Tricarbonyl iron complexes oxidation

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

Oxidation of tri-terr-butylcyclobutadiene(tricarbonyl)iron complex with ammonium ce-rium(IV) nitrate or iron(III) nitrate in acetone did not afford the expected dimer of ix i-tert-butylcyclobutadiene, but gave exclusively l,2-di-ter/-butyl-3-(2,2-dimethyl)propanoylcyclo-propene (4), in quantitative yield. ... 

Bicyclo[4.1.1] or [3.2.1]octenones and cyclopropanes have resulted from decomplexation of the iron tricarbonyl group from the alkyl-allyliron tricarbonyl complex, using oxidative (i.e., GO atmosphere) or carbonylative methods for the bicyclooctenones and ceric ammonium nitrate (GAN) for the cyclopropanes. Photolysis of analogous tricarbonyl iron complexes leads to monoolefmic hydrocarbons or aldehydes. The kinetics of GO substitution in reactions of 77 -cyclopropenyl complexes of iron is also reported. " A number of comprehensive reviews have appeared since 1992, illustrating the chemistry of ry -allyliron complexes. 

The mechanism involves formation of a reactive 16-electron tricarbonyliron species by sequential loss of two carbon monoxide ligands and coordination of allyl alcohol to pentacarbonyliron (Scheme 4-291). Oxidative addition under activation of the sp C-H bond adjacent to the hydroxy group leads to an Ti -allyl(hydrido)iron complex. Subsequent reductive elimination transfers the hydrido ligand to the 3-position of the allyl ligand to afford an ri -alkene(tricarbonyl)iron complex. Dissoziation of the enol ligand releases a 14-electron tricarbonyliron complex that will start the catalytic cycle de novo by complexation of allyl alcohol. The enol will finally tautomerize to the ketone. In total, a formal [l,3]-hydride shift is achieved at the allyl alcohol. ... 

The synthesis of 1 -benzothiepin 1 -oxide (23) can be achieved via complex formation with tricarbonyl iron, and quantitative oxidation of the coordination compound 22 with 3-chloroperoxy-benzoic acid. Subsequent irradiation at — 50 C provides 23, which crystallized as yellow needles after low-temperature (-40 C) chromatography, and was characterized by 1H NMR spectroscopy at — 30 C23 before loosing sulfur within one hour at 13°C to give naphthalene. 

The oxidative cyclization of chiral 2-pyrrolidino-l-ethanol derivatives is shown in the reaction of 251 with trimethyl-amine iV-oxide and a substoichiometric amount of cyclohexadiene iron tricarbonyl to produce the corresponding oxazolopyrrolidine ring 252. The mechanism of this reaction is unknown. Both amine oxide and iron complex are essential for the reaction (Equation 39) <2005TL3407>. 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

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). 

Over the past 15 years, we developed three procedures for the iron-mediated carbazole synthesis, which differ in the mode of oxidative cyclization arylamine cyclization, quinone imine cyclization, and oxidative cyclization by air (8,10,557,558). The one-pot transformation of the arylamine-substituted tricarbonyl(ri -cyclohexadiene) iron complexes 571 to the 9H-carbazoles 573 proceeds via a sequence of cyclization, aromatization, and demetalation. This iron-mediated arylamine cyclization has been widely applied to the total synthesis of a broad range of 1-oxygenated, 3-oxygenated, and 3,4-dioxygenated carbazole alkaloids (Scheme 5.24). 

In the quinone imine cyclization of iron complexes to carbazoles, the arylamine-substituted tricarbonyl(ri -cyclohexadiene)iron complexes 571 are chemoselectively oxidized to a quinone imine 574 prior to cyclodehydrogenation. This mode of cyclization is particularly applicable for the total synthesis of 3-oxygenated tricyclic carbazole alkaloids (Scheme 5.25). 

An alternative method for the oxidative cyclization of the arylamine-substituted tricarbonyl(r -cyclohexa-l,3-diene)iron complex (725) is the iron-mediated arylamine cyclization. Using ferricenium hexafluorophosphate in the presence of sodium carbonate provided hyellazole (245) directly, along with the complex 727, which was also converted to the natural product (599,600) (Scheme 5.71). 

Using a one-pot process of oxidative cyclization in air, the arylamine 780a was transformed to the tricarbonyl(ri -4b,8a-dihydro-9H-carbazole)iron complex 792. Finally, demetalation of 792 and subsequent aromatization gave carbazomycin A (260). This synthesis provided carbazomycin A (260) in three steps and 65% overall yield based on 602 (previous route four steps and 35% yield based on 602) (610) (Scheme 5.88). 

The arylamine 794 required for the improved total synthesis of carbazomycin B (261) was prepared in quantitative yield by hydrogenation of the nitroaryl derivative 793 (see Scheme 5.85). Oxidative coupling of the iron complex salt 602 and the arylamine 794 in air afforded the tricarbonyl(ri -4b,8a-dihydro-9H-carbazole)iron complex (795). Demetalation of 795, followed by aromatization, led to O-acetylcarbazomycin B (796). 

Reaction of the iron complex salt 602 with the arylamine 921 in the presence of air led directly to the tricarbonyl(ri -4b,8a-dihydro-9H-carbazole)iron complex (923) by a one-pot C-C and C-N bond formation. Demetalation of complex 923 and subsequent aromatization by catalytic dehydrogenation afforded 3,4-dimethoxy-l-heptyl-2-methylcarbazole (924), a protected carbazoquinocin C. Finally, ether cleavage of 924 with boron tribromide followed by oxidation in air provided carbazoquinocin C (274) (640) (Scheme 5.120). 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

Chemoselective oxidation of 4-methoxyanilines to quinonimines can be achieved in the presence of tricarbonyl(ri4-cyclohexadiene)iron complexes. This transformation has been used for the synthesis of carbazoles via intermediate tricarbonyliron-coordinated 4b,8a-dihydrocarbazol-3-one complexes (Scheme 1.24) [57]. 

Synthesis of the parent homotropone 34 was achieved starting from tricarbonyl(cycloocta-tetraene)iron complex 32 via protonation and formation of the bicyclo[5.1.0]octadienylium cation complex 33. Nucleophilic addition of hydroxide and oxidation was followed by oxidative decomplexation with cerium(IV). ... 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex . Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

In 1989 we reported an iron-mediated route for the construction of the tricyclic carbazole skeleton [72, 73]. This convergent method was applied to the total synthesis of the naturally occurring alkaloid carbazomycin A [72]. Key steps of our iron-mediated approach are the consecutive C C bond formation and oxidative cyclization (formation of the C N bond) between an electrophilic tricarbonyl(ri -cyclohexadienyhum)iron complex salt 30 and an arylamine 31 (Scheme 10). Subsequent oxidation and demetalation provides the aromatized carbazole 32. 

Alternatively, a mild and efficient one-pot electrophilic aromatic substitution/ oxidative cyclization without isolation of the intermediate complexes 36 has been achieved using air as oxidizing agent (mode B in Scheme 12). Thus, reaction via mode B leads to tricarbonyl(ri" -4a,9a-dihydrocarbazole)iron complexes 37, which on demetalation with trimethylamine A(-oxide and subsequent catalytic dehydrogenation provide the carbazoles 40. The naturally occurring carbazole... 

A third pathway leads via the quinone imine intermediates 38 to 3-hydro-xycarbazoles 41 (mode C in Scheme 12) [97, 98, 108, 109]. Oxidation of the complexes 36 with manganese dioxide afforded the quinone imines 38, which on treatment with very active manganese dioxide undergo oxidative cyclization to the tricarbonyl(ri" -4b,8a-dihydrocarbazol-3-one)iron complexes 39. Demetalation of 39 with trimethylamine iV-oxide and subsequent aromatization lead to the 3-hydro-xycarbazoles 41. The isomerization providing the aromatic carbazole system is a... 

On oxidation with MCPBA, l-benzothiepine(tricarbonyl)iron (29) was converted into sulfoxide (30). Further oxidation of (30) with MCPBA provided dioxide complex (31) (Scheme 11)... 

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