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

Area of Science:

• Organic Chemistry
• Chemical Biology
• Materials Science

Background:

• The thermal (3+2) dipolar azide-alkyne cycloaddition is an underutilized ligation method.
• This reaction offers potential applications in materials, bioorganic, and synthetic chemistry.
• Copper-free and strain-free cycloadditions are desirable for mild reaction conditions.

Purpose of the Study:

• To investigate the effects of alkyne substitution on the rate of the thermal azide-alkyne cycloaddition.
• To establish a reactivity scale for various alkyne substitutions.
• To guide the use of this ligation under mild conditions.

Main Methods:

• Experimental kinetic studies of azide-alkyne cycloaddition with varied alkyne substituents.
• Computational modeling to understand the reaction mechanism and substituent effects.
• Synthesis and characterization of substituted alkynes.

Main Results:

• Electron-withdrawing groups on the alkyne significantly accelerate the cycloaddition reaction rate.
• A reactivity scale was established, with rates varying by over 2100-fold.
• Unexpectedly, conjugated aryl groups on the alkyne retarded the reaction rate.
• A sulfonyl, ester-substituted alkyne reacted with azide at room temperature within hours.

Conclusions:

• Alkyne substitution provides a powerful handle to tune the rate of thermal azide-alkyne cycloaddition.
• The findings enable the rational design of alkyne components for efficient ligation under mild conditions.
• This study provides a valuable guide for employing this reaction in diverse chemical fields.