Cycloaddition of CO2 to epoxides by highly nucleophilic 4-aminopyridines: establishing a relationship between carbon basicity and catalytic performance by experimental and DFT investigations

Abstract

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.