What is non classical carbocation
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Definition of Nonclassical Carbocation. What is Nonclassical Carbocation? A carbocation the ground state of which has delocalized (bridged) bonding pi- or sigma-electrons. (N.B.: Allylic and benzylic carbocations are not considered nonclassical.) BARTLETT (1965).
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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.
Nonclassical charge delocalization in the transition state of a 1,2-alkyl shift
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.
Typical solvolysis kinetics involving nonclassical cations, illustrated with the most reactive compounds. kA/kB: rate constant ratio of epimers (X or Y= OTs); largest rate constant kA [ sec-1]; all measurements in HFIP
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]
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