Carbocations long-lived

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. 

Finding snch acids (called snperacids ) turned out to be the key to obtaining stable, long-lived alkyl cations and, in general, carbocations. If any deprotonation were still to take place, the formed alkyl cation (a strong Lewis acid) would immediately react with the formed olefin (a good TT-base), leading to the mentioned complex reactions. 

The chemistry of stable, long-lived (or persistent) carbocations, as they became known, thus began and its progress was fast and widespread. Publication of research done in an industrial laboratory is not always easy. 1 would therefore like to thank again the Dow Chemical Company for allowing me not only to carry out the work but eventually also to publish the results. 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

Silyl effects in carbocations which were predicted computationally and have been observed in the gas phase and in solvolysis reactions have been proven by NMR spectroscopic investigation of long-lived silyl-substftuted carbocations in... 

Siehl, H.-U. Excursions into Long-Lived Vipyl Cations NMR Spectroscopic Characterization a-Aryl Vinyl Cations, In Stable Carbocation Chemistry Prakash, G.K.S. Schleyer, P. v. R., Eds. Wiley New York, 1997 Chapter 5, p. 165 - 196. 

The authors succeeded in preparing (20-25) a series of long-lived carbocations bearing vinyl and related groups in the p-position to the carbocationic center (la-d, 3). Their structures were confirmed by and 13C NMR spectroscopy. These cations can be considered as a-complexes of C-centered electrophilic agents, namely vinyl and methyl-substituted vinyl cations derived from 9,10-dimethylphenanthrene and 1,2-dimethylacenaphthylene, respectively. Quite naturally, they had not been prepared by electrophilic vinyl-... 

Rearrangements of long-lived carbocations formed via ewfo-9-hydroxy-1,8,9,10,10,11,12-heptamethyltricyclo[6.2.2.02,7]dodeca-2(7),3,5,11 -tetraene (5) in FSO3H-SO2CIF-CD2CI2 at low temperature (-95 °C) were studied by H and... 

As mentioned above, persistent carbocation 9 underwent rearrangement into cation 10 which rearranged further into cation 11. To reveal general relations/factors governing cationic rearrangements in benzopentalene derivatives, the behavior of 5,5,10,10-tetramethyl-5,10-dihydroindeno[2,l-fl]indene (12) in superacids was studied (52). It had been expected that hydrocarbon 12 would transform into the long-lived 5,5,10,10-tetramethyl-4b,5,9b,10-tetrahydroindeno[2,l-a]inden-4b-yl cation (13). However, H and 13C NMR data showed that hydrocarbon 12 transformed firstly into isomeric ion 14 which transformed further into cation 15 (Scheme 11). 

Cyclobutenyl carbocations have been under study in our laboratory for some years (11). In (34), unusually strong temperature dependence of l3C chemical shifts (up to 0.077 ppm/K) was observed for long-lived cations 17a-c. The authors suggested that this resulted from a very low inversion barrier for the four-membered ring (Scheme 13). 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

Prior syntheses of long-lived perhalomethyl cations have been achieved by halide abstraction by use of either a strong Lewis acid (in superacidic or S02C1F solvent media) or Ag+ (vide supra), but no routes to such carbocations through oxidative removal of a halogen bound to carbon were known. The objectives of the current work have been to provide structural and spectroscopic data that, thus far, have been lacking for these systems, and to provide oxidative routes to... 

Carbocations have been prepared as long-lived species by the reaction RF -I- SbF, — R + SbF. SbF, a covalent liquid, is called a superacid because it is a stronger Lewis acid than R. ... 

The first long-lived fluorine-containing carbocation was discovered by Olah and coworkers.32 Thus, the fluorodimethylcarbcnium ion [Me2CF+] was obtained by protonation of 2-fluoropropene and also from 2,2-difluoropropane by reaction with antimony(V) fluoride. In the course of these investigations it was found that a-F stabilizes a cationic state, whereas fi-F is destabilizing. Attempts to prepare the simplest member of this class, the trifluoromethyl carbocation CF3+ failed. The ionization of trifluoromethyl halides with antimony(V) fluoride at — 80 C yielded only carbon tetrafluoride. 

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... 

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

Under the long-lived conditions of carbocations in superacid (Section 5.3, p. 235), 1,2-shifts interconverting ions of like stability also occur and are very rapid. For example, at — 180°C the five methyl groups of 2,3,3-trimethylbutyl cation have only one peak in the nmr. This observation implies that the methyl shift in Equation 6.6 occurs at the rate of 75 x 103 sec -1 with an activation barrier of <5 kcal mole-1.8... 

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