Rearrangements of 6-Tricyclo2,7>octyl Cations. Factors Influencing the Relative Stabilities of Bridged Carbocations

Article abstract of DOI:10.1021/jo00248a030

The objective of this work was to explore the effect of ring strain on ? delocalization of carbocations.The 6-tricyclo<3.3.0.0^2,7>octyl cation (3) incorporates 2-norbornyl and 2-bicyclo<2.1.1>hexyl structures in a highly strained molecular framework.Solvolyses of the epimeric brosylates 22 and 23, as well as nitrous acid deaminations of the analogues amines, 24 and 21, served to generate 3.The exo:endo rate ratios of the brosylates and the exo:endo product ratios of the tricyclo<3.3.0.0^2,7>octan-6-ols (19, 20) are close to unity.Product distributions and kinetic data suggest a weak k_a contribution at least for the endo brosylate 23.Several nondegenerate rearrangements of 3 were elucidated: Migration of C-2 from C-7 to C-6 (3 28) is followed, in part, by fragmentation (28 31).A minor fraction of 3 undergoes 4,6-hydride shifts (3 37 38).The degeneracy of 3 was probed with the aid of a 6-²H label.Migration of C-8 from C-7 to C-6 was found to be rapid, as compared to nucleophilic capture, whereas the norbornyl-type Wagner-Meerwein rearrangement (migration of C-4) was not observed.Product and label distributions indicate that the bridged structure (involving C-6, -7, and -8) 3c is nearly isoenergetic (<+/-> 0.5 kcal/mol) with the unsymmetrical ion 3a while products from the norbornyl-type delocalized ion (3b) are not observed, so 3b must be less stable by at least 3 kcal/mol.The exceptional order of the relative stabilities is explained in terms of "olefinic strain", i.e., the additional strain resulting from contraction of the basal bond in bridged carbocations.