Formation of cyclobutanes in thermal addition reactions

In reactions with tetracyanoethylene, the stereochemistry of the double bond of an enol ether is retained in the cyclobutane product when the reaction is carried out in nonpolar solvents. In polar solvents, cycloaddition is nonstereospecific in accordance with a longer lifetime for the zwitterionic intermediate.  

Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital-symmetry relationships and was discussed in detail in Part A, Chapter 10. 

The reaction types permitted by photochemical excitation that are particularly useful for synthesis are 2 + 2 additions between two carbon-carbon double bonds, and 2 + 2 addition of alkenes with carbonyl compounds leading to oxetanes. Many 

Intermolecular addition of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength sources. Relatively little preparative use has been made of simple alkenes, however. Dienes can be photosensitized using such materials as benzophenone, butane-2,3-dione, and acetophenone. Under these conditions, preparatively useful yields of stereoisomeric mixtures of dimers are obtained. Usually, some 4 + 2 adduct accompanies the dominant 2 + 2 adduct. 

The photodimerizations of cinnamic acids were among the earliest photochemical reactions to be studied.These compounds give good yields of dimers 

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Scheme 7.5. Formation of Cyclobutanes in Thermal Addition Reactions...

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