Ruthenium(II) Complexes of Heteroditopic N-Heterocyclic Carbene Ligands: Efficient Catalysts for C–N Bond Formation via a Hydrogen-Borrowing Strategy under Solvent-Free Conditions

Abstract

Both imidazol-2-ylidene (ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC) have evolved to be elite groups of N-heterocyclic carbene (NHC) ligands for homogeneous catalysis. To develop efficient ruthenium­(II)-based catalysts incorporating these ligands for C–N bond-forming reactions via hydrogen-borrowing methodology, we utilized chelating ligands integrated with ImNHC and mesoionic tzNHC donors connected via a CH2 spacer with a diverse triazole backbone. The synthesized ruthenium­(II) complexes 3 are found to be highly efficient for C–N bond formation across a wide range of primary amine and alcohol substrates under solvent-free conditions, and among all of the complexes studied here, catalyst 3a with a mesityl substituent displayed maximum activity. To our delight, catalyst 3a is also effective for the selective mono-N-methylation of various anilines utilizing methanol as a coupling partner, known to be relatively more difficult than other alcohols. Furthermore, complex 3a also delivers various substituted quinolines successfully via the reaction of 2-aminobenzyl alcohol with several secondary alcohols. Importantly, catalyst 3a exhibited the highest activity among the reported ruthenium­(II) complexes for both the N-benzylation of aniline [achieving a turnover number (TON) of 50000] and the realization of quinoline 8a by reacting 2-aminobenzyl alcohol with 2-phenylethanol (attaining a TON of 30000).

Nickel Complexes Bearing N,N,O-Tridentate Salicylaldiminato Ligand: Efficient Catalysts for Imines Formation via Dehydrogenative Coupling of Primary Alcohols with Amines

“…This process can be accomplished in one pot without using any oxidant or hydrogen acceptor and can be applied to the synthesis of various valuable nitrogen-containing compounds like amines, imines, amides, N -heterocycles, , etc. The ADC reactions are usually catalyzed by precious metals, where especially iridium and ruthenium-based catalysts have been predominantly investigated. ,, In the past few years, a trend has drifted toward the usage of earth-abundant and inexpensive metals such as iron, cobalt, and manganese . Very recently, nickel catalyst has risen to prominence in such transformation due to its inexpensive nature and unique redox property …”

Tunable mono- and di-methylation of amines with methanol over bimetallic CuCo nanoparticle catalysts

“…Among the methodologies developed for N -methylation with methanol, the borrowing hydrogen approach has been extensively studied. 18 More specifically, methanol is initially activated on a metal surface and dehydrogenated to yield the transient active formaldehyde, which spontaneously reacts with amine to form an imine and is finally reduced by a metal-hybrid. To this end, a number of elegant catalytic systems have been described and reviewed, 18 predominantly relying on the homogeneous transition metal complexes, with very few solid catalysts.…”

“…18 More specifically, methanol is initially activated on a metal surface and dehydrogenated to yield the transient active formaldehyde, which spontaneously reacts with amine to form an imine and is finally reduced by a metal-hybrid. To this end, a number of elegant catalytic systems have been described and reviewed, 18 predominantly relying on the homogeneous transition metal complexes, with very few solid catalysts. Surveying the literature on the N -methylation of amines with methanol, several appealing studies have come to our attention, using noble metal-based heterogeneous catalysts, such as NHC-Ir, 19 Ir@YSMCNs, 20 Pd/C, 21 and Pd@ s PS-NMe 2 , 22 to address this issue, which offered moderate to good catalytic performances.…”

Efficient methylation of anilines with methanol catalysed by cyclometalated ruthenium complexes

“…iridium [15][16][17] and specifically ruthenium. [18][19][20][21][22][23] However, more recently, first row transition metals like iron 24,25 or manganese [26][27][28][29] and others were developed, too. [30][31][32][33] Notably, most of the currently known catalysts rely on sophisticated ligands and only work above 100°C in the presence of large amounts of strong bases, which create additional environmental concern (see Scheme 1).…”