Aryl-substituted cyclobutadienes

Closely related to the dimerization of biphenylene (36) to tetraphen-ylene (37, Scheme XV) is the dimerization of an aryl-substituted cyclobutadiene to octadienes or cyclooctadienes by way of nickel complexes. A useful source of the cyclobutadiene group is its air-stable complex with NiBr2. Reduction of this complex with tert-butyllithium (electron-transfer agent) gives the tetraphenylcyclobutadiene-nickel(0)-triphenylphosphine complex (38), which isomerizes to the nickelole (39). The dimerization of 39 leads to 40, whose protonation yields the octadiene. Alternatively, at higher temperatures, 40 can extrude Ni(0) to produce 41 (26, Scheme XVI). 

Eq. (86) may be generalized to include aryl fused cyclobutadienes. In this case K should be substituted by ASC. However, a correction term for the presence of the four-membered rings should also be added to the equation. This was actually done by Wilcox [82],... 

The resonance between 55 and 56 (R = H, alkyl, aryl, ferrocenyl) in ferrocenyl-methyl cations including the diferrocenylmethyl cation has been used by Cais to explain the similar bending of the exocyclic C,C bonds toward the iron atom (Scheme 10.20) [65].Thefieldhas recently been reviewedbyGleiteref a/. [62],who included related isoelectronic boryl substituted systems. Wagner et al. [66] have prepared compounds of this type, and the authors proposed attractive interactions between the iron and boron atoms, because the C-B bonds are bent toward the iron atom. The analogy does not only apply to ferrocenes but also to isoelectronic (cyclobutadiene)(cyclopentadienyl)cobalt complexes and other related compounds [62]. 

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