Kinetics and mechanism of the reaction of [Et4N][HFe(CO)4] and alkyl halides. The unexpected formation of acetone

Article abstract of DOI:10.1021/om00136a025

When [Et₄N][HFe(CO)₄] is treated with methyl iodide in CH₃CN at 25.0 ± 0.2°C, a clean second-order reaction ensues as monitored via the change in the intensity of ν_CO of the metal carbonyl hydride. In all cases, yields of CH₄ were 85-100%. A second-order rate constant (first order in iron and in CH₃I) of (1.7 ± 0.1) × 10^-2 M^-1 s^-1 was determined, and activation parameters of E_a = 11.2 ± 1.0 kcal/mol, ΔH? = 10.6 ± 1.0 kcal/mol, and ΔS? = -33 cal/(mol K) were found over the temperature range 15-35°C. The iron product from this reaction is thought to be the unstable Fe(CO)₄(NCCH₃) , which can also be generated from Fe(CO)₅ in CH₃CN by irradiation or use of (CH₃)₃NO. Reaction of iron hydride with 6-bromo-1-hexene produced only 1-hexene and no detectable methylcyclopentane. Reaction with cyclopropylcarbinyl bromide yielded primarily methylcyclopropane and lesser amounts of 1-butene and cis- and trans-2-butene, although evidence is presented to suggest nonrigorously that this is a side reaction. A simple nucleophilic substitution is proposed as the rate-determining step. During this study acetone was detected and confirmed as a reaction product by isotope-labeling studies. A mechanism for this latter observation is discussed.