Scope and Mechanistic Limitations of a Sonogashira Coupling Reaction on an ­Imidazole Backbone

A Sonogashira coupling reaction method to join terminal alkynes to the imidazole backbone was developed and investigated. The method exhibits good functional group tolerance and provides target 4-alkynylated imidazoles in 70–93 % yield. The alkyne reagents were characterized by means of DFT calculations, from which electrostatic potential surfaces (EPS) were produced. A clear correlation between the EPS of the triple bond and the success of the coupling reaction was revealed. If the EPS is in range –94 to –105 kJ mol–1 the coupling is successful. An unsuccessful class of reagents (alkynols) was made compatible by means of an auxiliary group (tert-butyldimethylsilyl). EPSs of these modified reagents then resembled those of the model and these auxiliary-assisted reagents then coupled successfully in excellent yields.