Enantioselective Nitrene Transfer to Hydrocinnamyl Alcohols and Allylic Alcohols Enabled by Systematic Exploration of the Structure of Ion-Paired Rhodium Catalysts.

ABSTRACT

This work describes highly enantioselective nitrene transfer to hydrocinnamyl alcohols (benzylic C-H amination) and allylic alcohols (aziridination) using ion-paired Rh (II,II) complexes based on anionic variants of Du Bois' esp ligand that are associated with cinchona alkaloid-derived chiral cations. Directed by a substrate hydroxyl group, our previous work with these complexes had not been able to achieve high enantioselectivity on these most useful short-chain compounds, and we overcame this challenge through a combination of catalyst design and modified conditions. A hypothesis that modulation of the linker between the anionic sulfonate group and the central arene spacer might provide a better fit for shorter chain length substrates led to the development of a new biaryl-containing scaffold, which has allowed a broad scope for both substrate classes to be realized for the first time. Furthermore, we describe a systematic structural "knockout" study on the cinchona alkaloid-derived chiral cation to elucidate which features are crucial for high enantioinduction. De novo synthesis of modified scaffolds led to the surprising finding that for high ee the quinoline nitrogen of the alkaloid is crucial, although its location within the heterocycle could be varied, even leading to a superior catalyst. The free hydroxyl is also crucial and should possess the naturally occurring diastereomeric configuration of the alkaloid. These findings underline the privileged nature of the cinchona alkaloid scaffold and provide insight into how these cations might be used in other catalysis contexts.