EPR Evidence for the Origin of Nonlinear Effects in an Enantioselective Cu(II)-Catalyzed Spiroannulation

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 sp...

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Bibliographic Details
Published inACS catalysis Vol. 9; no. 2; pp. 1224 - 1230
Main Authors Armstrong, Brittany M, Sayler, Richard I, Shupe, Benjamin H, Stich, Troy A, Britt, R. David, Franz, Annaliese K
Format Journal Article
LanguageEnglish
Published American Chemical Society 01.02.2019
Subjects
reaction mechanism
Lewis acid catalysis
copper(II)−BOX
nonlinear effect
EPR spectroscopy
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Summary: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.
ISSN:2155-5435
2155-5435
DOI:10.1021/acscatal.8b03822