Iron, cyclopentadienyl nucleophilic substitution

The first example of iron-activated nucleophilic aromatic substitution on solid phase has been presented by Ruhland et al. [141], who attached [(cyclopentadienyl)-benzene Fe(II)] PF6 to polymer-bound piperazine (Scheme 55). The complex 68 was subjected to a variety of nucleophiles using different protocols. The decomplexation was achieved by irradiation in the presence of phenanthroline, and in the final step the resin-bound products were cleaved with methyl chloroformate to give corresponding carbamates in good yields. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The use of transition-metal arene complexes to facilitate nucleophilic aromatic substitution features in a route to derivatives of thiosalicylic acid and hence to thioxanthone. The cyclopentadienyl iron complex of 2-chlorobenzoic acid is converted into the benzamide prior to displacement of the chloride by thiophenoxide. Photolytic decom-plexation followed by directed remote metallation of the diaryl sulfide yielded the heterocycle (Scheme 207) <2000SL975>. 

Thus the iron atom in (Tr-cyclopentadienvl)iron dicarbonyl bromide forms additional bonds with the halide ligands on palladium. Carbonyl substitution on the generated intermediate (XV) then occurs. In this case the cyclopentadienyl group acts as a bridge. In general, such a mechanism corresponds to an A-type substitution, where the halogens in (7T-tetraphenyl-cyclobutadiene)palladium dichloride function as nucleophiles. 

It is interesting that in essence complex (XX) is the result of cyclopentadienyl transfer from nickel to the iron atom. In Maitlis s opinion (180) the reaction mechanism resembles the tetraphenylcyclobutadiene transfer from palladium to the iron atom. In terms of CO substitution in the Fe and Ni carbonyls these are nucleophilic reactions in which nickelocene plays the role of a nucleophile. 

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