Ru(ii) complexes containing dmso and pyrazolyl ligands as catalysts for nitrile hydration in environmentally friendly media

Abstract

The synthesis of two Ru-dmso complexes containing the ligands 2-(3-pyrazolyl)pyridine (pypz-H), and pyrazole (pz-H), [Ru(II)Cl2(pypz-H)(dmso)2], (2) and [Ru(II)Cl2(pz-H)(dmso)3], (3), has been described. Both complexes have been fully characterized in solution through (1)H-NMR and UV-Vis techniques and also in the solid state through monocrystal X-ray diffraction analysis. The redox properties of both complexes have also been studied by means of cyclic voltammetry. Exposure of 2 to visible light in acetonitrile produces a substitution of one dmso ligand by a solvent molecule generating a new complex, [Ru(II)Cl2(MeCN)(pypz-H)(dmso)] (4). Also, UV-visible spectroscopy points out that complex 2 presents a thermal and photochemical substitution of dmso ligands in aqueous solution. Finally, the reactivity of complexes 2 and 3 has been tested with regard to the hydration of nitriles using water as a single solvent, displaying good efficiency and selectivity for the corresponding amide derivatives. In general, better performance is achieved with complex 3. Reuse of these catalysts in water and glycerol has been explored for the first time in ruthenium-mediated nitrile hydration catalysis.

Metal-catalyzed nitrile hydration reactions: The specific contribution of ruthenium

“…However, longer reactions times and a higher temperature (160 ºC) were in this case needed to attain good conversions [34]. The suitability of glycerol, an emerging green solvent generated as byproduct in the biodiesel industries [35], for nitrile hydration reactions was further confirmed by Romero and co-workers [36].…”

Ruthenium-Catalyzed Amide-Bond Formation

“…Secondary coordination sphere activation of water, via hydrogen bonding with the OH group of the ligand, also explains the outstanding performances of the related phosphiniteruthenium(II) complex [RuCl 2 ( 6 -p-cymene){PMe 2 (OH)}] (12), which was also able to hydrate cyanohydrins [50,51]. Although other catalytic systems for the selective hydration of organonitriles in water have been described [52][53][54][55][56][57], none of them presented an activity and scope comparable to those of the bifunctional catalysts 4-12. Additional examples of nitrile hydration reactions catalyzed by homogeneous ruthenium catalysts in organic media are (Scheme 8): (i) The hydration of benzoxazolylacetonitrile by the arene-ruthenium(II) dimer [{RuCl(-Cl)( 6 -p-cymene)} 2 ], which led to benzoxazolylacetamide in high yield [58], and (ii) the asymmetric hydration of -benzyl--methylmalononitrile by the chiral catalysts 13 [59].…”

Efficient and selective hydration of nitriles to amides in aqueous systems with Ru(II)-phosphaurotropine catalysts

“…Of 16 substrates examined, 92-99% conversions of 14 nitriles was achieved in 1 h at reflux temperature. [12][13][14]. The most active catalyst was obtained in the reaction of 1 + 3 (pta-Bn)Cl (entry 6) which led to complete conversion of benzonitrile in 1 h (but see also Figure S1).…”

“…There are examples of catalytic hydrations in water with Pd(II) 3 and Rh(I) 4 complexes, however, the field is dominated by the use of ruthenium-based catalysts. 6,7 mtppms-Na (sodium salt of metamonosulfonated triphenylphosphine), 6,7 pta (1,3,5-triaza-7-phosphaadamantane), 6,7 P(NMe 2 ) 3 , 8,9 tris-(5-(2-aminothiazolyl)phosphine, 11 pyrazole, 12 or 2-(3-pyrazolyl)pyridine. 12 In some cases Ru-complexes of benzylated pta (pta-Bn) were used as catalysts.…”

“…6,7 mtppms-Na (sodium salt of metamonosulfonated triphenylphosphine), 6,7 pta (1,3,5-triaza-7-phosphaadamantane), 6,7 P(NMe 2 ) 3 , 8,9 tris-(5-(2-aminothiazolyl)phosphine, 11 pyrazole, 12 or 2-(3-pyrazolyl)pyridine. 12 In some cases Ru-complexes of benzylated pta (pta-Bn) were used as catalysts. 10 Heterogenized Ru(II)-arene-pta (RAPTA) complexes were shown active in hydration of nitriles both under conventional (thermal) conditions and with microwave irradiation.…”