Reduction reactions pentacarbonyliron

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

A plausible mechanism for the reaction includes several organometallic species that are sensitive to reactive moieties elsewhere in the molecule. If a chloro, chloromethyl, or mesyloxymethyl substituent is attached vicinal to the 1,1 -dibromo moiety, efficient ring opening occurs prior to carbonylation and P,y- and y, -unsaturated acid derivatives are formed. Reductive carbonylation has also been achieved with 1,1-dibromocyclopropanes using an excess of pentacarbonyliron in dimethylformamide with added methanol or sodium methoxide, or cobalt(II) chloride and nickel(ll) cyanide under phase-transfer conditions in a carbon monoxide atmosphere. However, the yield of cyclopropanecarboxylic acid derivatives is low, and when pentacarbonyliron is used the amount of monobromides is fairly high. ... 

Reduction of tertiary amides to tertiary amines can also be carried out under either thermal or photochemical conditions using 1,1,3,3-tetramethyldisiloxane (TMDS) as the reducing agent and pentacarbonyliron or dodecacarbonyltriiron as catalyst. The corresponding photo-assisted reaction promotes the reduction at room temperature. The reduction of aromatic nitro compounds to amines using carbon monoxide and water... 

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