Synthesis of (η6-arene)(η5-cyclopentadienyl) iron (II) complexes with heteroatom and carbonyl substituents. Part I: Oxygen and carbonyl substituents

A series of reactions have been used to introduce oxygen substituents into (η-arene)(η-cyclopentadienyl) iron (II) complexes. Photochemical ligand exchange led to the formation of the first recorded trioxygenated complex as well as mono- and di-oxygenated species. Using microwave techniques, reaction times for SNAr displacement reactions of halobenzene complexes by phenols were reduced from several hours to a few minutes. Phenols protected by either t-butylation or trimethylsilylation were found to give modest yields of the corresponding phenol complexes, using conventional thermal ligand exchange reactions. Without such protection, yields were extremely low. The above method led to the synthesis of the first example of a dihydroxybenzene complex. Some miscellaneous syntheses are also reported. The Nef reaction has been adapted to convert (η6-α-nitroalkylarene)(η5-Cp) iron (II) salts to corresponding aldehyde and ketone complexes. The α-nitroalkyl arene complexes were synthesised in good yields from (η6-halobenzene)(η5-Cp) iron (II) complexes using NaOtBu in DMSO. H/D exchange reactions with 2[H]6-DMSO in the presence of K2CO3 showed partial D incorporation in the methyl group for the unreacted α-nitroethylbenzene complex and complete exchange for the carbanion generated by deprotonation. Conversion of the α-nitroalkylarene complexes to the corresponding aldehyde and ketone complexes was accomplished in moderate yields using three methods: (A) H2O2 and NaOtBu in DMSO followed by reaction with CF3CO2H. (B) SnCl2/aq. HCl. (C) K2CO3 in DMF using microwave-mediated reactions. In addition, two one-pot syntheses are reported using methods B and C.