[65522-66-9] · C24Fe · Iron-Graphite · (MW 344.09)
Analysis of Reagent Purity: the determination of iron and potassium content by means of atomic absorption spectroscopy has been described.1
Preparative Method: Potassium-Graphite (C8K), prepared from 1.17 g of potassium (30 mmol) and graphite (3 g), was covered with THF (15 mL) and a solution of Iron(III) Chloride (1.62 g, 10 mmol) in THF (20 mL) added dropwise with stirring. After 30 min, the solution was heated to 70 °C for 10 min.
Handling, Storage, and Precautions: freshly prepared C24Fe is used in situ under argon. Air exposure results in complete loss of reactivity. Potassium-graphite reacts violently with water, and care should be taken to avoid such exposure.
Vicinal dibromides undergo highly stereospecific debromination to give alkenes when treated with a slurry of iron-graphite (C24Fe) in wet THF at reflux (1 mmol in 10 mmol C24Fe, THF 35 mL, H2O 1 mL). Elimination is anti, such that anti-dibromides give (E)-alkenes and syn-dibromides give (Z)-alkenes (eq 1). Based on a limited number of examples, diastereomerically pure dibromides give product alkenes of &egt;97.5% geometric purity. 1,2-Dibromoalkanes give terminal alkenes without detectable amounts of 2-alkenes.
C24Fe debrominates a-bromo ketones to the parent ketones in high yields under conditions nearly identical to those for dibromoalkanes. Replacement of water by D2O allows high levels of deuterium incorporation, while attempts to react other electrophiles with the presumed iron enolate intermediate gives only modest yields of condensation products (eq 2).
a,a´-Dibromo ketones give oxyallyl cations under exposure to iron-graphite in benzene, which have been reacted with enamines or furan to give the products of [3 + 2] (eq 3) or [3 + 4] (eq 4) cycloaddition, respectively.
James R. Green
University of Windsor, Ontario, Canada