Solution carbocation stabilities measured by internal competition for a hydride ion

Abstract

. Can. J. Chem. 60,2180Chem. 60, (1982. Although many techniques are known which allow one to compare the stabilities of solution carbocations, that involving the intermolecular competition for a hydride ion is conceptually (but not experimentally) the simplest procedure. This paper describes a variant of this which is experimentally more reliable and which uses intramolecular equilibria where the two competing systems are held together by a -(CH,),,-chain, e.g.By systematically varying " t~" in this example ()I = 0, 1,2, or 3), it has been found that a methylene chain of two or morecarbons is necessary in order to minimize steric interactions between the end "systems". It has also been found that all cycloalkyl rings studied + /CH3 (except cyclohexyl) stabilize a cation centre much better than an aliphatic equivalent, i.e. -C , in agreement with solvolysis \ C H~ rate studies. The same situation was found when comparing this aliphatic "system" against the 2-norbornyl cation (bicyclic) or against the tricyclic 2-adamantyl cation. In fact, in these cases the equilibria are too lop-sided to obtain numerical values for the equilibrium constants concerned. Finally, three carbocations were looked at where the 2-norbornyl cation structure was pitted against the structurally very related cyclopentyl, bicyclo[2.1. llhexyl, and bicyclo[3.2. lloctyl cations. In all cases, the 2-norbornyl cation is the more stable. I3C nmr spectroscopy was used as the analytical tool to measure (or attempt to measure) the equilibrium constants. Depending on the rate of the equilibration process, three different techniques are involved and the relative merits of these are discussed in the latter part of the paper. NANCY E. OKAZAWA et TED S. SORENSEN. Can. J. Chem. 60,2180Chem. 60, (1982. Parmi toutes les techniques connues qui permettent de comparer les stabilites des carbocations en solution, celle qui implique la competition intramolCculaire de l'ion hydrure est conceptuellement (mais non expkrimentalement) la plus simple. Ce travail decrit une variante de cette technique qui est plus fiable experimentalement et qui utilise les Cquilibres intramolCculaires lorsque les deux systemes en competition sont maintenus ensembles par une chaine de --(CH,),-, comme dans En faisant varier "n" d'une f a~o n systematique dans cet exemple (11 = 0, 1,2 ou 3) on a trouvi qu'une chaine contenant au moins deux unites methylenes est necessaire afin de minimiser les interactions steriques entre les systemes terminaux. On a egalement trouvt que tous les cycloalkyles CtudiCs (B l'exception du cyclohexyle) stabilisent mieux le centre cationique que les equivalents +/CH3 aliphatiques c'est-a-dire -C ; ceci est en accord avec les etudes de vitesse de solvolyse. On a trouve la m&me situation en \CH, comparant ce "systeme" aliphatique avec le cation norbornyle-2 (bicyclique) ou avec le cation tricyclique adamantyle-2. En fait, dans ces cas les equilibres sont trop dissymetriques pour obtenir les valeurs numeriques des constantes d'equilibres concernees. Finalemen...

Studies of 9-Fluorenyl Carbocations. Intramolecular Hydride Migration in a Substituted 9-Fluorenyl Carbocation

“…Carbocations undergo rearrangements by 1,2-alkyl or hydride migration, and we recently reported a photochemical pinacol rearrangement originating from a fluorenyl cation in neutral solutions . Carbocations are also known to undergo inter- as well as intramolecular hydride transfer reactions from carbon−hydrogen donors. These are thermodynamically driven, giving the more stable species .…”

Nondegenerate equilibrium in (2-propyl)cycloalkyl cations. Comparison of (2-propyl)cyclopentyl and (2-propyl)cyclohexyl cation using 13C NMR spectroscopy, equilibrium isotope effects and quantum chemical calculationsElectronic supplementary information (ESI) available: redundant coordinates for structures 1A, 1B, 1TS, 2A, 2B1, 2B2 and 2TS; van't Hoff plot for equilibrium 1A ⇋ 1B. See http://www.rsc.org/suppdata/p2/b1/b107756b/

“…On the contrary, for the equilibrium 2A 2B the structure 2A with the positive charge on the exocyclic carbon was found to be more stable. 3,4 The results we report in this study explain the different stabilities of alkyl-substituted fiveand six-membered cycloalkyl cations 1B and 2B compared to the isomeric cycloalkylsubstituted 2-propyl cations 1A and 2A and give additional information on the stabilization modes and the structural and conformational consequences in these type of carbocations.…”