Organocatalytic [6+4] Cycloadditions via Zwitterionic Intermediates: Chemo‑, Regio‑, and Stereoselectivities
The mechanisms and origins of chemo- and stereoselectivities of the organocatalytic [6+4] cycloaddition between 2-cyclopentenone and tropone have been investigated by a combined computational and experimental study. In the presence of a cinchona alkaloid primary amine catalyst and an acid additive,...
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Published in | Journal of the American Chemical Society Vol. 140; no. 42; pp. 13726 - 13735 |
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Main Authors | , , , , , , , |
Format | Journal Article |
Language | English |
Published |
WASHINGTON
American Chemical Society
24.10.2018
Amer Chemical Soc |
Subjects | |
Online Access | Get full text |
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Summary: | The mechanisms and origins of chemo- and stereoselectivities of the organocatalytic [6+4] cycloaddition between 2-cyclopentenone and tropone have been investigated by a combined computational and experimental study. In the presence of a cinchona alkaloid primary amine catalyst and an acid additive, 2-cyclopentenone forms a cross-dienamine intermediate that subsequently undergoes a stepwise [6+4] cycloaddition reaction via a zwitterionic intermediate. The rate-determining transition state features a strong hydrogen-bonding interaction between the tropone oxygen atom and the protonated quinuclidine directing the reaction course leading to a highly periselective [6+4] cycloaddition. The importance of the strong hydrogen-bonding interaction is also demonstrated by the influence of the concentration of the acid additive on the yields and enantioselectivities of the reaction. The corresponding [4+2] cycloaddition reaction has a much higher energy barrier. The enantioselectivity of the [6+4] cycloaddition originates from different repulsive hydrogen–hydrogen interactions that distinguish the diastereomeric transition states. |
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Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0002-7863 1520-5126 |
DOI: | 10.1021/jacs.8b07575 |