[17658-52-8] · C12Fe3O12 · Dodecacarbonyltriiron · (MW 503.67)
Physical Data: mp 140 °C dec.1
Solubility: partially sol organic solvents.
Form Supplied in: black air-sensitive solid, stabilized with ~10% methanol which can be removed by leaving on a vacuum pump at 25 °C for at least 5 h.2
Analysis of Reagent Purity: IR, 13C NMR.3
Purification: sublimation at 70 °C/0.1 mmHg.1b
Handling, Storage, and Precautions: store under an inert atmosphere in a freezer; old samples may ignite spontaneously upon exposure to air; emits acrid smoke and fumes when heated to decomposition.1b,4 Use in a fume hood.
Methanolic solutions of dodecacarbonyltriiron reduce aromatic nitro groups to primary aromatic amines while leaving a variety of functional groups unaffected (Table 1).5a Dienes are not reduced by this reagent, but do form a complex with an iron tricarbonyl fragment. These reactions occur under conditions which produce the hydridodecacarbonyltriferrate anion [HFe3(CO)11-], which is probably the active reducing agent.
Imines are also reduced in reasonable to good yields under the same conditions. Interestingly, the reagent will reduce phthalazine selectively to 1,2-dihydrophthalazine, constituting one of a few methods to accomplish this selective transformation (eq 1).5b
A number of improvements have been made to the original dodecacarbonyltriiron reduction. A significant rate acceleration was accomplished under phase-transfer conditions in which hydroxide rather than methanol is responsible for the formation of hydridodecacarbonyltriferrate anion (Table 2).6a In a similar manner, use of 18-Crown-6 in conjunction with 1 N Potassium Hydroxide in a two-phase reaction medium also leads to short reaction times and good yields of aromatic amines.6b A very convenient method for performing these reductions involves stirring the aromatic substrate in hexane solution overnight with dodecacarbonyltriiron adsorbed on basic alumina. Yields obtained from this procedure are good, conditions are significantly milder than the homogeneous reaction, and the workup is quite simple.6c
Dodecacarbonyltriiron has been reported to catalyze the cyclotrimerization of alkynes, forming substituted benzenes. For example, Diphenylacetylene forms hexaphenylbenzene upon heating with dodecacarbonyltriiron in a sealed tube (eq 2).7a,7b Hexacyclopropylbenzene has also been prepared in this manner from dicyclopropylacetylene in 20% yield with 2,3,4,5-tetracyclopropylcyclopentadienone being formed in ~30% yield as the major byproduct (eq 2).7c In fact, cyclopentadienones can be obtained using this reagent and they may be used as precursors to substituted benzenes (eq 3).7b,7c
Many metal carbonyl complexes are known to isomerize alkenes, often giving thermodynamic mixtures of alkene isomers. Usually unconjugated dienes are brought into conjugation.1f This latter property has been exploited in the isomerization of the cyclopentene double bond of prostaglandin precursors. The free diene is obtained upon oxidation with excess Collins reagent (Dipyridine Chromium(VI) Oxide) (eq 4).8a
More recently, it has been shown that dodecacarbonyltriiron will photochemically catalyze the isomerization of allyl alcohols to ketones or aldehydes and allyl ethers and acetates to the corresponding enol ethers and acetates (eqs 5-8).8b These isomerization reactions apparently involve a p-allyl-iron hydride intermediate.8c The inaccessibility of the allylic hydrogen may account for the exo-alcohol's relative inertness to the reagent (eq 8).8b
Episulfides are stereospecifically desulfurized by dodecacarbonyltriiron. Thus when heated at reflux in benzene with dodecacarbonyltriiron, cis- and trans-2-butene episulfide form cis- and trans-2-butene in 95:5 and 3:97 ratios, respectively.9
Thomas E. Snead
Emory University, Atlanta, GA, USA