Question

What is implied by the term “superaromatic”, as applied to the reactivity of ferrocene toward electrophilic aromatic substitution? How can this be used to explain the fact that no Lewis acid catalyst is required during the Friedel-Crafts acylation of ferrocene?   

Answer 1

As we all know about the ''Aromaticity'' well define by Huckel rule but the term ''Superaromaticity'' seems a term related to extra stabilisation that one could expect from a highly extended conjugated ring system over and above an extended linear one. More precisely , it seems to have been most applied to ring systems made exclusively of fused or interconnected benzene rings.

'' By Superaromaticity we mean aromatic stabilization due to cyclic motion of pi-electrons along the macrocyle.''

Ferrocene is a superaromatic (where delocalisation of e^- density in the pi-bonds of ring carbons occurs and Fe's e^- are also dispersed in the pi-orbitals for additional stabilization) , starting point of many synthesis, for instance of friedyl-crafts type.

The Friedyl-Crafts acylation of benzene derivatives requires aluminium chloride , a strong lewis acid as a catalyst. In contrast , Ferocene , a ''superaromatic'' compound, will readily react under milder conditions, with acetic anhydride and phosphoric acid as the catalyst. Since acylation reactions put a deactivating group on an aromatic ring, and the risk of getting di- or poly- acylated product should be minimal, but some diacetylferrocene might be formed.For proof, in this synthesis, you will isolate and purify unreacted ferrocene, acetylferrocene as well as any poly-acylated products.