Arene−Ruthenium(II) Complexes Containing Amino−Phosphine Ligands as Catalysts for Nitrile Hydration Reactions

Abstract

Three different series of novel mononuclear arene-ruthenium(II) complexes containing aminophosphine ligands, namely, [RuCl 2 {κ 1 (P)-2-Ph 2 PC 6 H 4 CH 2 NHR}(η 6 -arene)], [RuCl 2 {κ 1 (P)-3-Ph 2 -PC 6 H 4 CH 2 NHR}(η 6 -arene)], and [RuCl 2 {κ 1 (P)-4-Ph 2 PC 6 H 4 CH 2 NHR}(η 6 -arene)] (arene = C 6 H 6 , p-cymene, 1,3,5-C 6 H 3 Me 3 , C 6 Me 6 ; R = i Pr, t Bu; all combinations), have been synthesized and fully characterized. These readily accessible species are efficient catalysts for the selective hydration of organonitriles into amides under challenging reaction conditions, i.e., pure aqueous medium in the absence of any cocatalyst, being much more active than their corresponding nonfunctionalized triphenylphosphine counterparts [RuCl 2 (PPh 3 )(η 6 -arene)]. The results obtained in this study indicate that the (amino-phosphine)ruthenium(II) complexes operate through a "bifunctional catalysis" mechanism in which the ruthenium center acts as a Lewis acid, activating the nitrile molecule, and the P-donor ligand acts as a Brønsted base, the pendant amino group generating the real nucleophile of the hydration process, i.e., the OHgroup.

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

“…5). In accord with this proposal, rate enhancements were observed when the catalytic hydration of benzonitrile by complexes [RuCl 2 (η 6 -arene)(PPh 3 )] was performed in -14 -the presence of free PhCH 2 NH i Pr or PhCH 2 NH t Bu, amines that mimic the substituents of the phosphine ligands in complexes 17-19 [28]. In order to facilitate the solubility of the catalysts in water, remarkable efforts have been devoted in recent years to the study of ruthenium complexes bearing water-soluble phosphines.…”

“…5) [28]. All these complexes (5 mol%) were able to hydrate the model benzonitrile substrate in pure water as solvent, leading to the selective formation of benzamide in 43-98% yield after 24 h of heating at 100 ºC.…”

Ruthenium-Catalyzed Amide-Bond Formation

“…Although the activity shown by these complexes was only moderate (TOF up to 3 h -1 at 100 ºC), the hydration reactions proceeded significantly faster than those using the analogous triphenylphosphine derivatives [RuCl 2 ( 6 -arene)(PPh 3 )] under identical experimental conditions (TOF up to 0.1 h -1 ). However, when free amine PhCH 2 NHR (R = n-Bu or i-Pr) was added to the [RuCl 2 ( 6 -arene)(PPh 3 )]-catalyzed reactions the hydration rates increased, becoming similar to those observed with complexes 3 [39]. This fact suggested that the higher activity of [RuCl 2 ( 6 -arene)(Ph 2 PC 6 H 4 CH 2 NHR)] (3) vs [RuCl 2 ( 6 -arene)(PPh 3 )] is not due to a ligand-assisted activation of water by hydrogen-bonding.…”

“…Activation of water by the ancillary phosphine ligands was also evidenced when arene-ruthenium(II) complexes of general composition [RuCl 2 ( 6 -arene)(Ph 2 PC 6 H 4 CH 2 NHR)] (3 in Figure 2) were employed as catalysts [39]. Although the activity shown by these complexes was only moderate (TOF up to 3 h -1 at 100 ºC), the hydration reactions proceeded significantly faster than those using the analogous triphenylphosphine derivatives [RuCl 2 ( 6 -arene)(PPh 3 )] under identical experimental conditions (TOF up to 0.1 h -1 ).…”

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

“…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.…”