Organic and polymer synthesis using C1 feedstocks: The carbonylation of oxazolines and the alternating copolymerization of epoxides and carbon dioxide

Abstract

The use of C1 feedstocks in chemical synthesis is an atom-economical strategy for the production of polymers and small molecules. In particular, the utilization of carbon monoxide (CO) and carbon dioxide (CO₂) is of interest due to the ease of incorporation of the former and handling of the latter. Chemical transformations using biorenewable resources are also attractive, given the accessibility of these materials and the essentially inexhaustible supply. While CO only fits into the first category, CO ₂ fits into both, making it a very appealing starting material.

The synthesis of β-amino acids has garnered a great deal of attention, given the many uses of β-amino acids in pharmaceutical and medicinal applications. Typical syntheses rely on sophisticated starting materials or expensive catalyst systems. The carbonylation of oxazolines represents an atom-economical pathway to β-amino acid derivatives. The use of α-amino acids, biorenewable starting materials, in the synthesis of oxazolines allows the construction of a synthetic pathway that utilizes both biorenewable resources and C1 feedstocks, two goals in modern synthetic chemistry. The carbonylation of racemic or enantiopure oxazolines was accomplished using a silylcobalt precatalyst with benzyl alcohol as the activating agent. High conversion to product required short reaction times (6 h) and proceeded cleanly and efficiently.

The use of CO₂ in synthetic chemistry has expanded recently to include the alternating copolymerization of CO₂ and epoxides. The products are aliphatic polycarbonates, which are typically amorphous polymers. The physical properties of these materials lend them to certain applications, typically those requiring low decomposition temperatures. These materials have received a great deal of attention as possible alternatives to petroleum-based materials. Though CO₂ is a biorenewable resource, little work has focused on the use of a bio-derived epoxide. Using [(BDI)ZnOAc]-based catalysts, the alternating copolymerization of R- or S-limonene oxide (an epoxide derived from the biorenewable resource, limonene) and CO ₂ was realized. Selectivity for the trans-diastereomer in the copolymerization gives a highly regio- and stereoregular polymer. Molecular weight distributions are narrow for this copolymerization and moderate polymer molecular weights are obtained. Additionally, the combination of poly(R-limonene carbonate) with poly(S-limonene carbonate) in solution yields a polymer stereocomplex, with physical properties unlike either of the parent polymers.