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-d₁₂ gives ∼20% (S)-endo-2-chloro-5-norbornene [(S)-7] with ∼50% ee, 65−70% (R)-exo-2-chloro-5-norbornene [(R)-4] with >95% ee, and ∼12% (R)-3-nortricyclyl chloride [(R)-5] with ∼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_Ni 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.