Chiral Rhodium Nanoparticle-Catalyzed Asymmetric Arylation Reactions

Abstract

Conspectus The development of heterogeneous catalyst systems for enantioselective reactions is an important subject in modern chemistry as they can be easily separated from products and potentially reused; this is particularly favorable in achieving a more sustainable society. Whereas numerous homogeneous chiral small molecule catalysts have been developed to date, there are only limited examples of heterogeneous ones that maintain high activity and have a long lifetime. On the other hand, metal nanoparticle catalysts have attracted much attention in organic chemistry due to their robustness and ease of deposition on solid supports. Given these advantages, metal nanoparticles modified with chiral ligands, defined as “chiral metal nanoparticles”, would work efficiently in asymmetric catalysis. Although asymmetric hydrogenation catalyzed by chiral metal nanoparticles was pioneered in the late twentieth century, the application of chiral metal nanoparticle catalysis for asymmetric C–C bond-forming reactions that give a high level of enantioselectivity with wide substrate scope was very limited. This Account summarizes recent investigations that we have carried out in the field of chiral rhodium (Rh) nanoparticle catalysis for asymmetric arylation reactions. We initially utilized composites of polystyrene-based copolymers with cross-linking moieties and carbon black incarcerated Rh nanoparticle catalysts for the asymmetric 1,4-addition of arylboronic acids to enones. We found that chiral diene-modified heterogeneous Rh nanoparticles were effective in these reactions, with excellent enantioselectivities and without causing metal leaching, and that bimetallic Rh/Ag nanoparticle catalysts enhanced activity. The catalyst could be easily recovered and reused more than ten times, thus demonstrating the robustness of metal nanoparticle catalysts. We then developed a secondary amide-substituted chiral diene modifier designed as a bifunctional ligand that possesses a metal biding site and a NH group to activate a substrate through hydrogen bonding. This chiral diene was very effective for the Rh/Ag nanoparticle-catalyzed asymmetric arylation of various electron-deficient olefins, including enones, unsaturated esters, unsaturated amides and nitroolefins, and imines to afford the corresponding products in excellent yields and with outstanding enantioselectivities. The system was also applicable for the synthesis of intermediates of various useful compounds. Furthermore, the compatibility of chiral Rh nanoparticles with other catalysts was confirmed, enabling the development of tandem reaction systems and cooperative catalyst systems. The nature of the active species was investigated. Several characteristic features of the heterogeneous nanoparticle systems that were completely different from those of the corresponding homogeneous metal complex systems were found.

Rhodium-Catalyzed Asymmetric Conjugate Pyridylation with Pyridylboronic Acids

“…We began our study by investigating the asymmetric addition of 4-pyridylboronic acid (2a) to 2-cyclohexeneone (1a) under rhodium catalysis, and the results are summarized in Table 1. Consistent with the challenges encountered by others, 14 pyridylboronic acid resulted in poor yield when typical solvents 11 for rhodium-catalyzed conjugate arylation reaction were used (entries 1−3). Surprisingly, the use of ethanol as the solvent increased the yield from less than 13% to 42%, and the enantioselectivity was high with the rhodium/ chiral diene L1 catalyst 16a (entry 4, 85% ee).…”

Water-Soluble Noble Metal Nanoparticle Catalysts Capped with Small Organic Molecules for Organic Transformations in Water

“…Nanoscale metal nanoparticles have also gained considerable interest in recent years for their applications as catalysts for carbon–carbon coupling reactions attributable to the simplified separation and recycling processes involving, for example, centrifugation/decantation or (membrane) filtration. − The availability of Pd nanocatalysts with comparable activities to homogeneous Pd complexes offers an alternative approach for the reactions while avoiding the problems associated with purification and toxic waste production, which are significant concerns in large-scale synthetic processes. In addition, the developments in the synthetic protocols for water-soluble Pd nanocatalysts along with the significant benefits of catalysis applications in aqueous media allowed the investigations for water-soluble Pd nanoparticle catalysts on carbon–carbon coupling reactions to pick up the streams recently (Table ).…”

Synthesis of Rhodium Complexes with Chiral Diene Ligands via Diastereoselective Coordination and Their Application in the Asymmetric Insertion of Diazo Compounds into E−H Bonds

“…Chiral dienes (Scheme 1) have distinct electronic and steric properties which make them effective ligands for many reactions [1, 2] . In particular, rhodium complexes with chiral dienes are among the best catalysts for asymmetric addition of arylboronic acids to the enones, imines, and other polar double bonds [3–5] . Although these catalysts have shown a remarkable performance, their accessibility is still rather limited.…”

“…[1,2] In particular,r hodium complexes with chiral dienes are among the best catalysts for asymmetric addition of arylboronic acids to the enones,i mines,a nd other polar double bonds. [3][4][5] Although these catalysts have shown aremarkable performance,t heir accessibility is still rather limited. Only several chiral dienes are readily available via transformations of natural terpenes.…”