Mechanism aromatic iodination

Aromatic iodination can be carried out with a number of reagents, including iodine monochloride, 1C1. What is the direction of polarization of IC1 Propose a mechanism for the iodination of an aromatic ring with 1C). 

Chlorination is carried out in a manner similar to bromination and follows a similar mechanism to give aryl chlorides. Fluorination and iodination of arenes are rarely performed. Fluorine is so reactive that its reaction with benzene is difficult to control. Iodination is very slow and has an unfavorable equilibrium constant. However, iodine, in the presence of a powerful oxidizing agent can be used for electrophilic aromatic iodination. In the following example, the oxidant peroxyacetic acid reacts with iodine to... 

Aromatic cation-radicals can also react with NOj", giving nitro compounds. Such reactions proceed either with a preliminary prepared cation-radical or starting from nncharged componnd if iodine and silver nitrite are added. As for mechanisms, two of them seem feasible—first, single electron transfer from the nitrite ion to a cation-radical and second, nitration of ArH with the NOj radical. This radical is quantitatively formed when iodine oxidizes silver nitrite in carbon tetrachloride (Neelmeyer 1904). 

A number of l-aryl-2-thienylethylenes have been photocyclized in the presence of an oxidizing agent (usually iodine) to polycyclic aromatic compounds. Representative examples are given in Table 1. The mechanism, as with the conversion of stilbene to phenanthrene, probably involves conversion of the trans-alkene to the c/s-form, cyclization to the dihydro isomer, and oxidation of the latter to the fully aromatic compound. The yield of the cyclized product seems to decrease when the ethylene is attached to the /3-position of the thiophene. 

General support for this mechanism was found in the cyclization of bis(4-phenyl-3-butenyl disulfide) (a Scheme 4) which formed 3-iodo-2-phenyltetrahydrothiophene (e Scheme 4) when treated with iodine (71IJS(A)(1)39>. The intermediate carbenium ion (b) is not stabilized by loss of a proton, since it lacks the driving force of aromaticity, but rather reacts with... 

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... 

Recently it was found that iodine(v) compounds like 2-iodoxybenzoic acid (IBX) 7 can be used to affect selective oxidations at carbon atoms adjacent to aromatic systems. The mechanism of this transformation is believed to proceed via a SET (Single-Electron-Transfer) process. A postulated mechanism for the oxidation of benzylic positions is outlined in Scheme 32. This oxidation is quite general and proceeds efficiently in fluorobenzene/DMSO mixtures or in DMSO at 80 °C [ 135]. Starting from compounds 70, the corresponding aldehydes 71 can be obtained easily in good yields. 

In the first case [1] Y can be either a hydroxy, alkoxy or nitro group. The first two groups are important but variable constituents in coals and the last is probably minor or non-existent. The second active class of species are the alkyl-pyridines [2]. The final case [3] includes substituents on the benzyl carbon where X can be an ether or carbonyl functional group. The general mechanism of this reaction is most probably the base catalyzed iodination of the benzyl carbon with subsequent displacement of the iodide by the pyridine to form the pyridinium salt. In all three modes of activation, the single aromatic ring can be replaced with polycyclic rings. 

The parallels observed between CM) solubility and electrophilic substitution products are regular if C6o dissolution in aromatic hydrocarbons is considered as acid-base relationships. According to the theoretical research and experimental results, double bonds of aromatic hydrocarbons with mobile Tt-electrons are Lewis base. Consequently, they react with acids and Lewis acids to form complexes. It has been established that these complexes cannot be to a marked extent electrostatic. It has been found that they are often colored. Complexes with iodine (Lewis acid) give absorption bands at 300 nm in the UV region. These complexes are not true chemical compounds. According to Dewar, all the above facts are due to the formation of Tt-complexcs between an acid or Lewis acid and the entire Ji-electron system of an unsaturated compound which should be considered as Lewis base. Because in these complexes a double bond is an electron donor and Lewis acid is an electron acceptor, they are known as donor-acceptor complexes. The decrease in energy in complexing is conditioned by quantum-mechanical reasons. 

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. 

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