Indium-Catalyzed Heteroaryl-Heteroaryl Bond Formation through Nucleophilie Aromatic Substitution

摘要

Heteroaromatic molecules bearing heteroaryl-heteroaryl bonds are an important class of building blocks found in a variety of areas; for example, optoelectronic materials, liquid crystals, biological compounds, and ligands for asymmetric catalysis. Over the past 35 years, transition-metal-catalyzed cross-coupling reactions have been chiefly responsible for making (hetero)aryl-(hetero)aryl bonds. On the other hand, nucleophilic aromatic substitution (S_NAr) has actually been studied to construct such biaryl linkages since the 1940s. However, aromatic compounds, which are intrinsically electron-rich, are in general unreactive toward nucleophilic substitution. Therefore, two aryl substrates with entirely opposite electronic demands must be arranged to realize biaryl synthesis by the S_NAr reaction. Thus, electron-rich aryl nucleophiles with highly electropositive metals (e.g. Li~+, Mg~(2+), Zn~(2+)) and/or electron-poor aryl. electrophiles with one or more strong electron-withdrawing groups (EWGs; e.g. CF3, NO2, CN, CO2R) have each been the aryl substrate of choice. More than a stoichiometric amount of promoter is also often necessary. These requisites may have limited the widespread applicability of biaryl synthesis based on S_NAr. We envisioned that catalytic biaryl synthesis by S_NAr independent of such activated aryl substrates would be an attractive alternative to the transition-metal-catalyzed cross-coupling strategy. Herein, we report the first example of a catalytic heteroaryl-heteroaryl bond-forming reaction based on S_NAr without using both the heteroarylmetal nucleophile and heteroaryl electrophile substituted with EWGs.