Sustainable Synthesis of Cyclic Carbonates from Terminal Epoxides by a Highly Efficient CaI2/1,3-Bis[tris(hydroxymethyl)-methylamino]-propane Catalyst

Abstract

The nonstopping increment of atmospheric carbon dioxide (CO₂) concentration keeps harming the environment and human life. The traditional concept of carbon capture and storage (CCS) is no longer sufficient and has already been corrected to carbon capture, utilization, and storage (CCUS). CCUS involves significant CO₂ utilization, such as cyclic carbonate formation, for its cost effectiveness, less toxicity, and abundant C1 synthon in organic synthesis. However, the high thermodynamic and kinetic stability of CO₂ limits its applications. Herein, we report a mild, efficient, and practical catalyst based on abundant, nontoxic CaI₂ in conjunction with biocompatible ligand 1,3-bis[tris(hydroxymethyl)-methylamino]-propane (BTP) for CO₂ fixation under atmospheric pressure with terminal epoxides to give the cyclic carbonates. The Job plot detected the 1:1 Ca²⁺/BTP binding stoichiometry. Furthermore, formation of a single crystal of the 1:1 Ca²⁺/BTP complex was confirmed by single-crystal X-ray crystallography. The bis(cyclic carbonate) products exhibit potentials for components in the non-isocyanate polyurethanes (NIPUs) process. Notably, this protocol shows attractive recyclability and reusability.

Scheme 1. Synthesis of Cyclic Carbonate from Terminal and Internal Epoxides

Scheme 2. Chemical Structures of Complexing Ligands

Scheme 3. Model Reaction for the Optimization Table

Figure 1

Variation of the proton chemical shifts of the mixture of CaI₂ and BTP in the Job plot experiments. The total molar concentration of CaI₂/BTP was held constant at 0.167 M (CaI₂ + BTP (0.1 mmol) in DMSO-d₆ (0.6 mL)). The CaI₂/BTP equivalent ratios were (a) 0:10, (b) 3:7, (c) 6:4, and (d) 9:1.

Figure 2

Molecular structure of the Ca²⁺/BTP complex in the crystal. Displacement ellipsoids correspond to 50% probability.

Scheme 4. Substrate Scope for CO₂ Fixation to Various Epoxides

Reactions were performed respectively in the presence of CaI₂ (5 mol %) and BTP (10 mol %) as the catalyst for mono-epoxides and CaI₂ (10 mol %) and BTP (20 mol %) for di-epoxides in anhydrous DMSO (5 M). Completed reaction time is attached. Isolated yields are given. Recrystallized from the mixture of 2b and 2c.

Figure 3

Reactivity test of the CaI₂/BTP catalyst system.