A Kinetic and Thermodynamic Behavioral Comparison of Observable Secondary and Tertiary 2-Norbornyl Cations

Abstract

. Can. J. Chem. 53,1891Chem. 53, (1975. By analyzing various rearrangement processes, one can deduce the relative stabilities of the following 2-norbornyl cations (kcal/mol): 1,2-dimethyl, 0; 2-methyl, 1 ; 1-methyl, 6.5; and the parent 2-norbornyl, 8.5. Secondary 2-norbornyl cations have been assigned a "carbonium ion" structure and tertiary a "carbenium ion" structure. Using suitable model systems, an absolute rate comparison has been made of all three common rearrangement processes in the tertiary systems ("carbenium") with those already measured for the 2-norbornyl cation ("carbonium"). The activation free energies, AG*, (in kcal/mol) were: Wagner-Meerwein (WM) shift: tertiary, < 4; secondary, < 4; endo-6,2-hydride shift : tertiary, 7.2; secondary, 5.8; exo-3,2-hydride shift: tertiary, 6.6; secondary, 11.4. A discussion of the structure of tertiary and secondary 2-norbornyl cations emphasizes the following points: (i) a rationalization of the rapid endo-6,2-hydride shift observed in norbornyl cations does not necessitate a protonated nortricyclene ("carbonium ion") postulate; (ii) on the basis of the results for acyclic cations and the 1,2-dimethyl-2-norbornyl cation, one would not expect to "freeze out" the WM shift in an equilibrating 2-norbornyl cation structure; (iii) the formation of nortricyclenes may be related to a partial C G C 2 bond in the ions; and (iu) the structure of the observable secondary 2-norbornyl cation probably involves partial C G C 2 bonding but, in our opinion, this does not require a symmetrical static formulation. R. HASELTINE, N. WONG, T. S. SORENSEN et A. J. JONES. Can. J. Chem. 53,1891Chem. 53, (1975. En faisant I'analyse des divers processus de rearrangements, on peut deduire les stabilitks relatives des cations norbornyl-2 suivants (kcal/mol): dimethyl-1,2, 0; mkthyl-2, 1; methyl-1, 6.5; et le norbornyl-2 non-substituk, 8.5. On a attribuk au cation 'norbornyl-2 secondaire une structure de "carbocation" et au cation tertiaire une structure d"'ion carbenium". Faisant appel a des systemes modeles approprib, on a fait une comparaison entre les vitesses absolues des trois processus de rbrrangement commun dans les systemes tertiaires ("carb8nium") avec celles qui ont deja kt6 mesurees pour le cation norbornyl-2 ("carbocation"). Les knergies libres d'activation AG* (en kcal/mol) sont: rearrangements de Wagner-Meerwein (WM): tertiaire, < 4 ; secondaire, < 4 ; deplacement d'hydrure 6,2-endo: tertiaire, 7.2; secondaire, 5.8; dkplacement d'hydrure 3,2-exo: tertiaire, 6.6; secondaire, 11.4. Une discussion de la structure des cations norbornyl-2 tertiaires et secondaires met en bidence les points suivants: (i) une rationalisation du rkarrangement rapide d'hydrure 6,2-endo observe dans les cations norbornyles n'implique pas necessairement que I'on doive postuler I'existence d'un nortricyclene protone ("carbocation"); (ii) sur la base des rksultats obtenus avec des cations acycliques et ceux obtenus avec le cation dimkthyl-1,2 norbornyl-2, on ne devrait pas esp6er eliminer ("freeze-out") l...

Secondary 2‐Norbornyl Cation Intermediates Substituted at C(5) and C(7) by Electron‐withdrawing Groups. Addition of fluorosulfuric acid to unsaturated norbornane derivatives

“…These results are most simply inter reted by invoking the formation of the occur, yielding a mixture of the adducts 26 + 27. An energy barrier of 6-7 kcaUmol has been measured for the H- [6,2]-shift in 2-norbornyl cations in SbF5/SO2C1F [la] [26]. It must be higher than the energy barrier of the quenching of 28-Z+30-Z (or 29-Z&31-2)8) with FSO, or HS03F (-3 kcal/mol for ion-pairs, if diffusion limited [27]).…”

“…5). If one considers an energy barrier of 6-7 kcaVmol for the H- [6,2]-and H- [1,2]-shifts [la] [26] corresponding to the rearrangements 28-2 + 5-Z and 30-2 + 5-Z respectively, a free enthalpy diagram can be constructed in which the a-bridged ions 28-Z,30-Z are more stable than the rr-bridged ion 5-Z (Fig. 5).…”

Reaction Sequences Involving the Mutual Interconversion of All 10 Tertiary Dimethyl-2-norbornyl Cations. A Comparison of Predicted Products and Rates with Experiment

“…To make this distinction in the computerized treatment of activation energies seems too restrictive; there is already enough subdivision of steps for a general treatment. Such subtle thermodynamic factors are relevant to questions about the molecular structure of 2-norbornyl cations and this point is explored in an accompanying paper (5). Basically, the results of this analysis suggest that the 2HSS value used in this work (16.9) is appropriate only to a C1 methyl-substituted secondary ion and that the 2HTS value of 15.2 is only appropriate to a C1 hydrogen-substituted secondary ion.…”

Predicting the Most Favored Path and the Overall Rate of Multistep: 2-Norbornyl Cation Rearrangements. The Assignment of a Trial Activation Energy for Each Unique Step

“…line broadening in the 1,2-exo-3,4-tetramethyl-2-norbornyl cation (17) up to at least -10 "C, such as might have occurred from an endo-3,2-hydride shift (1 1) and we can assign a minimum AG* value of ca. 14.2 kcal/mol.…”

“…13 and footnote 2)), one can show by standard n.m.r. theory that a rate constant of > lo6 s-' (see accompanying paper (17)) is needed to produce the relatively sharp peak observed (14) for 2 at -140 "C. This leads to a maximum possible AG* value of 4 kcal/mol. Since this barrier is so small, one may wonder whether the ion might not have a symmetrical structure and hence there would be no 1TT process involved at all.…”