Stable-ion conditions

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. 

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. 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

Hydride-bridged ions of this type are evidently quite stable in favorable cases and can be observed under stable-ion conditions. The Itydride-bridged cyclooctyl and cyclononyl cations can be observed at — 150°C but rearrange, even at that temperature, to methylcy-... 

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

The case for the generality of the o-complex mechanism is further strengthened by numerous studies showing that benzenium ions (an alternative name for the o-complex) can exist as stable entities under suitable conditions. Substituted benzenium ions can be observed by NMR techniques under stable-ion conditions. They are formed by protonation of the aromatic substrate ... 

Recent results obtained by the authors and their co-workers in the field of organic cationic complexes generated under stable ion conditions are discussed. Complexes with C- and N-centered electrophiles are considered. 

Alkylated Fullerene Cations under Stable Ion Conditions... 

There are numerous reports of the direct NMR observation of the bishomocyclopropenyl cation [32 n = 2] under stable ion conditions. The first reports of the H NMR spectrum of [32 n = 2] appeared simultaneously (Brookhart et al., 1966 Richey and Lustgarten, 1966), and subsequently various accounts of the 13C NMR spectrum appeared, culminating in an extensive study by Olah and Liang (1975). The 13C data were taken as clear evidence for the bishomoaromatic nature of [32 n = 2] and to preclude the equilibrating classical ions [34] and [35] (Olah and Liang, 1975). 

In summary, the parent 1,3-bishomotropylium ion [43] is produced upon protonation of the bicyclo[6.1.0]nonatriene [44] and its NMR spectrum was recorded under stable ion conditions. Two different bridged 1,4-bishomo-tropylium ion skeletons [45] and [46] have been prepared from a multitude of different precursors. However, no unbridged 1,4-system has yet been characterized. 

The parent trishomocyclopropenyl cation [47], first proposed by Winstein et al. (1959), has been invoked as an intermediate in various solvolysis studies, observed under stable ion conditions by NMR spectroscopy, and studied theoretically (see Story and Clark, 1972 Paquette, 1978). Similarly, the related ions [48]—[51] have all been advanced as trishomoaromatics (see Story and Clark, 1972 Paquette, 1978). 

On the other hand, use of the kinetic expressions for the nmr data indicates that the 6,2-shift is about 8000 times faster than the 3,2 under stable ion conditions at 25°C. The discrepancy between the rates of the intramolecular shifts points strongly to the presence of controlling solvation effects which, as indicated, restrict the conclusions to be gained from these results. 

Again, solvolysis provides an insight into the nature of a transition state during the reaction. For a copious summary of these results the excellent review of Sargent (1972) should be consulted. By and large these studies seem to point to a classical structure for most of the derivatives but we will confine our remarks to studies of a few mono-or di-substituted 2-norbomyl ions under stable ion conditions. 

One thus comes to the conclusion that under stable ion conditions the mono- and dialkyl-substituted norbomyl ions are open or classical ions. It is not difficult to rationalize the structure of the monoalkyl ion on the basis that bridging would be expected to be unfavourable because it leads the system towards an inherently unstable secondary ion. The reason for a classical preference of the l,2-dimethyI-2-norbomyl ion is less obvious from the non-classical view because electron release from 2-methyl groups might have been expected to augment the stability of a bridged structure. 

The structure of the norbomyl ion under stable-ion conditions appears to be as elusive as that of the transition state reached during solvolysis. Theory suggests that the intrinsic energies of the classical and non-classical forms of the ion are extremely close and hence the structure to be found in solution will be governed by solvation effects. 

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. 

The trimethyl trihydroxy derivative of the trimethylenemethane dication (50) was observed under stable ion conditions." The dication 50 is the first Y-conjugated dication system experimentally observed. However, the question of the effect of the Y-conjugation... 

The bicyclo[2.2.2]octane-l,4-diyl dication, 61, and its tricyclopropane derivative, 62, could not be prepared under stable ion conditions even though they were attempted under various reaction conditions. The possible existence of these carbodications was supported by Modified Neglect of Differential Overlap (MNDO) calculations. 

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)... 

Even for a series with varying aromatic substituents, the correlations with deviate significantly from linearity. Typical behavior is illustrated with data for monosubstituted triarylmethyl cations in Figure 1.2. Significant deviations are observed in the points for the para n donors. Moreover, these deviations are in the direction that indicates that these substituents have kinetic stabilizing effects greater than indicated by ct+. In fact, there are good correlations with a parameter based on NMR chemical shifts of benzylic-type cations obtained 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... 

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