TiO₂‐Supported Re as a General and Chemoselective Heterogeneous Catalyst for Hydrogenation of Carboxylic Acids to Alcohols

Abstract

TiO2-supported Re, Re/TiO2, was found to promote selective hydrogenation of carboxylic acids having aromatic and aliphatic moieties to the corresponding alcohols. Re/TiO2 showed superior results compared to other transition-metal-loaded TiO2 and supported Re catalysts for selective hydrogenation of 3-phenylpropionic acid. 3phenylpropanol was produced in 97% yield under mild conditions (5 MPa H-2 at 140 degrees C). Contrary to typical heterogeneous catalysts, Re/TiO2 does not lead to the formation of dearomatized byproducts. The catalyst is recyclable and shows a wide substrate scope in the synthesis of alcohols (22 examples; up to 97% isolated yield)

Rhenium‐Loaded TiO2: A Highly Versatile and Chemoselective Catalyst for the Hydrogenation of Carboxylic Acid Derivatives and the N‐Methylation of Amines Using H2 and CO2

“…[17] Accordingly,w ee xamined the relative adsorption affinities of COOCH 3 and benzene moieties on the catalysts urfaceb ye mploying methyla cetate and benzene as probe molecules. We have previously reported that the selectivity of the hydrogenationo fc arboxylic acid derivatives strongly dependso n the relative adsorption affinitieso ftheir functional groups,specifically on the benzene rings and the COOH groups.…”

“…Consequently,ah ighly versatile heterogeneous catalyst for the reduction of ab road variety of carboxylic and carbonic acid derivatives represents ah ighly desirable research target. [17] Notably,t his heterogeneous catalytic system did not produce any dearomatized byproducts, despite the difficulty of hydrogenating the COOH group. [12] Even though various catalysts of this nature have been proposed, [8,12] heterogeneous catalysts normally suffer from competitive product formation as ar esult of dearomatization of the aromatic rings and/or the over-reduction to give alkane products, which diminishes their chemoselectivity compared with homogeneousc atalysts.…”

Catalytic transformation of functionalized carboxylic acids using multifunctional rhenium complexes

“…Although both low- and high-valent Re species in hetero-multimetallic catalysts such as Re 2 (CO) 10 -Ru 3 (CO) 12 39 , Re 2 (CO) 10 -Rh/Al 2 O 3 39 , Re 2 O 7 -OsO 4 40 , ReO x -Pd/SiO 2 41 , ReO x /TiO 2 42 , and Re/TiO 2 43 have been investigated for CA hydrogenation 23, 33 , the specific role of Re in these catalysts remains unclear. Recent examples of ReO x /TiO 2 and Re/TiO 2 catalysis have shown high alcohol selectivity with a rather limited substrate scope, with diverse Re entities (Re 0 , Re III , Re IV , Re VI , and Re VII , calcined and reduced at 400–500 °C with H 2 ) on TiO 2 determined by XPS analysis 42, 43 .…”

“…Recent examples of ReO x /TiO 2 and Re/TiO 2 catalysis have shown high alcohol selectivity with a rather limited substrate scope, with diverse Re entities (Re 0 , Re III , Re IV , Re VI , and Re VII , calcined and reduced at 400–500 °C with H 2 ) on TiO 2 determined by XPS analysis 42, 43 . Even though heterogeneous ReO 3 promotes CA hydrogenation under high H 2 pressure ( P H2  = ca.…”

“…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.…”