Diradicals and carbenes

The methods for generating these types of ions are the same as those described above for diradicals and carbenes. Eor example, the reaction of dichloromethane with O leads to the formation of CCl2. Thus, Jesinger and Squires have used CID of halocarbene anions to determine the thermochemical properties of carbynes (Eq. 5.16). ... 

A mechanistic explanation of this transformation of the epoxyhexenynes (325) to furo- and thienofurans (326) and (327) has been proposed <93CB975>. The reaction pathway leading to the products (326) and (327) presumably proceeds via the carbonyl ylides (330) which undergo a 1,7-dipolar cyclization to the allene derivatives (331). These are subsequently transformed into the furo[3,4-6]furans via a pathway involving diradical and carbene intermediates. 

In this chapter, we focus on the class of reactive intermediates that bear at least two unpaired electrons diradicals and carbenes. The exact definition of a diradical is somewhat in the eye of the beholder. Salem and Rowland provided perhaps the most general, yet effective, definition—a diradical is a molecule that has two degenerate or nearly degenerate orbitals occupied by two electrons. With this definition, carbenes can be considered as a subcategory of diradicals. In a carbene, the two degenerate molecular orbitals are localized about a single carbon atom. 

Similar to their open chain counterparts, fused thiirenes are highly reactive substances due to their antiaromatic character <1996CHEC-II(1)174>. They are thought to be in equilibrium with diradicals and carbenes (Scheme 1). In addition to behaving as dienophiles, they can therefore undergo typical carbene and radical reactions <1987TL2867>. 

A reaction scheme showing the individual steps of the low temperature photopolymerization reaction of the DR and AC reaction intermediates and of the SO, which are the final reaction products, are taken from the optical absorption spectra. All molecules shown in Fig. 16 have been observed in either the optical spectra or ESR-spectra or in both with the exception of only the short lived monomer species. The reactive chain ends are best represented by the diradical and carbene structures, shown in the Figure. 

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