Trihalomethyl cations

Chapter 18 by C. Chiappe focuses on the mechanism of bromination of alkenes, exploring the role of solvent on the formation of cyclic bromonium ion versus P-bromocarbemium ion, as key intermediates. In Chapter 19, H. P. A. Mercier et al. discuss the utility of a novel class of noble-gas onium salts as oxidants for generation and isolation of various trihalomethyl cation salts. 

Olah et al.532,533 studied trihalomethyl cations (CX3+, X = Cl, Br, I) under stable ion conditions. 13C NMR chemical shift values correlate well with the decreasing order of back-donation (Cl > Br > I). Similar correlation was also found for dimethylhalo-carbenium ions 258. The CF3+ fluoro analog, however, could not be observed under any conditions. This can be attributed to a combination of unfavorable thermodynamics (generation of CF3+ from CF4 is endothermic by about 20lstarting materials and a suitably strong Lewis acid.534... 

Further accumulation of fluorine substituents at carbon carrying positive charge results in significant destabilization, the trifluoromethyl cation being the least stable among halomethyl cations [58], Recently, a series of trihalomethyl cations CX 3 (X=C1, Br, I) has been generated in solution and characterized using NMR techniques however, all attempts to prepare trifluoromethyl cation have failed [58] ... 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

In the presence of superelectrophilic trihalomethyl cation, the carboca-tionic center is formed by hydride abstraction generating the distonic superelectrophile (81). Capture of the superelectrophile with fluoride leads to the fluorinated product (82). 

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