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Highly Diastereoselective Functionalization of Piperidines by Photoredox Catalyzed C–H Arylation and Epimerization

preprint
submitted on 06.12.2019 and posted on 13.12.2019 by Morgan Walker, Brian Koronkiewicz, Shuming Chen, Kendall N Houk, James Mayer, Jonathan Ellman
We report a photoredox catalyzed α-amino C–H arylation reaction of highly substituted piperidine derivatives with electron deficient cyano(hetero)arenes. The scope and limitations of the reaction were explored, with piperidines bearing multiple substitution patterns providing the arylated products in good yields and with high diastereoselectivity. In order to probe the mechanism of the overall transformation, optical and fluorescent spectroscopic methods were used. By employing flash-quench transient absorption spectroscopy, we were able to observe electron transfer processes associated with radical formation beyond the initial excited state Ir(ppy)3 oxidation. Following the rapid and unselective C–H arylation reaction, a slower epimerization occurs to provide the high diastereomer ratio observed for a large majority of the products. Several stereoisomerically pure products were re-subjected to the reaction conditions, each of which converged to the experimentally observed diastereomer ratios. The observed distribution of diastereomers corresponds to a thermodynamic ratio of isomers based upon their calculated relative energies using density functional theory (DFT).

Funding

NIH R35GM122473

NIH R01GM050422

NSF CHE-1764328

History

Email Address of Submitting Author

jonathan.ellman@yale.edu

Institution

Yale University

Country

United States

ORCID For Submitting Author

0000-0001-9320-5512

Declaration of Conflict of Interest

no conflict of interest

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