Heavier group 13 based frustrated Lewis pairs for catalytic C–H borylation

Abstract

Research into catalysis has always been of great interest to both academic researchers and in industry, due to the great utility that catalysts possess. In particular, research into catalysts that are based on more earth‐abundant elements has gained increasing attention, due to the advantages these have, namely lower cost and generally lower toxicity, relative to the currently most commonly used platinum group based catalysts. Frustrated Lewis pairs (FLPs) are one of the most researched class of molecules in this field of earth‐abundant element catalysis. They are highly reactive molecules comprised of a Lewis acid and a Lewis base that are unable to form an adduct due to steric constraints. This leads to remarkable reactivity when these pairs react with small molecules, activating many E–H bonds. They can then subsequently deliver these activated molecules to unsaturated molecules such as imines in a catalytic manner.

Despite the already significant amount or research into the field of FLPs, there is a notable lack of diversity in the elements chosen for the Lewis acidic and basic centres of the molecules, with more focus on the surrounding architecture of the FLP. Most FLPs utilise phosphorus or nitrogen as the Lewis basic element and boron as the Lewis acidic element. As such, the aim of this work is to investigate less utilised elements for the Lewis acidic portion of the FLP, in particular, other members of group 13.

Chapter 1 is an introduction that gives an overview of main‐group catalysis as a whole and also covers methods for C–H borylation of both aryl compounds and also terminal alkynes. This is to provide context to the work presented in the following chapters.

Chapter 2 introduces two target novel aluminium based FLP compounds and describes the synthetic steps on route towards them. This leads to the characterisation of several novel FLP compounds although the two target compounds were not synthesised due to issues with ligand exchange steps on the proposed synthetic routes. Computational analysis of these ligand exchange reactions is also presented, to explain the unexpected results of these reactions. Chapter 3 focuses on the usage of the novel FLP species synthesised in chapter 2 as precatalysts for the C–H borylation of both aryl compounds and terminal alkynes, examining how to generate a catalytically active species, optimisation of conditions for the catalytic reactions, and the scope of reactivity with both classes of substrate.

Chapter 4 investigates in detail the mechanism of the catalytic terminal C–H borylation of alkynes presented in chapter 3. Also examined is the competing mechanism of hydroboration of the alkyne triple bond, which is the usual reactivity for aluminium hydride compounds. By looking at both of these mechanisms an explanation of the catalystselectivity towardsthe terminal C–H borylation over the hydroboration is given.

Additionally, examination of how the catalytically active species are generated under reaction conditions is carried out. Additionally, experimental work was carried out to attempt to isolate some of the on cycle species to provide additional evidence for the proposed mechanism. However, these structures could not be isolated, presumably due to their highly reactive nature and instead products from overreaction were isolated.

Computational analysis is used to attempt to explain how these overreaction products form. From all of the above results an in depth model of the catalytic reaction is presented.

Chapter 5 presents a computational investigation into other potential new FLP species, with a focus on finding candidates to perform C–H borylation of the widest range of aryl compounds possible. Therefore the C–H activation step has been calculated for a wide range of potential FLP candidates, with changes in the steric bulk of the FLP, changes to the Lewis basicity of the FLP, and also changes to the Lewis acidity of the FLP examined to extract trends in how the reactivity changes and therefore propose a set of possible candidates for experimental synthesis and trialling.