Dibenzofurans substitution

There are reports of an increasing number of palladium-assisted reactions, in some of which the palladium has a catalytic function. Thus furan and thiophene undergo facile palladium-assisted alkenylation giving 2-substituted products. Benzo[6 Jfuran and TV- acetyl-indole yield cyclization products, dibenzofurans and carbazoles respectively, in addition to alkenylated products (8UOC851). The arylation of pyrroles can be effected by treatment with palladium acetate and an arene (Scheme 86) (81CC254). 

Dibenzofuran and carbazole gave yields of 69 and 80%, respectively, of the phosphinated derivative, but the position of substitution was not determined. ... 

Substituted dibenzofurans have also been obtained by cycloaddition of 3-vinylbenzofurans (81) as shown [82] in Scheme 2.33. 

As vinylbenzofurans allow a large variety of substituted dibenzofurans to be synthesized, 2- and 3-vinylbenzo[b]thiophenes allow an easy entry, by Diels Alder reaction with the appropriate dienophiles, to substituted dibenzo-thiophenes which are not easily accessible by other methods. Vinylbenzo-[bjthiophenes are less reactive than the corresponding vinylbenzo[b]furans. Some cycloaddition reactions of 2-vinylbenzo[b]thiophene (82) with various dienophiles are reported [83] in Scheme 2.34. 

Exposed to a complex mixture of radiolabeled PCDDs and dibenzofurans for 21 days Highest uptake was by 2,3,7,8-substituted isomers whole-body BCF values were log 5.24 for 2,3,7,8-TCDD and log 5.27 for 1,2,3,7,8-penta-CDD 21... 

Ankley, G.T., G.J. Niemi, K.B. Lodge, HJ. Harris, D.L. Beaver, D.E. Tillitt, R.R. Schwartz, J.P. Giesy, P.D. Jones, and C. Hagley. 1993. Uptake of planar polychlorinated biphenyls and 2,3,7,8-substituted polychlorinated dibenzofurans and dihenxo-p-dioxins by birds nesting in the lower Fox River and Green Bay, Wisconsin, USA. Arch. Environ. Contam. Toxicol. 24 332-344. 

Zitko, V. 1992. Patterns of 2,3,7,8-substituted chlorinated dibenzodioxins and dibenzofurans in aquatic fauna. 

Tolylsulfones of indoles, pyrroles, pyrazoles, furans, thiophenes, indolines, and dibenzofurans react with tributyltin hydride under radical conditions by t/wo-substitution. The most likely mechanism appears to be addition of the RjSiv radical to the ring, followed by elimination of the sulfonyl group, but an electron-transfer mechanism cannot be excluded (Equation (61)).199... 

Of additional concern is the thermal conversion of PCBs to polychlorinated dibenzofurans, especially when PCB—filled electrical transformers are involved (16). Further, PCB isomers with no chlorine atoms substituted in the ortho. ortho -biphenvl positions are particularly potent as inducers of aryl hydrocarbon hydroxylase activity. The potency of certain dibenzofurans and non-ortho, ortho chlorine substituted PCBs in inducing activity of these enzymes is similar to that of the highly toxic 2,3,7,8-tetrachloro-dibenzo- -dioxin (15. 17., 18). 

The half-wave reduction potentials (HWP) of dibenzothiophene and some of its derivatives have been measured for comparison with those of dibenzofuran and dibenzoselenophene. A shift to more negative HWP was observed for all of the methyl derivatives studied, the magnitude of which depended on the position of substitution. These shifts are in accord with LCAO molecular orbital theory predictions if the sulfur d orbitals are excluded from the calculations (Section III, A). ... 

Binuclear iron(II) complexes in which a hydroxide bridge is supported by the dinucleating bis-carboxylate ligand dibenzofuran-4,6-bis(diphenylacetate), (217), have proved useful models for hemerythrin. The nature of the binuclear iron center in hemerythrin itself, and in other metalloproteins, has been reviewed, the binding of O2, NO, N3, and NCS to the iron of hemerythrin discussed, " and the volume profile for hemerythrin reacting with O2 established. Bulky tolyl-substituted carboxylate ligands, both bridging and terminal, and... 

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 acid-catalyzed reaction (see Table II) has been applied to a range of substituted diphenyl ethers, particularly chlorinated compounds. " In general, where more than one mode of cyclization is possible, the less ste-rically hindered product results. Longer reaction times and larger amounts of palladium(II) acetate are required to produce acceptable yields of the more highly substituted dibenzofurans. 

Diphenyl ethers with an ortho substituent may undergo a similar photo-cyclization to produce dibenzofurans by expulsion of the substituent from an intermediate 9b-substituted 9a,9b-dihydrodibenzofuran of the same type as 40. Thus the diphenyl ethers 43 and 44 on irradiation lose methanol with the production of the dibenzofurans 45 and 46 (Scheme 4). With less highly... 

From limited data the generalization has been made that this method fails for the synthesis of 1-substituted dibenzofurans and for the synthesis of 4-substituted dibenzofurans unless the amino group and the potential 4-substituent are in the same ring. These generalizations are not substantiated in the synthesis of polychlorodibenzofurans. Polychloro-2-phenoxy-anilines are poorly soluble and nonbasic so that the usual method is precluded. In these cases aprotic diazotization with isopentyl nitrite in tetrachloroethylene at 80 C is the method of choice. Yields, as determined by GLC, are usually of the order of 40-50%. The yields of pure isolated... 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

When 2-allylbenzofuran (151, Scheme 39) is allowed to react with ethyl (dichloro)ethoxyacetate (152) in the presence of tin(IV) chloride at — 78°C, an electrophilic substitution occurs, and the intermediate 153, after elimination of hydrogen chloride, undergoes an electrocyclic reaction, producing ethyl dibenzofuran-l-carboxylate (154). ... 

Benzofuran in boiling dioxane and acetic acid, in the presence of equimolar proportions of styrene and palladium(II) aeetate, yields 2,3-diphenyl-dibenzofuran (26%) as well as 2-styryl- (16%) and 3-styrylbenzofuran (3%). The reaction presumably involves electrophilic substitution at the 2-position of benzofuran by palladium(II) acetate, followed by addition to the alkene and loss of palladium hydride. Further reaction at the 3-position of the resultant 2-styrylbenzofuran would yield an intermediate that could undergo... 

When benzyne is generated by aprotic diazotization of anthranilic acid in boiling 1,2-dimethoxyethane in the presence of substituted benzene-2-diazo 1-oxides (o-quinonediazides), dibenzofurans are obtained in moderate yields (Scheme 64). ... 

Phenol and alkyl-substituted phenols yield dibenzofuran and alkyldiben-zofurans by catalytic dehydration-dehydrogenation over thorium or cerium oxides. This reaction can also be achieved by thermal cracking of... 

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