Nucleophilic attack on the cyclohexadienyl ligand: From mono- to heterobifunctional (cyclohexadiene)iron complexes and the role of the electron-transfer pathway in hydride abstraction

Article abstract of DOI:10.1021/om00112a017

With the aim of synthesizing heterobifunctional cis-1,2-cyclohexadienes from benzene, we have investigated hydride abstraction from exo-substituted complexes [Fe(C₆H₆)(η⁴-C₆H₇R)] (3) and the second addition of a functional nucleophile to the functional complexes [Fe(C₆H₆)(η⁵-C₆H ₆R)]⁺ (2). Although hydride abstraction by [Ph₃C]⁺ from [Fe(η⁵-C₆H₆R)(CO)₃] (R ≠ H) was known to be sterically inhibited, we first attempted to remove a hydride from the known complexes 3 (R = CH₂Ph (3b), CHS(CH₂)₃S (3c), CH-(CO₂Et)₂ (3d), CN (3e)) according to an electron-transfer (ET) pathway. The ET between 3b and [Ph₃C]⁺ is exergonic (ΔG° = -5.5 kcal/mol^-1) and proceeds at -50°C, giving Ph₃C° and the 17e Fe^I complexes 3b⁺, both characterized by ESR spectroscopy. The species 3⁺ are stable at -50°C for R = CH₂Ph or CHS(CH₂)₃S, and slow H atom transfer to Ph₃C° occurs between -50 and -40°C to give the desired cations [Fe-(C₆H₆)(η⁵-C₆H ₆R)]⁺ (2) in high yields. However, the 17e Fe^I complex is not stable at -50°C with R = CH(CO₂Et)₂ and decomposes to R° and [Fe(C₆H₆)(η⁵-C₆H ₇)]⁺ (2a) in 70% yield. Above -40°C, 3b⁺ and 3c⁺ also decompose to R° and 2a faster than H atom transfer occurs. Addition of KCN to 2b and 2c in acetone at 20°C gives the heterobifunctional (cis-1,2-cyclohexadiene)iron(0) complexes 4b and 4c. The X-ray crystal structure of 4b has been reported and its decomplexation using FeCl₃ gives cis-1,2-PhCH₂CNC₆H₄ (5) free of any rearomatized product (the latter is easily formed thermally).