Mechanistic Understanding of the Heterogeneous, Rhodium-Cyclic (Alkyl)(Amino)Carbene-Catalyzed (Fluoro-)Arene Hydrogenation

Abstract

Recently, chemoselective methods for the hydrogenation of fluorinated, silylated, and borylated arenes have been developed providing direct access to previously unattainable, valuable products. Herein, a comprehensive study on the employed rhodium-cyclic (alkyl)­(amino)­carbene (CAAC) catalyst precursor is disclosed. Mechanistic experiments, kinetic studies, and surface-spectroscopic methods revealed supported rhodium(0) nanoparticles (NP) as the active catalytic species. Further studies suggest that CAAC-derived modifiers play a key role in determining the chemoselectivity of the hydrogenation of fluorinated arenes, thus offering an avenue for further tuning of the catalytic properties.

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“… 148 , 149 Moreover, CAAC-derived products adsorbed on Rh nanoparticles were shown to play a key role in providing high chemoselectivity of fluorinated arenes hydrogenation. 259 Remarkably, azolium cations can effectively stabilize anionic [PdX 3 ] – complexes released from Pd/NHC precatalysts in Mizoroki–Heck reactions at low Pd loadings (0.1 mM concentrations and below). 62 Ionic complex ([NHC–Ph] + ) 2 [Pd 2 X 6 ] 2– , formed by fast Ph–NHC coupling and isolated from the reaction mixture, was recognized as a new type of ionic palladium precatalysts for Mizoroki–Heck reaction.…”

Selective hydrogenation of fluorinated arenes using rhodium nanoparticles on molecularly modified silica

“…15 While the present study was in progress, the Glorius group reported that the Rh-carbene complex acts as pre-catalyst (in agreement with studies by Bullock et al), 16 decomposing under reaction conditions to form polydisperse Rh NPs (2-10 nm) on SiO 2 . 17 It was demonstrated that the presence of the carbene ligand in the precursor was required to obtain materials exhibiting high chemoselectivity. High yields of fluorinated cyclohexanes were obtained using the resulting Rh@SiO 2 material at Rh loading of 1-5 mol% (relative to substrate) and H 2 pressure of 50 bar within 24 h. The stability and recyclability of the catalytic system was not yet addressed in this study.…”

“…High yields of fluorinated cyclohexanes were obtained using the resulting Rh@SiO 2 material at Rh loading of 1-5 mol% (relative to substrate) and H 2 pressure of 50 bar within 24 h. The stability and recyclability of the catalytic system was not yet addressed in this study. 17 Despite these promising advances, the development of catalytic systems possessing high activity, selectivity, stability, and recyclability for the hydrogenation of fluoroarenes to fluorocyclohexanes remains a major challenge. In this context, working with metal NPs immobilized on molecularly modified supports seems particularly attractive since these materials were demonstrated to be versatile and suitable for the production of catalytic systems with tailor-made reactivity for challenging hydrogenation and hydrodeoxygenation reactions.…”

Magnetically separable and efficient platinum catalyst: Amino ligand enhanced loading and Fe2+ facilitated Pt0 formation

“…Noble metal catalysts are widely applied in several industrialized reactions such as CO oxidations, [1] C H oxidation reactions, [2] hydrogenation, [3] and hydrosilylation. [4] However, they suffer from problems of limited reserves and high costs.…”