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*RhCl] 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, RhCl·3HO, 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 RhCl·3HO outperforms [Cp*RhCl]. 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 Rh 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 RhCl·3HO and [Cp*RhCl].

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…Decomposition of M/NHC catalysts via C–NHC coupling was detected in Mizoroki–Heck 27 , 62 , 65 and Suzuki–Miyaura 66 couplings, in the oligomerizations of alkenes and alkynes, 67 – 70 in CH-functionalizations of heterocycles, 71 and in many other reactions. 23 , 25 The C–NHC coupling reactions accompanied by two-electron reduction of the metal center were observed experimentally for Pd II , 22 , 23 , 25 Ni II , 22 , 23 , 25 , 72 Ru II , 73 , 74 Rh III , 75 – 79 Fe III ( ref. 80 ) and Cu III ( ref.…”

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru‐, Rh‐, and Ir‐Complexes of CNHC^C Cyclometalated Ligands

“…RhCl 2 ] 2 by availing the in situ π-coordination of (hetero)aromatic reaction component to stabilize the orthometalated Rh intermediates (Scheme 35). [47] They have demonstrated the feasibility of this concept in 2-fold CÀ H/CÀ H cross-coupling reaction of N-(hetero)-arylimidazolium salts with various (hetero)arenes to access diverse beneficial heteroaromatic molecules. Notably, simple RhCl 3 • 3H 2 O performs better than the extensively used [(Cp*)RhCl 2 ] 2 in this reaction.…”

Recent advances in rhodium-catalysed cross-dehydrogenative-coupling between two C(sp2)–H bonds

“…A simple RhCl 3 ·H 2 O catalyst was employed for the CDC reaction of N -(hetero)-arylimidazolium salts with (hetero)arenes to obtain water-soluble and fluorescence-on planar polycyclic heteroaromatic molecules (Scheme 52c). 75 Comprehensive DFT computation and the control experiments suggested that the in situ heteroarene η 6 -coordination to Rh( iii ) center stabilises the organorhodium intermediates in this reaction, in a similar manner to [Cp*Rh( iii )] complexes. It was suggested to go through two cycles, the first cycle is a typical Rh( iii ) to Rh( i ) process while the second cycle involves an unprecedented Rh( i ) to Rh( iii ) to Rh( i ) process with the release of hydrogen.…”