Halides, alkyl, reaction with ferrate

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

A.iii. Sodium Tetracarbonyl Ferrate. An extremely useful organoiron reagent is sodium tetra-carbonyl ferrate [Na2Fe(CO)4], usually prepared by reduction of iron pentacarbonyl. The synthetic utility of this reagent lies in its ability to react with alkyl halides in a stepwise manner, including the reaction with two different alkyl halides, as reported independently by Cooke and by Collman and co-workers. - When sodium tetracarbonyl ferrate reacted with 1-bromooctane, a trigonal bipyramidal complex (715) was formed... 

Alternatively, acyliron complexes can be obtained in a two-step sequence either from dicarbonyl(cyclopentadienyl)ferrates or from dicarbonyl(cyclopentadienyl)iron halides via alkyl-Fp complexes. The first method makes use of the nucleophilicity of [CpFe(CO)2] anions. Their reaction with alkyl halides provides alkyl-Fp complexes that, upon treatment with phosphanes or phosphites, undergo a migratory insertion of a carbonyl ligand into the metal-alkyl bond leading to an acyliron complex (Scheme 4-38). " ... 

A good example of the carbonylation process is the reaction of the tetracarbonyl ferrate dianion [Fe(CO)4 ] with alkyl halides. This reagent is made by dissolving metal reduction of the 18-electron Fe(0) compound Fe(CO)5. Addition of two electrons would give an unstable 20-electron species but the loss of one of the ligands with its two electrons restores the stable 18-electron structure. 

Acyl carbonyl ferrates are involved as intermediates in two preparations of aldehydes. Alkyl bromides react with sodium tetracarbonyl ferrate(—ii) (from iron pentacarbonyl and sodium) by a process of oxidative addition to furnish the ferrate(—i) (124) this species rearranges on treatment with triphenylphosphine to a phosphonium-substituted acyl ferrate(—i) (125), which is subsequently cleaved by acetic acid to the homologous aldehyde, as outlined in Scheme 46. A related procedure employs reaction of an acid halide with sodium tetracarbonyl ferrate(—ii) to afford the acyl ferrate(—i) (126) directly this species is also cleaved by acetic acid, and affords yet another method for the reduction of acid halides to aldehydes. 

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