Iron reagents, oxidation with

An interesting alternative to the use of chromium(VI) oxidants for the conversion of 1 to 2 involves the use of a low-valent iron reagent prepared in situ by the action of hydrogen peroxide on an iron(II) complex of 1 (73). Vinblastine (as the free base) is treated with 2 equiv of perchloric acid in acetonitrile at -20°C. Ferrous perchlorate is then added, followed by the addition of excess 30% hydrogen peroxide. Work-up of the reaction mixture with a saturated solution of ammonium hydroxide gives 2 in yields of 35-50% after chromatography. 

Electrophilic substitution at the arylamine 709 using the complex salt 602, provided the iron complex 725 quantitatively. Sequential, highly chemoselective oxidation of the iron complex 725 with two, differently activated, manganese dioxide reagents provided the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727) via the non-cyclized quinone imine 726. Demetalation of the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727), followed by selective O-methylation, provided hyellazole (245) (599,600) (Scheme 5.70). 

The construction of the carbazole framework was achieved by slightly modifying the reaction conditions previously reported for the racemic synthesis (614). Reaction of the iron complex salt 602 with the fully functionalized arylamine 814 in air provided the tricarbonyliron-coordinated 4b,8a-dihydrocarbazole complex 819 via sequential C-C and C-N bond formation. This one-pot annulation is the result of an electrophilic aromatic substitution and a subsequent iron-mediated oxidative cyclization by air as the oxidizing agent. The aromatization with concomitant demetalation of complex 819 using NBS under basic reaction conditions, led to the carbazole. Using the same reagent under acidic reaction conditions the carbazole was... 

For phenol hydroxylation it is shown that Fe2+ and Fe3+ ion activity increases in the presence of other ions, among which the highest activity is displayed by salts and complexes of the following metals Co, Mn, Mo, Cu, Fe, etc. In aqueous solution Fenton s reagent oxidizes substrates according to the radical mechanism in which reactions with 0H radicals play the central role. In aprotic solvents oxidation with Fenton s reagent suggests the participation of different intermediates—complexes with iron ions, Fe=0, for example. 

Commercial zinc sulphate invariably contains a small amount of iron as an impurity. Since FeS04-7H20 crystallizes isomor-phously with ZnS04-7H20 a preparation of the latter cannot be freed of the former by recrystallization. By addition of chlorine, or its equivalent, to the solution of zinc sulphate, the iron is oxidized to ferric salt the ferric salt hydrolyzes somewhat, and, if the acid produced by the hydrolysis is neutralized as fast as formed, the hydrolysis proceeds to completion and all the iron is precipitated as Fe(OH)3. In this case, the reagent used to bring about the exact neutrality of the solution is a suspension of basic zinc carbonate. (Compare the similar procedure for removing traces of iron in the preparation of strontium chloride, Preparation 21.)... 

We have demonstrated recently that epoxidation and hydroxyl-ation can be achieved with simple iron-porphine catalysts with iodosylbenzene as the oxidant (24). Cyclohexene can be oxidized with iodosylbenzene in the presence of catalytic amounts of Fe(III)TPP-Cl to give cyclohexene oxide and cyclohexenol in 55% and 15% yields, respectively. Likewise, cyclohexane is converted to cyclohexanol under these conditions. Significantly, the alcohols were not oxidized rapidly to ketones under these conditions, a selectivity shared with the enzymic hydroxylations. The distribution of products observed here, particularly the preponderance of epoxide and the lack of ketones, is distinctly different from that observed in an autoxidation reaction or in typical reactions of reagents such as chromates or permanganates (15). 

Some natural products have been synthesized by means of oxidative coupling promoted by iron reagents. In 1995, Herbert and co-workers reported the formation of the alkaloid kreysigine (84) by intermolecular oxidative coupling of diaryl substrates 83a/b with iron(III) chloride followed by methanol work-up [67]. The yield for the free phenolic compound 83a was 53 %, whereas the benzyl-protected analogue 83b presumably cydizes and then de-benzylates, in an overall yield of 71 % (Scheme 19). 

II state, [PFe=0], is the only product detected spectroscopically. The kinetics of equations (50 56), including the formation of the Compound I precursor, have been studied in detail by several groups. The Compound I state is readily reached by reaction of an iron(ni) porphyrin with iodosylbenzene, potassium monopersulfate, or an amine N-oxide, and these reagents are frequently used for generation of the hydroxylation catalysts of the Compound I level that are discussed in Section 9. 

Oxidation (with such reagents as iron(III) chloride, potassium dichromate, silver oxide or nitrous acid) of 4,7-, 6,7-, and 5,6-dihydroxy-, and 5,6-dimethoxy-benzimidazoles gives the corresponding quinones. A 5-methyl group is oxidized by permanganate to carboxy (74CRV279). In the presence of copper(II)-piperidine or -dimethylamine complexes oxygen... 

Apart from acid derivatives, aldehydes and ketones are also versatile reagents. The (5-benz-y]ideneamino)pyrimidine which is formed initially undergoes ring closure on reaction with an oxidizing agent such as iron(III) chloride or iron(III) oxide. ... 

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