Carbocation equilibrating

Much effort was put into studying whether certain carbocations represent rapidly equilibrating or static (bridged, delocalized) systems (more about this in Chapter 9). 

The differentiation of bridged nonclassical from rapidly equilibrating classical carbocations based on NMR spectroscopy was difficult because NMR is a relatively slow physical method. We addressed this question in our work using estimated NMR shifts of the two structurally differing ions in comparison with model systems. Later, this task... 

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Non-Kolbe electrolysis may lead to a large product spectrum, especially when there are equilibrating cations of about equal energy involved. However, in cases where the further reaction path leads to a particularly stabilized carbocation and either elimination or solvolysis can be favored, then non-Kolbe electrolysis can become an effi-yient synthetic method. This is demonstrated in the following chapters. 

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

There is evidence that the configuration of the molecule may be important even where the leaving group is gone long before migration takes place. For example, the 1-adamantyl cation (17) does not equilibrate intramolecularly, even at temperatures up to 130°C, though open-chain (e.g., 5 50 and cyclic tertiary carbocations... 

In view of the observations of the ionic dissociation of nitro-cyano compounds, it is hardly surprising that even a hydrocarbon could dissociate ionically into a stable carbocation and carbanion, provided that the medium is polar enough to prevent the recombination reaction and to ensure equilibration. 

Rearrangement can also occur after the initial alkylation. The reaction of 2-chloro-2-methylbutane with benzene is an example of this behavior.35 With relatively mild Friedel-Crafts catalysts such as BF3 or FeCl3, the main product is 1. With A1C13, equilibration of 1 and 2 occurs and the equilibrium favors 2. The rearrangement is the result of product equilibration via reversibly formed carbocations. 

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

The concept of an intermediate phenonium ion was, at first, controversial, and its chief detractor was H. C. Brown.25 Although 3-phenyl-2-butyl tosylate showed the stereochemical behavior expected if an intermediate phenonium ion were formed, it did not, in his opinion, show the rate acceleration that should attend anchimeric assistance to ionization of the tosylate.26 Brown said that the stereochemical results could be accounted for by invoking rapidly equilibrating open carbocations (15). According to his explanation, ionization of the tosylate... 

This chapter begins with a short historic retrospect about the development of the carbocation concepts and covers the techniques used for their generation, observation, and characterization under superacidic long-lived conditions. This is followed by an extensive coverage of the multitude of trivalent (classical) and equilibrating (degenerate) and higher (five or six) coordinate (nonclassical) carbocations. 

The deuterium isotopic perturbation technique developed by Saunders and co-workers5 60 is capable of providing a convenient means to differentiate the rapidly equilibrating or bridged nature of carbocations. 

Core electron spectroscopy for chemical analysis (ESCA) is perhaps the most definitive technique applied to the differentiation between nonclassical carbocations from equilibrating classical species. The time scale of the measured ionization process is of the order of 10 16 s so that definite species are characterized, regardless of (much slower) intra- and intermolecular exchange reactions—for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. 

EQUILIBRATING (DEGENERATE) AND HIGHER (FIVE OR SIX) COORDINATE (NONCLASSICAL) CARBOCATIONS... 

The cross-polarization, magic-angle spinning method (CP MAS) has been applied by Myhre and Yannoni50 to cation 19 in the solid state at very low temperatures using 13C NMR spectroscopy. In the initial study, no convincing evidence for a frozen 2-butyl cation was obtained even at — 190°C. However, subsequently they managed to freeze out the equilibration of the 2-butyl cation (19) at —223°C.62 It behaves like a normal secondary trivalent carbocation. 

Many more cyclic and polycyclic equilibrating carbocations have been reported. Some representative examples, namely, the bisadamantyl (499),859 2-norbornyl (500),40 7-perhydropentalenyl (501),188 9-decalyl (502),188 and pentacylopropylethyl (503)860 cations, are given in Scheme 3.19. All these systems again involve hypercoordinate high-lying intermediates or transition states. 

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