Robust bifunctional aluminium–salen catalysts for the preparation of cyclic carbonates from carbon dioxide and epoxides

Abstract

SummaryTwo new one-component aluminium-based catalysts for the reaction between epoxides and carbon dioxide have been prepared. The catalysts are composed of aluminium–salen chloride complexes with trialkylammonium groups directly attached to the aromatic rings of the salen ligand. With terminal epoxides, the catalysts induced the formation of cyclic carbonates under mild reaction conditions (25–35 °C; 1–10 bar carbon dioxide pressure). However, with cyclohexene oxide under the same reaction conditions, the same catalysts induced the formation of polycarbonate. The catalysts could be recovered from the reaction mixture and reused.

Highly Active Salen‐Based Aluminum Catalyst for the Coupling of Carbon Dioxide with Epoxides at Ambient Temperature

“…Many aluminum complexes containing salen‐based ligands used as catalysts for the coupling of CO 2 with epoxides have been reported in the literature . However, compared to the conditions developed in this study, previously reported reaction conditions for this transformation are much harsher.…”

“…Generally, salen‐based aluminum complexes have poor solubility in common organic solvents , . Thus, many salen‐based aluminum catalysts for the coupling of CO 2 and epoxides have a tert ‐butyl group at the 3‐position of the phenolate ring to improve their solubility in epoxides, which are common solvents for this transformation . Until now, aluminum complexes containing a methyl group at the 3‐position of the phenolate ring of the salen ligands have never been reported as catalysts for the cycloaddition of CO 2 to epoxides.…”

“…Finding new catalyst systems for the coupling of CO 2 with epoxides to make cyclic carbonates, which have broad applications as aprotic solvents, electrolytes for lithium‐ion batteries, monomers for polymers, and pharmaceutical intermediates, is one of the most active research areas in CO 2 conversion A variety of catalyst systems have been reported for this transformation, among which nontoxic catalysts based on earth‐abundant aluminum are among the most significant . Bidentate benzotriazole phenoxides, bidentate pyrazole‐based scorpionates, bidentate 8‐hydroxyquinoline,, tetradentate triphenolateamine, tetradentate porphyrin, tetradentate salen, tetradentate salenac, salalen,, and bidentate/tridentate/tetradentate aliphatic amino alcohols, have all been used extensively as chelating ligands ...…”

“…Bidentate benzotriazole phenoxides, bidentate pyrazole‐based scorpionates, bidentate 8‐hydroxyquinoline,, tetradentate triphenolateamine, tetradentate porphyrin, tetradentate salen, tetradentate salenac, salalen,, and bidentate/tridentate/tetradentate aliphatic amino alcohols, have all been used extensively as chelating ligands for aluminum‐based catalysts. Among these ligands, tetradentate salen compounds are the most popular ligands for aluminum‐based catalysts for the coupling of CO 2 with epoxides …”

“…Figure shows the simplified structure of the salen‐based aluminum catalysts for the coupling of CO 2 with epoxides. Salen ligands are very versatile due to the many opportunities for facile modification at R 1 , R 2 , X, and Y. Theoretically, many substituents and groups could be introduced into salen ligands; however, most of the modifications in salen‐based aluminum catalysts have been confined to the R 1 , R 2 , and X positions (Figure ) . In addition, systematic tuning at bridging site Y is limited to –CH 2 CH 2 –, 1,2‐C 6 H 4 , and (1 R ,2 R )‐cyclohexyl groups.…”