolysis of alkenes, especially in the presence of water vapour. 

The goals set with respect to the spectroscopy and kinetics of the simple peroxy radicals was mainly achieved. A detailed review on the chemistry and role of the peroxy radicals in the photo-oxidation of VOC was written within the LACTOZ project [25]. Further studies on quantum yields determination of photolabile carbonyl compounds and on the mechanism of the ozonolysis of alkenes are required. 

Pyridine hydrochloride in pyridine is a mild reagent for the nucleophilic cleavage of a conjugated (but not necessarily strained) cyclopropyl ring furnishing /S-chloromethyl ketones. Photo-oxidation of mono- and di-substituted alkenes in the presence of iron (ill) chloride affords a-chloro-ketones whereas tri- and tetra-substituted alkenes suffer cleavage to form dichloro-ketones [equations (45) and (46)]." °... 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

An extremely interesting feature of these mechanisms is the fact that superoxide and the alkene radical cation are both formed in the reduction (Fig. 20) and also in the Frei oxidation (Fig. 19). In the Frei photo-oxidation, however, they are formed concurrently in a tight ion pair and collapse to product more rapidly than their diffusive separation. In the reduction (Fig. 20), the formation of the radical cation and superoxide occur in independent spatially separated events allowing the unimpeded diffusion of superoxide which precludes back-electron transfer (BET) and formation of oxidized products. The nongeminate formation of these two reactive species provides the time necessary for the radical cation to abstract a hydrogen atom from the solvent on its way to the reduced product. 

---