Two-Fold C-H/C-H Cross-Coupling Using RhCl3·3H2O as the Catalyst: Direct Fusion of N-(Hetero)arylimidazolium Salts and (Hetero)arenes.

Abstract

[Cp*RhCl2]2 is the most prevailing catalyst employed for rhodium-catalyzed chelation-assisted C-H/C-H cross-coupling reactions due to the special ligand effect of Cp*. In this article, a novel concept of using a simple inorganic rhodium salt, RhCl3·3H2O, as the catalyst by taking advantage of in situ π-coordination to Rh with a (hetero)aromatic reaction component to stabilize Rh intermediates is proposed and evaluated. Our studies not only prove the feasibility of this concept but also disclose a novel 2-fold C-H/C-H cross-coupling reaction of N-(hetero)arylimidazolium salts with various (hetero)arenes to access water-soluble, fluorescent, cationic, and planar polycyclic heteroaromatic molecules, in which RhCl3·3H2O outperforms [Cp*RhCl2]2. Mechanistic experiments and DFT calculations reveal that this successive quadruple C-H activation reaction consists of two different C-H activation modes, i.e., concerted metalation-deprotonation (CMD) and oxidative addition. Notably, this is the first report of a C-H bond activation via oxidative addition to RhI in a bi(hetero)aryl formation with hydrogen evolution. Finally, the different ligand electrochemical parameters of neutral (hetero)arenes and anionic Cp* are used to explain the different catalytic behaviors of RhCl3·3H2O and [Cp*RhCl2]2.