Iron-catalyzed halogenation reactions

Dry methylene chloride does not react with the common metals under normal conditions however, a reaction with aluminum can be initiated, sometimes explosively, by the addition of small amounts of other halogenated solvents or an aromatic solvent (7). Iron catalyzes the reaction, and this can be significant in the handling and storage of methylene chloride and in the formulation of products, eg, in aluminum aerosol containers of pigmented paints, where the conditions necessary for the reaction are commonly found. A typical reaction in this process is shown in equation 2. 

An iron-catalyzed multicomponent reaction forms poly-substituted pyridines in good yields (Scheme 5).The reaction involves a benzaldehyde, malono-nitrile, ethyl acetoacetate, and aniline in a nucleophilic addition followed by intermolecular cyclization. A number of groups are tolerated on the aldehyde including nitro, halogens, methyl, and methoxyls. Methyl and nitro groups were... 

When used with A-chloro-, A-bromo-, and A-iodosuccinimide, iron(III) chloride catalyzes the introduction of halogens into arenes. The reaction works well even with deactivated aromatic rings but in some cases the regioselective course is difficult to control (Scheme 39) [49]. 

Unlike the alkanes, however, the reaction of benzene with the halogens is catalyzed by iron. The relative lack of reactivity in aromatic hydrocarbons is attributed to delocalized double bonds. That is, the second pair of electrons in each of the three possible carbon-to-carbon double bonds is shared by all six carbon atoms rather than by any two specific carbon atoms. Two ways of writing structural formulas which indicate this type of bonding in the benzene molecules are as follows ... 

This iron-sulfur oxygenase [EC 1.14.12.11] catalyzes the reaction of molecular oxygen with toluene and NADH to produce (15 ,27 )-3-methylcyclohexa-3,5-diene-l,2-diol and NAD. This reductase is an iron-sulfur flavo-protein (FAD) that contains ferredoxin. Ethylbenzene, 4-xylene, and some halogenated toluenes can likewise undergo conversion to the corresponding cw-dihydro-diols. 

With unsymmetrical methyl ketones arylation occurred at the primary carbon. Where two yields are stated the figure in parentheses is the isolated yield and the other is analytical (e.g. determined by GLC or H NMR). Reaction in the dark, catalyzed by iron(II) sulfate. dBoth halogens replaced. 

SAFETY PROFILE Moderately toxic by skin contact, inhalation, intravenous, and intraperitoneal routes. Mildly toxic by ingestion. Experimental teratogenic and reproductive effects. A skin and eye irritant. Less toxic than dimethylformamide. Mutation data reported. Combustible when exposed to heat and flame. A moderate explosion hazard. Violent reaction with halogenated compounds (e.g., carbon tetrachloride, hexachlorocyclohexane) when heated above 90°C. Iron powder catalyzes the reaction so that it initiates at 71 °C. 

In this reaction, one polymer chain forms per molecule of the organic halide (initiator), while the metal complex serves as a catalyst or as an activator, which catalytically activates, or homolytically cleaves, the carbon—halogen terminal. Therefore, the initiating systems for the metal-catalyzed living radical polymerization consist of an initiator and a metal catalyst. The effective metal complexes include various late transition metals such as ruthenium, copper, iron, nickel, etc., while the initiators are haloesters, (haloalkyl)benzenes, sulfonyl halides, etc. (see below). They can control the polymerizations of various monomers including methacrylates, acrylates, styrenes, etc., most of which are radically polymerizable conjugated monomers. More detailed discussion will be found in the following sections of this paper for the scope and criteria of these components (initiators, metal catalysts, monomers, etc.). 

Copper or iron phthalocyanines encapsulated inX or Y zeolites [25g], which catalyze the oxidation of cyclohexane to Ol/One and to AA with oxygen (in the presence of small amounts of t-BuOOH) at near-ambient conditions. The catalyst remains in the solid phase throughout the reaction, and can be easily filtered off. Moreover, the solvent type affects performance best selectivity to AA (41%) is achieved with methanol [25g], at 12.7% cyclohexane conversion, with a halogen-substituted phthalocyanine of Fe encapsulated in an X zeolite. Cyclohexanone and cyclohex-andione are hypothesized to be the intermediate compounds of the reaction. Incorporation of the zeolite-encapsulated Fe phthalocyanine inside a polymer matrix can serve to enhance catalyst stability and limit leaching phenomena [25h[. 

The oxidation of cyclohexene with dioxygen in SCCO2 has also been reported. The iron(m)-halogenated-porphyrin-catalyzed reaction produced epoxide and allylic oxidation products (Scheme 14). The turnover number (TON) reached up to 1530 for cyclohexene in 24 h at 80°C. The activity is lower in SCCO2 than in organic solvents, but the selectivity for epoxidation (up to 34%) is higher in SCCO2. 

Unfortunately, all of the above-described synthetic approaches towards HBC (1) suffer from serious drawbacks, such as harsh reaction conditions, a complicated experimental work-up, and low yields. Furthermore, under aluminum(III) chloride catalysis, dealkylation, migration of the alkyl substituents - or even chlorination of the aromatic system- occurs, which clearly limits the accessibility of functionalized HBC derivatives for further investigations and applications. In order to overcome these problems, the weaker Lewis acid iron(III) chloride in nitromethane was used instead of AICI3, and the reaction conditions were carefully optimized [55, 56]. In this way, access was obtained to a multitude of HBC derivatives 8 and 9 with diverse substitution patterns and symmetries bearing solubilizing alkyl chains and halogen substituents, starting from functionalized hexaphenylbenzenes. The sixfold symmetric hexaphenylbenzenes 10 were synthesized by the Co2(CO)g-catalyzed cyclotrimerization of substituted diphenyl-acetylenes 11 (Scheme 13.4a) [57], whereas the intramolecular Diels-Alder reaction... 

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