4.7 Article

Relative Rates of Metal-Free Azide-Alkyne Cycloadditions: Tunability over 3 Orders of Magnitude

Journal

JOURNAL OF ORGANIC CHEMISTRY
Volume 84, Issue 21, Pages 13615-13623

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.joc.9b01887

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Funding

  1. NSF [DMR 1404550]
  2. Development and Promotion of Science and Technology Talents Project, Thailand
  3. UCSC Undergraduate Research in Science and Technology Award

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The thermal (3 + 2) dipolar azide-alkyne cycloaddition, proceeding without copper or strained alkynes, is an underutilized ligation with potential applications in materials, bioorganic, and synthetic chemistry. Herein, we investigate the effects of alkyne substitution on the rate of this reaction, both experimentally and computationally. Electron-withdrawing groups accelerate the reaction, providing a range of relative rates from 1.0 to 2100 between the slowest and fastest alkynes studied. Unexpectedly, aryl groups conjugated to the alkyne significantly retard the reaction rate. In contrast, a sulfonyl, ester-substituted alkyne is reactive enough that it couples with an azide at room temperature in a few hours. This reactivity scale should provide a guide to those who wish to use this ligation under mild conditions.

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