Two is better than one : photoswitchable [2.2]paracyclophane catalysts : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry, Massey University

Abstract

Traditional catalysts are inherently static, typically designed to adopt a fixed structure that facilitates a single chemical transformation. While this rigidity is suitable for the synthesis of known targets, it presents limitations in applications requiring adaptability. This project explores an alternative strategy: the development of dynamic catalysts whose activity can be modulated by light. By integrating photoswitchable azobenzene units into a [2.2]paracyclophane framework, catalysts were designed to change shape upon irradiation in a reversible manner. This enables a single catalyst to access multiple conformations, and potentially distinct reactivity profiles, through non-invasive light control, offering a powerful approach for programmable catalysis.

An azo coupling methodology was developed for this project, which enabled the synthesis of six analogues of gold(I) carbene complexes. The initial generation of catalysts were characterised in terms of their photophysical properties and the steric environment around the active site. The trans and cis isomers were tested in the cyclisation of a model substrate, revealing limited differences in selectivity between isomeric states. In response, a further five catalyst families were synthesised aimed at improving photo-responsivity and catalytic performance. These new systems were similarly evaluated, and their catalytic performance was compared to that of the original generation.

Limitations of the gold catalysts prompted a shift toward rhodium paddlewheel complexes. A racemic [2.2]paracyclophane-based acid was synthesised from an amine intermediate, and methods for enantiomeric resolution were investigated to access enantiopure complexes. Ultimately, attempts to complex with rhodium failed, prompting an investigation that suggested an azo switch interferes with metal coordination. However, the value of the methodology was shown by synthesising a gold(I) phosphine complex and analysing it in catalysis, showing similar results to the carbene ligands.

In conclusion, this PhD thesis details the synthesis and investigation of photoswitchable catalysts, supported by robust methodologies to access diverse catalyst families. Through structural, photophysical, and catalytic studies, this work lays the groundwork for light-responsive systems with dynamic reactivity. By harnessing light as a non-invasive trigger, it shines new light on the future of catalyst design.