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Synthesis, structure and reactivity of phosphorus and arsenic heterocycles

Ould, Darren 2020. Synthesis, structure and reactivity of phosphorus and arsenic heterocycles. PhD Thesis, Cardiff University.
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The work described in this thesis relates to the synthesis and exploration of benzo-fused phosphole and benzo-fused arsole complexes, which have been modified by the inclusion of sulfur, nitrogen and oxygen heteroatoms in the pnictole ring, along with the inclusion of a halogen co-ligand. Chapter 2 synthesised the dithiaphospholes, dithiarsoles, diazaphospholes, diazarsoles and dioxaphospholes by adding the appropriate ligand to phosphorus trihalides (PCl3, PBr3 or PI3) or arsenic trichloride. Structural determination found that these compounds have an envelope geometry and an exocyclic halogen coligand. Chapter 3 then made use of these compounds to firstly synthesise an intriguing looking N-centred paddlewheel complex. After this, halide abstraction was performed to acquire phosphenium and arsenium cations. UV-visible spectroscopy performed on the diazaphospheniums and diazarseniums revealed the shape and positioning of the absorption bands is highly dependent on the choice of cation (P or As) and counterion (AlCl4 −, GaCl4 − or OTf−). Lastly, Chapter 3 synthesised a P–P dimer from the diazaphosphole, which by using EPR spectroscopy underwent homolytic cleavage at 350 K in toluene solvent. Interestingly, the corresponding As–As dimer had formed but underwent oxidative addition to form 2-iodo-1,3-diisopropyl-benzodiazarsole. Having established fundamental insight into the structure and reactivity of these heterocycles, Chapter 4 used these compounds as pre-catalysts for hydroboration reduction catalysis of aldehydes. The benzyloxy-diazarsole was found to be highly effective for this, as using 5 mol% pre-catalyst loading effectively hydroborated a wide variety of electron donating and electron withdrawing substrates, as well as more sterically demanding ones. When using the phosphole and phosphenium compounds as pre-catalysts, the diazaphosphenium triflate proved to be the most effective for the catalysis, but required higher catalytic loading (10 mol%) and longer reaction times for quantitative conversion to occur. Comparing the arsenic and phosphorus pre-catalysts found that the benzyloxy-diazarsole was superior to the diazaphosphenium triflate.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 22 June 2020
Last Modified: 16 Mar 2021 02:28

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