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2-norbornyl == Nonclassical Ions == Nonclassical carbocations are stabilized by charge delocalization from contributions of neighbouring C-C or C-H bonds, which can form bridged intermediates or transition states. Nonclassical ions have been extensively studied with the 2-norbornyl system, which as “naked” cation unambiguously exhibit such a bridged structure. The landmark of nonclassical ions are unexpectedly fast solvolysis rates and large differences between epimeric esters. Such behaviour is not restricted to 2-norbornyl esters, as exemplified with several cyclopentyl and steroidal esters with the tosyloxy leaving group.

Substitution reactions of secondary esters occur by SN2- or SN1 –like mechanisms; only in highly polar solvents such as hexafluoroisopropanol (HFIP) of low nucleophilicity one can expect a nearly same uniform SN1 –like mechanism. The examples shown here illustrate that large solvolysis rates and differences between epimers can occur which surpass those of the 2-norbornyl system. In these cases a vicinal C-C or C-H- bond can lead to significant delocalization of the positive charge, if these  bonds are close to antiperiplanar to the leaving group. The reaction of epimeric esters can be severely slowed by steric hindrance of solvation. Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press:  New York /Springer 1977 https://www.springer.com/de/book/9781461341208 Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014, 27, 374-379 https://onlinelibrary.wiley.com/doi/full/10.1002/poc.3290

a) Sykes P. A guide book to mechanism in organic chemistry 6th Ed., New Delhi: Orient Longman, 1986, p. 111 ff; http://about.mdma.ch/picproxie_docs/000536454-Guidebook_to_Mechanism_in_Organic_Chemistry.pdf Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976; https://link.springer.com/content/pdf/bfm%3A978-1-4684-0826-3%2F1.pdf Good overview : Reusch, W. http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/rearrang.htm Schneider, H.-J.  The Controversy about Nonclassical Ions – Abandoned too Early ? J. Phys. Org. Chem. 2018,  https://onlinelibrary.wiley.com/doi/10.1002/poc.3846