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Bifunctional porphyrin catalysts for the synthesis of cyclic carbonates from epoxides and CO2: structural optimization and mechanistic study.
J Am Chem Soc. 2014 Oct 29; 136(43):15270-9.JA

Abstract

We prepared bifunctional Mg(II) porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br(-), Cl(-), and I(-) counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1j-m, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized Mg(II) porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h(-1)). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1-6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with (18)O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br(-) on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the Mg(II) and Br(-) and a conformational change (induced-fit) of the quaternary ammonium cation.

Authors+Show Affiliations

Division of Chemistry and Biotechnology, Graduate School of Natural Science and Technology, Okayama University , Tsushima, Okayama 700-8530, Japan.No affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article

Language

eng

PubMed ID

25268908

Citation

Ema, Tadashi, et al. "Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates From Epoxides and CO2: Structural Optimization and Mechanistic Study." Journal of the American Chemical Society, vol. 136, no. 43, 2014, pp. 15270-9.
Ema T, Miyazaki Y, Shimonishi J, et al. Bifunctional porphyrin catalysts for the synthesis of cyclic carbonates from epoxides and CO2: structural optimization and mechanistic study. J Am Chem Soc. 2014;136(43):15270-9.
Ema, T., Miyazaki, Y., Shimonishi, J., Maeda, C., & Hasegawa, J. Y. (2014). Bifunctional porphyrin catalysts for the synthesis of cyclic carbonates from epoxides and CO2: structural optimization and mechanistic study. Journal of the American Chemical Society, 136(43), 15270-9. https://doi.org/10.1021/ja507665a
Ema T, et al. Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates From Epoxides and CO2: Structural Optimization and Mechanistic Study. J Am Chem Soc. 2014 Oct 29;136(43):15270-9. PubMed PMID: 25268908.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Bifunctional porphyrin catalysts for the synthesis of cyclic carbonates from epoxides and CO2: structural optimization and mechanistic study. AU - Ema,Tadashi, AU - Miyazaki,Yuki, AU - Shimonishi,Junta, AU - Maeda,Chihiro, AU - Hasegawa,Jun-ya, Y1 - 2014/10/16/ PY - 2014/10/1/entrez PY - 2014/10/1/pubmed PY - 2014/10/1/medline SP - 15270 EP - 9 JF - Journal of the American Chemical Society JO - J. Am. Chem. Soc. VL - 136 IS - 43 N2 - We prepared bifunctional Mg(II) porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br(-), Cl(-), and I(-) counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1j-m, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized Mg(II) porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h(-1)). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1-6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with (18)O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br(-) on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the Mg(II) and Br(-) and a conformational change (induced-fit) of the quaternary ammonium cation. SN - 1520-5126 UR - https://www.unboundmedicine.com/medline/citation/25268908/Bifunctional_porphyrin_catalysts_for_the_synthesis_of_cyclic_carbonates_from_epoxides_and_CO2:_structural_optimization_and_mechanistic_study_ L2 - https://dx.doi.org/10.1021/ja507665a DB - PRIME DP - Unbound Medicine ER -
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