Catalytic transformation of functionalized carboxylic acids using multifunctional rhenium complexes

Abstract

Carboxylic acids (CAs) are one of the most ubiquitous and important chemical feedstocks available from biorenewable resources, CO2, and the petrochemical industry. Unfortunately, chemoselective catalytic transformations of CHnCO2H (n = 1–3) groups into other functionalities remain a significant challenge. Herein, we report rheniumV complexes as extremely effective precatalysts for this purpose. Compared to previously reported heterogeneous and homogeneous catalysts derived from high- or low-valent metals, the present method involves a α-C–H bond functionalization, a hydrogenation, and a hydrogenolysis, which affords functionalized alcohols with a wide substrate scope and high chemoselectivity under relatively mild reaction conditions. The results represent an important step toward a paradigm shift from ‘low-valent’ to ‘high-valent’ metal complexes by exploring a new portfolio of selective functional group transformations of highly oxygenated organic substrates, as well as toward the exploitation of CAs as a valuable biorenewable feedstock.

Catalytic Hydrogenation of N‐protected α‐Amino Acids Using Ruthenium Complexes with Monodentate Phosphine Ligands

“… Ru complexes for the catalytic hydrogenation of carboxylic acids. Ru‐ 1 : this work; Ru‐ 2 and Ru‐ 3 : previous work …”

“…We report herein that ruthenium complexes of the type (PP)Ru, which contain two molecules of a monodentate phosphine, especially tris( para ‐fluorophenyl)phosphine (e. g., Ru‐ 1 in Figure ), effectively catalyze the hydrogenation of N‐protected‐α‐amino acids at relatively low temperature (100–120 °C). Although several examples of molecular catalyst systems based e. g. on [(Ph 2 P)CH 2 ] 3 CMe (Triphos)−Ru– and Co complexes ((PPP)M complexes), as well as our (PP)Ru and (PP)Re complexes, have been reported to promote the hydrogenation of carboxylic acids selectively to alcohols, these require relatively high hydrogenation temperatures (140–200 °C), which often results in racemization (epimerization) at the α‐chiral centers of natural α‐amino acids. Another notable issue associated with these methods is the involvement of notorious α‐amino aldehydes and/or their hydrate forms as short‐lived reaction intermediates, which undergo racemization at their α‐stereogenic center more readily than the parent α‐amino acids.…”

“…However, the chirality of the stereogenic center in 1 a – 1 g was consistently well preserved in all cases. Authentic (racemic and scalemic) samples of 2 a – g were obtained by (PP)Re‐catalyzed hydrogenation of 1 a – g (SI, Schemes S1 and S2).

…”

“…Although several examples of molecular catalyst systems based e. g. on [(Ph 2 P) CH 2 ] 3 CMe (Triphos)À Ru [8a-e] and Co [9] complexes ((PPP)M complexes [8] ), as well as our (PP)Ru [10] and (PP)Re [11] complexes, have been reported to promote the hydrogenation of carboxylic acids selectively to alcohols, [12] these require relatively high hydrogenation temperatures (140-200°C), which often results in racemization (epimerization) at the α-chiral centers of natural α-amino acids. Another notable issue associated with these methods is the involvement of notorious α-amino aldehydes [13] and/or their hydrate forms as short-lived reaction intermediates, which undergo racemization at their α-stereogenic center more readily than the parent α-amino acids.…”

Diferentes Perspetivas para a Valorização de CO2 a Álcoois Utilizando Catálise Heterogénea

“…Por exemplo, Braca et al [38] testaram diferentes catalisadores baseados em rénio na hidrogenólise de ésteres fórmicos a metanol. Por outro lado, os catalisadores de rénio têm sido igualmente utilizados na hidrogenação de grupos carbonilo e carboxílico, tanto em catálise heterogénea como homogénea, apresentando ótimos resultados em termos de conversão [39][40][41][42][43]. Mais recentemente, Ting et al [44] descreveram a conversão de CO 2 a metanol utilizando um catalisador de Re/TiO 2 [44].…”