Positive iodination

The interpretation of the above data on iodination has been questioned by Buss and Taylor217, and by Grovenstein et a/.218,219. The former workers studied the iodination of 2,4-dichlorophenol at about 25 °C using a stirred flow reactor, the advantages of which are that once a steady state has been reached there is no change in the concentration of the reactive species in the reactor with time and the rate of reaction is simply a product of extent of reaction multiplied by the reciprocal ol the contact time hence it is possible to use unbuffered solutions and low iodide ion concentrations. They found general catalysis by the base component of added phosphate buffers and the observed rate coefficients varied with [H+ ] according to 

Grovenstein and Kilby218 showed that the kinetic isotope effect kH/kD is 3.97 for the iodination 2,4,6-trideuterophenol by iodine in aqueous buffer at 25 °C, and this is in accord with the base catalysis described above. However, this large isotope effect means that the intermediate is in fairly rapid equilibrium with the reactants, so that it is difficult to determine from kinetic studies which iodinating species is involved. Thus it might be positive iodine, equilibria (112), (113), (115) 

Subsequently, Grovenstein and Aprahamian219 investigated the iodination of 4-nitrophenol and its 2,6-dideuterated derivative, by iodine in aqueous solution 

Although this kinetic work appears to show that the iodinating species is I+ 

A positive iodinating species was postulated to account for the kinetics and isotope effect observed in the iodination of some amines by iodine in aqueous potassium iodide (in some cases in the presence of acetate, lactate, or phosphate ion). The isotope effects (kH/kD values in parenthesis) for these compounds studied were 2,4,6-trideutero-m-dimethylaminobenzenesulphonate ion, 25 °C (1.0) 2,4,6-trideutero-m-dimethyIbenzoate ion, 30 °C (1.4) 2,4,6-trideutero-dimethylaniline, 30 °C, lactate (3.0) 2,4,6-trideuteromethylaniline, 25 °C, acetate (3.2) 2,4,6-trideuteroaniline, 25 °C (3.5), phosphate (4.0) 2,4,6-trideutero-metanilate ion, 35 °C (2.0) 2,4,6-trideutero-m-aminobenzoate ion, 30 °C (4.8), phosphate (3.0) 2,6-dideutero-l-dimethylaminobenzene-4-sulphonate ion, 25 °C, phosphate (1.0) 4-deutero-l-dimethylaminobenzene-3-sulphonate ion, 25 °C, phosphate (1.0). The kinetics of these reactions was given by 

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Grimison and Ridd225 also suggested the preformation of a positive iodinating species as a means of explaining the results obtained in the iodination by iodine in aqueous potassium iodide, of imidazole at 25 °C. The kinetics of this reaction follow equation (121)... 

The possibility that a number of iodinations with iodine and iodine monochloride may involve the molecular species rather than the positive species are formerly believed has been considered under positive iodination (see pp. 92-97). Briefly, iodination rates have been found to be proportional to [I-]-2 and [I ] 1, generally with a mixed dependence, and the former term has been interpreted as involving iodination by hypoiodous acid (or more likely, positive iodine) and the latter term attributed to iodination by molecular iodine. 

Vinyl iodonium salts 1 are reactive due to the high leaving ability of the positive iodine group, and the stability is inversely dependent on the stability of the carbocation 2 formed upon departure of the iodonio group (eq 1). ... 

The anodic chlorination in some cases allows one to achieve better regioselec-tivities than chemical alternatives (p/o ratio of chlorotoluene in chlorination of toluene anodic 2.2, chemical alternative 0.5-1.0) [215]. Anodic oxidation of iodine in trimethyl orthoformate afforded a positive iodine species, which led to a more selective aromatic iodination than known methods ]216]. Aryliodination is achieved in good yield, when an aryhodide is oxidized in HOAc, 25% AC2O, 5% H2SO4 in the presence of an arene ]217, 218]. Alkyl nitroaromatic compounds, nitroaromatic ketones, and nitroanihnes are prepared in good yields and regioselectivity by addition of the corresponding nucleophile to a nitroarene and subsequent anodic oxidation of the a-complex (Table 13, number 11) ]219, 220]. 

Tetrahydrofuran freshly distilled from lithium aluminum hydride should be used. A commercial product with a peroxide content giving a positive iodine test must be treated with about 0.3% of cuprous chloride (boiling for 30 minutes and distillation) before the addition of the hydride. 

Nitrosonium ion was found to promote iodination of cyclohexene when AcOH was used as solvent, the trans-iodo acetoxy derivative was formed in a good yield. Solvolysis of the latter, followed by saponification, led to a cA-diol so that this method can serve as an alternative to the wet-Prdvost reaction. The NO+ cation is believed to promote the formation of some positive iodine species (equation 1). Oxidation by oxygen leads to the regeneration of iodine and NO+ from nitrosyl chloride189. 

Trifluoromethyl is a wonderful group with numerous substituent effect surprises84. Within the context of discussions of positive halogen , it is well-established85 that solution-phase nucleophilic reactions of trifluoromethyl iodide are consistent with the presence of formally positive iodine, e.g. equation 53. ]... 

Anodic oxidation of iodine or Mel in trimethyl orthoformate (TMOF) gives a new positive iodine active species ( I /TMOF), which makes possible a unique rearrangement of aryl alkyl ketones (28) to methyl arylalkanoates (29 equation 72). ... 

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