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Network topology and cavity confinement-controlled diastereoselectivity in cyclopropanation reactions catalyzed by porphyrin-based MOFs

Abstract

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.

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Publication details

The article was received on 09 May 2019, accepted on 08 Oct 2019 and first published on 09 Oct 2019


Article type: Paper
DOI: 10.1039/C9CY00893D
Catal. Sci. Technol., 2019, Accepted Manuscript

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    Network topology and cavity confinement-controlled diastereoselectivity in cyclopropanation reactions catalyzed by porphyrin-based MOFs

    K. Epp, B. Bueken, B. Hofmann, M. Cokoja, K. Hemmer, D. E. De Vos and R. A. Fischer, Catal. Sci. Technol., 2019, Accepted Manuscript , DOI: 10.1039/C9CY00893D

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