Allylic hydroperoxides photooxygenations

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

Selective fomation of allylic hydroperoxides via singlet oxygen ene reaction 837 TABLE 2. Syn/anti regioselectivity in the photooxygenation of aUyUc alcohols 10- and 10-Z... 

IV. SELECTIVE FORMATION OF ALLYLIC HYDROPEROXIDES BY DYE-SENSITIZED INTRAZEOLITE PHOTOOXYGENATION... 

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. 

For geminal dimethyl trisubstituted alkenes, such as 35 or 39, photooxygenation in solution gives a nearly solvent independent ratio of secondary to tertiary allylic hydroperoxide of ca In addition, for the case of 1-methyl-l-cycloalkenes °, such as 123 and... 

The intrazeolite photooxygenation in a series of isobutenylarenes (20, 137-146), bearing either electron-withdrawing or electron-deficient substituents on the aryl ring, affords rapidly the ene allylic hydroperoxides as the major or even exclusive products (Table 22). The relative yield of the ene adduct was always higher than 85%. 

Highly regioselective ene reactions of singlet oxygen with alkyl-substituted alkenes can also be used for synthetic purposes. For example, in the photooxygenation ofl-t-butylcycloheptene only one allylic hydroperoxide, by the exclusive geminal hydrogen... 

SCHEME 27. Transition states leading to the major and the minor allylic hydroperoxides for the self-photooxygenation reactions of [60]fullerene adducts 102-106... 

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