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Synthesis of a mechanically planar chiral rotaxane ligand for enantioselective catalysis

Synthesis of a mechanically planar chiral rotaxane ligand for enantioselective catalysis
Synthesis of a mechanically planar chiral rotaxane ligand for enantioselective catalysis

Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed “mechanical chirality,” have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral rotaxanes remain scarce. Here, we describe a mechanically planar chiral rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction. Molecules that exist in non-identical mirror image forms are referred to as chiral. Chirality can arise because of various molecular features in which atoms are held in fixed orientations that are themselves chiral, and typically such “stereogenic units” are maintained by direct bonds between atoms. Molecular chirality can also arise by threading a dumbbell-shaped molecule through a molecular ring to generate a rotaxane. However, these molecules have not been investigated significantly because until recently they were extremely hard to make in one mirror image form. Here, we report the first example of a catalyst based on such a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in healthcare and other areas. Our results demonstrate that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We report an enantioselective catalyst based on a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in many areas. Our results demonstrate, for the first time, that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We achieve enantioselectivities for the Au I-catalyzed Ohe-Uemura cyclopropanation of benzoate esters comparable to previously reported covalent catalysts.

SDG9: Industry, innovation, and infrastructure, catalysis, chirality, gold, rotaxanes, stereoselective
2451-9308
994-1006
Heard, Andrew, W.
f01893de-d170-48db-8804-b0a83bc4f192
Goldup, Stephen
0a93eedd-98bb-42c1-a963-e2815665e937
Heard, Andrew, W.
f01893de-d170-48db-8804-b0a83bc4f192
Goldup, Stephen
0a93eedd-98bb-42c1-a963-e2815665e937

Heard, Andrew, W. and Goldup, Stephen (2020) Synthesis of a mechanically planar chiral rotaxane ligand for enantioselective catalysis. Chem, 6 (4), 994-1006. (doi:10.1016/j.chempr.2020.02.006).

Record type: Article

Abstract

Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed “mechanical chirality,” have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral rotaxanes remain scarce. Here, we describe a mechanically planar chiral rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction. Molecules that exist in non-identical mirror image forms are referred to as chiral. Chirality can arise because of various molecular features in which atoms are held in fixed orientations that are themselves chiral, and typically such “stereogenic units” are maintained by direct bonds between atoms. Molecular chirality can also arise by threading a dumbbell-shaped molecule through a molecular ring to generate a rotaxane. However, these molecules have not been investigated significantly because until recently they were extremely hard to make in one mirror image form. Here, we report the first example of a catalyst based on such a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in healthcare and other areas. Our results demonstrate that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We report an enantioselective catalyst based on a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in many areas. Our results demonstrate, for the first time, that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We achieve enantioselectivities for the Au I-catalyzed Ohe-Uemura cyclopropanation of benzoate esters comparable to previously reported covalent catalysts.

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Accepted/In Press date: 10 February 2020
e-pub ahead of print date: 9 March 2020
Published date: 9 April 2020
Additional Information: Funding Information: S.M.G. thanks the European Research Council (consolidator grant agreement no. 724987 ) and Leverhulme Trust ( ORPG-2733 ) for funding and the Royal Society for a Research Fellowship. S.M.G. is a Royal Society Wolfson Research Fellow. The authors would like to thank Dr. Marzia Galli and Dr. Jorge Meijide Suarez for helpful discussions and the latter for preparation of starting material S12. The authors thank Dr. Graham Tizzard of the National Crystallographic Service for helpful discussions around the SC-XRD data. The authors acknowledge the use of the IRIDIS High Performance Computing Facility and associated support services at the University of Southampton , in the completion of this work. Publisher Copyright: © 2020 The Author(s) Copyright: Copyright 2020 Elsevier B.V., All rights reserved.
Keywords: SDG9: Industry, innovation, and infrastructure, catalysis, chirality, gold, rotaxanes, stereoselective

Identifiers

Local EPrints ID: 439033
URI: http://eprints.soton.ac.uk/id/eprint/439033
ISSN: 2451-9308
PURE UUID: 364d1ca6-597c-4b08-9381-4b1b7c504a1f
ORCID for Stephen Goldup: ORCID iD orcid.org/0000-0003-3781-0464

Catalogue record

Date deposited: 01 Apr 2020 16:32
Last modified: 19 Mar 2021 02:45

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Contributors

Author: Andrew, W. Heard
Author: Stephen Goldup ORCID iD

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