Copper-Catalyzed Conjugation between Nitrile Imines and Terminal Alkynes

Area of Science:

• Organic Chemistry
• Synthetic Chemistry

Background:

• Nitrile imines and nitrile oxides are both 1,3-dipoles with often assumed similar reactivity.
• Nitrile oxides react with terminal alkynes via copper catalysis to yield isoxazoles.
• Nitrile imines do not directly yield pyrazoles under similar copper-catalyzed conditions.

Purpose of the Study:

• To investigate the distinct reactivity of nitrile imines compared to nitrile oxides in copper-catalyzed cycloadditions.
• To develop an efficient synthetic route to pyrazoles utilizing nitrile imines.
• To explore alternative pathways for pyrazole synthesis beyond direct cycloaddition.

Main Methods:

• Copper-catalyzed conjugation of nitrile imines with terminal alkynes to form alkynyl hydrazones.
• Preparation of copper acetylides from terminal alkynes with 1,10-phenanthroline (phen) ligand.
• Base-promoted 5-endo-dig cyclization of alkynyl hydrazones using 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU).
• Electrophilic interception of alkynyl hydrazone cyclization intermediates.

Main Results:

• Copper-catalyzed reaction of nitrile imines and terminal alkynes yields alkynyl hydrazones, not pyrazoles.
• Alkynyl hydrazones are efficiently converted to 1,3,5-trisubstituted pyrazoles using DBU.
• This DBU-mediated cyclization offers an improvement over existing metal-catalyzed methods.
• Electrophilic trapping allows for the synthesis of functionalized alkynyl hydrazones and tetrasubstituted pyrazoles.

Conclusions:

• Nitrile imines exhibit unique reactivity in copper-catalyzed reactions with alkynes, leading to alkynyl hydrazones.
• A novel, efficient, and broadly applicable method for synthesizing 1,3,5-trisubstituted pyrazoles from nitrile imines has been established.
• The developed methodology provides a valuable alternative to existing pyrazole synthesis routes and allows for further functionalization.