Ferrocenes boryl-substituted

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]. 

However, in contrast to benzene, ferrocene is sensitive to oxidation, and the ferrocenium cation, FeCpj, a paramagnetic 17-electron species, is readily formed in the presence of various oxidants. The ferrocenium cation is reluctant to undergo electrophilic substitution, and therefore reactions such as halogenation and nitration, which are important routes to substituted benzene derivatives, cannot be used for the synthesis of substituted ferrocenes. Only electrophilic substitution under nonoxidizing conditions (e.g., Friedel-Crafts acylation, Mannich reaction, borylation, lithiation or mercuration), and radical substitution are available as an entry into the chemistry of substituted ferrocenes. 

The dihaloboryl-substituted ferrocenes can be used to prepare various other boryl derivatives thus, FC-BX2 compounds (X = Cl, Br or I) have been converted into FC-BR2 (R = NMe2 [4, 7], NEt2, OEt, SMe, Me [4, 5]). As an example, the synthesis... 

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