Mitchell Nascimento

Topic

Ring-Expansion and Ring-Opening of Phosphorus-Containing Heterocycles

Department of Chemistry

Date & location

• Thursday, May 2, 2024
• 9:00 A.M.
• Elliott Building, Room 226/228

Examining Committee

Supervisory Committee

• Dr. Scott McIndoe, Department of Chemistry, University of Victoria (Supervisor)
• Dr. Neil Burford, Department of Chemistry, UVic (Member)
• Dr. Mohsen Akbari, Department of Mechanical Engineering, UVic (Outside Member)

External Examiner

• Dr. Roland Roesler, Department of Chemistry, University of Calgary

Chair of Oral Examination

• Dr. Lincoln Shlensky, Department of English, UVic

Abstract

This thesis describes the synthesis of phosphorus-rich heterocyclic frameworks through the ring-expansion of cyclic phosphanes, which we demonstrate as a method to access structure-colour correlation and switchability as well as newly accessible P₃C and P₃CO frameworks. Investigations into the synthesis of and physical properties of hybrid phosphonate-siloxane heterocycles for the design of new P-O-Si-O polymers through ROP are also described, alongside investigations into the electronic and steric stabilization of phosphonate moieties with the goal of minimizing dynamic covalent behaviour in phosphonate-containing rings and polymers.

Chapter 1 gives an overview of silicon and phosphorus chemistry from elements to heterocycles, covering ring expansions and ring-opening polymerizations of silicon and phosphorus heterocycles, and reviewing strategies to accessing new main group polymers.

Chapter 2 describes the acid-mediated ring-expansion of sterically encumbered cyclic phosphanes with nitriles, the structural characterization of the bent rings, and the change of the electronic structure induced by the ring-bending which allows for a charge transfer to occur.

Chapter 3 describes the investigation into the substrate scope of acid-mediated ring-expansion of (tBuP)₃, which was found to be limited to substrates which bear an exocyclic nitrogen which acts as a Lewis base to activate substrates upon protonation. Structural characterization of cationic and neutral cycloaddition products of xylyl isonirile and tert-butyl isocyanate, the latter of which forms a novel P₃CO framework, are described, as are electronic effects on substrate specificity and a discussion of substrates which did not provide selective addition.

Chapter 4 describes the synthesis and characterization of hybrid phosphonate-siloxane heterocycles, and their physical characterization as relevant to accessing the ability of the rings to undergo thermal ROP. Rings lacking kinetic stabilization are found to be dynamic in solution. The least sterically hindered of these rings were found to form polymeric material upon heating, while more sterically hindered derivatives did not undergo ROP. Our preliminary studies into preventing observed dynamic behaviour through electronic modification of phosphonate moieties are also described, as are preliminary studies into the synthesis of an isolable RPO2 monomer.

Chapter 5 provides an outlook for the work in this thesis, with an overview of potential routes for future studies.