MU-HYDRIDO BRIDGING IN TRICYCLOALKYL CARBOCATIONS

From 2-adamantanone, a series of tricyclic alkenes 5 (n = 5-8) were synthesized in 10 steps. The analogous alkene 5 (n = 4) could not be prepared by the same route. These structures, in each case, were designed to have an inwardly pyramidalized bridgehead hydrogen, which is suitably positioned so that the formation of a cationic center on the opposite bridgehead could then potentially lead to a μ-hydrido-bridging interaction. The nature of the hydrido bridge was found to be very structure-dependent, and one can correlate this behavior with the distance d between the terminal bridge-supporting carbons (C--H-C+) in the tricycloalkane framework. For the shortest distances, one develops a fully symmetric μ-H bridge, while larger d result in a gradation toward a normal tertiary carbocation structure. The cation 4 (n = 5) is quite stable in a pKa sense (pKa ≈ -1), and the pKa relationship between this cation and the n = 6 case was computed with semiempirical MO theory. The calculated pKa was -5.3, in good agreement with an experimental bracketing of the pKa between -3 and -8. One also predicts from the MO theory that the unknown cation 4 (n = 4) will have pKa ≈ +10, i.e., stable in neutral water. © 1990, American Chemical Society. All rights reserved.