Oxygen singlet-state: cycloaddition

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

If the alkene or alkadiene has at least one hydrogen on the carbon adjacent to the double bond, reaction with singlet oxygen may give hydroperoxides. The mechanism of this reaction is related to [4 + 2] cycloadditions and is presumed to occur through a HLickel pericyclic transition state (see Section 21-10D) ... 

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