Fluoronium ion

To explain the stereospecificity of the reaction a transition state (6) is proposed rather than the bridged fluoronium ion (7),... 

DFT study of the fluorinated DB[a,A]ACR-8,9-epoxides lead to the same conclusions regarding fluoronium ion character and stabilization at positively charged sites of the parent DB[a,/ ]ACR, and destabilization via inductive effect at positions bearing negative charge density. 

A four-center molecular syn addition ([2 + 2]-cycloaddition) is also consistent with experimental observations.202 Molecular-orbital calculations and experimental observations by NMR spectroscopy in superacidic systems rule out the existence of the bridged fluoronium ion.204-206... 

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). 

P-Fluonne or fluorine further removed from the cation center always inductively destabilizes carbocations [115, 116] No simple P-fluoroalkyl cations have been observed in either the gas phase or solution, and unlike the cases of the other halogens, there is no evidence for formation of alkyl fluoronium ions (5) in solution [117, 118], although (CH3)2F+ is long-lived in the gas phase [7791 The only P-fluonnated cations observed m solution are those that benefit from additional conjugativc stabilization, such as a-trifluoromethylbenzyl cations [772] and per-fluonnated allyl [729], cyclopropenium [772], and tropyliiim [727] ions... 

Molecular fluorine, because of its very low bond dissociation energy, usually reacts uncontrollably with organic compounds.43 Merritt, however, has observed electrophilic addition of F2 to cis- and trans-i -phenylpropenes at low temperatures. The mode of addition is predominantly syn. A fluoronium ion, in which the fluorine is positively charged, would be very unstable and apparently does not form.44 Attempts to form a three-membered ring fluoronium ion in superacid medium have also failed.45... 

The occurrence of intramolecular fluorine transfer via a bridged fluoronium ion was demonstrated by means of the EBFlow technique136. The ion 49a in the El generated... 

Direct observation of a number of bridged halonium ions (X = Br, Cl, I) is possible [74] when, for example, 2,3-dihalo-2,3-dimethylbutanes are ionised in SbFs/SOi mixtures however, as yet, no analogous fluoronium ions have been observed in solution (Figure 4.27). 

However, mass-spectral breakdown patterns of PI1OCH2CH2F suggest that a cyclic fluoronium ion can be observed as an ion-neutral complex in the gas phase [76], whilst calculations indicate that the fluoronium ion is more stable than the isomeric CH2CH2F ion [54]. Participation by fluorine remote from the reaction centre has been postulated to account for the product obtained from the reaction between 5-fluoro-l-pentyne and trifluoroacetic acid [46] (Figure 4.29). 

---