2.3- Diphenyl-2-butenes, stereochemistry

Another characteristic of photocycloaddition to electron-rich alkenes is the loss of any stereochemistry of the starting alkene in the oxetane structure. An example is the formation of practically the same mixture of geometric isomers of 2,2-diphenyl-3,4-dimethyloxetane from benzophenone and either cis- or trans- 2-butene (equation 103). This is understandable on the basis of the diradical intermediate having a sufficiently long lifetime for bond rotations to occur. 

With few exceptions, the stereochemistry of the hydrogenation of a carbon-carbon double bond is the result of syn addition of two atoms of hydrogen from the catalyst to the proximal face of the double bond. A classical demonstration is given by the hydrogenation (Pd, AcOH, 1 atm) of the (Z)- and ( )-isomers of 2,3-diphenyl-2-butene (15) and (16) which yield respectively the meso (98%) and ( ) (98%) isomers of 2,3-diphenylbutane (equations 14 and 15). Likewise the hydrogenation of dimethyl 1,2-cyclohexenedicarboxylate yields only the cis saturated isomer. Such stereospecificity is not always found. 

The sfereochemisfry of the more highly substituted alkenes is difficult to define using the cis and trans designations. Therefore, a more systematic manner of indicating stereochemistry in these systems has been developed that uses the E and Z nomenclature system. Draw the structures of the E and Z stereoisomers of l,4-diphenyl-2-butene-l,4-dione used in this experiment. In this case, which is cis and which is trans ... 

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