Bond relocation, aromaticity

Irradiation of the biscyclopropenylmethane la in the presence of thioxanthone as a triplet sensitizer gave tetraphenylbicyclo[2.2.1]heptadiene 3a by bond relocation in an intermediate strained quadricyclane 2a.A similar cyclization occurred for the 1,2-biscyclo-propenylethane derivative lb, while in the case of 4 aromatization was observed either on photolysis in diethyl ether or thermolysis for 24 hours at 175 C. In the same manner, the sulfone 5 was converted into a hexasubstituted benzene. ... 

Formation of crosslinks, typically accompanied by elimination of associated moieties, is a common occurrence and serves as the basis for the formation of graphite, carbon and boron-containing fibers. This is particularly true when nonaromatic double and triple bonds are present such as 7 or where aromaticity can be gained through bond relocation. 

Ni° complex leads to M-C bonds that are close in length to a single CT-bond [2]. It should be noted that such an effect involving the relocation of roughly 0.5 electrons within the aromatic ring is connected with the acceptor strength of nickel, since the bond-length distribution in the isoelectronic copper complex is quite different [2]. 

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