Alkenes photosensitized oxygenation

When alkenes are treated with oxygen that has been photosensitized (p. 316), they are substituted by OOH in the allylic position in a synthetically useful... 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

Photolysis, in the presence of oxygen, of alkenes containing an ally lie hydrogen atom leads to the formation of hydroperoxides. The sensitized process is more efficient, and often yields photoproducts different in structure from those obtained by nonsensitized photooxidation. Cyclohexadiene and related dienes on photolysis in the presence of oxygen yield the transannular peroxides. Thus, on photosensitized oxidation, a-terpinene (410) is converted into ascaridole (411).435 The equivalent process is not, in general, observed in acyclic dienes. Certain polynuclear aromatic hydrocarbons, such as anthracene and naphthacene and including the heterocycles 5,10-diphenyl-1-... 

This is a useful route for the preparation of aldehydes and ketones from alkenes, and is covered in this section. Photosensitized oxidative cleavage of alkenes occurs in reasonable yield using p-dimethoxybenzene in the presence of oxygen (equation 31)157. The products are aldehydes or ketones depending upon substrate structure. 

Singlet molecular oxygen (lC>2) is a very small electrophilic reagent that can produced by photosensitization and react easily with dienes and alkenes in [4 + or [2 + 2] cycloadditions or ene-reactions. In order to develop synthetic applic tions of photooxygenations, numerous studies have recently been developed wi chiral substrates and especially in the presence of chiral auxiliaries. 

Other Addition Reactions.—The influence of conformation on the steric course of the photosensitized oxidation of steroidal and other endocyclic alkenes has been discussed. The major products of singlet oxygen reactions of 19-nor-A -steroids were the A -Sa-hydroperoxides resulting from preferred a-face... 

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