Olefin photo-oxidation

Furthermore, silica-based systems received attention as effective catalysts for several photochemical reactions, e.g olefin photo-isomerisation [12], olefin photo-oxidation [13-15], photomethatesis [16] and methane coupling [17]. Recently, it was also reported that silica mesoporous materials are more active photocatalysts than amorphous silica [18]. Among them, Mg/Si02 systems [13] were found to act as catalysts for the photo-oxidation of propene to propene oxide in the presence of molecular oxygen, which is an attractive path for the production of this industrially important chemical. 

Optically active olefin (Z)-(S)-( )-l-deuterio-3,4-dimethyl-l-phenyl-2-pentene, 1 (Scheme 3), was photo-oxidized in acetone at 10°C. The tertiary al-lylic hydroperoxide was obtained in high yield and only in the trans configuration. 

Photo-oxidation of trimethylethylene was frequently used as evidence to support or contradict various mechanistic possibilities for the ene reaction. For example, the equal amounts of photo-oxidized products (olefin 4) led to the conclusion that the ene reaction proceeds without any regioselectivity (Scheme 5). 

The stereospecific deuterium labeling and subsequent photo-oxidation of the olefin 5 and similar olefins 6 and 7 revealed the hidden regioselectivity of the... 

Figure 43 Result on the enantioselective photo-oxidation of an olefin. Products were analyzed by converting the hydroperoxide to alcohol by treatment with triphenylphosphine.

Photo-oxidation of olefins in pyridine solution in the presence of FeCIs has been found to take one of three courses, depending on the substitution pattern of the substrate olefin. These routes lead to formation of a-chloroketones, gem-dichloroketones, and a,-dichloroketones, and have all been illustrated by their use in actual syntheses. a-Chloroketones have also been produced by irradiation of pyridine solutions of vinylsUanes or vinyl sulphides under similar conditions. DCA-sensitized photo-oxidation of l,2-diphenyl-3,3,4,4-tetramethylcyclobutene in MeCN leads to the corresponding ozonide in a process which occurs by initial formation of an oxirane. Jasmine lactone (12 ... 

Since Reactions 1, 2, and 3 are much faster than competing reactions involving olefins, the equilibrium relationship 4 must hold even in the presence of a-pinene (15). The buildup of ozone in Figure 1 above the steady state concentration it shows when no a-pinene is present is therefore accompanied by an increase in the N02. N0 ratio. This increase is effected by reactions which convert NO to NO2 as shown by a few reactions from Wayne s (11) mechanism for the N02-initiated photo-oxidation of an olefin ... 

Moller, K. J. Am. Chem. Soc in press.)(17). This is of interest from the standpoint that such small pieces of a bulk semiconductor lattice cannot fully develop the normal semi- conductor band structure and so reside in the so called size-quantized or quantum-confined regime. This is where the electron-hole pair of an excited semiconductor particle has a radius(18) larger than the actual particle size. The electron then behaves as a particle in a box and novel optical properties result. We decided, therefore, to look at preparation of CdS inside the zeolite cavities and then to explore the photo-oxidation chemistry of these species with absorbed olefins. 

Since all of the CdS clusters reside in the sodalite cages of the zeolite Y framework, the larger supercages of the structure are still available for absorption of substrate molecules - in this case olefins for photo- oxidation via electron transfer. Colloidal CdS in free solution has been used for such oxidations previously(19) and in a competitive oxidation of styrene and 1,1-diphenylethylene we find that unconfined bulk CdS will effect oxidation in a ratio of 1 2 for these two olefins (irradiation at 365nm). In the zeolite confined system we find however that the ratio becomes 1 1 ie a slight shift in selectivity toward the smaller substrate as may be expected on the basis of size/diffusion effects. From the viewpoint of the enzyme mimic, we have here a system... 

A number of other photo-oxidations involving aromatic substrates have been reported, some of which again have been carried out in non-traditional reaction environments. A light-promoted oxidative cleavage of the olefinic bond in aromatic alkenes, which takes place in mesoporous Si02 (FSM 16) and involves a catalytic amount of I2, has been reported (Scheme 23). The photochemical involvement appears to extend no further than the formation of the iodine atoms. The suggestion that an a-iodoketone is an intermediate in the process is based on NMR evidence from the reaction where R = Bu, but it is not clear how this intermediate is converted to the final product. A similar transformation of aromatic alkenes has been carried out in zeolite NaY. Irradiation of styrene,... 

Formation of an olefinic fragment from NOR was proposed as an explanation of the process. Its generation was, however, not confirmed in photo-oxidized PO. O-sec-Alkylhydroxylamines (126) characteristic of species formed in PE, were used in a recent mechanistic study [68,191,195-197]. A new mechanism was proposed for the regenerative process involving reactivity with O-centred radicals ROO and RC(0)00 (Scheme 20). The reactivity with the alkylperoxyl... 

In the presence of FeClj, photo-oxidation of trisubstituted cyclic olefins in pyridine-benzene has been reported to give gem-dichloroketones from which methyl ketones possessing a terminal triple bond can be obtained by dehydrochlorination. For example, under these conditions 1-methylcyclohexene has been converted into (43), an important intermediate in the preparation of brevi-comin (44). These reactions have been interpreted in terms of a long-range... 

Reactions involving excited states of benzene include the photoaddition of benzene to furan,41 to olefins,42 and to amines,43 the photochemical chlorination of benzenes,44 the photo-oxidation of benzene,48 and the photo-reactions of tetrafluoroiodobenzene.48 These papers will be discussed in detail in Part 1 of this volume. The gas-phase structures of benzvalene and hexamethylprismane and hexamethylbenzene have been evaluated.47... 

The photo-oxidation of SOa to S03T in the presence of H20 and Oa or NaO has been monitored by e.s.r.841 The photo-oxidation of carbon monoxide by oxygen may be catalysed by illumination of a number of inorganic oxides,842 and in another study it has been shown that the catalytic activity of ZnO for this reaction depends on the contact time of the catalyst with the gases prior to irradiation.843 Flash illumination of TiOz initiates the dehydrogenation of methanol or ethanol to formaldehyde and acetaldehyde, respectively.844 The oxidation of paraffins and olefins to ketones takes place on irradiation of Ti02 in the presence of these... 

A matter of much dispute has been whether the photo-oxidation of olefins to allylic hydroperoxides involves an ene-reaction or a perepoxide intermediate (see... 

Vol. 6, p. 535). In the past year no more evidence has been produced which strongly supports a perepoxide as an intermediate in this reaction. Jefford and Boschung121 report interesting results on the mechanism of 2 attack on olefins from a study of the methylene blue-sensitized photo-oxidation of the norbornenes... 

Electron-donor (nucleophilic) olefins react with 1Oa to give dioxetans in competition with, or to the exclusion of, formation of allylic hydroperoxides. Foote and co-workers have reported that photo-oxidation of indenes apparently... 

In conclusion, singlet oxygen plays a role in the photo-oxidative degradation of polymers containing olefinic unsaturations. Polymers that do not contain these groups, e.g. poly(vinyl chloride), poly(methyl methacrylate), polystyrene, etc., are unreactive [24]. 

Ring-contraction procedures for generating a four-membered ring fused to a six-membered ring include the application of the Ramburg-Backlund reaction to the synthesis of the hetero-propellanes (357 X = O or SOj), electrocyclic ring closure in the synthesis of the propellanes (358 X = O or NMe), and sensitized photo-oxidation of the olefin (359) in the synthesis of (360). 

The photodegradation and photo-oxidative degradation of different poly-olefins have been subjects of many publications (Table 3.1) and reviews [570, 853, 856, 866, 1999, 2268]. 

In poly(vinyl chloride) subjected to low temperature (170°C) processing or short processing times, mechano-chemically formed hydroperoxides are primarily responsible for the initiation of photo-oxidation [483,1939-1942]. Poly(vinyl chloride) processed for excessive times at high temperatures (210°C) appears to be more sensitive to carbonyl and olefin as well as peroxide photoactivators. Chemical removal of peroxides in processed poly(vinyl chloride) films effectively increases their initial photostability below that of a compression moulded poly(vinyl chloride) film but under these conditions the effect of carbonyl initiation can be seen. 

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