Ligand Assisted Rhodium Catalyzed Selective Semi‐hydrogenation of Alkynes Using Syngas and Molecular Hydrogen

Abstract

This work reports the Rhodium catalyzed selective semi‐hydrogenation of alkynes to (E) and (Z)‐alkenes in the presence of various Buchwald phosphine ligands such as S‐Phos (L1), t‐Bu XPhos (L2), Ru‐Phos (L3), Johnphos (L4) and DavePhos (L5). Selective formation of E‐alkenes in high activity with use of H2 gas (5 bar) is an alternative method for conventional Birch reduction. High selectivity obtained towards Z‐alkenes with use of syngas (10 bar) is an alternative method for Lindlar reaction. (E)‐alkenes were obtained in high yield due to the steric crowding present in L4 ligand. The carbon monoxide present in syngas controls over‐hydrogenation of alkynes and (Z)‐alkenes were obtained as major product due to cis addition of molecular hydrogen to alkynes. The use of additives, base, co‐catalyst and excess use of chemicals is prevented with this protocol for semi‐hydrogenation reaction. With this simple and effective catalyst system and mild reaction conditions, internal as well as terminal alkynes were smoothly converted to respective alkenes in high yield (>90%).

cis‐Selective Transfer Semihydrogenation of Alkynes by Merging Visible‐Light Catalysis with Cobalt Catalysis

“…To date, several homogeneous catalytic systems have been developed for semihydrogenation of alkynes. The transition metals involved in these systems are Pd, Ru, Rh, Ir, V, Ni, Co, Cu, Mn, and even Fe . Despite these significant achievements, many of the above mentioned procedures face limitations, such as lengthy synthesis process for catalysts, elevated reaction temperature, by‐product formation due to over‐reduction, and tedious operation of H 2 usage.…”

Recent Progress in Pd-Based Nanocatalysts for Selective Hydrogenation

“…Besides, H + /H – pairs tend to kinetically reduce polar groups, which greatly improves the chemical selectivity of hydrogenation. 26 For example, Zheng’s group found that the Pd/Cu 2 O catalyst showed excellent performance for semihydrogenation of terminal alkynes. This was mainly because the unique interface structure of Pd/Cu 2 O and Pd 1+ promoted the heterolysis of H 2 at Pd–O–Cu sites.…”

“…Due to the noncompetitive adsorption of the substrate and H 2 , there is sufficient space for the activation of H 2 . Besides, H + /H – pairs tend to kinetically reduce polar groups, which greatly improves the chemical selectivity of hydrogenation . For example, Zheng’s group found that the Pd/Cu 2 O catalyst showed excellent performance for semihydrogenation of terminal alkynes.…”

Cu Decorated Rh/TiO2 Catalyst for Selective Hydrogenation of Phenylacetylene to Styrene

“…Selective hydrogenation of alkynes such as phenylacetylene (PA) to styrene (ST) constitutes a great significance in both industry and scientific research [1–4] . It is an effective method to remove the by‐product during the production process of polyethylene and polystyrene [5,6] .…”