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Free keywords:
2-norbornyl; asymmetric catalysis; ACDC; carbonium ions; non-classical carbocation
Abstract:
Utilizing carbocationic intermediates
in asymmetric transformations is a challenge for catalysis. While recent studies have provided promising approaches to systems proceeding through classical, trivalent carbenium ions, we were particularly intrigued by the possibility of imparting stereochemical control onto the 2-norbornyl cation, a small, fully aliphatic and positively-charged hydrocarbon that is known to adopt a non-classical structure. We now show that strong and confined imidodiphosphorimidate (IDPi) catalysts
are competent acids for carrying out a
n asymmetric stereoablative C–C bond forming reaction, utilizing racemic exo- or
endo-norbornyl derivatives to exclusively
obtain the highly enantioenriched
exo-product. In addition, structurally-different substrates can be converted to the same product with remarkable enantioselectivities
by leveraging the reactivity of a variety of functional groups. Mechanistic and kinetic studies suggest that all employed substrates proceed through a common cationic intermediate.
