Catalytic synthesis of heterocycles from carbon dioxide

Background
My group have developed world-leading catalysts based on metal(salen) complexes for the synthesis of cyclic carbonates from CO2 and epoxides and have extended this work to the synthesis of oxazolidinones from CO2 and aziridines. In 2010 we obtained the crystal structure of our most active catalyst ((salen)Al-O-Al(salen)), but earlier this year obtained a second structure in which the bridging oxygen atom was protonated. This led to the originally C2-symmetric catalyst becoming unsymmetrical, with one of the two aluminium atoms acting as a Lewis acid (towards a water molecule in the crystal structure) and being six-coordinate. This forced the salen ligand attached to this aluminium ion to sterically hinder the other aluminium ion, preventing it from also acting as a Lewis acid.

Objectives
In this project, the new structural information obtained from the latest crystal structure will be used to guide the mechanism based development of new and even more active metal(salen) based catalysts for cyclic carbonate and oxazolidinone synthesis.

Experimental Approach
The ability of various acids (including carbonic acid) to protonate the bridging oxygen in complex 1 will be investigated and the influence of steric and electronic effects within the salen ligand on the structural changes observed on protonation determined. Other reactions of the bridging oxygen atom such as alkylation and silylation will also be studied. The approach will be extended to related catalysts based on other Earth-crust abundant metals and to relatewd types of salen ligands.

Reaction kinetics will be monitored by NMR, HPLC or GC as appropriate. Hammett plots based on relative reaction rates will be constructed to investigate the build-up of charge in the transition state by incorporating substituted aromatic rings on either the carbon of an epoxide and either the carbon or nitrogen atom of aziridine substrates. Deuterated epoxides and aziridines will be prepared and used to investigate the stereochemistry of reactions and the magnitude of any secondary kinetic isotope effect. Finally, there is the opportunity for the student to carry out DFT calculation on the catalytic cycle and thus obtain training in computational mechanistic chemistry.

Novelty
The structural changes observed on protonation of complex 1 are completely novel and unexpected, especially the concept that one metal ion acting as a Lewis acid can sterically shut down the Lewis acidity of an otherwise identical metal ion at the opposite end of the catalyst.

Training
The student will receive training in the synthesis of metal salen complexes and aziridines on a multi gram scale. Catalyst testing will be carried out both at 1-50 bar CO2 pressures, providing experience of handling compressed gases and high pressure equipment. Reaction products will be purified by chromatography or distillation and analysed by 1H and 13C NMR, mass spectrometry and IR spectroscopy as well as by 19F NMR and X-ray crystallography when appropriate. The student will receive training in the interpretation of these data and in running NMR and IR spectra. The student will receive training in the analysis of kinetic data and Hammett plots (including error analysis) using excel or origin based spreadsheets. There is an opportunity for the student to gain experience of the use of DFT calculations to analyse catalytic cycles.

The student will deliver weekly presentations (with PowerPoint slides) at my group meetings and will give two presentations each year to the whole of the GCCE. The student will also be encouraged to attend relevant national and international conferences and present their results as lectures / posters.

All Chemistry research students have access to our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills: https://www.york.ac.uk/chemistry/postgraduate/idtc/

The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. The Department strives to provide a working environment which allows all staff and students to contribute fully, to flourish, and to excel: https://www.york.ac.uk/chemistry/ed/. This PhD project is available to study full-time or part-time (50%).

This PhD will formally start on 1 October 2020. Induction activities will start on 28 September.