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Strong and Confined Acid Catalysts Impart Stereocontrol onto the Non-Classical 2-Norbornyl Cation

preprint
submitted on 23.11.2019 and posted on 23.11.2019 by Roberta Properzi, Philip S. J. Kaib, Markus Leutzsch, Gabriele Pupo, Raja Mitra, Chandra Kanta De, Peter R. Schreiner, Benjamin List
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.

Funding

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.

History

Email Address of Submitting Author

list@mpi-muelheim.mpg.de

Institution

Max-Planck-Institut für Kohlenforschung

Country

Germany

ORCID For Submitting Author

0000-0002-9804-599X

Declaration of Conflict of Interest

B.L., P.S.J.K. and R.P. are inventors on patent WO2017037141 (A1) filed by the MPI für Kohlenforschung covering the IDPi catalyst class and its applications in asymmetric synthesis.

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