This review provides an overview of the catalytic activity of iron complexes of nitrogen ligands in driving carbene transfers towards CC, C–H and X–H bonds. The reactivity of diazo reagents is discussed as well as the proposed reaction mechanisms.

The protonation of commercially available porphyrin ligands yields a class of bifunctional catalysts able to promote the synthesis of N‐alkyl oxazolidinones by CO₂ cycloaddition to corresponding aziridines. The catalytic system does not require the presence of any Lewis base or additive, and shows interesting features both in terms of cost effectiveness and eco‐compatibility. The metal‐free methodology is active even with a low catalytic loading of 1 % mol, and the chemical stability of the protonated porphyrin allowed it to be recycled three times without any decrease in performance. In addition, a DFT study was performed in order to suggest how a simple protonated porphyrin can mediate CO₂ cycloaddition to aziridines to yield oxazolidinones.

The cycloaddition of CO₂ to N‐alkyl aziridines was efficiently promoted by the convenient TPPH₂/TBACl binary catalytic system. The metal‐free procedure was effective for the synthesis of differently substituted N‐alkyl oxazolidin‐2‐ones in yields up to 100 % and excellent regioselectivities (up to 99 %). The mechanism of the reaction was proposed based on a DFT study which indicated the formation of an adduct between TPPH₂ and TBACl as the effective catalytic active species.