Continuous diradical transition state

Gilbert Higley Tetrahedron Lett. 1973, 2075 Caramella Huisgen Schmolke /. Am. Chem. Soc. 1974, 96, 2997, 2999 Mazzocchi Tamburin /. Am. Chem. Soc. 1975, 97, 555 Zimmerman Fleming /. Am. Chem. Soc. 1983, 105, 622 Klumpp Schakel Tetrahedron Lett. 1983, 24, 4595 McGafhn de Meijere Walsh Chem. Ber. 1991, 124, 939. A "continuous diradical transition state has also been proposed Doering Sachdev J. Am. Chem. Soc. 

The cyclic orbital interaction of p and q with o or with o can significantly occur at the transition state of the ring closure of 1,3-diradicals. The continuous orbital phase for the cyclic orbital interaction with o implies effective stabilization of the transition states when the o bonds are electron acceptors. 

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

Doering, W. v. E. Sachdev, K. Continuous diradical as transition state. Internal rotational preference in the thermal enantiomerization and diastereoisomerization of cis- and trans-l-cyano-2-isopropenylcyclopropane, 7. Am. Chem. Soc. 1974, 96, 1168-1187. 

A homolytic cyclopropane C-C bond rupture that would furnish in turn a 1,3-diradical is also conceivable. However, it is always difficult to establish whether a purely diradical or ionic mechanism is in operation. Between these two extremes there exists a graded continuum of polarized diradicals of which the zwitterion represents the end of the spectrum. In addition, the continuous development of radical character during the formation of the transition state of a homolytic bond scission, called the continuous diradical, has been postulated to explain the behavior of some reactions. Alternatively, the contribution of a truly concerted transformation cannot be overlooked. ... 

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