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1.7.7.11 Application of Collman's Reagent

DOI: 10.1055/sos-SD-001-00654

Stephenson (Product Subclasses 17), G. R.; Perseghini, M.; Togni (Product Subclass 8), A.Science of Synthesis, (20011865.

Reduction of pentacarbonyliron produces a dianion, Na2Fe(CO)4, which has become known as Collman's Reagent. Originally,[‌1034‌] sodium amalgam was used, but now sodium benzophenone ketyl is preferred.[‌1035‌] A phase-transfer procedure with sodium hydroxide,[‌1036‌] a nickelbipyridyl catalyzed electrosynthesis procedure,[‌1037‌] and a direct preparation from iron(III) chloride and sodium/naphthalene under a carbon monoxide atmosphere[‌1038‌] have been described. This nucleophilic organometallic complex is generated in situ for use with organic electrophiles, which can be utilized in turn to make unsymmetrical ketones, or aldehydes when the second electrophile is a proton. Aldehydes have been prepared from alkyl bromides in this way.[‌1034‌] The reaction proceeds by formation of σ-bonded organometallic intermediates. Oxidation in the presence of methanol (see Section 1.7.7.8.2), after reaction with the first electrophile, affords carboxylic acid esters. Typically, alkyl halides or tosylates are used as the electrophiles, but there are many other possibilities. For example, reaction with diethyl cyclopropanedicarboxylate[‌1039‌] can be followed by protonation, alkylation with iodomethane, or oxidation in an alcoholic solvent to form aldehydes, ketones, or esters. When iodomethane is used, both the anionic metal complex and the diester-stabilized anion are alkylated.[‌1039‌] When the second electrophile is an α,β-unsaturated ketone, transfer of the acyl anion component to the β-carbon is observed (Scheme 117).[‌1040‌] Reaction of the acyliron intermediate with copper(I) chloride affords symmetrical 1,2-diketones.[‌1041‌]

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