Rainer Siegfried

Rearrangements of Spirocyclobutane‐Substituted 2‐Norbornyl Cations

2-Norbornyl cations with spiroannellated cyclobutane rings were generated for comparison with the previously studied cyclopropane analogues. Starting with the Diels-Alder reaction of cyclopentadiene with methylenecyclobutane, spiro- [bicyclo[2.2.1]heptane-2,1′-cyclobutan]-6-one (11) was prepared. The tosylhydrazone 12 of 11 was photolyzed in NaOD/D2O to give the analogous alcohol 13 with a ca. 1:1 distribution of deuterium. Ring expansion was not observed, in contrast to the cyclopropane analogue. − The tosylhydrazone 22 of spiro[bicyclo[2.2.1]heptane-2,1′-cyclobutan]-3-one (21) and the related tosylates (28, 32) rearranged, in part, to afford derivatives of spiro[bicyclo[2.2.1]heptane-7,1′-cyclobutane] (29, 33, 34). In both series, ring expansion of the spiroannellated cyclobutane, by exo-3,2-C shift, was the major reaction, giving rise to a uniquely endo-selective tertiary cation (36). Analogously positioned cyclopropane rings remain intact, due to stabilizing interactions with the neighboring positive charge which are lacking in the cyclobutane systems. − In CDCl3 solution, the tosylate 32 produced mixtures of isomeric tosylates by way of ion pair recombination. We observed that exo exo shifts of the counterion proceed with little scrambling of 18O whereas complete equilibration of the tosylate oxygens is attained in exo endo shifts.