A mechanistic study of the carbocation route from tetrahydrodicyclopentadiene to the adamantane ring

Abstract

The energy profile for the transformation of the endo-tetrahydrodicyclopentadienyl cation A into the 1-adamantyl cation R has been probed by means of six separate experiments employing superacid techniques. Of the approximately 14 cations between A and R, required as intermediates for the above transformation, it can be shown that only one of these is capable of direct observation. This cation, the tricyclo[5.2.1.03.8]dec-3-yl (homobrendyl) cation J, is characterized in detail in one of the six experiments. The rate of the rearrangement of J into the 1-adamantyl cation has been measured. Cation J also undergoes a degenerate rearrangement, the dynamics of which can be probed using NMR line-broadening and selective inversion–recovery experiments. The relationship between the exo and endo tetrahydrodicyclopentadienyl cations D and A has been studied. Only A is directly observable, even though in some experiments direct precursors of D were used as cation sources. Several experimental probes were made in an attempt to detect the presence of a small equilibrium concentration of D, without success, and a rationale is provided for the much greater stability of A vs. D. The remaining four experiments involved the addition of neutral precursors to superacid solutions, but the initially formed (expected) cations could not be observed, only cations A, J, or R being observed instead. These "partitioning"-type experiments prove whether the corresponding left-hand or right-hand transition-state barriers are the lower in energy. In the case of starting with transient cation intermediate E, only observable cation J is obtained, showing that the barrier between B and E is the high point on the overall carbocation reaction profile.