A Natural Bifunctional Catalyst for CO2 Valorization

Sensitivity for climate change and related issues, e.g. the impact of greenhouse gas emissions, has significantly grown in the last decades and consequently the scientific community has increased its attention to research fields associated with this topic. A relevant example is the use of CO2 as a C1 building block for producing fine chemicals. In this context, the cycloaddition of CO2 to three-membered heterocycles, like epoxides and aziridines, has particularly attracted researchers because it allows the synthesis of high added value products (cyclic carbonates and oxazolidinones) with a 100% of atom economy. Our research group has widely explored this research field, looking for sustainable catalytic systems. Recently, we found that protonated porphyrins like TPPH4Cl2 (TPP = dianion of tetraphenyl porphyrin) efficiently catalyze the cycloaddition of CO2 to N-alkyl aziridines. These bifunctional, metal-free systems are effective in the absence of any additive, but they still need quite harsh conditions (100 °C, 1.2 MPa CO2 pressure). Hence, a more economically and environmentally sustainable catalyst is desirable. Trying to take a step forward into this direction, we considered the possibility to employ a catalyst derived from a natural compound and we have lately discovered that cinchonine hydrochloride, easily obtained by treating the natural alkaloid cinchonine with HCl, is highly appealing from this point of view. Indeed, the abovementioned system can promote the cycloaddition of CO2 to various N-alkyl aziridines even at room temperature and atmospheric pressure, with yields up to 80% and > 96% A/B regioselectivity. These preliminary results are promising, even if additional optimization is needed because none of the tested substrates has been completely converted. Furthermore, considering that cinchonine hydrochloride is a chiral compound, the possibility to exploit it for achieving enantiopure oxazolidinones should be explored.