Generation of Stable Carbocations

A wide variety of aliphatic tertiary and secondary alcohols can be ionized to the corresponding alkyl cations by use of magic acid [46-52]. Formation of the t-butyl cation (Eq. 23) [46] and a cyclopropyl-stabilized di-cation [53] are representative examples. Primary (and some secondary) alcohols are protonated only at temperatures lower than -60 °C [54]. At more elevated temperatures, they might cleave to give the corresponding carbocations, which, however, immediately rearrange to the more stable tertiary cations [49,50]. 

Similar to alcohols, aliphatic ethers [54], thiols [55], and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in magic acid carbocations are subsequently formed upon raising the temperature. Promoted sulfides, excluding tertiary alkyl, are resistant to cleavage up to -i-70 °C [56]. 

Alkyl chlorides, fluorides, and bromides are convenient and frequently used precursors for the generation of alkyl cations in HSOsF-SbFs systems [55]. It should be noted, however, that the HS03F-SbF5 system is less suitable than SbFs for the generation of alkyl, especially secondary alkyl, cations from the corresponding alkyl halides. 

Carbocations can be generated by the protonation of unsaturated hydrocarbons such as alkenes and cycloalkenes [49,52], cyclopentadienes [57], benzenes and naphthalenes (Eq. 24) [58], pyrenes and cyclophanes [59], unsaturated heterocycles [60], and their derivatives with carbon-heteroatom multiple bonds [2], including carbonyl and nitrile compounds and diazoalkanes [61]. 

Magic acid can abstract hydride from saturated alkanes, including straight-chain alkanes and branched and cyclic alkanes, at -125 to 25 °C to give alkyl cations [47,62], Its corrosive and toxic nature makes HF-SbFs a less frequently used acid system for the preparation of carbocations, compared to HSOsF-SbFs. It is, however, preferred in the generation of arenium ions, because high acidity is required for their formation (Eq. 25) [63], 

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Generation of Stable Carbocations. Thanks to its high acidity, Magic Acid can be used for the generation of such reactive carbocations as the t-butyl cation and other alkyl cations, while fluorosulfuric acid itself is suitable only for the generation of more stable cations such as aryl- or cyclopropyl-stabihzed carbocations. 

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

The electrophile, an acyl cation, is generated in a manner similar to that outlined in Figure 17.4 for the generation of the carbocation electrophile from an alkyl halide. First the Lewis acid, aluminum trichloride, complexes with the chlorine of the acyl chloride. Then A1C14 leaves, generating an acyl cation. The acyl cation is actually more stable than most other carbocations that we have encountered because it has a resonance structure that has the octet rule satisfied for all of the atoms ... 

Most benzylic solvolyses generating relatively stable carbocations belong to a category to which the Brown constants are effectively applicable. Extensive data on a,a-dialkylbenzyl solvolyses are available from Brown s original studies, and a wide set of benzylic substituent effects were included in Johnson s (1978) compilation of Brown correlations. Although all these... 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

The reactions of stable carbocations with water are generally base catalyzed,109,110 and their reactions with hydroxide ion are slower than their reactions with azide ion and sulfhydryl ions. The less favorable reaction of hydroxide ion with carbocations has been attributed to the fact that deprotonation of a water molecule by hydroxide ion is not a thermodynamically favorable reaction, and activation energy to generate a desolvated hydroxide ion is required.110 These factors would also account for the less favorable reaction of hydroxide ion as a nucleophile with 82 to form tetrols instead of as a base to bring about epoxide ring closure. 

The authors have generated a stable carbocation by the interaction of nonanol 617c with SbFj—SOjFCl at —130 °C. Judging by the H and NMR spectra the ion obtained should have the pyramidal structiu e 619. Cited below are the chemical > hifts of the signals of the hydrogens (parenthesiad) and carbons of this ion. 

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. 

Protonation and subsequent loss of water should generate a earbocation. Examine all of the carbocations derived from protonation of (3-D-glueose. Identify the most stable carboeation (this is the one that will form most readily), and draw whatever resonance eontributors are needed to describe the geometry, energy, and atomie charges in this cation. Can you explain why substitution oeeurs selectively at Ci ... 

Note that in the S l reaction, which is often carried out under acidic conditions, neutral water can act as a leaving group. This occurs, for example, when an alkyl halide is prepared from a tertiary alcohol by reaction with HBr or HC1 (Section 10.6). The alcohol is first protonated and then spontaneously loses H2O to generate a carbocation, which reacts with halide ion to give the alkyl halide (Figure 11.13). Knowing that an SN1 reaction is involved in the conversion of alcohols to alkyl halides explains why the reaction works well only for tertiary alcohols. Tertiary alcohols react fastest because they give the most stable carbocation intermediates. 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

Substituted allenyl cations 47 have been generated from propargyl alcohols 48 under stable carbocation conditions (Sbf s/f SOsII in SO2CIF) (equation 17). On the basis of 13C-NMR chemical shifts, the positive charge has been found to be extensively delocalized with the mesomeric allenyl cations contributing highly to the total ion structure36,37. 

Another reaction that has been applied to the generation of highly functionalized polymers is cationic polymerization [12-15]. Catalysts for cationic polymerizations are aprotic acids, protic acids, or stable carbocation salts. In these processes, the catalyst generally reacts with a cocatalyst to form an active initiated species. Initiation takes place by protonation of the monomer (Fig. 2A). Monomers that possess cation stabilizing groups, such as electron rich olefins, are preferred as they more readily undergo the desired polymerization process... 

Generation and NMR studies of the carbocations from various classes of PAHs under stable ion conditions, in combination with computational studies, provide a powerful means to model their biological electrophiles. These approaches allow the determination of their structures, relative stabilities, charge delocalization modes, and substituent effects, as a way to understand structure/reactivity relationships. 

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