SUBSTITUTED 2-NORBORNYL CARBENES

A series of endo-6-substituted bicyclo[2.2.1]hept-2-yl carbenes have b een generated by pyrolysis of the sodium salt of the tosylhydrazone pr ecursors. The endo-6-trimethylsilyl and endo-6-thiomethoxy substituted carbenes give 1,3-migration of these substituents to the carbene cent er. However groups such as methyl, CH(2)SiMe(3), phenyl, and methoxy a re ineffective 1,3-migrating groups. The facile 1,3-migration of trime thylsilyl and thiomethoxy has been rationalized in terms of stabilized transition states where the migrating group interacts effectively wit h the carbene vacant orbital. In the cases of endo-6-thiomethoxy and m ethoxy derivatives, the carbene also inserts effectively into the meth yl group of the substituent. Heteroatom stabilization of the transitio n state for C-H insertion facilitates these processes. Certain exo-6-s ubstituted bicyclo[2.2.1]hept-2-yl carbenes have also been generated. While the exo-6-thiomethoxy and carbomethoxy systems give exclusive 1, 3-hydrogen migration, the exo-6-sulfonyl derivative gives, in addition to 1,3-hydrogen migration, an alkene product derived from 1,2-hydroge n migration. The lowered propensity for 1,3-hydrogen migration is attr ibuted to the strong electron-withdrawing sulfonyl group, which decrea ses the hydridic 1,3-interaction with the carbene vacant orbital.