Carbon monoxide as a reagent: A report on the role of N-heterocyclic carbene (NHC) ligands in metal-catalyzed carbonylation reactions

This report is a focused and critical essay on the effectiveness of M–NHC catalysts in reactions that specifically utilize carbon monoxide as a C1-carbon source. NHC ligands are touted as excellent trialkylphosphine (PR₃) mimics and are purported to improve existing phosphine based catalysts. One premise for using NHCs is that the need for excess ligand could be obviated in certain reactions. If true, then reactions involving CO as a reagent should be improved when a M–NHC complex is employed. Herein is a compilation of results that feature CO as a reagent in reactions such as: hydroformylation, hydroaminomethylation, carbonylation of aryl halides, oxidative carbonylation of amino and phenolic compounds, and the copolymerization of alkenes and CO. The aim of the report is to highlight reactions in which the ancillary NHC ligand is beneficial, detrimental, ineffective, or inconclusive in promoting the desired chemistry.