Iron carbonyls, exchange reactions

As seen from Table III, iron pentacarbonyl reacts satisfactorily in spite of its inertness towards carbon monoxide substitution under the normal conditions 189). In benzene at 80° C, however, Fe(CO)5 dissociates rapidly (190). The Fe(CO)4 generated displays a nucleophilic reactivity which should promote an A-type mechanism. In spite of the specificities discussed, Maitlis et al. 177) have proposed the following mechanism for the metal carbonyl exchange reactions. 

In contrast to thermal uncatalyzed reactions of /V-acylsulfinylamines with aryl isocyanates which give rise to azoarenes,188 the cobalt or iron carbonyl-catalyzed process gives additionally 3,5-dioxo-l,2,4-triphenyl-1,2,4-triazolidine (Scheme 124).189 The only possible restriction on this simple urazole synthesis would be the expectation that the substituents on the reactants must be the same to prohibit exchange. 

Carbonyl exchange at (FeH(CO)4] in tetrahydrofuran solution is catalysed by sodium ions. Formation of ion pairs (1) is thought to be responsible for the catalysis. A kinetic study of diene exchange at five-co-ordinate iron complexes of the type [( 7 -enone)Fe(CO)3] shows that the reaction proceeds via competing associative and dissociative pathways. ... 

Like the Pb—N bond (see Section IV,B) the Pb—O bond is also easily affected by exchange reactions. Hydrogenation to Pb—H with B2H6 (11) was already mentioned in Section III,A,l,b. Proton-donating substances, including iron carbonyl hydrides (104), react at once (42, 44). 

Excellent examples of metal exchange reactions are provided by complexes of tetraphenylcyclobutadiene. When either iron pentacarbonyl or nickel carbonyl is reacted with tetraphenylcyclobutadiene palladium bromide, the corresponding metal is exchanged with the release of palladium(O) metal. The reaction proceeds only in aromatic solvents and seems to be generally applicable to a variety of transition metal carbonyls. As illustrated in Fig. 6-8, the first step is proposed to involve the formation of uncomplexed tetraphenylcyclobutadiene, a mixed metal halide carbonyl, carbon monoxide. 

The use of iron carbonyl derivatives in organic synthesis is often limited by the difficulty of separation of the iron complexes from the desired product at the end of the reaction. In the synthesis of aldehydes from alkyl halides using tetra-carbonylhydridoferrate, [HFe(CO)4] , this problem has been overcome by attaching the iron complex to an ion-exchange resin [equation (9)]. The resin converts alkyl halides into the homologous aldehyde in over 90% yield, and at the end of the reaction the iron complex is retained on the polymer. Allyl chlorides fail to react, and secondary alkyl halides undergo elimination. 

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