Highly Versatile Catalytic Hydrogenation of Carboxylic and Carbonic Acid Derivatives using a Ru-Triphos Complex: Molecular Control over Selectivity and Substrate Scope

Abstract

The complex [Ru(Triphos)(TMM)] (Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane, TMM = trimethylene methane) provides an efficient catalytic system for the hydrogenation of a broad range of challenging functionalities encompassing carboxylic esters, amides, carboxylic acids, carbonates, and urea derivatives. The key control factor for this unique substrate scope results from selective activation to generate either the neutral species [Ru(Triphos)(Solvent)H2] or the cationic intermediate [Ru(Triphos)(Solvent)(H)(H2)](+) in the presence of an acid additive. Multinuclear NMR spectroscopic studies demonstrated together with DFT investigations that the neutral species generally provides lower energy pathways for the multistep reduction cascades comprising hydrogen transfer to C═O groups and C-O bond cleavage. Carboxylic esters, lactones, anhydrides, secondary amides, and carboxylic acids were hydrogenated in good to excellent yields under these conditions. The formation of the catalytically inactive complexes [Ru(Triphos)(CO)H2] and [Ru(Triphos)(μ-H)]2 was identified as major deactivation pathways. The former complex results from substrate-dependent decarbonylation and constitutes a major limitation for the substrate scope under the neutral conditions. The deactivation via the carbonyl complex can be suppressed by addition of catalytic amounts of acids comprising non-coordinating anions such as HNTf2 (bis(trifluoromethane)sulfonimide). Although the corresponding cationic cycle shows higher overall barriers of activation, it provides a powerful hydrogenation pathway at elevated temperatures, enabling the selective reduction of primary amides, carbonates, and ureas in high yields. Thus, the complex [Ru(Triphos)(TMM)] provides a unique platform for the rational selection of reaction conditions for the selective hydrogenation of challenging functional groups and opens novel synthetic pathways for the utilization of renewable carbon sources.

Multifaceted catalytic hydrogenation of amides via diverse activation of a sterically confined bipyridine–ruthenium framework

“…Cole-Hamilton 12, 13 , Leitner/Klankermayer 13–15 , and Beller 16 have developed different (triphos)Ru catalyst systems (triphos = CH 3 C[CH 2 PPh 2 ] 3 ; Ph = C 6 H 5 ) for selective cleavage of the C=O bond of amides. When combined with catalytic Yb(OSO 2 CF 3 ) 3 , hydrogen pressure ( P H2 ) can be reduced as low as ca.…”

“…Use of a combination of ( P , C , P )Ir pincer complex and stoichiometric B(C 6 H 5 ) 3 was also proved to be effective for inducing the C=O bond cleavage 17 . In contrast, Ru complexes for the selective cleavage of the C–N bond of amides were intensively studied by Ikariya (( P , N H)Ru) 18–21 , Milstein ([ P ,( N , N ) bpy ]Ru, where bpy = bipyridine; ( N , N ) bpy  = bipyridine nitrogens) 22–25 , Bergens ([( P , N H 2 )( P , N H 2 )]Ru) 26–28 , Leitner/Klankermayer, ((triphos)Ru) 15 , and Beller (( P , N H, N )Ru) 29 . Milstein’s milestone discovery, in which deprotonation at the 2-(pyridyl)methylene unit in the ( P , N py , P)Ru pincer complexes induces an active catalyst 30 , has had a great impact on the ensuing molecular design of hydrogenation catalysts (py = pyridine; N py  = pyridine nitrogen).…”

Catalytic transformation of functionalized carboxylic acids using multifunctional rhenium complexes

“…While aliphatic CAs generally represent excellent substrates for the present hydrogenation strategy, the hydrogenation of benzoic acid (CA- w ) has rarely been examined 18–21, 43 . Indeed, hydrogenation of CA- w with Re- a proceeded sluggishly even under slightly harsher conditions ( P H2  = 4 MPa, 180 °C, 24 h), affording benzyl alcohol (AL- w ) in 29% yield (Fig.…”

“…For comparison, AL- w is generated in 93% (95% selectivity) and 62% yield under harsher conditions using a Ru-triphos (2 mol%, P H2  = ca. 5 MPa, 220 °C, 24 h) 21 and Co-triphos complex (2.5 mol%, P H2  = ca. 8 MPa, 100 °C, 22 h) 19 , respectively.

Figure 3Improved chemoselective hydrogenation of CAs and hydrogenolysis of the resulting α-alkoxy alcohols using Re-b.

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Homogeneous Catalyzed Reactions of Levulinic Acid: To γ‐Valerolactone and Beyond

“…[52] The TMM ligand appears to be very labile and readily decomplexes under reducing conditions generating the active catalyst. Near quantitative yields of 1,4-PDO were obtained from levulinic acid and methyl levulinate at 50 bar H 2 and 140 o C (table 2, entry 10).…”