- Network topology and cavity confinement-controlled diastereoselectivity in cyclopropanation reactions catalyzed by porphyrin-based MOFs
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In this work, we show that the stereoselectivity of a reaction can be controlled by directing groups of substrates, by network topology and by local cavity confinement of metal-organic framework (MOF) catalysts. We applied the porphyrin-based PCN-224(Rh), which contains no stereocenters in the cyclopropanation reaction using ethyl diazoacetate (EDA) as carbene source. When styrene and other non-coordinating olefins are used as substrates, high activity, but no diastereoselectivity is observed. Interestingly, conversion of 4-amino- and 4-hydroxystyrene substrates occurs with high diastereomeric ratios (dr) of up to 23 : 1 (trans : cis). We attribute this to local pore confinement effects as a result of substrate coordination to neighboring Rh-centers, which position the olefin with respect to the active site, causing a break of local symmetry of the coordinated substrate. The effect of local pore confinement was improved by using PCN-222(Rh) as catalyst, which is a structural analog of PCN-224(Rh) with characteristic Kagomé topology featuring shorter Rh-Rh distances. A remarkable dr of 42 : 1 (trans : cis) was observed for 4-aminostyrene. In this case, the length of the substrate corresponds to the average distance between two neighboring Rh centers within the pores of PCN-222(Rh), which drastically boosts the diastereoselectivity. This work showcases how diastereomeric control can be achieved by favorable substrate-catalyst interactions and thoughtful adjustment of confined reaction space using porphyrin-based MOFs, in which stereocenters are inherently absent.
- Epp, Konstantin,Bueken, Bart,Hofmann, Benjamin J.,Cokoja, Mirza,Hemmer, Karina,De Vos, Dirk,Fischer, Roland A.
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p. 6452 - 6459
(2019/11/20)
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- Highly Enantioselective Intermolecular Cu(I)-Catalyzed Cyclopropanation of Cyclic Enol Ethers. Asymmetric Total Synthesis of (+)-Quebrachamine
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A set of cyclic enol ethers derived from 2,3-dihydrofuran 35 and 3,4-dihydropyran 8 with a varying substitution pattern at the olefinic system were synthesized. Evans's ligand 5 with Cu(I)OTf was found to be an effective catalyst in the cyclopropanation reaction between cyclic enol ethers 14, 19, 28-31, and 33 and ethyl diazoacetate 6 to give diastereoselectivities up to exo/endo = 95:5 and enantioselectivities higher than 95% in nearly all cases. Because of the selective building of a quarternary carbon center and good yields in the formation of bicyclic structures 34c-h, the reaction was used as a key step in the asymmetric synthesis of (+)-quebrachamine 7, an indole alkaloid of the Aspidosperma family. After acid-induced ring opening of bicyclic compound 34f to lactone 40 followed by LiAlH4 reduction to the masked aldehyde 41, a reaction with tryptamine gave intermediate 42. This alcohol was efficiently converted into the indole alkaloid (+)-quebrachamine 7 in an overall yield of 37% starting from the chiral synthon 34f. Moreover it revealed the absolute configuration of the quarternary center of the cyclopropanation product 34f to be S.
- Temme, Oliver,Taj, Shabbir-Ali,Andersson, Pher G.
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p. 6007 - 6015
(2007/10/03)
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- EXCEPTIONALLY EFFECTIVE CATALYSIS OF CYCLOPROPANATION REACTIONS BY THE HEXARHODIUM CARBONYL CLUSTER
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Hexadecacarbonylhexarhodium exhibits exceptional activity as a cyclopropanation catalyst in reactions between ethyl diazoacetate and alkenes that are remarkably free of competing processes.
- Doyle, Michael P.,Tamblyn, William H.,Buhro, William E.,Dorow, Roberta L.
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p. 1783 - 1786
(2007/10/02)
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- Transition-Metal-Catalyzed Reactions of Diazo Compounds. 1. Cyclopropanation of Double Bonds
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Rhodium(II) and palladium(II) carboxylates are efficient catalysts for the cyclopropanation of olefins by diazo esters.Intramolecular competitions within diolefins and intermolecular competitions between pairs of monoolefins showed quite different cyclopr
- Anciaux, Andre J.,Hubert, Andre J.,Noels, Alfred F.,Petiniot, N.,Teyssie, Philippe
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p. 695 - 702
(2007/10/02)
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