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

Abstract

AbstractThis 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%).