The Norbornyl Cation: Prototype of a 1,3‐Bridged Carbocation

Abstract

The protracted discussion of whether 2-norbornyl cations are bridged or unbridged, i. e. nonclassical or classical, has lost most of its meaning, because these terms apply to the extremes of a graded series of structures. This is shown by a study of the solvolysis of stereoisomeric 6-substituted 2-norbornyl toluenesulfonates. Norbornyl and other bicyclic cations, to which nucleophiles have limited access, are stabilized by bridging between the cationic center and a saturated carbon atom in the @-position. Bridging is reduced by electron-attracting substituents and by bridging.

Carbon Participation in the Solvolysis of 6‐ and 7‐Substituted 2‐Norbornyl p‐Toluenesulfonates. Norbornanes, Part 11

“…their inductivities differ widely. It is concluded that through space induction involves graded bridging of the substituent-bearing C-atom to the incipient cationic center at C (2) and that this involves differential bridging strain. The different reactivities of unsubstituted 2-exo-and 2-endo-norbornyl derivatives can then be ascribed to a stereoelectronic effect.…”

“…On the other hand 2-exo-and 2-endo-norbornanols were obtained when R was an acceptor, indicating that unbridged cations 5 are attacked from both sides of C (2) [ 11 [5]. Finally, reversible Wagner-Meerwein rearrangements of the cations 3 and 4 were fast relative to their capture by solvent when R was a donor, but slow when R was an acceptor, as in 5 and 6 [6], a sign that bridging and rearrangement are related.…”

Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

“…The large p-value for 5 indicated an unusually strong inductive interaction between C (6) and the incipient cationic center at C (2) and led to the conclusion that induction involves graded C-participation, i.e. 1,3-bridging, as illustrated in 8 [5] [13]. On this basis bridging appears to be less favorable in the adamantyl cation 9 than in the norbornyl cation 8.…”

Carbocations in Superacid Systems

“…The rapid equilibrium in cations 495 and 496 has been frozen out in the solid state at À165 C and À160 C, respectively, by Yannoni and co-workers. The controversy [27][28][29][30][31][32][33][34][35][36][37]40,[862][863][864][865][866] about it is well known, and the question has been whether the ion has a symmetrically bridged nonclassical structure 504 with a pentacoordinate carbon atom or is a rapidly equilibrating pair of classical trivalent ions 505a and 505b. Some representative examples, namely, the bisadamantyl (499), 859 2-norbornyl (500), 40 7-perhydropentalenyl (501), 188 9-decalyl (502), 188 and pentacylopropylethyl (503) 860 cations, are given in Scheme 3.19.…”