Coupling between epoxides and $CO_2$ catalyzed by Fe(III) and Al(III) complexes

Abstract

Although Carbon dioxide ($CO_2$) is considered to be a green house gas, it is abundant, easily available and relatively nontoxic C1 source. The contribution of $CO_2$ on global warming will be reduced by $CO_2$ utilization. Moreover, if $CO_2$ utilization reaction provides value-added products, the process will be used in industrial chemical production. One way to utilize $CO_2$ is the coupling reaction with epoxides. It is one of successfully industrialized $CO_2$ utilization methods. Among the coupling catalysts, metal catalysts with tripodal ligands showed remarkable reactivity. Inspired by the key concepts of the tripodal ligands, new $N_1O_3$ ligands were designed to have an exceptional reactivity and selectivity between poly- or cyclic carbonates. Additionally, epoxy amines were used as substrates to synthesize amino carbonates or oxazolidinones. As the newly designed catalysts can be synthesized in large scale without difficulty, it will be applicable for industrial applications.

Chapter 1. Selective Synthesis of Cyclic Carbonates Using Newly Designed Fe(III) Complexes Cyclic carbonates can be produced from the coupling reaction between epoxides and $CO_2$. They can be used as fuel additives, electrolytes or polar aprotic solvents. They are commercialized in 1950’s and their demand is gradually increasing. Therefore, the catalysts for cyclic carbonate production have been constantly developed. Among the catalysts, salen complexes of Co, Cr or Al are reported to show high reactivities. In 2013, an aluminum complex with a tripodal $N_1O_3$ ligand was reported to exhibit the highest activity. DFT calculation indicated that the cis-coordiantion site of the metal center stabilizes the transition state when the metal binds the intermediate formed from $CO_2$ and ring-opened. In order to maximize the ligand effect for the selectivity, a new type of $N_1O_3$ ligand was developed.

Chapter 2. Selective Synthesis of Polycarbonate Using Newly Designed Al(III) Complexes Polycarbonate is an easily processible biodegradable polymer with high heat resistance. Poly(cyclohexene)carbonate is known to have similar properties with polystyrene. It can also further functionalized with monomers containing isocyanate or lactone groups. The reaction mechanism of polycarbonate synthesis is similar to that of cyclic carbonate synthesis. The selectivity for poly or cyclic carbonate synthesis can be controlled by the choice of co-catalysts. In this study, newly designed ligands, used for cyclic carbonate synthesis, were used to synthesize aluminium complexes to selectively synthesize polycarbonates.

Chapter 3. Chemoselective Conversion of Epoxy Amine into Amino Carbonate or Hydroxyl Oxazolidinone The epoxy amine compounds contain two reactive functional groups, amine and epoxide groups. One way to convert the epoxy group is to form a carbonate by the coupling reaction with $CO_2$. Also, the amine group is nucleophilic to attack $CO_2$ to form an unstable carbamic acid. Intramolecular reaction between the short-lived carbamic acid and the epoxy group provides a oxazolidinone. Both products, the cyclic carbonate and the oxazolidinone, can be hydrolyzed to form aminodiols abundant in pharmaceuticals or natural products. In this study, chemoselective conversion of epoxy amines to amino carbonates or oxazolidinones was demonstrated. Additionally, as the chirality plays a crucial role in biological processes, stereochemistry of the products was clearly verified by using enantioenriched epoxy amines. This method can be applicable to developing new drug derivatives or natural products.