Cycloaddition via a Tetramethylene Intermediate

Computations [12] show that concerted [ 2 2a]-cycloaddition of ethylene is disfavored relative to biradical formation even when the recommended orientation is adopted. The qualitative argument has been made [13] that the HOMO-LUMO interactions along this pathway are less favorable than in the orientation leading to a transoid biradical. All in all, it is hardly surprising that [t 2s +7r2a] cycloaddition of alkenes has only been observed in exceptional circumstances - if at all. For example, Kraft and Koltzenburg [14] observed that bicyclo[4.2.2]-deca- raTis-3,czs-7,9-triene dimerizes to a product with one cis and one trans junction  

The unusual stereochemistry of the product was taken to be compelling evidence for concerted [t,2s- -t,2q cycloaddition [2, p. 75 ff.], until the reaction was shown [15] to proceed by a stepwise mechanism. 

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