Fluorinated carbocations

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of unsaturated compounds, although the highly electron-deficient double bond in fluoroalkenes is not usually susceptible to electrophilic attack. Only the most powerful electrophilic reagents... 

The chemistry of highly fluorinated carbocations, preparations, and reactions is die subject of an extensive review.82 Also available is a discussion on die use of intrinsic 19F NMR isotope shifts for die determination of carbocation structures.83... 

A large number of simple fluorinated carbocations have been die subject of theoretical calculation, and die resulting calculated 19F and 13C chemical shifts correlate... 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

Olah GA, Mo YK (1972) Fluorinated Carbocations. In Tatlow JC, Peacock RD, Hynman HH, Stacey M (eds) Advances in Fluorine Chemistry. Butterworth, London, vol 7, p 69... 

The Influence of Fluorine or Fluorocarbon Groups on some Reaction Centres 103 Table 4.9 Stable fluorinated carbocations observed in solution by NMR spectroscopy... 

The stability of fluorinated carbocations [1] is determined by a delicate equilibrium between inductive destabilization and mesomeric stabilization of the positive charge. a-Fluoro substituents stabilize the positively charged carbon by TT-donation... 

Scheme 2.138 Stabilizing and destabilizing effects for fluorinated carbocations.

Despite the undeniable impact that NMR has had on progress in carbocation chemistry, it seemed to us surprising that there were no previous reviews in Annual Reports on NMR Spectroscopy that were entirely devoted to providing, under a certain umbrella, some of the NMR characteristics of certain classes of carbocations. Volume 14 (a comprehensive collection of l9F NMR data by V. Wray) contains 19F data for representative fluorinated carbocations. In volume 15, in a chapter on isotope effects on nuclear shielding (by P. E. Hansen), examples of deuterium isotope effects in classical and non-classical carbocations were included. However, there has not been a previous review entirely devoted to compiling NMR data on a large class or group of carbocations for comparative purposes. 

Contents The Vibrational Spectra of Inorganic Fluorides (D. J. R nolds) Fluorinated Carbocations (G. A. Olah and Y. K. Mo) Transition Metal Pentalluorides and Related Compounds (R. D. Peacock) The Dissociation Energy of Fluorine (J. B kowitz and A. C. Wahl) Fluoroxyfluorcalkanes and Perfluoroacyl and Inorganic Hypofluorites (M. Lust and J. M. Shreeve) Treatment of Hydrogen Fluoride Injuries fA. J. Fi el). 

C, indicate that the mesomeric effect e.g. MejC- -F Me2C=F ) of the halogens involved decreases in the order F > Cl > Br. All this and related work by Olah s group on fluorinated carbocations has been reviewed, and the results of MO studies of fluorinated methyl, allyl, or trans-2 vinylcyclopropylmethyP cations and radicals have been presented in detail. The electronic structure of difluoromethane and the partial double-bond character of C— F bonds in CHjF, CHjFj, and CHF, have received attention at the hands of MO pundits. 

C. Douvris, E. S. Stoyanov, F. S. Tham, C. A. Reed, Isolating fluorinated carbocations Chem. Commun. 2007, 1145. 

Douvris C, Stoyanov E, Tham FS, Reed CA (2007) Isolating fluorinated carbocations. Chem Commun 1145... 

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