Cyclic iodonium ion

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Both the reactions are essentially the additions of iodine carboxylate (formed in situ) to an alkene, i.e., the reaction of an alkene with iodine and silver salt. The Prevost procedure employs iodine and silver carboxylate under dry conditions. The initially formed transiodocarboxylate (b) from a cyclic iodonium ion (a) undergoes internal displacement to a common intermediate acylium ion (c). The formation of the diester (d) with retention of configuration provides an example of neighbouring group participation. The diester on subsequent hydrolysis gives a trans-glycol. 

Attack by Cl, 111 I, 19 and RS + 2il is similar to that by Br there is a spectrum of mechanisms between cyclic intermediates and open cations. As might be expected from our discussion in Chapter 10 (p. 312), iodonium ions compete with open carbocations more effectively than bromonium ions, while chloronium ions compete less effectively. There is... 

The addition of iodine azide to double bonds gives p-iodo azides.674 The addition is stereospecific and anti, suggesting that the mechanism involves a cyclic iodonium ion interme-... 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

The initial addition of iodine leads to a cyclic iodonium ion, which is opened through nucleophilic substitution by benzoate anion ... 

The difference in reactivity between alkylated and acylated pentenyl glycosides can be rationalised as follows the elctrophilic iodonium ion will add to the double bond of the pentenyl moiety to give a cyclic iodonium ion. Nucleophilic attack by the oxygen will lead to an oxonium ion intermediate which then forms an oxocarbenium ion and an iodo-tetrahydrofuran derivative. The aglycone oxygen will be of low... 

Iodine isocyanate, preformed or made in situ from AgNCO and h, adds to alkeites - with the regio- and stneo-chemistry expected of reactions proceeding via cyclic iodonium ions. When the INCO is made in situ, a competing mechanism also occurs (except with the most reactive alkenes) in which the alkene complexes with the iodine, and the complex then reacts with the isocyanate ion to generate the same P-iodoisocyanate as obtained firom INCO direct The reaction can be carried out at -35 to +20 C in Et20, CHzCh, THF, pentane or excess alkene as solvent. Dichloromethane or ether are... 

The addition of iodine azide to double bonds gives p-iodo azides. The reagent can be prepared in situ from KI—NaNa in the presence of Oxone -wet alumina. The addition is stereospecific and anti, suggesting that the mechanism involves a cyclic iodonium ion intermediate. The reaction has been performed on many double-bond compounds, including allenes and a,p-unsaturated ketones. Similar reactions can be performed with BrNa and CfNa. 1,4-Addition has been found with acyclic conjugated dienes. In the case of BrNa, both electrophilic and free-radical mechanisms are important, whUe with CIN3 the addi-... 

The regiospecificity of the addition reactions has been accounted for by assuming the intermediacy of a cyclic iodonium ion 16. This would be expected to undergo ring-opening in such a way as to provide maximum stability for the incipient carbonium ion (i.e. to... 

The first step of the Provost reaction is the reaction of the alkene with iodine to form the cyclic iodonium ion. Next, the iodonium ion is stereospecifically opened by the silver carboxylate to form the corresponding frans-1,2-iodo carboxylate. The iodine is displaced intramolecularly by the carbonyl group of the carboxylate (anchimeric assistance) to form a cyclic cationic intermediate. In the absence of water, this cation is opened with the inversion of configuration by the second equivalent of silver carboxylate to afford the frans-1,2-dicarboxylate. However, in the presence of water Woodward-Brutcher modification) the common intermediate is converted to a c/s-orthocarboxylate which is hydrolyzed to the corresponding c/s-1,2-diol. 

NaHCC>3, 0 °C) of 46 resulted in a 70 30 mixture of 47 and 48 [27, 28]. The major product 47 is formed through the six-membered cyclic transition state wherein the iodonium ion is formed under the steric guidance of the methyl substituent, i.e., on the anti face of the double bond as in 47a. The minor product 48 could be envisioned to be derived from the transition state resembling 48a. The transition state 47a is favored over the transition state 48a on account of the absence of A( L3) strain for having the methyl substituent oriented axial, though the strain is small. 

The alkene 11 reacts with electrophiles on the less-hindered (exo) face of the double bond. Thus, catalytic osmium tetroxide andNMO (seeScheme 5.80) or KMn04 provide the exo-cis-diol resulting from approach of the osmium from the more accessible face of the molecule. To prepare the isomeric endo-cis-diol, the Woodward-Prdvost reaction may be used (iodine and silver acetate in the presence of water). In this case, iodine should approach the exo face, but subsequent attack on the exo-iodonium ion by acetate anion would occur from the opposite (endo) face (see Scheme 5.93). Formation and hydrolysis of the cyclic intermediate cation gives the endo-cis-d o. ... 

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