Biradical cyclobutadiene like

Photolysis of tetrakis(trifluoromethyl)thiophen gives (447), which adds to furan at room temperature to give (448). Desulphurization with PhjP gives (449), which on irradiation with a low-pressure Hg lamp gives (450). Three mechanisms for this conversion are considered. A route via a biradical (452) is discounted since the triplet scavenger pentadiene has no effect on the reaction course. Retro-Diels-Alder reaction to give the furan and a cyclobutadiene followed by readdition is one possibility, and isomerization to (451) and reclosure seems even more likely. ... 

A comparison of the three dimerizations discussed in the latter part of this chapter illustrates nicely the interplay between symmetry and energy The presence of an additional tt bond in cyclobutadiene offers enough energetic advantage to concerted closure of the four-membered ring to syn-TCOD for it to take precedence over the more general - and no less allowed - stepwise pathway via a transoid biradical. Cyclopropene, with just the one 7r-bond, behaves like a normal alkene and finds the latter pathway more convenient. Silacyclopropene starts off along a similar pathway, but makes use of the relative weakness of the CSi bonds to react in an entirely different manner, but one that is still consistent with the requirements of orbital symmetry conservation. 

---