Carbocation under stable ion conditions

The first reaction provides a route for the reduction of alkyl halides since the carbo-cation (isopropyl, in Rl) may be prepared from action of AICI3 on the corresponding alkyl halide. Reactions of the type Rl are also important in the process, catalytic cracking, in the manufacture of gasoline. They have also been studied in mass spectro-metric experiments [235]. Reaction R2 is one route to the preparation of carbocations under stable ion conditions. Reaction R3 is employed in the laboratory synthesis of the tropylium cation. Reaction R4, the (crossed) Cannizzaro reaction, is unusual in that it takes place under strongly basic conditions. The oxy dianion is an intermediate in the reaction of concentrated hydroxide with the aldehyde, R HO. None of R1, R2, or R3 may have hydrogen atoms a to the carbonyl groups. Formaldehyde (R1 = H) is readily... 

Indeed, many examples are now known of such rapidly equilibrating carbocations under stable ion conditions (see Table 13.1, Ref.11)). The question to be resolved is whether the behavior of the 2-norbornyl cation under solvolytic conditions is best interpreted in terms of such a pair of rapidly equilibrating classical carbocations or ionpairs, or as the stabilized a-bridged species. 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

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. 

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. 

As is apparent in the last step, isobutane is not alkylated but transfers a hydride to the Cg+ carbocation before being used up in the middle step as the electrophilic reagent (tert-butyl cation 4). The direct alkylation of isobutane by an incipient tert-butyl cation would yield 2,2,3,3-tetramethylbutane,142 which indeed was observed in small amounts in the reaction of ferf-butyl cation with isobutane under stable ion conditions at low temperatures (vide infra). 

In recent years the pioneering work of Olah, Saunders, Brouwer, and Hogeveen has made possible the direct spectroscopic observation of many carbocations20. One remarkable development from such studies has been the conclusion that many carbocations, such as 2,3,3-trimethyl-2-butyl, which can be captured in solvolysis without equilibration17, undergo very rapid equilibration under stable ion conditions. Such equilibration often cannot be frozen out even at temperatures as low as -150 °C. Under such stable ion conditions it appears that rapid equilibration is the norm. 

As noted earlier, in exceptionally weakly nucleophilic media the NMR method is used to observe directly many nonclassical ions — intermediates postulated in explaining unusual rates, products and stereochemistry of the above solvolysis reactions. This enables research under stable-ion conditions may result in dis-coverii new, earlier unknown kinds of carbocation rearrangements illustrated by the 7-norbomenyl and 7-norbomadienyl cations. 

Despite having made these analogies to other systems, the exact nature of the C4H7 system is less clear than that of norbornyl, and it is still under investigation. Considerable evidence exists in support of both the bicyclobutonium and cyclopropylcarbinyl carbenium ions as important contributors to C4H7. Under stable ion conditions ese two appear to be in equilibrium. As with so many carbocations, a very flat potential energy surface is implied, with structures of similar energy and low barriers to interconversion. 

Pertinent reviews this year deal with a reappraisal of the structure of the norbornyl cation under stable ion conditions, theoretical approaches to the structure of carbocations, pyramidal mono- and di-cations, and dynamic n.m.r. studies of carbonium ion rearrangements. ... 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Under superacidic, low nucleophilicity so-called stable ion conditions, developing electron-deficient carbocations do not find reactive external nucleophiles to react with thus they stay persistent in solution stabilized by internal neighboring group interactions. 

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)... 

The pathway of these alkylations was clearly demonstrated by Olah et al from their extensive work on the alkylation of the lower alkanes by stable carbocations under superacidic, stable ion conditions. They found that the order of reactivity of C—C and C—H bonds reflected their donor abilities and was in the order tertiary C—H > C—C > secondary C—H primary C—H, although various specific factors, such as steric hindrance, can influence the relative rates. 

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