Development of Silicon Carbide-Supported Palladium Catalysts and Their Application as Semihydrogenation Catalysts for Alkynes under Batch- and Continuous-Flow Conditions

Abstract

Silicon carbide (SiC)-supported palladium (Pd) catalysts [3% Pd/SiC and a 3% Pd-diethylenetriamine (DETA)/SiC complex] for chemoselective hydrogenation under batch- and continuous-flow conditions were developed. The alkyne, alkene, azide, nitro, and benzyloxycarbonyl-protected aromatic amine (N-Cbz) functionalities were chemoselectively reduced in the presence of 3% Pd/SiC. By contrast, benzyl ether, alkyl N-Cbz, epoxide, aromatic chloride, aromatic ketone, and tert-butyldimethylsilyl ether were tolerant to the 3% Pd/SiC-catalyzed hydrogenation. The combined use of 3% Pd/SiC and DETA demonstrated excellent chemoselectivity toward the semihydrogenation of various mono- and disubstituted alkynes under batch- and continuous-flow conditions. Furthermore, compared with the separate use of 3% Pd/SiC and DETA, the newly developed 3% Pd(DETA)/SiC-packed in a cartridge showed higher chemoselectivity toward the continuous-flow semihydrogenation of alkyne over 24 h.

Comparative Study of Batch and Continuous Flow Reactors in Selective Hydrogenation of Functional Groups in Organic Compounds: What Is More Effective?

“…Obtaining (Z)-alkenes from alkynes by their hydrogenation is an important reaction for constructing the precursor of bioactive or industrially valuable functional materials [ 68 , 69 ]. Yamada et al [ 69 ] developed a Pd catalyst supported on silicon carbide (3%Pd/SiC), which provides high chemical stability.…”

“…Obtaining (Z)-alkenes from alkynes by their hydrogenation is an important reaction for constructing the precursor of bioactive or industrially valuable functional materials [ 68 , 69 ]. Yamada et al [ 69 ] developed a Pd catalyst supported on silicon carbide (3%Pd/SiC), which provides high chemical stability. Chemoselective hydrogenation of alkynes was investigated over this catalyst at an ambient H 2 pressure and temperature (25 °C) in methanol (0.25 M) under batch and continuous flow conditions.…”

“…However, a quite low isolated yield of 4-aminostyrene (67%) was obtained in 2 h of the reaction. Yamada et al [ 69 ] assumed that this happened due to the competitive coordination of the aromatic amino group of the substrate and product to Pd species instead of DETA. Therefore, to avoid competitive coordination of amino substrates to Pd species, another catalyst with immobilized DETA on 3%Pd/SiC (3%Pd(DETA)/SiC) was developed.…”

“…Semihydrogenation of alkynes in a continuous flow reactor packed with 50 mg of catalyst (3%Pd/SiC) was carried out by pumping a solution of alkyne (0.5 mmol) and DETA (3.0 equivalents) in MeOH (0.25 M) into the catalyst cartridge. There is no explanation for using a higher amount of DETA in flow experiments in the paper [ 69 ]. It was found that disubstituted and monosubstituted alkyne can be selectively hydrogenated to Z-alkynes with excellent-to-high yields (80–95%) with a residence time of 1–2 min.…”

“…However, after that, it started to drop and reached 65% in 24 h. It was postulated that the catalyst deactivation could be associated with excess amounts of DETA, which strongly coordinates with 3% Pd/SiC, during the flow reaction, resulting in blocking active sites of the catalyst. Using 3% Pd(DETA)/SiC as the catalyst allowed obtaining 90% of the desired Z-alkene and remained unchanged for 24 h. Thus, Yamada et al [ 69 ] showed that when performing semihydrogenation of alkenes, the possible coordination of functional groups of additives or substrates with active catalyst species should be taken into account, both in batch and continuous flow reactors. The reaction conditions should be chosen in such a way that Z-alkene will be close to 90–100%.…”