Effects of axial C-donor ligands on metalloporphyrin-catalyzed carbene and nitrene transfer reactions

Metalloporphyrin complexes are well-known catalysts for atom and/or group transfer reactions that can be used to construct CC, CN or CO bonds. Much effort has been directed to the modification of porphyrin ligands, while the effect of axial ligands has received little attention. This review focuses on the effect of axial C-donor ligands, such as carbene and methyl ligands on metalloporphyrin-catalyzed carbene and nitrene transfer reactions. Metalloporphyrin complexes bearing axial carbene ligands were initially prepared for the study of reactive intermediates of carbene transfer reactions, but the presence of axial carbene and/or methyl ligands was found to significantly affect the rate of catalysis. In this work, the synthesis and characterization of metalloporphyrin complexes bearing axial carbene or methyl ligand(s) are described, followed by their reactivity in stoichiometric carbene transfer reactions (for those compounds bearing [an] axial carbene ligand[s]) and catalytic carbene and nitrene transfer reactions. A brief discussion of the participation of this axial C-donor ligand effect in the catalytic carbene transfer reactions is also presented.