Cycloaddition of CO2 to epoxides by highly nucleophilic 4-aminopyridines: establishing a relationship between carbon basicity and catalytic performance by experimental and DFT investigations†
The development of single-component halogen-free organocatalysts in the highly investigated cycloaddition of CO2 to epoxides is sought-after to enhance the sustainability of the process and reduce costs. In this context, the use of strongly nucleophilic single-component catalysts has been generally restricted to a limited selection of N-nucleophiles. In this study, predictive calculations of epoxide-specific carbon basicities suggested that highly nucleophilic 3,4-diaminopyridines possess suitable basicity to serve as active single-component catalysts for the cycloaddition of CO2 to epoxides. Indeed, experimentally, the most active compounds of this class performed efficiently for the conversion of epoxides to carbonates under atmospheric pressure outperforming the catalytic activity of traditional N-nucleophiles. Importantly, the 3,4-diaminopyridino scaffold could be easily supported on polystyrene and used as a recyclable heterogeneous catalyst under atmospheric CO2 pressure. Finally, the mechanism of the cycloaddition reaction catalyzed by several N-nucleophiles was investigated highlighting the importance of the 3,4-diaminopyridine nucleophilicity in competently promoting the crucial initial step of epoxide ring-opening without the addition of nucleophiles or hydrogen bond donors.