Nonclassical carbocation problem

The nonclassical carbocation problem and the inclusion of solvent effects... 

For historical overviews on the 2-norbornyl cation, the bicyclobutonium ion and related hypercoordinated carbocations see (a) Nonclassical Ions, Reprints and Commentary, P. D. Bartlett, W.A. Benjamin, Inc, New York and Amsterdam 1965 (b) The Nonclassical Ion Problem, Brown, H.C. with comments by Schleyer, P.v.R. Plenum Press, New York, 1977... 

The bond-type connected may be used whenever a treatment of bonds with all participating electrons delocalized is adequate to the problem, e.g. as is the case with nonclassical carbocations [37] ... 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

As discussed in Section 7.4, conformational control in deamination of open-chain amines is difficult to evaluate because the activation energies of conformational changes are often smaller than those of the steps in deamination reactions. Alicyclic amines are more suitable for such mechanistic investigations. In addition, the con-formers of such amines can be locked if they contain bulky substituents (tert-hvXyX) or if the amines are based on bi- and polycyclic hydrocarbons (decalinamines, cholestaneamines, norbornylamines, etc.). We shall therefore concentrate first on the deamination of the epimeric 4-( er butyl)cyclohexylamines. Then, we will discuss the structural problems of cyclic carbocations formed in deamination of norbor-nylamine, cyclopropylmethylamine, and cyclobutylamine, i. e., compounds that are at the center of interest in the debate on classical versus nonclassical carbocations. 

This review attempts to discuss the various points of view on the nonclassical carbocations. The main point is to establish the relative role of nonclassical and classical ions in various chemical processes. The author has followed P. Bartlett s advice that when setting forth the achievements of the human mind one should see how we came to the modern understanding of a given problem ( ... how we know what we know ). 

In the chemistry of carbocations there are many unsolved problems. So far no general experimental technique has been worked out to establish the detailed structure of carbocations in the gas phase or in superacid media. The energetic characteristics have been determined but for a very small number of ions. We are still unable to predict exactly the rates of solvolysis for a wide range of substrates. There also remain specific problems to be solved in the field of precisely nonclassical carbocations ... 

The racemization of optically active 2-norbornyl derivatives in the course of solvolysis can be accounted for, as Winstein himself appreciated8, either in terms of a rapidly equilibrating pair of classical carbocations (18) or in terms of the formation of a symmetrical nonclassical species (19-25). Consequently, the problem cannot be resolved solely on the basis of such racemization. 

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