Antimony pentafluoride carbocations

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

Vinyl and phenyl mfluoromethyl groups are reactive in the presence of aluminum chloride [10] Replacement of fluorine by chlorine often occurs Polyfluori-nated trifluoromethylbenzenes form reactive a,a-difluorobenzyl cations in antimony pentafluoride [11] 1 Phenylperfluoropropene cyclizes in aluminum chloride to afford 1,1,3-trichloro 2 fluoroindene [10] (equation 10) The reaction IS hypothesized to proceed via an allylic carbocation, whose fluoride atoms undergo halogen exchange... 

The use of antimony pentafluoride as a component of superacids for the generation of carbocations from various organic compounds was reviewed recently [16],... 

Olah, G. A. From Boron Trifluoride to Antimony Pentafluoride in Search of Stable Carbocations. 80,19-88 (1979). 

The carbocations being reactive intermediates have short life and so they are unstable. But they have been rendered stable after being trapped in strongest super acids which are mixtures of fluorosulphuric and antimony pentafluoride dissolved in S02 or chlorofluorosulphuric acid S02CIF. Under these conditions their structure and stability have been correlated. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

More recently, development of the superacid solvent systems has permitted the preparation at low temperature of stable solutions of carbocations of many structural types. The solvents ordinarily used consist of the strong Lewis acid antimony pentafluoride with or without an added protonic acid, usually hydro-... 

The main feature of the proton NMR spectra of alkyl fluorides in antimony pentafluoride is the substantial deshielding of the protons in the carbocations as compared with the starting alkyl fluorides (Figure 3.3 and Table 3.1). 

Antimony pentafluoride itself (neat or in S02 or S02C1F solution) also ionizes alcohols to form alkyl carbocations [Eq. (3.15)]. 

Solution of antimony pentafluoride in anhydrous HF was found to be an efficient medium for alkylation reactions of fluoroolefins. Compounds, known to give stable carbocations in superacidic media are excellent alkylating agents. For example, f-butyl chloride (57) reacts with TFE, giving the corresponding alkane 58 ... 

From Boron Trifluoride to Antimony Pentafluoride in Search of Stable Carbocations... 

Superacids such as Magic acid (42), a system containing antimony pentafluoride and fluorosulfonic acid and designated a superacid because it is a more ready electron pair acceptor than anhydrous aluminium chloride (43), have useful catalytic properties in synthetic organic chemistry where the reaction involves a carbocation intermediate. The extremely low nucleophilicity of the counter ion of such acids make it possible to prepare cations such as the tertiary butyl (( 3)30" " with an appreciably long life time in superacid solutions whereas such cations are too reactive... 

Miller and coworkers treated 1-chloro- and l-bromO 2-phenylethyne with antimony pentafluoride in liquid sulphur dioxide benzene was added as a carbocation scavenger. Instead of carbon-halogen heterolysis they unexpectedly observed formation of 2-halo-3-phenylbenzothiophene 5 -oxide (41). On closer examination and depending upon substituent X various other products were observed, e.g. 1-halo-2,2-diphenylvinylsulphinic acids (42). The initial step in the formation of these products is probably electrophilic addition of sulphur dioxide, assisted by the Lewis acid. Trapping of the carbon cation by benzene and elimination of antimony pentafluoride explained the various products. 

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