Hydrogenation of Polar Bonds Catalysed by Ruthenium-Pincer Complexes

Abstract

Catalytic hydrogenation of polar bonds using molecular hydrogen is an important, atom-economical synthetic reaction. Classical reduction methods of polar bond often require reactive metal-hydride reagents in stoichiometric amount and produce copious waste. Hydrogenation of carbonyl compounds in particular provides 'green' approaches to synthetically important building blocks, such as alcohols and amines. We have designed and synthesized several ruthenium-based pincer catalysts for unprecedented hydrogenation reactions including: (1) amides to alcohols and amines, (2) biomass-derived di-esters to 1,2-diols and (3) CO 2 and CO derivatives to methanol. These atom-economical reactions operate under neutral, homogeneous conditions, at mild temperatures, mild hydrogen pressures, and can operate in absence of solvent with no generation of waste. The postulated mechanisms involve metal-ligand cooperation (MLC) by aromatization-dearomatization of the heteroaromatic pincer core.

Metal–ligand cooperation by aromatization–dearomatization as a tool in single bond activation

“…Our studies indicate that the mechanism of this reaction does not involve H 2 oxidative addition to the Ru(II) centre; rather, H 2 binds to the vacant coordination site trans to the hydride ligand, followed by proton transfer to the side arm. This bond activation process has led to the development of several ruthenium-catalysed hydrogenation and dehydrogenation reactions [11][12][13][14]16], the first ones being alcohol dehydrogenative coupling to yield esters and H 2 [17], the first example of hydrogenation of esters to alcohols under mild conditions (5 bar H 2 ) [19], and the unprecedented intermolecular dehydrogenative coupling of alcohols with amines to form amides and H 2 [20].…”

“…Pincer-type complexes of several transition metals exhibit MLC based on aromatizationdearomatization, including complexes of Ru, Fe, Co, Rh, Ir, Ni, Pd, Pt, Mn and Re, leading to facile activation of various chemical bonds. Upon deprotonation of pyridine-based pincer complexes at the methylenic group, dearomatization of the pyridine core takes place [11][12][13][14][15][17][18][19][20], as judged from crystal structures, which exhibit an exo-cyclic double bond. Activation of chemical bonds (Y-H = O-H, N-H, C-H, B-H, Si-H) by cooperation between the metal and the dearomatized ligand results in ligand aromatization, with no formal change in the oxidation state of the metal (scheme 2).…”

“…We have reported a new mode of metal-ligand cooperation involving aromatizationdearomatization processes of pyridine-based pincer complexes derived from PNP and PNN ligands (scheme 2), as summarized in recent reviews [11][12][13][14][15], leading to unusual bond activation processes and to novel, environmentally benign catalysis, including hydrogenation reactions of polar double and triple bonds, and several acceptorless dehydrogenation reactions. Applications of acceptorless dehydrogenation reactions in synthesis were recently reviewed [16].…”

Editorial of Special Issue Ruthenium Complex: The Expanding Chemistry of the Ruthenium Complexes

“…In an innovatory undertaking, graphene oxide was used to support ruthenium catalysts in order to activate self-healing in multifunctional materials that are able to simultaneously integrate the healing process with the advantageous properties of graphene-based materials [159]. Part of a large body of work concerns fundamental reactions studied anew with Ru complexes including hydrogenation and transfer hydrogenation; oxidation and hydroxylation; C–C, C–X and N–X bond formation; olefin metathesis and related C–C couplings; alkylation; arylation; isomerization; epimerization; condensation; cyclization; atom transfer radical reactions; oligomerization and polymerization [160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193]. Among non-metathetical reactions, the versatile and easy to handle transfer hydrogenation is often chosen as standard method for appraising and comparing the catalytic activity of ruthenium promoters [167,168,169,170,171,172,173].…”

Manganese catalyzed selective hydrogenation of cyclic imides to diols and amines

“…In the past, we and others have reported the catalytic hydrogenation of a variety of carbonyl compounds such as ketones, esters, acids, anhydrides, amides, carbonates, carbamates and urea derivatives, as reviewed. [1][2][3][4][5][6][7][8][9] Although examples of hydrogenation of cyclic imides were reported, selective hydrogenation of cyclic imides to diols and amines have received scant attention. Partial hydrogenation of cyclic imides to hydroxyamides were reported by Ikariya [10][11] and Bergens 12 using ruthenium catalysts.…”