Lewis Acid-Promoted Heterogeneous Platinum Catalysts for Hydrogenation of Amides to Amines

Abstract

For the hydrogenation of a tertiary amide (N-acetylpiperidine), Lewis acidic support materials (Nb 2 O 5 and MoOx/TiO 2 ) increase the catalytic activity of Pt metal nanoparticles by a factor of > 100. In situ infrared (IR) study shows that coordination of the C=O group of amide to surface Lewis acid sites weakens the C= O bond, and the activated amide undergoes hydrogen transfer from H species on Pt sites. Among 19 types of heterogeneous catalysts tested, Pt/Nb 2 O 5 shows the highest catalytic activity. Pt/Nb 2 O 5 is generally effective for the selective hydrogenation of various tertiary amides and e-caprolactam to the corresponding amines under additive-free and solventless conditions and shows an order of higher turnover number (TON) than previously reported catalysts. Pt-MoOx/TiO 2 is the secondary most effective and is reusable 10 times after the reaction.The hydrogenation of carboxylic acids and their derivatives is a key transformation in the pharmaceutical and fine chemical industries and biomass conversion. [1, 2] The reaction is more difficult than the hydrogenation of carbonyl compounds because of the lower electrophilicity of the C=O group in carboxylic acid derivatives. Among the derivatives of carboxylic acid, carboxamides have relatively low electrophilicity of the C=O bond, [2a] and thus catalytic hydrogenation of amides by molecular hydrogen has been a challenging reaction. [1, 2] The synthesis of amines by reduction of the corresponding amides has been carried out by an unsustainable method using excess amount of a dangerous reductant such as LiAlH 4 . [1] Reduction of amides without the hydride reagents is a key research area in pharmaceutical industry. [3] Homogeneous catalysts for hydrogenation of amides by H 2 have been reported, [4][5][6][7][8][9][10][11][12][13] but most of them require acidic or basic additives in the reaction mixture as well as an expensive organic ligand. More importantly, most of the homogeneous systems lead to cleavage of CÀN bonds of amides to give alcohols and amines. [5][6][7][8][9][10][11][12][13] Klankermayer, Cole-Hamilton and co-workers [4] recently developed the first homogenous catalytic system, a Ru-Triphos complex with an acidic additive, CH 3 SO 3 H or HN(SO 2 CF 3 ) 2 , for the hydrogenation of amides to amines with preservation of the CÀN bonds. Early heterogeneous catalysts, such as Cu/Cr oxides, selectivity catalyzed hydrodeoxygenation of amides to amines under harsh conditions (20 À 30 MPa H 2 , ca 250 8C). [1] Ru/Re, Rh/Re and Ru/Mo nanoparticles reported by Fuchikami [14] and Whyman [15] and Rh-MoOx/SiO 2 + CeO 2 [16] are effective catalysts for this reaction under milder conditions, but these catalysts suffer from problems such as need of high H 2 pressure (7-10 MPa) and limited substrate scope. The TiO 2 -supported Pt/Re catalyst [17] was reported to catalyze the hydrogenation of some amides under low H 2 pressure (2 MPa) both in batch and in continuous flow reactors, but the system suffers from limited substrate scope and deactivation in t...

The Catalytic Reduction of Carboxylic Acid Derivatives and CO2 by Metal Nanoparticles on Lewis‐Acidic Supports

“…We have recently reported an example that could be used to rationalize catalytic activities on the basis of in‐situ FT‐IR techniques, and the thus obtained results could potentially be used as catalyst design guidelines . As mentioned in the introduction, given the low electrophilicity of the C=O group in the carboxylic acid derivatives, the catalytic activity of the C=O group should be increased upon activation with Lewis acids.…”

Mechanistic study of the selective hydrogenation of carboxylic acid derivatives over supported rhenium catalysts

“…In particular, most catalytic systems are limited because they preferentially promote hydrogenation of aromatic systems over carboxylic acid groups. [10][11][12][13][14] Because aromatic systems are present in many organic substances, this problem seriously limits the use of those hydrogenation processes in synthetic organic chemistry.…”

Palladium doping of In2O3 towards a general and selective catalytic hydrogenation of amides to amines and alcohols

“…Early catalysts based on copper-chromium oxides, 15 ReO 3 , 16 Raney catalysts, 17 PtO 2 , 18 and Pd-Re/high surface area graphite (HSAG) 19 catalyzed the hydrogenation of amides to amines under harsh conditions. Mitigation of reaction conditions was accomplished by applying more efficient bimetallic/bifunctional Rh-Re, 20 Ru-Mo, 21 Rh-Mo, 22 Ru-Re, 23 Pt-Re/TiO 2 , 24 Pd-Re/graphite, 25 Rh/MoO x -SiO 2 , Ni/LaAlSiO, 26 Pt/Nb 2 O 5 , 27 Pt/MoO x -TiO 2 27 and Ir/Mo-mesoporous SiO 2 (KIT-6) 28 catalysts. With this aim, a large screening of bi-and trimetallic systems was also performed, which allowed the identification of a relatively active catalyst based on Pt-Re-In supported on silica (or carbon).…”