Dimethylhalonium ions

Note added in proof Recently, the ter -butylium ion has been identified in alkyl bromide solutions by NMR (P. Brueggeler, E. Mayer, Z. Naturforsch. Teil B, 34, 891 (1979)), and some dimethylhalonium ions have been identified in alkyl halide solvents by the same method (P. Brueggeler, E. Mayer, Z. Naturforsch. Teil B, 34, 896 (1979)). 

Methyl bromide and methyl iodide in HS03F-SbF5-S02ClF at -78°C give both dimethylhalonium ions (127, R = CH3, X = Br, I) as well methylhydridohalonium ions (126, R = CH3, X = Br, I).349 These hydridohalonium ions have been characterized by 13C NMR spectroscopy.349 Methyl chloride does not give methylhydrido-chloronium ion under similar conditions. 

Ions 129-133 are some of the representative dialkylhalonium ions that have been prepared and characterized." (Trimethylsilyl)methylhalonium ions 134 have also been obtained under stable ion conditions and characterized by1H, 13C, and 29Si NMR spectroscopy.67 The Raman and IR spectroscopic studies of dimethylhalonium ions 129 seem to indicate that these species exist in a bent conformation. 

The intrinsic acidity of dimethylhalonium ions has been determined, both by theoretical methods and by gas-phase reactions of the isolated ions with pyridine bases.135 The calculated geometry of the dimethylhalonium ions shows a bent structure with the C-X-C angle decreasing in the order Cl > Br > I. Thermochemical calculations for the reaction of the dimethylhalonium ions with pyridine, 2,6-dimethylpyridine, and 2,6-di-r-butylpyridinc indicated that proton transfer, with the formation of the dimethylhalonium ylide, is endothermic, whereas methyl transfer, with formation of methyl halide, is exothermic. The endothermicities for proton transfer are, nevertheless,... 

When CT4 is allowed to decay for several months in gaseous CH3F at 660 Torr, tritiated methyl fluoride is the only product detectable by radio GLC, with a 68% yield. Its formation has been traced to formation of a dimethylhalonium ion, follo>ving electrophilic attack to the n-center of the substrate (Equation 13) while the absence of labeled ethyl... 

The condensation of methyl chloride or bromide to ethylene proceeds by a related mechanistic path involving initial acid-catalyzed dimethylhalonium ion (or related catalyst complex) formation with subsequent proton elimination to a reactive methyl-halonium methylide, which then is readily methylated by excess methyl halide. The ethyl-halonium ion intermediate gives ethylene by jS-elimination (see Scheme 22). 

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