Iodine aromatic compound reactions

This reaction can be used to prepare brominated or iodinated aromatic compounds or dihaloalkanes the latter is produced from the mixture of metal halide and halogen with alkenes. " ... 

Chlorine monofluoride (which is commercially available) acts as a powerful fluorinating and oxidizing agent (e.g. reaction 17.22) oxidative addition to SF4 was shown in Figure 16.13. It may behave as a fluoride donor (equation 17.23) or as a fluoride acceptor (equation 17.24). The structures of [C12F] (17.9) and [C1F2] (17.10) can be rationalized using the VSEPR model. Iodine monochloride and monobromide are less reactive than CIF, but of importance is the fact that, in polar solvents, ICl is a source of I" " and iodinates aromatic compounds. 

Aromatic Ring Reactions. In the presence of an iodine catalyst chlorination of benzyl chloride yields a mixture consisting mostly of the ortho and para compounds. With strong Lewis acid catalysts such as ferric chloride, chlorination is accompanied by self-condensation. Nitration of benzyl chloride with nitric acid in acetic anhydride gives an isomeric mixture containing about 33% ortho, 15% meta, and 52% para isomers (27) with benzal chloride, a mixture containing 23% ortho, 34% meta, and 43% para nitrobenzal chlorides is obtained. 

Cyanogen Iodide (ICN) has been used extensively for the cyanation of alkenes and aromatic compounds [12], iodination of aromatic compounds [13], formation of disulfide bonds in peptides [14], conversion of dithioacetals to cyanothioacetals [15], formation of trans-olefins from dialkylvinylboranes [16], lactonization of alkene esters [17], formation of guanidines [18], lactamization [19], formation of a-thioethter nitriles [20], iodocyanation of alkenes [21], conversion of alkynes to alkyl-iodo alkenes [22], cyanation/iodination of P-diketones [23], and formation of alkynyl iodides [24]. The products obtained from the reaction of ICN with MFA in refluxing chloroform were rrans-16-iodo-17-cyanomarcfortine A (14)... 

The halogenation of a wide variety of aromatic compounds proceeds readily in the presence of ferric chloride, aluminum chloride, and related Friedel-Crafts catalysts. Halogenating agents generally used are elemental chlorine, bromine, or iodine and interhalogen compounds (such as iodine monochloride, bromine monochloride, etc.). These reactions were reviewed554 and are outside the scope of the present discussion. 

The mechanism of this reaction shows that excitation of the substrate gave an n,n triplet state, but this excited state was unable to dissociate the carbon-iodine bond. This was demonstrated by showing that the n,n triplet state, when sensitized by chrysene, did not produce coupling products. Probably, the reaction occurred in an excited a,a triplet state mainly localized on the carbon-iodine bond, and the interaction between this triplet state and aromatic compounds led to homolytic cleavage of the C-I bond with the formation of both a 5-thienyl radical and a complex between the aromatic compound and the halogen atom. The formation of this complex was demonstrated by the presence of a short-lived transient with Amax = 510 nm, showing a second-order decay kinetics and a half-life of ca. 0.4 (is in laser flash photolysis. The thienyl radical thus formed... 

The purpose of present review is to summarize the application of different classes of iodine(III) compounds in carbon-carbon bond forming reactions. The first two sections of the review (Sects. 2 and 3) discuss the oxidative transformations induced by [bis(acyloxy)iodo] arenes, while Sects. 4 through 9 summarize the reactions of iodonium salts and ylides. A number of previous reviews and books on the chemistry of polyvalent iodine discuss the C-C bond forming reactions [1 -10]. Most notable is the 1990 review by Moriarty and Vaid devoted to carbon-carbon bond formation via hypervalent iodine oxidation [1]. In particular, this review covers earlier literature on cationic carbocyclizations, allyla-tion of aromatic compounds, coupling of /1-dicarbonyl compounds, and some other reactions of hypervalent iodine reagents. In the present review the emphasis is placed on the post 1990s literature. 

Alkenyl(phenyl)iodine(III) compounds can also serve as starting materials in rearrangements. Allenyl(aryl)iodine(III) compounds of type 86 can be synthesized from (diacetoxyiodo) derivatives 85 and propargylsilanes [145]. It depends on the leaving group ability of the aromatic substituent on iodine in 86 as to whether the reaction proceeds via nucleophilic substitution to compounds of type 87 or by an iodonio-Claisen rearrangement to compounds 88, Scheme 37 [146,147]. The easy access to propynyl compounds 87 has been shown [148] and solvent effects in these reactions have been investigated as well [149,150]. 

The title reactions offer a possibility for exchanging the halogen atom in aryl halides (Hal = Cl, Br, I) first with a metal (MgHal, Li) and then with an electrophile. It is generally easier to introduce bromine than chlorine or iodine into aromatic compounds. Accordingly, functionalizations of aryl bromides are the preparatively most important examples of the title reaction. 

Earlier investigations reported on the gas-phase bromination and iodination of aromatic compounds of the type C6H5X (X = F, Cl, Br, Me) with Br+ and I+ produced by a radiolytic method44. It was proposed that the reaction with Br+ involves, as the first step, the exothermic charge exchange of equation 1045. 

Dinitro-6-phenyliodonium phenolate (146) is a stable iodonium zwitterion484. It reacts under photolytic conditions with various alkenes, alkynes and aromatic compounds to afford 2,3-dihydrobenzo[ ]furans, benzo[6]furans and 6-aryl-2,4-dinitrophenols. The mechanism involves hypervalent iodine compounds (iodinanes, 147) and is illustrated for the reaction with an aromatic compound (equation 127). Compounds 148 are the major products when ArH = PhH, PhOCH3 or 1,4-dimethoxybenzene. With furan and thiophene, 149 is the principal product. The reaction does not proceed with chlorobenzene and nitrobenzene. 

Diaryl tellurium dichlorides, obtainable from aromatic compounds and tellurium tetrachloride, react with excess bromine or iodine in refluxing DMF or acetonitrile in the presence of a metal fluoride to give aryl halides in low yields. The reactions with chlorinating agents are very sluggish. The reaction of bromine and bis[4-methoxyphenyl] tellurium dichloride formed 2,4-dibromomethoxybenzene in 76% yield. Reactions of these tellurium dichlorides with copper(I) cyanide in DMF produced aryl cyanides in yields of less than 8%3. 

Activation of aromatic compounds by transition-metal complexes was initially studied with Cr(CO)3 complexes. Nucleophilic addition of 2-lithio-l,3-dithianes to arene-chromium(O) complexes 185 followed usually by iodine-promoted decomplexation affords the corresponding 2-arylated 1,3-dithianes 186. The reaction of //-(toluene)- and (anisole)tricarbonylchromium (185) with compound 161 gave mixtures (52 46 and 10 90, respectively) of ortho and meta substituted derivatives (186) (Scheme 54)244. The meta directing effect was also observed (mainly better than 95%) with amino and fluoro substituted complexes245. 

Reactions with sulphur atoms alone give a variety of cyclic products, depending on the conditions [325], but when iodine is also present the potentially aromatic compound... 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

The first step of the reaction involves iodination of the aromatic compound with the triiodide salt in the presence of water as a solvent. The water should contain from 0.7 to 1.25 molar equivalents of a hydroxide, preferably an alkali metal hydroxide, and from 1-2 molar-equivalents of an alkali metal triiodide (e.g. iodine plus sodium iodide). The aqueous solvent should also contain from 0.1 to 20 mole % of an acid catalyst, which may be a mineral acid such as sulfuric, hydrochloric or phosphoric acid. Reaction is carried out at temperatures ranging from 20°-120° C. If the starting compound contains a nuclear substituent, iodination will occur in the ortho or para position on the nuclear ring. 

Subsequent protic workup releases the aromatic compound. The metalative Reppe reaction can also be used to prepare iodo-substituted or homologated aromatics by treatment of the titanium aryl compound with iodine or an aldehyde, respectively. This procedure has recently been extended to include pyridine derivatives (254 and 255), where the titanacyclopentadiene intermediate can be treated with sulfonylnitriles to afford pyridines after protic workup.192 As with the alkyne cyclotrimerizations, treatment with the appropriate electrophiles affords iodo- and homologated pyridines. 

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