Remote Inductive Effects in the Solvolysis of Secondary and Tertiary 2-Norbornyl Derivatives. Evidence for Nonclassical Stabilization of the 2-Norbornyl Cation

Article abstract of DOI:10.1021/jo00238a033

This paper deals with evidence for the structure of the 2-norbornyl cation, when it occurs as an intermediate in solvolysis reactions in relatively nucleophilic media.Seen in a wider context, the 2-norbornyl cation has been shown unequivocally to have a bridged structure in isolation and in nonnucleophilic media.Thermochemical comparisons in the gas phase and in superacid media show the secondary 2-norbornyl cation to be exceptionally stable; at high levels of ab initio theory, the symmetrically bridged structure is the only minimum and the classical cation does not even exist.Spectroscopic evidence (e.g., NMR at 4 K, 1H and 13C chemicalshift analyses, ESCA, etc.) in superacids clearly favors the nonclassical structure.The same is true for the 2-norbornyl cation as a reactive intermediate.Solvolysis of 2-exo-norbornyl tosylate in the very weakly nucleophilic hexafluoroisopropyl alcohol is exceptionally fast, whereas the endo isomers react at a normal rate.While high exo/endo ratios also are found for 2-methyl and 2 aryl-substituted norbornyl systems, effects other than bridging must be responsible, since these tertiary systems are shown to respond differently than the secondary systems with regard to secondary deuterium isotope effects or when additional substitutuents are introduced.Thus, electronegative substituents at C5 as well as at C6 reduce the exo/endo ratio significantly; e.g., a dichloro group at C5 reduces the parent value of 860 to 9.4.While substituents at C6 suffer from the potential disadvantage of electronic interaction with the leaving group at C2, substituents at C5 are free from this complication.The reduction of exo/endo ratios by deactivating substituents is shown to be stronger in weakly nucleophilic solvents, compared to 60percent ethanol.Dipolar interactions between substituents at C5 and the leaving group at C2 can be excluded by an analysis of the spatial arrangement of the 5-CN group relative to the brosylate group; different orientations do not affect the relative rates significantly.The rate constans of deactivated 2-norbornyl sulfonates can be correlated by linear free energy relationships (LFER), as also has been shown more extensively by Grob.Rates of exo-sulfonates are unique in their exceptionally large response to polar substituents.Since no break is observed in the LFER, including the parent 2-exo-norbornyl sulfonate, a graded set of mechanisms proceeding from strong, to weak, to no participation is the most likely explanation for the behavior.The possibility of rapid equilibration between partially bridged 2-norbornyl cation geometries as well as heavy atom tunneling is analyzed in detail and rejected by structural considerations based both on high level ab initio calculations and on Laube's X-ray structure of a partially bridged 1,2-dimethyl-2-norbornyl cation derivative.