Reactivity of co-ordinated ligands. Part VI. Electrophilic substitution and addition reactions of co-ordinated cyclo-octatetraene

Abstract

Tricarbonyl(cyclo-octatetraene)iron undergoes both electrophilic addition and substitution reactions. Under Friedel–Crafts conditions, with acetyl chloride and aluminium trichloride, acetylation occurs to give two derivatives, viz. a bicyclic addition product, the tricarbonyl(8-acetylbicyclo[5,1,0]octadienylium)iron cation, and a neutral substituted complex, tricarbonyl(acetylcyclo-octatetraene)iron. The cationic complex is related to that obtained by the protonation of tricarbonyl(cyclo-octatetraene)iron with sulphuric acid. On reaction with methoxide ion, nucleophilic addition occurs to give tricarbonyl-(8-acetyl-6-methoxybicyclo[5,1,0]octa-2,4-diene)iron. The neutral acetyl derivative is readily reduced by sodium borohydride to the corresponding secondary alcohol tricarbonyl-(1-hydroxyethylcyclo-octatetraene)iron. Reaction of this alcohol with hexafluorophosphoric acid produces a carbonium ion complex which reacts subsquently with methoxide ion to give tricarbonyl(methoxymethylcyclo-octatetraene)iron. Possible structures of this carbonium ion are discussed. Formylation of tricarbonyl(cyclo-octatraene)iron produces tricarbonyl(formylcyclo-octatetraene)iron. The chemical properties of the aldehyde group appear to be normal and it undergoes a series of reactions similar to those described for the acetyl derivative.