C−H Bond Activation/Arylation Catalyzed by Arene–Ruthenium–Aniline Complexes in Water

Abstract

Water-soluble arene-ruthenium complexes coordinated with readily available aniline-based ligands were successfully employed as highly active catalysts in the C-H bond activation and arylation of 2-phenylpyridine with aryl halides in water. A variety of (hetero)aryl halides were also used for the ortho-C-H bond arylation of 2-phenylpyridine to afford the corresponding ortho- monoarylated products as major products in moderate to good yields. Our investigations, including time-scaled NMR spectroscopy and mass spectrometry studies, evidenced that the coordinating aniline-based ligands, having varying electronic and steric properties, had a significant influence on the catalytic activity of the resulting arene-ruthenium-aniline-based complexes. Moreover, mass spectrometry identification of the cycloruthenated species, {(η -arene)Ru(κ -C,N-phenylpyridine)} , and several ligand-coordinated cycloruthenated species, such as [(η -arene)Ru(4-methylaniline)(κ -C,N-phenylpyridine)] , found during the reaction of 2-phenylpyridine with the arene-ruthenium-aniline complexes further authenticated the crucial roles of these species in the observed highly active and tuned catalyst. At last, the structures of a few of the active catalysts were also confirmed by single-crystal X-ray diffraction studies.

Ligand‐Tuned C–H Bond Activation/Arylation of 2‐Arylpyridines over Pyridine‐Based N,O/N,N Ligated Ruthenium–Arene Complexes

“…The 1 H NMR spectra of complexes [Ru]‐1 – [Ru]‐10 depicts an analogous trend, where a downfield shift in the chemical shifts of the protons associated with the ligands is observed, compared with the free ligands, suggesting the coordination of the ligand to the ruthenium metal center. The 1 H NMR resonances for protons corresponding to the ruthenium‐coordinated η 6 ‐arene ring are also observed in the expected region for all complexes , , . The FTIR and UV/Visible spectra recorded for the synthesized complexes are also in accordance with the previous reports , .…”

“…The 1 H NMR spectroscopic analysis of the reaction mixture, using the stoichiometric ratio of catalyst [Ru]‐2 and substrate ( 1a ) (C/S = 1:5), after 3 h, shows the formation of 2‐phenylpyridine coordinated cyclometallated species {(η 6 ‐ p ‐cymene)Ru(κ 2 ‐ C,N ‐phenylpyridine)} + [Ru]‐A . Earlier reports inferred that such organometallic species [Ru]‐A are indeed a crucial species involved in several ruthenium–arene catalyzed ortho C–H bond‐activation reactions , , . The presence of cycloruthenated species ( [Ru]‐A ) is also confirmed by mass spectrometric investigation ( m/z 390.1) of the stoichiometric reaction of complex [Ru]‐2 with 2‐phenylpyridine ( 1a ), even in the absence of base K 2 CO 3 (Figure ), suggesting the possible role of the ruthenium‐coordinated 2‐acetylpyridine in the C–H activation reaction.…”

“…In particular, catalytic systems based on ruthenium–arene complexes for the C–H bond activation/functionalization of various heteroarenes have also been vastly explored, where the role of ligands has been found to be a crucial factor . For instance, Inoue et al have used bulky phosphines to tune the selectivity of monoarylated products during ortho arylation of heteroarenes.…”

“…[12i] In our recent studies with ruthenium–arene complexes containing O,O and O , N donor tropolone/aminotropolone ligands for C–H bond arylation of 2‐phenylpyridine in water, we also investigated the interesting role of carboxylate additives, driven by steric bulkiness, in tuning the selectivity of C–H arylation products . Later, we also explored aniline‐based ruthenium–arene complexes for C–H bond arylation in water, where the strength of the aniline–ruthenium bond and substitution on the aniline ligand were found to be crucial factors for tuning the selectivity of mono‐ versus biarylated products during the reaction …”

“…Further, in our deliberate efforts to develop an efficient catalytic system for C–H activation/functionalization reactions,, herein we have synthesized a series of ruthenium(II)–arene complexes containing pyridine‐based N , O and N , N donor ligands, and we have systematically investigated their catalytic performance for ortho C–H bond activation/arylation of 2‐phenylpyridine with several aryl halides in water. The synthesized ruthenium–arene complexes have been well‐characterized using NMR spectroscopy and mass spectrometric analysis, and the molecular structure of a few of the representative complexes have been authenticated by single‐crystal X‐ray diffraction studies.…”