Chiral (η⁶-p-Cymene)ruthenium(II) Complexes Containing Monodentate Acylthiourea Ligands for Efficient Asymmetric Transfer Hydrogenation of Ketones

Abstract

The new chiral ligands (R)-/(S)-N-((1-phenylethyl)carbamothioyl)benzamide (L1/L2), (R)-/(S)-N-((1-phenylethyl)carbamothioyl)thiophene-2-carboxamide (L3/L4), and (R)-/(S)-N-((1-phenylethyl)carbamothioyl)furan-2-carboxamide (L5/L6) were synthesized, characterized, and used to prepare novel chiral Ru(II) complexes. The chiral Ru(II) complexes 1–6 were obtained from reactions between the chiral ligands L1–L6 and [RuCl2(p-cymene)2]2. The complexes were characterized by analytical and spectroscopic (NMR, FT-IR, electronic) techniques. The solid-state structures of the ligands L1 and L3 and complexes 1, 4, and 6 were determined by single-crystal X-ray diffraction methods. In all of the complexes, the ligand is bound to the Ru(II) center only via the sulfur donor atom. This monodentate coordination of the acylthiourea ligands was observed for the first time with ruthenium. The Ru(II) complexes 1–6 all act as efficient catalysts for the asymmetric transfer hydrogenation of aromatic ketones in the presence of 2-propanol and KOH to produce chiral alcohols. All of the catalysts showed excellent conversions of up to 99% and enantiomeric excesses of up to 99%.

Half-sandwich Ru(η6-C6H6) complexes with chiral aroylthioureas for enhanced asymmetric transfer hydrogenation of ketones – experimental and theoretical studies

“…49 Various methodologies have been adopted for preparing chiral alcohols with high enantiomeric excess (ee). 52 The Ru-benzene complexes exhibited higher activity compared to previously reported Ru-p-cymene catalysts containing the same ligands. 51 In the present paper, we report the synthesis of the new Ru-benzene complexes, [RuCl 2 (η 6 -C 6 H 6 )L] (1-6), from [RuCl 2 (η 6 -C 6 H 6 )] 2 (1)(2)(3)(4)(5)(6) for ATH of ketones to their corresponding chiral alcohols is investigated and compared with similar η 6 -p-cymene complexes.…”

“…52 The air stable half-sandwich Ru(II) complexes (1)(2)(3)(4)(5)(6) were prepared as orange solids in high yields (80-88%) from the reactions between [RuCl 2 (η 6 - 52 The air stable half-sandwich Ru(II) complexes (1)(2)(3)(4)(5)(6) were prepared as orange solids in high yields (80-88%) from the reactions between [RuCl 2 (η 6 - …”

“…Conversions and ee up to 99 % were obtained (Table 1, entries [19][20][21][22][23][24]. 52 In general, Ru-benzene catalysts required only 14 h for the formation of chiral alcohols (up to 99% conversion and 99% ee) from ketones while Ru-p-cymene catalysts required 24 h for the same conversion. Chiral benzhydrol is widely used in the synthesis of various pharmaceuticals.…”

Strong in Vitro Cytotoxic Potential of New Ruthenium–Cymene Complexes

“…This strongly suggests that the ligand is bound to the ruthenium center via the sulfur atom from the thiourea group. 30 In complex C2 a strong band at 1600 cm −1 from pyridine ν(CN) of the complex shows a downward shift of up to 10 cm −1 relative to the multiple bands observed in the free ligand. The electronic absorption spectra of complexes C1 and C2 were recorded in acetonitrile in the range 800−200 nm.…”

Catalytic asymmetric transfer hydrogenation of ketones: recent advances

“…37 For the same process, thiourea-derived ligands of the type 33 were successfully used, the corresponding intermediate complexes also being characterized by the same technique. 38 …”