REARRANGEMENT OF BICYCLO[3.2.1]OCT-2-EN-6-YL AND BENZOBICYCLO[3.2.1]OCTENYL CATIONS

The ketones 9, 27, and 51 were prepared by modified or novel routes. The analogous tosylhydrazones 10, 28, and 52 were photolyzed in 0.5 N NaOH (NaOD) to generate the carbocations 15, 34, and 53, respectively, by way of diazonium precursors. 2,3-Unsaturation (15) and 2,3-benzoanellation (34) affected the course of 4,6-(4,5-)hydride shifts, as elucidated by deuterium labeling. While the parent system 3 strongly prefers C-5 over C-6 (95:5) as the terminus of hydride shifts, 15 and 34 (55:45) approach the equivalence of C-5 and C-6 that is indicative of rapid Wagner-Meerwein rearrangements or symmetrically bridged intermediates. This trend is attributed to flattening of the six-membered ring in 15 and 24, as compared to the chair conformation of 3. On the other hand, predominant nucleophilic substitution at C-6 is unaffected by 2,3-unsaturation. We suggest that sigma participation (k(DELTA)) by alkyl groups is not sufficient to eliminate inverting displacement of nitrogen from the diazonium precursors by the solvent (k(s)). Equivalence of C-5 and C-6 toward external nucleophiles is achieved only by pi-participation, i.e. with the 3,4-unsaturated and 3,4-benzoanellated bicyclo[3.2.l]oct-6-yl systems (53).