Fluoronium cations

Basically, two different routes are conceivable for their asymmetric construction 1) nucleophilic substitution reaction with a fluoride anion and 2) electrophilic addition of fluoronium cations to activated or masked carbanions. First attempts on enantioselective nucleophilic fluorination date back to the pioneering work of Hann and Sampson [3]. In an ambitious dehydroxylation/fluorination sequence the authors reacted a racemic a-trimethylsiloxy ester with a half molar equivalent of an enantiomerically pure proline-derived aminofluorosulphurane in hope to achieve a kinetic resolution. Unfortunately, the fluorinated product was obtained without significant enantiomeric excess. 

SCHEME 40.36. Synthesis of aziridines using a fluoronium cation as an organocatalyst. 

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... 

Even electrophilic fluorination of alkanes has been reported. F2 and fluoroxytrifluoro-methane have been used to fluorinate tertiary centers in steroids and adamantanes by Barton and coworkers ". The electrophilic nature of a reaction involving polarized but not cationic fluorine species has been invoked. Gal and Rozen have carried out direct electrophilic fluorination of hydrocarbons in the presence of chloroform. F2 appears to be strongly polarized in chloroform (hydrogen bonding with the acidic proton of CHCI3). However, so far no positively charged fluorine species (fluoronium ion) is known in solution chemistry. 

Hehre and Hiberty have also made ab initio calculations on the C2H4F and C2H4CU cations. When the 4-31G basis set was used the 1-haloethyl cations were calculated to be the most stable with both halogens (see Table 5). The bridged fluoronium ion was calculated to be much less stable than the... 

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