Strong and Confined Acid Catalysts Impart Stereocontrol onto the Non-Classical 2-Norbornyl Cation
2019-11-23T07:04:59Z (GMT) by
In 1949, S. Winstein and D. Trifan proposed that the 2-norbornyl cation adopts a bridged, non-classical structure with a 3-center, 2-electron unit much like the bonding in diborane. While some embraced this proposal, others firmly refuted it, giving rise to the vituperative 2-norbornyl cation controversy. After 60 years of debate, the long-sought crystallographic proof was eventually collected in 2013. Several decades after the first non-classical formulation, we became interested in imparting stereocontrol over the 2-norbornyl cation, a positively charged, simple hydrocarbon that, due to its structural features, offers a major challenge to asymmetric catalysis. Our investigation began by reversing the original experiment by Winstein. Specifically, we found that IDPi catalysts are competent chiral acids for carrying out a diastereo- and enantioselective C-C bond forming reaction between racemic exo- and endo-norbornyl trichloroacetimidates and 1,3,5-trimethoxybenzene to exclusively deliver the exo-product with excellent enantioselectivities. We also found that several structurally-different substrates can be converted to the same product with good to excellent enantioselectivities, pointing to the existence of a common cationic intermediate. Mechanistic and kinetic studies were conducted to elucidate relevant aspects of the reaction.