Efficient and low-cost metal-free Porphyrin/TBACl system for the CO2 valorization into N-alkyl and N-aryl oxazolidin-2-ones

Carbon dioxide is the primary greenhouse gas responsible for one-quarter of the atmospheric emissions. The need to reuse and valorize wastes to produce added value compounds has rendered the abundant CO2 one of the most attractive C1 source for the synthesis of fine-chemicals and pharmaceuticals [1]. Considering the importance in developing eco-friendly synthetic processes, high attention has been devoted to the CO2 cycloaddition to three-membered ring compounds, such as epoxides and aziridines to synthesize respectively cyclic carbonates and oxazolidin-2-ones with 100% of atom economy [2]. Unlike the commonly reported procedures, that usually involve harmful metal catalysts or promoters, we developed an eco-compatible, commercially available, and low-cost methodology able to efficiently promote the synthesis of N-alkyl and N-aryl oxazolidin-2-ones by the CO2 cycloaddition to aziridines [3]. The combination of TBACl (tetrabutyl ammonium chloride) and 1% mol of TPPH2 (tetraphenyl porphyrin) catalyzed the synthesis of a wide range of oxazolidin-2-ones in yields up to 99% and A/B regioselectivities up to 99:1, by employing a moderate CO2 pressure and temperature (1.2 MPa and 125 °C) (Scheme 1). Collected data revealed that the catalytic performance was only slightly influenced by the porphyrin steric features and was not affected by the electronic properties of the catalyst. In addition, the combination of experimental results and DFT calculations allowed suggesting a reaction mechanism in which the TPPH2/TBACl adduct played the key-role. The exoergonically formed adduct activated the aziridine ring towards the nucleophilic attack of the halogen atom by reducing the free energy barrier which is required for the uncatalyzed CO2 coupling to N-alkyl and N-aryl aziridines.