Transfer Hydrogenation of Ketones by Ruthenium Complexes Bearing Benzimidazol‐2‐ylidene Ligands

Abstract

A series of benzimidazolium ligand precursors were metalated with [RuCl2(p‐cymene)]2 to give ruthenium(II) N‐heterocyclic carbene complexes. All compounds were fully characterized by elemental analyses, 1H NMR, 13C NMR, and IR spectroscopy. The new benzimidazol‐2‐ylidene complexes have been found to be effective catalysis for the transfer hydrogenation of ketones by using 2‐propanol as the hydrogen source in the presence of KOH.

Catalytic activity of Ru/tetrahydropyrimidinium salts system for transfer hydrogenation reactions

“…It is observed that NaOH and KOH show good conversions when compared to Cs 2 CO 3 , K 2 CO 3 , t ‐BuOK and NaOAc in the hydrogenation reactions. As in previous studies, the best results are obtained with KOH . In the absence of a base no transfer hydrogenation of ketones is observed.…”

“…The reduction of acetophenone to 1‐phenylethanol was initially used as a model reaction with in situ prepared Ru(II) systems with 1a , 1b , 1c , 1d , 1e , 1f as catalysts in the transfer hydrogenation. In order to ensure complete formation of the active catalyst, a 2‐propanol solution of 1 mol% [RuCl 2 ( p ‐cymene)] 2 and 2 mol% 1a is stirred in the presence of 2 mmol of KOH at room temperature for 0.5 h. According to the literature during the formation of Ru–carbene complexes with [RuCl 2 ( p ‐cymene)] 2 , one carbene molecule bonds to the Ru center . Then acetophenone (1.00 mmol) is added and the reaction performed at 80°C for 1 h. When a lower catalyst molar ratio is used, conversion decreases (Table , entries 10 and 11).…”

“…[30] We have previously reported imidazolidine, benzimidazole-2-ylideneruthenium(II) complexes and an in situ formed tetrahydropyrimidineruthenium(II) system which exhibit high activity. [31][32][33] In order to find more efficient ruthenium catalysts we have prepared a series of new 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts, 1a-f (Scheme 1), and we now report the use of the in situ generated catalytic system composed of [RuCl 2 (p-cymene)] 2 as ruthenium source, 1a-f as carbene precursors and KOH as a base for the transfer hydrogenation of aryl ketones in 2-propanol for 1 h. All synthesized compounds were characterized using 1 H NMR, 13 C NMR and IR spectroscopy and elemental analysis, the results of which support the proposed structures. The molecular and crystal structure of 1,3-bis(4-methylbenzyl)-3,4,5,6tetrahydropyrimidinium chloride salt was determined using the singlecrystal X-ray diffraction technique.…”

Novel ruthenium(II)–N-heterocyclic carbene complexes; synthesis, characterization and catalytic application

“…According to Scheme 1, 1aee were synthezised in good yields by quaternization of 1-alkyl/aryl imidazole in DMF with corresponding aryl chlorides [29,31,32]. 1a and 1b compounds have already been reported by Günay et al [32].…”

Synthesis of an Ionic Paramagnetic Ruthenium(III) Complex and Its Application as an Efficient and Recyclable Catalyst for the Transfer Hydrogenation of Ketones

“…amine-bis(phenolate)s, chiral ligands, and heterocyclic ligands [5][6][7][8][9][10][11][12][13][14][15][16] are the most prominent catalysts and exhibit high activities in the transfer hydrogenation of ketones. In contrast to electron-rich ligands as strong σ-donors, [17] electron-deficient ligands are less explored.…”