Endo Entry to the Nortricyclyl−Norbornenyl Cation System: Stereochemistry in the Fragmentation of endo-5-Norbornenyl-2-oxychlorocarbene
journal contributionposted on 14.10.2005, 00:00 by Robert A. Moss, Xiaolin Fu, Ronald R. Sauers, Peter Wipf
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 SNi 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.