Endo entry to the nortricyclyl-norbornenyl cation system:: Stereochemistry in the fragmentation of endo-5-norbornenyl-2-oxychlorocarbene

Fragmentation of (S)-endo-5-norbornenyl-2-oxychlorocarbene [(S)-8] in cyclohexane-d(12) gives similar to 20% (S)-endo-2-chloro-5-norbornene [(S)-7] with similar to 50% ee, 65-70% (R)-exo-2-chloro-5-norbornene [(R)4] with > 95% ee, and similar to 12% (R)-3-nortricyclyl chloride [(R)-5] with similar to 22% ee. (Analogous stereochemical results were also obtained starting with the enantiomeric carbene (R)-8.) The (S)-8 to (S)-7 and (S)-8 to (R)-4 conversions are ascribed mainly to retention and inversion S(N)i transition states, respectively. These have been located by computational methods and are nearly isoenergetic. In more polar solvents (CDCL3 and CD3CN), the fragmentation of (S)-8 increasingly occurs via competitive ion pair pathways in which steroselectivity is diminished, and escape to the norbornenyl-nortricyclyl cation directs the products away from endo-2-chloro-5-norbornene toward exo-chloride 4 and nortricyclyl chloride 5.