Turning capsule catalysis inside out

Abstract

Coordination capsules offer a simple synthetic route to complex structures. These systems can be used to encapsulate a variety of different species, giving them potential uses in biology, as single molecule magnets, in gas storage as well as in catalysis. Almost all current catalytic approaches utilise binding the substrate in the capsule cavity in an analogous fashion to biology’s catalysts, enzymes, which activate substrates upon binding in an active site.

Substrate encapsulation allows the cavity’s microenvironment to influence various reaction factors such as enantio- and regioselectivity. The drawbacks of this approach include narrow substrate scope and the frequent occurrence of product inhibition, which limits genuine catalysis to quite specific reaction types.

In biology, some enzymes require the binding of a second species, a cofactor, to generate the active catalyst. Rather than the encapsulated guest acting as the substrate, the bound species can instead be treated as a cofactor. Varying the functionality of the guests generates a modular approach to accessing a multitude of capsule-guest systems with different, emergent catalytic properties.

Chapter 2 outlines the initial attempts at using p-quinones as cofactor mimics, whereby alteration of the bound guest led to the transformation of the bound p-quinone into a hydroxylated species. The formation of this species, along with the stabilisation of its conjugate base, means that each host-guest system generates two equivalents of protons, which are liberated into the bulk for catalysis.

Chapter 3 covers using the Pd2L4 capsules to stabilise the conjugate bases of weak acids to promote Brønsted acid catalysed reactions in the bulk, primarily the cyclisation of citronellal.

The hydrogen bond donor ability of the guest is enhanced by encapsulation, while also moving the substrate into the cage’s microenvironment.

Chapter 4 focusses on increasing the redox properties of electron deficient p-quinones upon formation of host-guest complexes with Pd2L4 capsules to promote electron transfer catalysis in the bulk. This is further built upon in Chapter 5, whereby p-quinones with extended conjugation, encapsulated in Pd2L4 capsules, generate complexes that can be activated by light to act as photoredox catalysts.