Nonclassical ion
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.[1][2] Nonclassical ions have been extensively studied with the 2-norbornyl system, which as “naked” ion 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 has been shown with some cyclopentyl and steroidal esters with the tosyloxy leaving group.[3]
Substitution reactions of secondary esters occur by SN2- or SN1-like mechanisms.[4] [5][6] Only in highly polar solvents such as hexafluoroisopropanol (HFIP) of low nucleophilicity one can expect a nearly same uniform SN1-like mechanism. The solvolysis of several cyclopentyl and steroidal esters show 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, and the migration leads to a more stable tertiary carbocation.
![](http://178.128.105.246/host-http-upload.wikimedia.org/wikipedia/commons/thumb/a/a3/Nonclass_1_2_shift_TS_B.jpg/220px-Nonclass_1_2_shift_TS_B.jpg)
The reaction products in these cases always result from the migration of the neighbouring bond. The reaction of epimeric esters can be severely slowed by steric hindrance of solvation.
![](http://178.128.105.246/host-http-upload.wikimedia.org/wikipedia/commons/thumb/8/8f/Kinetics_via_Nonclass_cations_B.jpg/220px-Kinetics_via_Nonclass_cations_B.jpg)
Solvolysis of cyclopropylcarbinyl, cyclobutyl and homoallyl esters are also characterized by very large rates, and have been shown to occur via a common nonclassical ion structure in the form of a bicyclobutonium ion.[7] [8]
See also
[edit]References
[edit]- ^ Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press: New York /Springer 1977 [1]
- ^ Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014 , 27, 374-379 [2]
- ^ Schneider, H.-J. The Controversy about Nonclassical Ions – Abandoned too Early? J. Phys. Org. Chem. 2018,[3]
- ^ Anslyn, E.V., Dougherty, D.A Modern Physical Organic Chemistry University Science Books 2005[4]
- ^ Sykes P. A guide book to mechanism in organic chemistry 6th Ed. , New Delhi: Orient Longman, 1986, p. 111 ff[5]
- ^ Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976 [6]
- ^ Saunders, M., Laidig, K.E., Wiberg, K.B. , Schleyer Structures, energies, and modes of interconversion of C
4H+
7 ions J. Am. Chem. Soc., 1988, 110, 7652–7659 [7] - ^ Siehl , H. U. The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations Adv. Phys. Org. Chem. , 2018, 52, 1-47 [8]