The Design and Application of Ion-paired Ligands to Address Selectivity Challenges in Transition Metal Catalysed Oxygen and Nitrogen Transfer Reactions

Asymmetric transition metal catalysed reactions are commonly used to access enantioenriched compounds. Classical approaches in this field involve the use of chiral ligands on the metal to induce asymmetry and whilst a well-proven approach to enantiocontrol, it is not universally applicable. Our group has developed an alternative approach in which achiral anionic ligands are ion-paired to chiral cations based on readily available cinchona alkaloids and have been shown to induce enantioselectivity in challenging reaction types: iridium- catalysed C-H borylation, and rhodium-catalysed C-H amination and alkene aziridination. Chapter 2 explores the synthesis of novel ion-paired metalloporphyrins. Porphyrins are a privileged class of ligand that are challenging to render chiral. In this work, we executed the synthesis of novel porphyrin scaffolds featuring anionic sulfonate groups, which were ion- paired to chiral cations based upon cinchona alkaloids. We later utilised manganese and iron salts to form the corresponding metalloporphyrins. As initial proof-of-concept, we chose to investigate enantioselective epoxidation, and pleasingly good reactivity was seen for a range of substrates containing hydrogen-bond donor groups although poor enantioinduction was observed in the products (<5% ee). Various troubleshooting such as alternative substrates, linkers and metals were trialled, but unfortunately, no enantioselectivity was observed. Chapter 3 explores ion-paired rhodium tetracarboxylate ligands, developed within the group, in an enantioselective and chemoselective allylic amination reaction of trans alkenyl alcohols. Interestingly, the reactions with the state-of-the-art rhodium catalyst Rh2(esp)2 favour aziridination of the alkene. However, upon subjecting our anionic rhodium ligands paired to cinchona alkaloid derived chiral cations, the allylic amination product is favoured in typically high chemoselectivity (>20:1) and high enantiomeric excess (>85% ee) for a range of carbon length substrates featuring pendant alcohol groups. This work displays a rare example of chiral cations controlling both enantioselectivity and chemoselectivity in a transition metal catalysed reaction. A substrate scope was explored featuring differing electronic and steric profiles for the reaction (29 examples, up to 97% ee), as well as preliminary work exploring post functionalisation of the allylic amine products and extension to propargylic C-H amination of alkynols.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Adams, Hannah

Advisor dc:contributor.advisor

• Phipps, Robert

• Asymmetric Catalysis
• Ion-pairing
• Nitrogen Transfer
• Oxygen Transfer
• Porphyrin Ligands
• Transition Metal Catalysis

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/c9ede404-ca8c-4b23-9de1-e4bf8e4ae9f5/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.116312

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/380866