Phosphinous Acid‐Assisted Hydration of Nitriles: Understanding the Controversial Reactivity of Osmium and Ruthenium Catalysts

Abstract

The synthesis and catalytic behavior of the osmium(II) complexes [OsCl (η -p-cymene)(PR OH)] [R=Me (2 a), Ph (2 b), OMe (2 c), OPh (2 d)] in nitrile hydration reactions is presented. Among them, the best catalytic results were obtained with the phosphinous acid derivative [OsCl (η -p-cymene)(PMe OH)] (2 a), which selectively provided the desired primary amides in excellent yields and short times at 80 °C, employing directly water as solvent, and without the assistance of any basic additive (TOF values up to 200 h ). The process was successful with aromatic, heteroaromatic, aliphatic, and α,β-unsaturated organonitriles, and showed a high functional group tolerance. Indeed, complex 2 a represents the most active and versatile osmium-based catalyst for the hydration of nitriles reported so far in the literature. In addition, it exhibits a catalytic performance similar to that of its ruthenium analogue [RuCl (η -p-cymene)(PMe OH)] (4). However, when compared to 4, the osmium complex 2 a turned out to be faster in the hydration of less-reactive aliphatic nitriles, whereas the opposite trend was generally observed with aromatic substrates. DFT calculations suggest that these differences in reactivity are mainly related to the ring strain associated with the key intermediate in the catalytic cycle, that is, a five-membered metallacyclic species generated by intramolecular addition of the hydroxyl group of the phosphinous acid ligand to the metal-coordinated nitrile.

Half-Sandwich Arene-Osmium(II) Complexes with Phosphinite Ligands

“…pseudo-octahedral three-legged piano-stool geometry, with metrical parameters around the metallic core comparing well with those previously described for the related complexes [OsCl2(η 6 -p-cymene)(PPh3)] [14] and [OsCl2(η 6 -p-cymene)(PPh2OH)] [15]. The Os(1)-P(1) and P(1)-O(1) bond distances (2.318(2) and 1.611(4) Å, respectively) are also comparable to those found in the structure of [OsCl2(η 6 -p-cymene)(PPh2OH)] (Os-P = 2.321(1) Å and P-O = 1.603(4) Å) [15].…”

Cymene‐Osmium(II) Complexes with Amino‐Phosphane Ligands as Precatalysts for Nitrile Hydration Reactions

“…However, it is also known that amino‐phosphanes, in both free state or coordinated to a metal fragment, are prone to undergo hydrolytic cleavage of the P−N bond . Starting from [OsCl 2 ( η 6 ‐ p ‐cymene){PPh 2 (NMe 2 )}] ( 5 a ) such a hydrolysis process would lead to the formation of the phosphinous acid derivative [OsCl 2 ( η 6 ‐ p ‐cymene){PPh 2 (OH)}] ( 18 ), a compound previously described by us and also active in the hydration of nitriles . In line with this, it was very striking to find that, under identical reaction conditions, the activities of 5 a and 18 were very similar.…”

“…Thus, using only 1 mol% of this complex and performing the reactions in pure water, a large variety of organonitriles could be selectively converted into the corresponding primary amides in high yields and short times, without the requirement of any acidic or basic additive. The nitrile hydration process was compatible with the presence of common functional groups on the substrates, and, more importantly, the performance shown by [OsCl 2 ( η 6 ‐ p ‐cymene){PPh 2 (NMe 2 )}] surpassed those of related Ru(II), Ru(IV), Rh(I) and Pt(II) species featuring the same amino‐phosphane ligand PPh 2 (NMe 2 ), thus giving additional evidence of the enormous potential of osmium in this catalytic transformation . On the other hand, the observations made in this work suggest a rethinking of the role really played by the amino‐phosphane ligands in the metal‐catalyzed hydration of nitriles (precursors of phosphinous acids instead of simple H‐bond acceptors).…”

“…In the context of these studies with phosphinous acids, we were also surprised to find that the osmium(II) complex [OsCl 2 ( η 6 ‐ p ‐cymene)(PMe 2 OH)] exhibits a catalytic performance similar (or even superior in the case of the less reactive aliphatic nitriles) to its ruthenium counterpart [RuCl 2 ( η 6 ‐ p ‐cymene)(PMe 2 OH)] ,. We must stress here that the slow ligand exchange kinetics in Os complexes has been for long time considered as a major drawback for the application of osmium derivatives in homogeneous catalysis .…”

“…We must stress here that the slow ligand exchange kinetics in Os complexes has been for long time considered as a major drawback for the application of osmium derivatives in homogeneous catalysis . A clear example of the underestimation of the catalytic potential of this metal is the fact that, contrary to the case of ruthenium, to date the number of studies on nitriles hydration using osmium compounds can be counted on the fingers of one hand ,. The remarkable catalytic activity featured by [OsCl 2 ( η 6 ‐ p ‐cymene)(PMe 2 OH)] supports the exploration of new osmium complexes with cooperative ligands as potential catalysts for nitrile hydration reactions.…”