2-Fluoroethyl cation

Fig. 6.1. PES of the isomerization Ila - lib (X = F) calculated by the MNDO method and represented as a function of two angles. (Adapted from Ref. [5]) Transition vectors are shown of two transition state structures that link the isomers of the 2-fluoroethyl cation (AE = 22.5 and 7.4 kcal/mol relative to lib)...

Cyclic aliphatic halonium ions (I, Br, Cl) have been observed directly in superacid solution by NMR spectroscopy (B-75MI11900). Cyclic halonium ions with ring size three, five and six are formed from open chain dihalides by reaction with strong Lewis acids such as SbFs. Although numerous iodonium, bromonium and chloronium ions are known, no fluoronium ion has been directly observed. NMR spectra of a solution of 2,3-difluoro-2,3-dimethyl-butane (12) in SbF5-S02 at — 90 °C provide evidence for a rapid interconversion of the two open-chain, substituted /3-fluoroethyl cations (67JA4744). The open-chain cation is about 48.2 kJ mol-1 more stable than the closed fluoronium ion (74JA2665). 

Attempts to obtain the tetramethylfluoronium ion 166-F has, however, been unsuccessful.340 Ionization of 2,3-difluoro-2,3-dimethylbutane in SbF5-S02 at —60°C gave an equilibrating i-fI uorocarbenium ion 167. The equilibration was shown to occur through the intermediacy of -tert--butyl-1-fluoroethyl cation 168 by a recent 13C NMR spectroscopic study.391... 

Recent energy calculations by Hehre favor the 1 -haloethyl form of the cation (38). Based upon the temperature dependence of the observed nmr spectrum, Olah (39) has postulated the formation of the 1-fluoroethyl cation... 

In the next work, Freitas and co-workers explored the conformation of 3-fluoropiperidinium cation and its analogues (Fig. 3a). To analyse how XF,N) and y(F,H(N)) couplings would behave if subjected to strong H-bond or F/N attraction, the authors computed these couplings as the dependence on shortening the N-C-C and C-C-F angles in (2-fluoroethyl)amine and (2-fluoroethyl)ammonium cation from the equilibrium structures. The calculations were performed at the BHandH/ EPR-III theory level. 

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