Allylic photo-sensitized oxidation

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] ... 

Photocatalytic activation of allyl and benzyl ethers results either in carbon-carbon coupling or oxygenation [148-151]. The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt, by preformed catalysts, or by particles supported within surfactant vesicles. Radical intermediates formed by hydrogen abstraction by photogenerated hydroxyl or hy-droperoxyl radicals may also be important in the photoelectrochemically induced oxidation of hydrocarbons. In the Ti02-sensitized photooxidation of toluene to cresols, for example, a photo-Fenton (radical) type mechanism has been suggested [150]. ... 

Reusability is a characteristic of the sensitizers prepared by stirring Ceo-fullerene with aminomethylated poly(styrene/vinylbenzene). They have been used to promote the standard O2 oxidation processes such as ene and Diels-Alder reactions (Scheme 7), and catalysts suitable for photoxidations in water have been prepared from them by reaction with poly(allylamine). The same reactions have been carried out using a novel solvent-free procedure which involves loading a porphyrin into solvent-swollen polystyrene beads and carrying out the photo-oxidation in neat liquid substrate. The formation of the allylic hydroperoxide (89) from p-pinene, with complete conversion and in 84% yield, is particularly noteworthy, as the standard liquid-phase reaction can be problematic. It is suggested that the possibility of using this approach under solar conditions is further evidence of the sustainable, green chemistry potential of synthetic photochemistry. 

The photooxygenation of y- and 6-pyronenes produces the expected hydroperoxides and endoperoxides (Scheme 10), further reaction of which (thermolysis, acid and base heterolysis, etc.) leads to the formation of materials with potential as perfumery or therapeutic agents. An endoperoxide hemiacetal and two allylic hydroperoxides were obtained from the methylene blue sensitized photo-oxidation of racemic phythol (Scheme 11). °... 

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