Halonium compounds

The 127I NQR spectra of an extensive series of iodonium compounds, [R2I]+BF, have been reported52,53. In addition to the fact that iodine is the only halogen that forms an extensive series of halonium compounds, the study of 127I has the advantage that, since its spin is equal to 5/2, there are two transitions and therefore both the coupling constant and the asymmetry parameter may be obtained. This is particularly important for a polycoordinated nucleus, since we may anticipate that the asymmetry parameter will be far from negligible. A selection of the NQR parameters for these compounds is shown in Table 9. Also studied were the iodonium halides, whose structures differ from those of the... 

Since water meets these requirements for many compounds, it is an excellent ionizing solvent. Frequently the replacement of ligands by water molecules takes place so readily that no intermediate steps such as halonium compounds are formed. 

Cocatalysts of two types occur (/) proton-donor substances, such as hydroxy compounds and proton acids, and (2) cation-forming substances (other than proton), including alkyl and acyl haUdes which form carbocations and other donor substances leading to oxonium, sulfonium, halonium, etc, complexes. 

Bromine and chlorine convert the 1- and 2-butenes to compounds containing two atoms of halogens attached to adjacent carbons (vicinal dihahdes). Iodine fails to react. In this two-step addition mechanism the first step involves the formation of a cation. The halonium ion formed (a three-membered ring) requires antiaddition by the anion. 

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

The anomalous iodoacetamide-fluoride reaction violates this rule, in that a less stable -halonium complex (18) must be involved, which then opens to (19) in the Markownikoff sense. This has been rationalized in the following way estimates of nonbonded destabilizing interactions in the possible products suggest that the actual product (16) is more stable than the alternative 6)5-fluoro-5a-iodo compound, so the reaction may be subject to a measure of thermodynamic control in the final attack of fluoride ion on the iodonium intermediate. To permit this, the a- and -iodonium complexes would have to exist in equilibrium with the original olefin, product formation being determined by a relatively high rate of attack upon the minor proportion of the less stable )9-iodonium ion. 

A detailed spectroscopic examination should settle the question of whether the ion has the open or the cyclic structure. In general halo-chromic salts lose their color when a covalent bond is established to the central carbon atom, but the bromonium ion might resemble the carbonium ion. Compounds of similar color but which are certainly not cyclic halonium ions are also known ... 

Peroxidases catalyze not only the formation of halohydrines, but also the halo-genation of 1,3-dicarbonyl compounds. In CPO-catalyzed halogenation of eno-lizable substrates, the halonium ion is trapped by the enolate to afford the corresponding mono- and dihalogenated products (Eq. 9, Table 10). 

Treatment of either cis- or rrans-stilbene-2-carboxylic acids with chlorine or bromine leads to 4-halogeno-3,4-dihydro-3-phenylisocoumarins (58T<4)393). The reactions are stereospecific and are thought to involve intramolecular attack by the carboxyl group on a halonium ion. Ring closure to the corresponding 4-hydroxy compound also occurs stereo-specifically using peroxyphthalic acid (59JOC934). 

In general, addition of more polar interhalogen compounds (ICI, IF, BrF, ClF) to fluoroolefins proceeds as an ionic process, starting with attack of halogen bearing positive charge on the C=C bond and formation of a cyclic halonium cation (Eq. 2) [10,18]. 

The synthesis of chlorine (I) and bromine (I) trifluoromethanesulfonates (triflates) was reported by DesMarteau. Stability and reactivity of these materials are similar to those of perfluoroalkyl hypohalites. Both compounds readily react at low temperature with a variety of fluoroolefins. Based on NMR analysis of the products of adding CF3S020X to pure cis- or trans-isomers of 1, 2-difluoroethylene, it was concluded that the reaction proceeds as syn-addition [35]. This statement was later criticized [18], since the assignment of stereoisomers was found to be incorrect. According to [18], addition of CF3S020X to haloolefins, as well as reactions of ClF, BrF and IF proceed as anti-addition via cyclic halonium cationic intermediates. 

The halonium structure 17 was also attributed107 to protonated bromo- and chloromethane, in agreement with theoretical predictions105,106. However, in these two ions an additional interaction (stronger for the bromo than for the chloro compound) is present between the hydrogen bonded to the halogen and the carbon atom107. Protonated iodomethane, on the contrary, structurally resembles protonated methane with the proton bonded to the carbon atom. These conclusions are supported by both MI and CID data for protonated and deuterated halomethanes and by thermochemical data. 

The simplest and, unfortunately, perhaps most generally perceived mechanism suggests that the halonium ion, X+, itself is the halogenating agent which is either innately present in the positive halogen compound or formed in situ from such compounds (equation 1). While substantial data now support the existence of X+ in the gas phase1, there is... 

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