Trifluoromethyl cation

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] ... 

Among the simple transient species of carbon and fluorine are included trifluoromethyl cation, CF3+, which has a very transient existence trifluoromethyl anion, CF3, which is stable at low temperature and can be generated from CF3TMS (trimethylsilyltrifluoromethane) CF3-, a free radical available from irradiation of CF3I and difluorocarbene, CF2 , which exists as a ground-state singlet. 

Although free trifluoromethyl cation salts have not yet been isolated and characterized, it is possible to generate a mixture of chlorofluoromethyl cations (CFyClj., ) in situ from carbon tetrachloride and strong Lewis acids [8]. These systems can be used for electrophilic trihalomethylation of electron-rich aromatic substrates. The remaining chlorine substituents are substituted by fluorine with 70% HF-pyridine (Scheme 2.141). 

An alternative mechanism for the formation of the esters is the dissociative mechanism involving RfS02 + and Rf + which was considered to be unlikely104 because of failure to detect Rf + directly and the observation of only a symmetrical anhydride upon the equilibration of a mixture of perfluoroalkanesulfonic acid and triflic anhydride, or a mixture of triflic acid and a perfluoroalkanesulfonic anhydride. However, recent work showed that the unsymmetrical anhydride indeed exists98. The preferential formation of trifluoromethyl esters in the reactions of mixtures of triflic and other perfluoroalkanesulfonic anhydrides in the presence of the corresponding sulfonic acids is consistent with the dissociation mechanism, since trifluoromethyl cation is much more easily formed than its higher analogues. Therefore, the dissociative mechanism (equation 84) cannot be ruled out unequivocally. 

The only example of the cationic perfluoroalkylation of furan by trifluoromethyl cation (generated by radiolysis of CF4) resulted in a mixture of 2- and 3-substituted... 

Bucci, R. Laguzzi, G. Pompili, M.L. Speranza, M. Gas-phase heteroaromatic suhstitution. 12. Reaction of free trifluoromethyl cation with simple five-memhered heteroarenes in the gas phase. J. Am. Chem. Soc. 1991, 113, 4544-4550. 

The direct trifluoromethylation of thiophene can be performed under electro-phihc and radical conditions. The electrophilic reaction proceeded in the gas phase using trifluoromethyl cations obtained from CF4 under radiolysis ( °Co y-rays) [44], The selectivity trend for thiophene in the gas phase follows the order C2>C3>S1. The major products of this transformation were found to be monosubstituted trifluo-romethylthiophenes 78 and 79 (<20 % yield) [45], It has been proposed that the trifluoromethylation proceeds through electrophilic substitution and single-electron transfer mechanisms. 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

When we examine the cyclohexadienyl cation intermediates involved m the nitra tion of (trifluoromethyl)benzene we And that those leading to ortho and para substitu tion are strongly destabilized... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Recently developed trifluoromethylatmg agents capable of transferring the trifluoromethyl group as a cation to strongly nucleophilic compounds such as carbanions and sulfur and phosphorus nucleophiles are prepared from o-biphenyl trifluoromethyl sulfoxide [164] and are shown in equation 141... 

Bischler-Napieralski reaction conditions can be attributed, again, to the destabilizing ability of the trifluoromethyl group to the cationic transition state of the acid catalyzed elimination. Formation of compound 29 was ultimately accomplished by base catalyzed methanol elimination-conditions conditions that are quite unusual for isoquinoline formation. 

Experience in PTC with cationic catalysts showed that, in general, the most suitable compounds have symmetrical structures, are lipophilic, and have the active cationic charge centrally located or sterically shielded by substituents. For anionic catalysis sodium tetraphenylborate fulfills these conditions, but it is not stable under acidic conditions. However, certain derivatives of this compound, namely sodium tetra-kis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB, 12.162) and sodium tetrakis[3,5-bis-(l,l,l,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]borate (HFPB) are sufficiently stable to be used as PTC catalysts for azo coupling reactions (Iwamoto et al., 1983b 1984 Nishida et al., 1984). These fluorinated tetraphenylborates were found to catalyze strongly azo coupling reactions, some of which were carried out with the corresponding diazotization in situ. 

An unprecedented carbene insertion reaction was observed on reaction of the cationic re-arene ruthenium amidinates with trimethylsilyldiazo-methane (Scheme 145, TFPB = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). 

Suitable electrolytes for the aqueous phase (w) comprise, for example, LiCl, HCl, MgCl2, MgS04, while suitable electrolytes for the organic phase (o) comprise salts of, for example, tetrabutylammonium, tetraphenylarsonium, or bis(triphenyl-phosphoranylidene)ammonium cations with tetraphenylborate, tertrakis(4-chloro-phenyl)borate, or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion. 

RTILs consist of large, unsymmetrical ions, such as 1,3-dialkyl-imidazolium, 1-alkylpyridinium, 1-alkylpyrazolium, tetralkylammonium or tetralkyl-phosphonium cations and tetrachloroaluminate, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate or bis((trifluoromethyl) sulfonyl)imide anions. 

In contrast, 13C nmr spectra of ring-substituted /J-bromocumyl cations [2] can be unambiguously interpreted in terms of open /1-bromocarbocations, since the ring substituent effects on the chemical shifts are similar to those on the corresponding non-brominated cations [1], even for the electron-attracting p-trifluoromethyl group (Olah et al, 1972). 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

Keywords Cation-radical salts, Molecular conductors, Organic superconductors, Organometallic anions, Tetrakis(trifluoromethyl)metallates, Tetrathiafulvalene... 

---