The Nonclassical Ion Problem

Beginning in 1951, there appeared in the literature an alternative explanation, o-participation, for enhanced rates of solvolysis of highly branched derivatives (21, 22). 

It was puzzling to me that the phenomenon appeared to be important chiefly in strained systems, systems where steric eflFects could also be involved. 

Consequently, these must be the properties of the norbornyl system, rather than a consequence of cr-participation under solvolytic conditions. 

The m-dimethyl substituents in the 3-position of 2-anisylcamphenilyl result in a major increase in the exo/endo rate ratio. This can only be a steric phenomenon. The results with the trimethylnorbornyl derivatives supports the importance of steric effects in this rigid system (25, 26). 

It is of interest to point out that the original three proposals for enhanced rates due to o-participation have been withdrawn. 

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The arguments against nonclassical ions are summed up in Brown, H.C. The Nonclassical Ion Problem Plenum NY, 1977. This book also includes rebuttals by Schleyer, P.v.R. See also Brown, H.C. Pure Appl. Chem., 1982, 54, 1783. 

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... 

H. C. Brown, The Nonclassical Ion Problem, Plenum Press, New York, 1977. 

The norbornyl cation is at the heart of the nonclassical ion problem . The argument over the stable cation concerns whether it is a rapidly equilibrating pair of classical ions or rather a symmetrical ion (Brown, 1977). The isotopic perturbation studies by Saunders and Kates (1980) show that the postulated rapid Wagner-Meerwein rearrangement (4) is not consistent with the results but is a static structure like [4],... 

Proposal No. 3. In the nonclassical ion problem , Professor Paul von R. Schleyer has advanced a new interpretation11. He postulates carbon participation in secondary exo-norbornyl and comparable steric hindrance to ionization in the tertiary e/zdonorbornyl derivatives. However, he argues that such steric hindrance to ionization should not be important in the secondary 2-norbornyl system, since the smaller 2-H should provide far less steric hindrance to the departure of the endo anion than the larger 2-R44. ... 

Fig. 9. Absence of a free-energy correlation for the solvolysis of isopropyl tosylate in representative solvents. I Reproduced from Brown, H. C. The nonclassical ion problem. New York Plenum Press 1977.1...

Brown, H. C. The nonclassical ion problem. New York Plenum Press 1977... 

Brown, H.C. "The Nonclassical Ion Problem" Plenum Press New York, 1977. 

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