Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 86, Issue 6, Pages 4598-4606Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.joc.0c03054
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- Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic
- University of Colorado Denver
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This study reveals a plausible mechanism for the formation of all-carbon quaternary stereocenters through the Sc-catalyzed C-H cycloaddition of imidazoles to 1,1-disubstituted alkenes. The Sc complex activates the C2-H bond in the imidazole substrate and undergoes intramolecular cyclization to generate the quaternary center. The findings also highlight the electronic and steric effects on catalyst activity and enantioselectivity.
This density functional theory (DFT) study reveals a detailed plausible mechanism for the Sc-catalyzed C-H cycloaddition of imidazoles to 1,1-disubstituted alkenes to form all-carbon quaternary stereocenters. The Sc complex binds the imidazole substrate to enable deprotonative C2-H bond activation by the Sc-bound alpha-carbon to afford the active species. This complex undergoes intramolecular cyclization (C=C into Sc-imidazolyl insertion) with exo-selectivity, generating a beta-all-carbon-substituted quaternary center in the polycyclic imidazole derivative. The Sc-bound a-carbon deprotonates the imidazole C2-H bond to deliver the product and regenerate the active catalyst, which is the rate-determining step. Calculations illuminate the electronic effect of the ancillary Cp ligand on the catalyst activity and reveal the steric bias caused by using a chiral catalyst that induce the enantioselectivity. The insights can have implications for advancing rare-earth metal-catalyzed C-H functionalization of imidazoles.
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