Iridium complexes of N-heterocyclic carbenes in C–H borylation using energy efficient microwave technology: influence of structure, ligand donor strength and counter ion on catalytic activity

Bridged and unbridged N-heterocyclic carbene (NHC) ligands were metalated with [Ir(COD)Cl]₂ to give iridium(I) mono- and biscarbene substituted catalysts [Ir(COD)NHC(Cl)] and [Ir(COD)(NHC)₂][X] (X: I, PF₆, BF₄, CF₃COO, OTf). The prepared NHC-complexes were tested in the C–H borylation reaction of aromatic carbons with bis(pinacolato)diboron (B₂pin₂) and pinacolborane (HBpin). The use of microwave technology in this study not only facilitates a time efficient screening of a wide range of influences such as ligand σ-donor strength and structural motif as well as the effects of the complex counter ion, but also provides an energy efficient heating source. Catalyst 6^TFA, which features a chelating NHC ligand, proved to be most effective catalyst and further investigations with this complex in the borylation of mono- and disubstituted benzene derivatives exploring chemo- and regioselectivity were undertaken.