Halonium ions stabilities

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) ... 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

The N atom so strongly stabilizes cations that a (3-halocarbocation is the likely intermediate, not a halonium ion. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

The question of intermediates in thiophene substitutions may not always be so straightforward as portrayed so far. The sulfur atom itself can be the site of attack by an electrophile, i.e. (14) and also Section 3.13.2.4. One can postulate that selectivity in thiophenes could result from attack of an electrophile, for example a halonium ion, at sulfur followed by 1,5-sigmatropic rearrangement and deprotonation as shown in equation (6). A determination of the relative stabilities of (23) and (17) for different substituents X is needed for assessment of the validity of this possibility. Note that the rearrangement type represented by (23) to (17) is known in a photochemical variant (equation 7) (73TL3929). From ab initio calculations on thiophene it also appears that initial attack of an electrophile on sulfur is the pathway of higher electron density (72MI31302). 

These and other facts point to a mechanism of the type envisaged by Hobart and Kimball,1470 and frequently invoked by Winstein and, co-workers,165 4-1165 18s7- i i. ie < jn which an intermediate halonium ion ( A ) is formed, aud subsequent attack by a nucleophile occurs at the site of greatest incipient c rbonium ion stabilization. Alternative suggestions have also been advanced by Dewar4 1 and by de la Mare.1104 Tho former postulated a v-complex intermediate ( ), the Utter an 1 open carbon tun ion intermediate ( C . ... 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

Heteroaromatic Halophenium Ions. Halophenium ions are a class of halonium ions analogous to thiophene, furan, and pyrrole. To date, no parent halophenium ions 189 are known, but many stabilized tetraphenyl iodophenium, benzoiodophenium, and dibenzoiodophenium ions have been prepared by Beringer and co-workers.419,420 Some of them have been analyzed by X-ray crystal structure investigations. Representative examples are ions 190-192. 

Miscellaneous Halonium Ions. The Si-containing ring system 194 analogous to 188 has been reported by Muller and co-workers.422 The silyl cation is stabilized by intramolecular interactions with the F atoms increasing the coordination number of Si and, consequently, Si becomes considerably shielded. In the 29Si NMR spectrum, the resonance at 829 Si 77 indicate that cation 194 has only a small silicenium ion character... 

A number of distonic halonium-based dications are also known and their properties are often superelectrophilic.81 The chemistiy of halonium ions was thoroughly reviewed, thus only a few aspects are described here. Alkylation of dihaloalkanes with methyl and ethyl fluoroantimonate (CHsF-SbFs-SCF and CHsCE F-SbFs-SCh) gives the monoalkylated halonium ions and/or the dialkylated dihalonium ions, depending on the reaction conditions. Iodine shows an unusual ability to stabilize positive... 

In the gas phase, addition of an ionic electrophile to a neutral (M) is usually accompanied by elimination of a neutral molecule from the reagent ion. This elimination process stabilizes the reaction products by removing excess energy from the initial ion-molecule adduct. Typically, protonation and alkylation reactions normally used in chemical ionization (Cl) and radiolytic experiments are of this type, as shown in equations 3 (HA+ = CH5+, C2H5+, NH4+, etc.) and 4 (R2X+ = an halonium ion, vide infra), respectively. 

The stabilities of halonium ions (RXR )+ were established in a study of the equilibria of equation 8 where R+ is Et+, z-Pr+, r-Bu+, 2-methyl-2-butyl+, cyclopentyf and 2-norbornyl+ and XR is MeCl, CH2C12, CHC13, EtCl, CHF3, CF4 and S02F214. 

In Section 3.5.1 it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated olefins react with Br2 but not with Cl2 via halonium ions. Because C—Cl bonds are shorter than C—Br bonds, chloronium ions presumably have a higher ring strain than bromonium ions. Accordingly, a /3-chlorinated tertiary carbenium ion is more stable than the isomeric chloronium ion, but a /3-brominated tertiary carbenium ion is less stable than the isomeric bromonium ion. 

Reaction at the meta position gives a sigma complex whose positive charge is not delocalized onto the halogen-bearing carbon atom. Therefore, the meta intermediate is not stabilized by the halonium ion structure. The following reaction illustrates the preference for ortho and para substitution in the nitration of chlorobenzene. 

In an analogous fashion, the decomposition of the halonium ion 25 to provide the 3-halo product would be fostered by the superior ability of the adjacent S or NH to stabilize positive character at C-2 over that of phenyl to stabilize a similar deficiency at C-3 [Eq. (25)] (cf. the electrophilic orientation patterns in dibenzothiophen and in carbazole98) ... 

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