Dibenzofuran metallation

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

Regioselective cleavage of dibenzofuran derivatives has been achieved with lithium metal, as exemplified by the preparation of 3-methyl-2-phenylphenol (108) (80S634). 

Kanemasa et al.63 reported that cationic aqua complexes prepared from the /ram-chelating tridentate ligand (i ,f )-dibenzofuran-4,6-diyl-2,2,-Mv(4-phcnyloxazolinc) (DBFOX/Ph) and various metal(II) perchlorates are effective catalysts that induce absolute chiral control in the Diels-Alder reactions of 3-alkenoyl-2-oxazolidinone dienophiles (Eq. 12.20). The nickel(II), cobalt(II), copper(II), and zinc(II) complexes are effective in the presence of six equivalents of water for cobalt and nickel and three equivalents of water for copper and zinc. 

All of the organohalogen compounds studied were commercial products obtained from various manufacturers and used as received. Only the DBDPO was purified further by recrystallization for some of the chromatography and thermal analysis experiments. Samples of antimony trioxide and antimony pentoxide were also obtained from commercial sources. The ultrapure antimony trioxide, bismuth trioxide, bismuth metal, antimony metal, dibenzofuran and diphenyl ether were all obtained from Aldrich Chemicals. The poly(propylene) (PP) resin was 0.7 mfi, food grade from Novamont and the poly(ethylene) was unstabilized, high molecular weight, HDPE from American Hoechst. 

Specific considerations of metabolism and accumulation are not necessary in the application of a TTC provided that the substances are not likely to show very large species differences in accumulations such as, e.g., polyhalogenated-dibenzo-p-dioxins, dibenzofurans, and biphenyls and related compounds, as well as nonessential heavy metals in elemental, ionic or organic forms. Such substances are known to accumulate in the body, and the traditionally employed safety factors (Section 5.2.1) may not be high enough to account for species differences in rates of elimination of such chemicals. Therefore, the TTC approach should not be used for such substances. 

Polyhalogenated dibenzo-p-dioxins, -dibenzofurans, or -biphenyls and similar substances nonessential, heavy metals... 

The main concern of the early work (before about 1955), much of which was due to Gilman, was the examination of the basic chemistry of dibenzofuran with particular reference to electrophilic substitution and metallation. A later development of dibenzofuran chemistry owed its impetus to the discovery of dibenzofuranoid lichen metabolites and the consequent synthetic efforts that this initiated. 

The 4-sodio and 4-potassio derivatives are known but are less convenient to obtain than the 4-lithio derivative, which is readily available by metallation of dibenzofuran with butyllithium. Metallation can also be achieved with cross-linked poly(p-lithiosytrene) but offers no advantage in yield. The 4,6-dimetallo derivatives of dibenzofuran can be made by treatment of dibenzofuran with butyllithium or butylsodium. Lithiodiben-zofurans and dilithiodibenzofurans are also available by bromine-lithium exchange with butyllithium. " ... 

The metallation of substituted dibenzofurans has been little studied. Lithiation of 4-methoxydibenzofuran occurs at the 3- and 6-positions. Similarly, 2-methoxydibenzofuran is lithiated at the 1- and S-positions. 1,3-Dimethoxydibenzofuran, however, is lithiated at the 4-position, ° ... 

Dibenzofuran and dimethyldibenzofurans ° yield radical anions on treatment with alkali metals. Their ESR spectra have been studied. The rate constant for the production of the radical anion of dibenzofuran at a stationary mercury electrode has been determined. ... 

Alkyllithium compounds metallate dibenzofuran, dibenzothiophene and IV-alkylcarbazoles (in increasing order of difficulty) to form compounds of type (297) substitution occurs ortho to the heteroatom as expected from benzene chemistry. 

The nucleation of parallel (3-sheet structure via metal ion chelation in aqueous solution was investigated by incorporating an ethylene glycol spacer between a dibenzofuran-4,6-dipro-panoic acid template and the two peptide strands (Scheme 6).[7 The flexibility of the ethylene glycol spacer allows the peptide to sample a variety of largely unordered conformations in the absence of metal ions. Upon binding of palladium(II), nickel(II), or copper(II) ions, the peptidomimetic adopts a well-defined (3-sheet conformation as discerned from biophysical and spectroscopic studies (i.e., CD, IR absorption, and H NMR spectrometry).17 The synthesis of the dibenzofuran-4,6-dipropanoic acid template was discussed... 

The use of metal-complex mediators was applied by Nunnecke and co-authors (Nunnecke and Voss 1999) to the dechlorination of chlorinated benzenes and diben-zofurans. The main products were chlorobenzene and unsubstituted dibenzofurans, respectively. 

Harnly M, Stephens R, McLaughlin C, Marcotte J, Petreas M, Goldman L (1995), Environ. Sci. Technol. 29 677-684. Polychlorinated dibenzo-p-dioxin and dibenzofuran contamination at metal recovery facilities, open burn sites, and a railroad car incineration facility"... 

Watanabe I, Kawano M, Tatsukawa R (1993) Consumption trend and environmental research on brominated flame retardants in Japan and the formation of polyhalogenated dibenzofurans at the metal reclamation factory. Document distributed at the OECD Workshop on the Risk Reduction of Brominated Flame Retardants, Neuchatel, Switzerland... 

Halogens and their covalent compounds Metal carbonyls Organometallic compounds Oxidizing agents Polychlorinated dibenzofuran Polychlorinated dibenzo-p-dioxin... 

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