Aluminium/boron exchange reactions for catalysis

Abstract

The multi-trillion-dollar chemical industry is reliant on catalysts to efficiently transform bulk materials into higher value chemicals. However, the most commonly used catalysts are made from precious heavy metals which are expensive, scarce and toxic. The need to switch to more Earth-abundant elements is becoming increasingly important for sustainability and economic reasons. Aluminium is the third most abundant element in the Earth’s crust – it is cheap, readily available and recyclable. Yet, the use of aluminium in catalysis is still currently limited as it is less thoroughly developed than transition metal-based systems.

Recently, aluminium catalysts have found applications in the borylation of unsaturated substrates. In these reactions, exchange of substituents between aluminium and boron is hypothesised to be a key step. Despite this, there is limited detailed insight into the processes that underpin these transformations and this hinders the development of the next generation of improved aluminium catalysts. In this thesis, I set out to gain detailed insight into this key Al/B exchange, providing useful applications as a result of these revelations. Chapter 1 gives context to the rest of the thesis by providing an overview of relevant stoichiometric and catalytic reactivity at aluminium.

In Chapter 2, aluminium catalysts that mediate alkene hydroboration with pinacolborane under mild conditions are developed. A surprising link between catalyst size and catalyst performance is revealed.

In Chapter 3, the mechanism of this protocol is examined in depth. A synergistic effect between aluminium and boron is discovered, which explains the dependence on catalyst size with the observed catalytic performance.

In Chapter 4, the potential of a favourable Al-C(sp3)/B-H exchange step is highlighted by switching to a borane with diminished Lewis acidity. This enables the first example of alkene hydroboration with HBdan. The mechanism is distinct to the analogous reaction with pinacolborane.

Finally, in Chapter 5, Al/B exchange is progressed beyond hydridoboranes. Compatible exchange pathways at aluminium with silylboranes are discovered, which enables convenient access to silylalanes. The properties of these silylalanes are investigated, and promising catalytically relevant reactivity is uncovered.