Iron pentacarbonyl-promoted reaction

Analogous carbonylation reactions using nickel and iron carbonyl based systems also produce alkanecarboxylic acids [11, 13, 14]. The mechanism of the conversion of benzyl halides into arylacetic acids using iron pentacarbonyl is not as well defined as it is for reactions promoted by nickel or molybdenum carbonyl complexes. Iron... 

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. 

Corey-Winter elimination dibenzobarralene (442). In a similar way, spirohex-4-ene was obtained in 80% yield from (443). Daub and his co-workers have synthesized several uncommon olefins, among these (442), from parent cyclic thionocarbonates, using iron pentacarbonyl as the efficient elimination promoter. Semmelhack and Stauffer have reported that bis-(cyclo-octa-l,5-diene)nickel (444) converts thionocarbonates of vic-diols into the corresponding olefins in excellent yields and with a high degree of stereospecificity. In connection with a discussion of the mechanistic pathways of this reaction, the authors presented evidence for the intermediacy of the carbene complex (445). 

Pentacarbonyl iron is fairly inert to substitution reactions, and attempts to prepare Fe(CO)5- (CNR) (n = 1-5) by the direct reaction of Fe(CO)5 with isocyanides in Carius tubes has produced only the complexes Fe(CO)s (CNR) (n = 1 and 2). The products were obtained as mixtures that required separation. Other syntheses, including photochemical and trimethylamine N-oxide-promoted displacement of carbonyl groups, or other means, give the same products in variable yield. Procedures based on diiron nonacarbonyl and triiron dodecacarbonyl have produced similar results. The only zero-valent iron complex Fe(CO)5 (CNR) where n > 2 is the complex Fe(CNR)5 prepared either by metal vapor synthesis techniques or by sodium amalgam reduction of iron(II) bromide in the presence of isocyanide. ... 

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