Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Ragogna, Paul J.

Abstract

The study of low-coordinate species and the pursuit of interesting bonding motifs has revealed new reactivity modes for main-group compounds, akin to transition metals, in the past two decades. An underexplored class of two-coordinate phosphorus compound is a phosphinidene sulfide, which has yet to be isolated in the solid-state with an accompanying crystal structure. This dissertation explores strategies towards the synthesis and reactivity of low-coordinate phosphinidene chalcogenide species, and related compounds supported by strongly pi-donating or weakly pi-donating environments. While the primary synthetic targets remained elusive, significant discoveries were made towards new methods of preparing electron rich phosphines, and the dual-reactivity modes of bis(diisopropylamino)cyclopropenone to generate a chlorophosphonate or cyclic phosphonates. Chapter 2 outlines the strategies pursued for the generation of a phosphinidene sulfide utilizing a π-donating N-heterocyclic imine (NHI) ligand. Although synthesis of the desired species was not achieved, solid-state structures of new phosphorus-chalcogenide species were serendipitously obtained. These results suggested that the NHI was not adequate to stabilize a phosphinidene sulfide, and would require a bulkier ligand. Chapter 3 explores a novel family of bis(azido)phosphines and their participation in chemoselective Staudinger reactions with secondary or tertiary phosphines to produce chiral phosphines. A tautomeric equilibrium of species was analyzed by 31P-31P nuclear Overhauser effect spectroscopy (NOESY). Thermogravimetric analysis (TGA) provided insights into the thermal stability of these compounds. Density functional theory (DFT) suggested that the observed chemoselectivity was a result of energetically inaccessible lowest unoccupied molecular orbitals (LUMOs) with appropriate N3 π* character. Chapter 4 introduces a new synthetic approach for synthesis of a chlorophosphonate (4.3BAC) and cyclic phosphonates (4.18Ph and 4.18Mes), using C3O and PCl3 or RP(O)Cl2. Pasteur separation allowed for structural characterization of 4.3BAC and [C3Cl]Cl by-product, but an efficient separation method was not discovered. An intermediate [4.6BAC]+ was detected and hydrolysis species were investigated. Reactivity of 4.3BAC with various Lewis acids was screened, and the in situ generation of monomeric dioxophosphoranes was reported.

This thesis highlights the complexities of isolating low-coordinate phosphorus chalcogenide species, while offering a new synthetic strategy for the synthesis of chiral phosphines and α-cationic dioxophosphoranes.

Summary for Lay Audience

Phosphorus is an element that is ubiquitous all around us. It is found in biological systems, in everyday products like detergents, and in important chemicals such as herbicides, pesticides, and fertilizer. It is also important in the production of fine chemicals found in pharmaceuticals, and tailored ligands in catalysis. One reason that phosphorus is so versatile is the ability to adopt a diverse set of structural motifs, and easily bonds with group 16 elements like oxygen, sulfur, and selenium (known as chalcogens). Development of new methods for producing phosphorus-containing molecules can therefore impact many industries. To achieve this, such advancements can come from a greater understanding of the influence of structure and bonding on reactivity, which is why the isolation of highly reactive species can be of value. These highly reactive species are often fleeting (short-lived) intermediates, but can be challenging to detect. Strategies have thus been developed, using either bulky protecting groups or electron-rich groups to stabilize (and isolate) many highly reactive molecules.

In this thesis, a series of underexplored of phosphorus-chalcogenide motifs supported by either electron-rich or electron-poor groups were investigated. Although no evidence of the formation of the primary synthetic target of this work was found, two different motifs were each explored. The first motif was built of a phosphorus atom connected to groups of three-nitrogen atoms, in series, called an azide (N3). Although two azide groups were present, only one of the azides could be converted by a known reaction. This selectivity was observed for each combination of reagents screened, therefore offering an easy method for tunability of the products. The final motif explored contained a phosphorus atom attached to two oxygen atoms, but was prepared by an unexpected exchange of an oxygen with two chlorine atoms. The method developed was also used for the preparation of other known and valuable products.

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