[36654-49-6] · C9H21Cl6Fe2N3O3 · Iron(III) Chloride-Dimethylformamide · (MW 543.73)
Alternate Name: ferric chloride-DMF.
Physical Data: mp 220 °C.
Preparative Method: N,N-Dimethylformamide (1.5 mol) is added with stirring to a solution of Iron(III) Chloride (1 mol) in dry ether (1.6 L) to yield a precipitate which is filtered and dried in vacuo to yield a yellowish green powder (95%) which is sufficiently pure to use for oxidation reactions.2
Purification: recrystallize from dichloromethane and ethanol to give needles.
Handling, Storage, and Precautions: stable to air and moisture; FeCl3 is hygroscopic and corrosive; inhalation or ingestion may be fatal; DMF is mildly toxic by skin contact and inhalation; it is a skin and eye irritant. Use in a fume hood.
Oxidative aryl-aryl coupling represents a challenging synthetic transformation. Iron(III) chloride in DMF or [Fe(DMF)3Cl2][FeCl4] is a good reagent for intramolecular (eq 1) and intermolecular (eq 2) phenol coupling reactions.1,2 As the redox potential of substrates differ, many alternate reagents exist for this transformation (intiated by one-electron abstraction from the substrate by the oxidant). Alternative reagents used are Vanadyl Trichloride,7 Thallium(III) Trifluoroacetate,8 manganese(II) acetylacetonate,9 Potassium Ferricyanide,10 copper(II) amine complexes,11 photolysis,12 (Diacetoxyiodo)benzene,13 electrochemical anodic oxidation,1a and FeCl3 in the solid state.14
Addition of anhydrous FeCl3 in DMF to an enolate in THF yields the 1,4-ketone (eq 3).3 This reaction has also been applied in an intramolecular fashion (eq 4).15
Lithium salts of allylic sulfones are oxidized by addition of [Fe(DMF)3Cl2][FeCl4] at 0 °C (eq 5) to give 1,6-disulfones by 3-3´ coupling.4 These products can undergo Cope rearrangement to 1,2-disulfones.
Trimethylsilyloxybicyclo[n.1.0]alkanes (available by Simmons-Smith reaction of enolates of cycloalkanones) can be oxidized by FeCl3 in DMF at 0 °C to afford the one-carbon ring homologation product (eq 6).5 This method has been utilized to synthesize 3-methoxycycloheptatrienones (eq 7).16
[Fe(DMF)3Cl2][FeCl4] is an excellent reagent for oxidative workup of chromium tricarbonyl complexes to obtain the arene ligand or aryl product.6 Cerium(IV) Ammonium Nitrate represents a viable alternative.17
Andrew D. White
Parke-Davis Pharmaceutical Research, Ann Arbor, MI, USA