Journal
ACS CATALYSIS
Volume 9, Issue 2, Pages 1224-1230Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.8b03822
Keywords
nonlinear effect; copper(II)-BOX; Lewis acid catalysis; EPR spectroscopy; reaction mechanism
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Funding
- National Science Foundation [CHE-1363375]
- ACS-PRF [49I8I-DNI1]
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Herein, we describe an enantioselective Cu(II)-catalyzed spiroannulation of N-Boc-iminooxindoles with allylsilanes where a significant positive nonlinear effect (NLE) is observed. EPR spectroscopic studies of the copper(II) species present under synthetically relevant conditions reveal explicit spectroscopic evidence based on analysis of the metal center for the species responsible for the positive NLE in a metal -catalyzed system. EPR spectroscopy indicates that formation of a heterochiral ML2 species under scalemic conditions enriches the effective enantiopurity of the catalytically active species, leading to the asymmetric amplification observed in the spiroannulation. Mathematical analysis of the positive NLE reveals a high thermodynamic preference toward formation of the heterochiral ML2, which has a low relative reactivity when compared to the homochiral ML2.
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