Reactivity of co-ordinated ligands. Part VII. Electrophilic substitution and addition reactions of co-ordinated cycloheptatriene

Abstract

Under Friedel–Crafts conditions, with acetyl chloride and aluminium trichloride, tricarbonyl(cycloheptatriene)iron undergoes both electrophilic addition and substitution to give tricarbonyl(2–6-η-1-acetylcycloheptadienium)iron, isolated as the hexafluorophosphate salt, and tricarbonyl(1-acetylcyclohepta-1,3,5-triene)iron. Reaction of tricarbonyl(cycloheptatriene)iron with acyl tetrafluoroborates (RCO)BF₄(R = Me or Ph) yields only addition salts of the tricarbonyl(2–6-η-1-acetylcycloheptadienium)iron type. These salts react with methoxide ion to give the ethers, tricarbonyl(1-acyl-6-methoxycyclohepta-2,4-diene)iron, which readily undergo 1,2-elimination of methanol on silica gel to yield substitution products. Formylation of tricarbonyl(cycloheptatriene)iron produces tricarbonyl(formylcycloheptatriene)iron. This aldehyde is readily reduced to the corresponding primary alcohol. This alcohol on treatment with hexafluorophosphoric acid gives tricarbonyl(2–6-η-1-methylenecycloheptadienium)iron hexafluorophosphate which contains an exocyclic double bond. The acetyl and formyl derivatives undergo protonation in acid media yielding ring and oxygen protonated species respectively. Reaction of tricarbonyl(cycloheptatriene)iron with the Simmons–Smith reagent produces hexacarbonyl[bi(cyclohepta-2,4,6-trienyl)]di-iron by hydrogen abstraction. The stereochemistry of the electrophilic addition to tricarbonyl(cycloheptatriene)iron is discussed.