Exploring the Chemistry of Bis(azido)phosphine Chalcogenides and the Pursuit of Isolable Phosphinidene Chalcogenides

John Lortie, Western University

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 (4.3_BAC) and cyclic phosphonates (4.18_Ph and 4.18_Mes), using C₃O and PCl₃ or RP(O)Cl₂. Pasteur separation allowed for structural characterization of 4.3_BAC and [C₃Cl]Cl by-product, but an efficient separation method was not discovered. An intermediate [4.6_BAC]⁺ was detected and hydrolysis species were investigated. Reactivity of 4.3_BAC 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.