Hydration of Aliphatic Nitriles Catalyzed by an Osmium Polyhydride: Evidence for an Alternative Mechanism

Abstract

The hexahydride OsH6(PiPr3)2 competently catalyzes the hydration of aliphatic nitriles to amides. The main metal species under the catalytic conditions are the trihydride osmium­(IV) amidate derivatives OsH3{κ2-N,O-[HNC­(O)­R]}­(PiPr3)2, which have been isolated and fully characterized for R = iPr and tBu. The rate of hydration is proportional to the concentrations of the catalyst precursor, nitrile, and water. When these experimental findings and density functional theory calculations are combined, the mechanism of catalysis has been established. Complexes OsH3{κ2-N,O-[HNC­(O)­R]}­(PiPr3)2 dissociate the carbonyl group of the chelate to afford κ1-N-amidate derivatives, which coordinate the nitrile. The subsequent attack of an external water molecule to both the C­(sp) atom of the nitrile and the N atom of the amidate affords the amide and regenerates the κ1-N-amidate catalysts. The attack is concerted and takes place through a cyclic six-membered transition state, which involves Cnitrile···O–H···Namidate interactions. Before the attack, the free carbonyl group of the κ1-N-amidate ligand fixes the water molecule in the vicinity of the C­(sp) atom of the nitrile.

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

“…The coordination of the nitrile to the metal center enhances the electrophilicity of its C­(sp) atom, which makes it more susceptible to undergo an intra- or intermolecular nucleophilic attack of the hydroxide group. The attack leads to metal-κ 1 - N -amidate derivatives, which are the true catalysts of the hydration . According to this, it seems reasonable to think that the first step in the formation of 2 and 3 is the coordination of a nitrile molecule to the metal center of the unsaturated cation 1 to afford the six-coordinate intermediate A .…”

“…The attack leads to metal-κ 1 -N-amidate derivatives, which are the true catalysts of the hydration. 38 According to this, it seems reasonable to think that the first step in the formation of 2 and 3 is the coordination of a nitrile molecule to the metal center of the unsaturated cation 1 to afford the six-coordinate intermediate A. Thus, the nucleophilic attack of the hydroxide group to the coordinated nitrile could give the κ 1 -N-amidate intermediate B in equilibrium with the κ 2 -N,O and κ 1 -O counterparts C and D. Subsequently, the electrophilic alkylidyne ligand would trap the free NH arm of D to yield 2 and 3.…”

Hydration of Nitriles Catalyzed by Ruthenium Complexes: Role of Dihydrogen Bonding Interactions in Promoting Base-Free Catalysis

“…Amides belong to an essential and versatile category of organic compounds constituting a variety of biological molecules, polymers, and synthetic compounds and intermediates. − Various synthetic methods have been evolved for the preparation of amides. − Hydration of nitriles is one of the most convenient and desirable methods for the synthesis of amides as there is no generation of any toxic chemical or byproduct. − However, traditional methods for the hydration of nitriles involve harsh reaction conditions including highly alkaline or acidic media, which are not only environmentally and practically unsustainable but often lead to overhydrolysis of amides to the corresponding free carboxylic acids or ammonium carboxylates. − To circumvent these problems and to stop hydration of a cyanide substrate at the amide stage, several catalysts have been developed. ,− Nitrile hydratase enzymes, a family of biocatalysts that selectively promote hydration of nitriles to amides, also suffer from a limited substrate scope, thus restricting their widespread utilization. , In that context, transition metal-based catalysts have been found quite successful not only for their high activity but also for the better substrate scope. − As a result, various catalysts have been developed for promoting the direct nucleophilic addition of a water molecule to a nitrile bond, thus producing an amide. ,, The ruthenium complexes have also been explored as the efficient catalysts for the hydration of nitriles, including promotion of the metal–ligand cooperative catalysis. ,− …”

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

“…Metal fragments with these characteristics are the polyhydrides of the platinum-group metals. 16 Among them, the osmium polyhydrides occupy a predominant position due to their proven ability to catalyze interesting organic synthesis reactions, 17 in addition to the dehydrogenation of liquid organic hydrogen carriers 18 and boranes. 19 Promising acidic ligands capable of donating σ-electron density to a platinum-group metal in a high oxidation state are the heavier tetrylenes, 20 heavier counterparts of Fisher-type carbenes.…”