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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.
Generous support from the Max Planck Society, the Deutsche Forschungsgemeinschaft (Leibniz Award to B.L. and Cluster of Excellence Ruhr Explores Solvation (RESOLV, EXC 1069) and the European Research Council (Advanced Grant “C–H Acids for Organic Synthesis, CHAOS”) is gratefully acknowledged.