Halonium ions alkylations with

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) ... 

Therefore, it must be concluded that earlier attempts to prove the existence of stable, well-defined alkyl cations were unsuccessful in experiments using sulfuric acid solutions and inconclusive in the interaction of alkyl halides with Lewis acid halides. Proton elimination reactions or dialkyl halonium ion formation may have affected the early conductivity studies. 

The method has been extended by the use of reactive intermediate oxonium ions such as sulfonyl cation-ether adducts, dialkyloxycarbenium salts with ether, and so on. The more reactive dialkyl halonium ions also readily alkylate dialkyl ethers to the corresponding oxonium ions."... 

A typical fragmentation of alkyl chlorides and bromides is the loss of an alkyl group with the formation of a halonium ion. The ion forms the base peak in... 

Calculated equilibrium geometries, bond lengths, and charge densities have been compared for halonium ions (1-5 X = Cl or Br) derived from the addition of halogen electrophiles to fluoro-substituted terminal alkenes (R = alkyl or perfluoroalkyl with... 

Halonium ions are an important class of onium ions.43 The dialkylchloro, bromo, and iodohalonium ions can be prepared and even isolated as stable salts (i.e., 46), as shown by Olah et al. by reacting an excess of haloalkane with strong Lewis acid halides in solvents of low nucleophilic-ity (eq 14). In superacid solution, dialkylhalonium ions show enhanced alkylating reactivity.44 It is considered that this enhanced reactivity is due to further protolytic (or electrophilic) activation involving the non-bonded... 

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... 

These last two studies led us to investigate the possibility of using halonium ions to mediate the reactions of simple alkyl and arylamides with glycals. 

Identification of chloro and bromo compounds is a relatively simple matter because of the unique isotopic pattern. The presence of fluoro and iodo compounds, although not easy, can be inferred from the conspicuously low [M - -1]/[M] ratio, which is due to the fact that F and I are monoisotopic. The molecular ion in aliphatic chlorides is visible only in lower monochlorides. With an increase in the number of chlorine atoms, the abundance of the molecular ion decreases further. The t-cleavage to expel a halogen atom often produces an abundant ion (e.g., the base peak in the mass spectrum of r-butyl chloride is C4H9+). The a-cleavage is of low consequence in alkyl chlorides, but the loss of an alkyl radical can be prominent when the alkyl chain is longer than four carbons the product is a flve-membered ring halonium ion ... 

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