Exploiting chirality in mechanically interlocked molecules for new applications in nanotechnology

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In pioneering publications in Science(1) and J. Am. Chem. Soc.(2) molecular knots have been shown by the Leigh group to have promising catalytic properties due to their well-defined cavities and well-expressed chirality.(1,2) Pentafoil knots, in particular, show very strong affinities for halide anions as a combined result of the multiple hydrogen bond and coulombic interactions that can be utilised for anion binding catalysis.(3) Furthermore, the catalytic activity can be allosterically switched on and off by the addition or removal of metal ions that template the cavity formation in a manner reminiscent of allosteric control in enzymes.(1)This project will aim to prepare a range of molecular knots that contain photoredox-active Ir(III) centres in place of the previously used first row transition metal centres.(1,4 )The iridium centres will play a dual role: Firstly, the relative inertness of Ir(III) allows the isolation of the starting materials as single enantiomers.(5) Due to the required symmetry of the circular helicates, the use of an enantiopure Ir(III) complex should result in the stereoselective synthesis of the knot precursor. Secondly, cyclometallated Ir(III) complexes bearing a bipyridine ancillary ligand are well documented as effective visible light photoredox catalysts.(6)By incorporating photoredox active components into a helicate of single-handedness we aim to achieve stereoselective dual photoredox/anion binding catalysis. As an example, the two step nucleophilic attack of tetrahydroisoquinolines can be promoted by a combined mixture of a photoredox catalyst and an enantioselective hydrogen bond donor catalyst.(7) This process could be achieved in a one pot procedure using Ir(III) knots. We propose controlling the reaction via allosteric interactions using different metal ions as signalling systems. The removal of first row transition metal ions will prevent the system from acting as an anion binding catalyst whilst the photoredox properties will be unaffected. The addition of Fe(II) to template the cavity will allow anion binding but quench the photoredox process. Finally, the addition of Zn(II) should allow both photoredox and anion binding catalysis to work in tandem.This project aims to merge a range of important and emerging areas including molecular knots, photoredox catalysis and switchable catalysis to provide systems capable of catalysing reaction pathways with strict control over both the stereochemistry and regiochemistry in a manner reminiscent of biological systems. Molecular knots are an emerging field with promising potential for switchable catalysis. Chemical topology provides a to date untapped strategy for retaining molecular connectivity whilst altering function. Any progress in using chemical topology to alter reaction pathways would be at the forefront of the field. Whilst the use of photoredox catalysis as a green and mild synthetic method is rapidly on the rise, stereoselective photoredox processes are still in their infancy. Any enantioselectivity achieved using photoredox catalysis would be a significant addition to the current state-of-the-art. Nature is able to finely tune the reaction rate of many complex pathways via allosteric interactions with a wide range of different signalling molecules. The ability to alter synthetic reaction pathways opens the possibility of creating smart systems capable of responding to different stimuli. This project aims to combine a range of emerging technologies to create new and exciting functional catalysts for future applications.1. V. Marcos et. al., Science 2016, 352, 1555-1559.2. G. Gil-Ramirez et. al., J. Am. Chem. Soc. 2016, 138, 13159-13162.3. J.-F. Ayme et. al., J. Am. Chem. Soc. 2015, 137, 9812-9815.4. D. A. Leigh, et. al., Nat. Chem. 2014, 6, 978-982.5. O. Chepelin et. al., J. Am. Chem. Soc. 2012, 134, 19334-19337.6. C. K. Prier et. al., Chem. Rev. 2013, 113, 5322-5363.7. G. Bergonzini et. al., Chem. Sci. 2014, 5,1-60