Developing the Catalytic Applications of 1,3,2-Diazaphospholenes

The development of new catalytic methods for organic synthesis is a critical endeavour in order to obviate the economic and ecological drawbacks of stoichiometric organic synthesis. While catalysis can offer significant advantages in terms of atom-economy, step-economy and energy input, the catalysts themselves may suffer from pitfalls such as toxicity or scarcity. Consequently, there is a growing need to develop catalysts based on more sustainable materials such as phosphorus. This thesis is an account of the work done in developing the catalytic applications of 1,3,2-diazaphospholenes. The first chapter provides an introduction to the canonical reactivity of phosphorus-based catalysts, fol-lowed by a review of the known reactivity of 1,3,2-diazaphospholenes, in order to provide context for experimental work presented in this thesis. Chapter 2 describes the development of a new methodology to effect enantioselective 1,4-reductions of α,β-unsaturated carbonyl compounds. In order to overcome issues of reactivity and selectivity, a new class of chiral 1,3,2-diazaphospholenes, based around a rigid backbone, were designed. Various α,β-unsaturated acyl pyrroles, ketones and amides were reduced with moderate to excellent levels of enan-tioselectivity. Chapter 3 presents the development of a cascade reaction triggered by a 1,3,2-diazaphospholene-catalysed 1,4-reduction of allylic acrylate esters. The intermediary enolate that is thus formed undergoes a [3,3]-sigmatropic rearrangement, thereby generating a new carbon-carbon bond. We demonstrate that the diastereoselectivity of this process can be modulated through different catalysts and solvent choices, while a proof-of-principle for an enantioselective variant is given. Chapter 4 demonstrates the viability of α,β-unsaturated carboxylic acids as substrates for 1,3,2-diazaphospholene catalysed reductions. It is shown that the presence of the acidic proton on the carbox-ylate group does not inhibit the reactivity. An example demonstrating the possibility for enantioselectivity at the α-position is also provided. Finally, a view on the opportunities that still exist in the field of diazaphospholene catalysis is offered.