We show here that attaching -NH2, -NHCH3, or -N(CH3)2 to ethylene oxide can dramatically reduce the CO2 cycloaddition barrier, from 69.5 kcal/mol (R = -H) down to 22.1 kcal/mol [R = -N(CH3)2], which may enable CO2 fixation under milder conditions without the help of catalysts. A joint analysis of local charges, frontier orbital energies, molecular electronegativity, and partial electron transfer explains how these substituents facilitate CO2 cycloaddition to ethylene oxide. The distortion/interaction-activation strain model (D/I-ASM) simulation reveals that the computed low reaction barrier results from the decreased activation strain energy and increased intermolecular interaction energy in the transition state. Density functional theory calculations show that -N(CH3)2-monosubstituted ethylene oxide (NEO) can greatly lower the energy threshold for CO2 sequestration. NEO can also work with the common organic catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) that assists CO2 for further conversion into dimethyl carbonate (via alcoholysis) and N,N'-dimethylurea (via ammonolysis) with maximal barrier heights as low as 24.2 and 21.9 kcal/mol, respectively. The facile coupling of NEO with CO2 and the undemanding alcoholysis/ammonolysis of NCC with TBD would promise the inclusion of amino functionalities to small-molecule-based epoxides, or polymeric epoxy resins, in the fixation and further economic conversions of CO2.
© 2023 The Authors. Published by American Chemical Society.