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MECHANISTIC INVESTIGATIONS INTO VISIBLE-LIGHT DRIVEN HYDRODESULFURIZATION REACTIONS FOR C‒C COUPLING
Stewart, Shea
Stewart, Shea
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Thesis/Dissertation
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2025-05
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Chemistry
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https://doi.org/10.34944/nxkg-tb69
Abstract
Using light energy to generate radical intermediates rather than toxic or otherwise hazardous reagents for the synthesis of fine chemicals is a pillar of green chemistry. Photon driven hydrodesulfurization of mercaptans by phosphines yields carbon-centered radicals through the β-fragmentation of phosphoranyl radical intermediates. The β-fragmentation yields carbon-centered radicals which can be intercepted by alkenes to form new C‒C bonds. Such chemistry was first described in the 1950s whereby thiyl radicals were produced via ultraviolet (UV) light illumination. However, employment of UV-light is not as attractive as visible-light, as many species in reaction systems may absorb the UV-light used, leading to competing side reactions. As such, visible-light mediated methods for hydrodesulfurization of mercaptans have been developed in the literature. These methods, though, generally rely on expensive/homogeneous catalysts such as Ir-, Au-, and Ru-centered complexes which can be difficult to isolate for recycling after reaction completion. Therefore, the development of visible-light driven hydrodesulfurization schemes which do not rely on expensive metals or which can utilize easily recycled heterogeneous catalysts has been studied in this thesis. In Chapter 2, it is displayed that hydrodesulfurization of mercaptans by triphenylphosphine (TPP) to enable C‒C coupling can be driven via visible-light illumination without the requirement of adding a photocatalyst. This is despite the lack of visible-light absorption by the reagents. Further study revealed that triphenylphosphine oxide (TPPO) is the key species which enabled the visible-light photoreaction to occur. Spectral study then led to the hypothesis of a TPP-TPPO complex in solution to impart light-absorption, though later experimentation ruled this out. Instead, a tri-molecular thiol-TPP-TPPO complex is proposed whereby the TPPO acts as a base to coordinate with the thiol S‒H. It was then shown that other bases, such as pyridine, can also enable the visible-light photoreaction.
In Chapter 3, TiO2 nanoparticles are used as a photocatalyst to accelerate visible-light driven thiol hydrodesulfurization for downstream C‒C coupling. The substrate scope is expanded on from Chapter 2 by coupling thioacetic acid with various different styrenes. TiO2 has a band-gap of 3.0-3.2 eV, meaning it practically does not absorb visible-light. Therefore, diffuse reflectance UV-Visible spectroscopy (DR-UV/Vis) was used to study the action of the TiO2 as a photocatalyst. It was found that all reactants form so-called interfacial charge-transfer transition complexes (ICTTCs) which absorb visible-light. Further, it was revealed that reagent combinations form unique complexes, and that co-adsorbed TPP/thiol likely represents the key visible-light absorbing species to initiate the reaction. Attenuated total reflectance infrared absorption spectroscopy (ATR-IR) was also used to analyze the functionalized powders, and helped confirm the interactions of the molecules on the TiO2 surface through the modulation of reactant signals for co-functionalized powders relative to singly functionalized powders. UV-Vis and IR were also taken of the functionalized powders after visible-light illumination revealing some information about changes to the particles and adsorbates upon excitation. Additionally, cryogenic EPR experiments employing in-situ illumination were conducted, giving insight into the charge separated states soon after illumination. Phosphorous K-edge XANES was used to investigate TPP and TPPO on TiO2 nanoparticles, providing further evidence for their complexation on the surface. Lastly, wavelength dependent C‒C coupling studies revealed that oxygen vacancies formed TPP on the TiO2 surface cause a switching in mechanistic action of the TiO2.
Chapter 4 talks about the development of a simple, routine, Extended X-ray Absorption Fine Structure (EXAFS) protocol which is sensitive to changes at nanoparticle surfaces. Other surface sensitive EXAFS techniques already exist; however, the experimental setups are typically complicated or require meticulously synthesized materials, making them not user friendly such that much prior experience is needed to implement them. The protocol developed ameliorates this, requiring only brushing particles onto a piece of tape before measurement. Then through analysis of difference data, whereby data from a standard is subtracted from that of the analyte, bulk contributions to spectra are removed leaving behind surface specific signals. The Fourier analysis of these signals allows one to gain information about changes in local coordination around the absorber (e.g. Ti) in the particles probed at the surface brought about by treatment of the particles such as functionalization with adsorbates or photo-illumination. This method is similar to the Δµ XANES technique developed by Ramaker, but goes a step further by modeling the residual EXAFS oscillations left behind after subtraction. The merits of this protocol are illustrated through the resolution of surface lattice-oxygen vacancy formation on P25 TiO2 nano-powders via treatment with NaBH4 or TPP/light.
This dissertation provides the first concrete explanation for “photocatalyst free” visible-light driven thiol desulfurization by TPP for downstream C‒C coupling. The use of TiO2 nanoparticles to drive this reaction is also something which is not found in the literature, either. Lastly, a novel method for analysis into happenings local to nanoparticle surfaces via difference EXAFS fitting is given. It is expected that the mechanistic insights gained regarding the catalyst free and TiO2 enabled desulfurization systems will open up opportunity for the development of other organic transformations. Meanwhile, the positive results from the difference EXAFS fitting show warrant its further development.
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