Bicyclo deca-3,7,9-triene dimerization

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 ... 

Largely similar behavior is evident in the dimerizations of c ,franj-l,3-cyclooctadiene (38), tranj,ciJ-2,4-cyclooctadienone (39) ° and bicyclo[4.2.2]deca-3-franj,7,9-triene (40). ... 

The dimerization of unstrained alkenes is not known to occur by the pathway shown in Figure 11.70, but there was a report that the dimerization of bicyclo[4.2.2]deca-frflMS-3,cis-7,9-triene (68, equation 11.26) gave as the major product 69, in which the stereochemistry of the product is consistent with that predicted in Figure 11.70. (Two other, unidentified adducts were also formed in the reaction.) Padwa and co-workers found, however, that the thermal dimerization of cis,frans-l,3-cyclooctadiene (70) produced three dimers plus other products (equation 11.27). The major dimer, 71, was the product predicted by the [ 2 -1- pathway, but dimers 72 and 73 were also foimd. The results suggested that the formation of all of the dimers could be explained on the basis of closure of a biradical intermediate, 74. Therefore, it appears that a concerted pathway is not required to explain formation of 69, and there are no clear examples of concerted [ 2 -I- 2 ] dimerization. ... 

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