Cations halonium, stabilities

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... 

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 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. 

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. 

Halonium ions have received a great deal of attention in the last decade and a wdiole book has already been devoted to these species The stability of dialkyl-halonium ions decreases in the order R2l > R2Br > R2C (thefluoroniumhomolo-gues have never been observed), and iodonium and bromcmium hexafluoroantimonates have been prepared and characterised in Olah s laboratory Under specific conditions these salts are active initiators of the cationic polymerisation of cdefins, and this still little-explored field will be briefly analysed in Chap. IV. 

The methoxymethyl cation can be obtained as a stable sohd, MeOCHj SbF, Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. In super acid solution, ions such as CX (X = Cl, Br, I) have been prepared. Vinyl-stabilized halonium ions are also known. ... 

Wieting, R. D., Staley, R. H., Beanchamp, J. L. (1974). Relative stabilities of carbonium ions in the gas phase and solution. Comparison of cyclic and acyclic alkylcarbonium ions, acyl cations and cyclic halonium ions. Journal of the American Chemical Society, 96, 7552-7554. 

Halogenation (Section 22.1 A) The electrophile is a halonium ion formed as an ion pair by interaction of chlorine or bromine with a Lewis acid. The mechanism involves an initial reaction between Clj and FeClj to generate a molecular complex that can rearrange to give a C1+, FeCl " ion pair. The C1+ reacts as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the aryl chloride product. 

Staley RH, Wieting RD, Beauchamp JL. Carbenium ion stabilities in the gas phase and solution. An ion cyclotron resonance study of bromide transfer reactions involving alkali ions, alkyl carbenium ions, acyl cations, and cyclic halonium ions. J Am Chem Soc. 1977 99 5964-72. 

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