Cyclohexadienylium-tricarbonyliron complex

Cyclohexadienylium-tricarbonyliron complexes are readily available by hydride abstraction from cyclohexadiene-tricarbonyliron complexes using triphenylcarbe-nium tetrafluoroborate [61]. They are stable and can be handled in air. The hydride ion is removed at one of the non-coordinated carbon atoms from the face opposite to the metal fragment. The resulting cyclohexadienylium system is stabilized as an r 5-ligand by the tricarbonyliron fragment (Scheme 1.26). 

Cyclohexadienylium-tricarbonyliron complexes represent the most versatile iron complexes applied as building blocks in synthetic organic chemistry. Because of their positive charge, a large variety of nucleophiles undergo nucleophilic attack at the... 

Tricarbonyl(ri -cyclohexa-l,3-diene)iron complexes are intermediates in the reaction of tricarbonyI(T -cyclohexadienylium)iron complexes with substituted anilines in the presence of air (Scheme 4-131). Under these conditions, concomitant oxidative cyclization of the intermediate aniline-functionalized (ri -cyclohexa-l,3-diene)iron complexes occurs to give tricarbonyliron-complexed dihydrocarbazoles. The latter are... 

Full stereocontrol is achieved for the addition of protons at the exocyclic double bond of (Ti -5-alkylidenecyclohexa-l,3-diene)tricarbonyliron complexes to afford tri-carbonyl(ri -cyclohexadienylium)iron complexes. This demonstrates that the stereodirecting influence of the complex moiety in this conjugate system is even effective for the position P to the diene ligand with the proton entering anti to the tricarbonyliron fragment (Scheme 4-149). ... 

The iron-mediated construction of the carbazole framework proceeds via consecutive C-C and C-N bond formation as key steps [70,71]. The C-C bond formation is achieved by electrophilic substitution of the arylamine with a tricarbonyliron-coordinated cyclohexadienyl cation. The parent iron complex salt for electrophilic substitutions, tricarbonyl[/j -cyclohexadienylium] iron tetrafluoroborate 6a, is readily available by azadiene-catalyzed complexation and subsequent hydride abstraction (Scheme 9). 

Tricarbonyliron-coordinated cyclohexadienylium ions 569 were shown to be useful electrophiles for the electrophilic aromatic substitution of functionally diverse electron-rich arylamines 570. This reaction combined with the oxidative cyclization of the arylamine-substituted tricarbonyl(ri -cyclohexadiene)iron complexes 571, leads to a convergent total synthesis of a broad range of carbazole alkaloids. The overall transformation involves consecutive iron-mediated C-C and C-N bond formation followed by aromatization (8,10) (Schemes 5.24 and 5.25). 

The tricarbonyliron-coordinated cyclohexadienylium salts are readily available on a large scale by azadiene-catalyzed complexation of the corresponding cyclo-hexadienes with pentacarbonyliron [23] and subsequent hydride abstraction using trityl tetrafluoroborate [24]. Alkylation of methyl lithioacetate with the iron complex... 

Electrophilic aromatic substitution of 2,3-dimethyl-4-methoxyani]ine by reaction with the tricarbonyliron-coordinated cyclohexadienylium salt generates the aryl-substituted tricarbonyliron-cyclohexadiene complex. Treatment of this complex with very active manganese dioxide results in oxidative cyclization and aromatization with concomitant demetallation to afford directly 4-deoxycarbazomycin B, a degradation product of the antibiotic carbazomycin B [32]. Using ferricenium hexafluorophos-... 

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