Stereochemistry of the Photoadduct

It was indicated earlier that a diene intermediate is proposed to form on photochemical cycloaddition of MA to benzene. The diene subsequently reacts with another mole of MA to form the observed diadduct. Thus, even if the 

DA reaction was to give exclusively an endo product, two possible products, 14 and 15, could form. On the basis of extensive spectroscopic and chemical 

In the reaction of phenanthrene derivatives, although syn and anti geometries are possible, originally no stereochemistry of the adduct was specified. Recently, Creed et have confirmed the phenanthrene MA adduct to be cis-exo, hence, anti geometry was preferred. Shields et showed that with acenaphthene, adduct formation, which is solvent dependent, gave more anti product. 

It was recognized quite early that the mechanism of the sensitized and unsensitized photoreactions are different. Bryce-Smith and Lodge showed that uhsensitized reaction is not affected by the presence of O2 while, on the other hand, the sensitized reaction is inhibited by O2. Thus, the unsensitized reaction was believed to be singlet in character whereas the sensitized process was considered to proceed through a triplet. 

The presence or absence of the diene intermediate has caused some confusion in the past. Hardham and Hammond failed to trap the diene 2 

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