Axially Chiral Salts for Sensing and Catalysis

Axial chirality is typically exemplified by homobiaryl compounds such as BINAP and BINOL. These structures are ubiquitous in asymmetric catalysis, where they function as ligands or organocatalysts. The analogous hetero-biaryl compounds containing a N-C axis are formally cationic at nitrogen due to its trivalency. These are interesting since they bear the axial chirality of neutral homobiaryls, as well as the ability to form ion pairs and participate in supramolecular charge-charge interactions. We recently reported the first enantioselective synthetic methodology approach towards this class of compound. The N-arylquinolinium salts displayed high barriers to racemisation (>~40 kcal/mol) and solvatochromism in their UV-vis and fluorescence spectra.

We will synthesise a novel library of diverse cationic N+-C heterobiaryl atropisomers and explore their sensory and catalytic applications. Neutral axially chiral frameworks will be synthesised as previously reported in the literature, followed by quaternization to afford the corresponding novel salts. Benzimidazoles will be alkylated on N(3) to afford the corresponding benzimidazolium cations, while those substrates bearing a pyridone functionality will either be directly alkylated on oxygen or activated prior to cross-coupling at this position (e.g. Tf2O then Suzuki/Stille). We have already performed a proof-of-concept reaction using Meerwein’s salt, and the quaternization proceeds with complete retention of axial chirality. Temperature is crucial in these asymmetric syntheses, so where appropriate we will use our expertise in continuous flow chemistry to achieve thermal control. We will also explore novel approaches to N-C axes using Rh(II)-catalysed carbene insertion. An interesting extension would be to form axially chiral C+-C axes, which are known, stable intermediates in the synthesis of numerous triangulenes. We have shown that N-arylquinolinium salts display solvatochromism in their fluorescence spectra. We will explore this effect across the library of cationic compound classes using fluorescence and UV-vis plate readers to enable rapid data acquisition, allowing structure-activity relationships to be established. Chiroptical Properties. Of particular interest is chirosolvatochromism, where the cationic molecules display different absorption or emission behaviour in the presence of one or other enantiomer of a chiral solvent, or in the presence of chiral additives. This could provide a method for high-throughput optical measurement of ee, which can be a bottleneck in the development of catalytic asymmetric reactions. Solid (ssNMR, XRD) and solution-phase (NMR) techniques will be used to understand the structure of the complexes formed.

Eligibility

·       Candidates must have, or expect to obtain, a 2:1 or first-class degree in Chemistry or closely related discipline.

·       Due to funding restrictions, the position is only available for UK-resident candidates (UK and ROI nationals, or EU nationals that have

obtained settled status in the UK).

·       Full eligibility information can be viewed via: https://www.nidirect.gov.uk/articles/department-economy-studentships.

·       Candidates must be available to start the post by October 2022.

Closing date

Friday 25th February 2022.

How to Apply

Applications must be submitted online via the University's Postgraduate Application Portal:  Login - Applications Portal (qub.ac.uk)

For more information please contact: Dr Peter Knipe ([Email Address Removed]).