Fundamental Palladium Catalyzed Oxidative Addition Reactions

Simple item page
dc.contributor.advisorEsterhuysen, Catharineen_ZA
dc.contributor.advisorBickelhaupt, F Matthiasen_ZA
dc.contributor.authorMoloto, Bryan Phutien_ZA
dc.contributor.otherStellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.en_ZA
dc.date.accessioned2023-11-28T09:23:07Zen_ZA
dc.date.accessioned2024-01-08T20:30:50Zen_ZA
dc.date.available2023-11-28T09:23:07Zen_ZA
dc.date.available2024-01-08T20:30:50Zen_ZA
dc.date.issued2023-11-28en_ZA
dc.descriptionThesis (PhD)--Stellenbosch University, 2023.en_ZA
dc.description.abstractENGLISH ABSTRACT: This thesis focuses on investigating fundamental oxidative addition (OA) reactions catalysed by palladium (see Chapter 1). OA, being the first and rate determining step in cross-coupling reactions, is a reaction of vital importance in synthetic chemistry. Palladium-catalysed crosscoupling reactions are widely used in industrial applications, such as in catalytic converters and the synthesis of pharmaceuticals. Besides these applications, palladium is widely used as a versatile catalytic reagent in many different chemical processes. Considering the importance of oxidative addition reactions catalysed by palladium, a deep understanding of the underlying mechanism is crucial to designing new catalysts and improving the existing ones. In a nutshell, the main focus is on understanding the mechanism behind the oxidative addition step and the trends in activation barriers upon variation of either the catalyst or substrate structure. The following summary will discuss only the most important findings from the chapters involved. As explained in Chapter 2, the findings in this thesis were successfully obtained using the Activation Strain Model of chemical reactivity (ASM, discussed in section 2.3) in combination with computations based on Density Functional Theory (DFT) as implemented in the ADF program. The ASM model is a fragment-based approach that characterizes reactions in terms of the rigidity and the bonding capabilities of the original reactants , and the extent to which the reactants must deform along the reaction pathway of a particular reaction mechanism. Thus, the total energy profile of a particular chemical reaction can be decomposed into contributions from the deformation of the reactants (the strain energy) and their mutual interaction (the interaction energy). The interaction energy can then be further decomposed using the canonical energy decomposition analysis (EDA) of ADF into electrostatic interactions, destabilizing Pauli repulsion, and stabilizing orbital interactions. In Chapter 3, with the aim of understanding the underlying mechanism and trends found by the oxidative addition, we detailed our quantum chemical exploration of the palladiummediated activation of C(spn )–X bonds (n = 1–3; X = F, Cl, Br, I) in the archetypal model substrates H3C–CH2–X, H2C=CH–X, and HC≡C–X by a model bare palladium catalyst. First and foremost, we investigated the bond dissociation enthalpies (BDEs) of the bonds to be activated. So, we started from the C(sp3 )–X moving to C(sp2 )–X and then to C(sp)–X bonds for each of the selected set of X atoms above. We found that as we move down group 17, the C(spn )–X bond becomes weaker and as such easier to break. Based on our state-of-the-art analyses, we discovered that as we vary the substituent X, going down Group 17 from X = F to Cl to Br to I on the C(spn )–X substrate, the oxidative addition barriers drastically decrease. This favorable activation barrier stabilization originates from two factors: (i) a less destabilizing activation strain; and remarkably (ii) a more favorable electrostatic attraction between the catalyst and the substrate. When changing the substrate from C(spn )–F to C(spn )– I, consequently, the electrostatic interaction between the catalyst and substrate also becomes more favorable. Iodine, being the largest halogen of the selected substituents, has a more diffuse and electron-rich density and a higher nuclear charge that in turn engage in favorable electrostatic attractions with the palladium nucleus and electron density, respectively. This effect makes the oxidative addition reaction involving the C(spn )–X bond with a larger halogen atom correspond to a more stabilizing interaction and hence lower reaction barrier. Next, in Chapter 4 we have quantum chemically investigated the palladium-mediated activation of HnA–AHn bonds (AHn = CH3, NH2, OH, F) by catalysts PdLn with Ln = no ligand, PH3, (PH3)2. Herein, we found that as we move from C to F along the period, i.e., from H3C– CH3 to H2N–NH2 to HO–OH to F–F, the activation barriers decrease and more interestingly the activation of the F–F bond is even barrierless. As we move from C to F on the selected substrates, the number of the substituents around the A–A bond become less and as such enabling the catalyst to approach the substrate with ease, thereby resulting in a decreasing activation barriers. The causal effects of this barrier stabilizations stem from: (i) a reduced activation strain due to a weaker HnA–AHn bond; (ii) a decreased Pauli repulsion as a result of a difference in steric shielding of the HnA–AHn bond; and (iii) an enhanced backbonding interaction between the occupied 4d atomic orbitals of the palladium catalyst and * acceptor orbital of the substrate. The findings in this thesis have the potential to equip experimentalists with detailed mechanistic insight that can facilitate a deep understanding into the trends in reactivity of palladium-mediated oxidative addition reactions.en_ZA
dc.description.abstractAFRIKAANS OPSOMMING: Hierdie tesis fokus op die ondersoek van fundamentele oksidatiewe addisie (OA) reaksies wat deur palladium gekataliseer word (sien Hoofstuk 1). OA, synde die eerste en tempobepalende stap in kruiskoppelingsreaksies, is 'n reaksie van deurslaggewende belang in sintetiese chemie. Palladium-gekataliseerde kruiskoppelingsreaksies word alomgebruik in industriële toepassings, soos by voorbeeld in katalitiese omsetters en die sintese van geneesmiddels. Benewens hierdie toepassings, word palladium alomgebruik as 'n veelsydige katalitiese reagens in vele verskillende chemiese prosesse. Met inagneming van die belangrikheid van oksidatiewe addisiereaksies wat deur palladium gekataliseer word, is 'n diepgaande begrip van die onderliggende meganisme van kardinale belang om nuwe katalisators te ontwerp en die bestaandes te verbeter. In 'n neutedop is die hooffokus op die begrip van die meganisme agter die oksidatiewe-addisiestap en die neigings in aktiveringsenergiëe by variasie van óf die katalisator óf substraatstruktuur. Die volgende opsomming sal slegs die belangrikste bevindinge uit die betrokke hoofstukke bespreek. Soos in Hoofstuk 2 verduidelik, is die bevindinge in hierdie tesis suksesvol verkry deur gebruik te maak van die 'Activation Strain Model' van chemiese reaktiwiteit (ASM, bespreek in Afdeling 2.3) in kombinasie met berekeninge gebaseer op Digtheidsfunksionaalteorie (DFT) soos geïmplementeer in die ADF program. Die ASM model is 'n fragment-gebaseerde benadering wat reaksies klassifiseer in terme van die rigiditeit en die bindingsvermoë van die oorspronklike reaktante, en die mate waartoe die reaktante langs die reaksieroete van 'n spesifieke reaksiemeganisme moet vervorm. Dus kan die totale energieprofiel van 'n bepaalde chemiese reaksie ontbind word na bydraes vanaf die vervorming van die reaktante (die vervormingsenergie) en hul onderlinge interaksies (die interaksie-energie). Die interaksieenergie kan dan verder ontleed word met behulp van die kanoniese energie-dekomposisieanalise (EDA) van ADF in elektrostatiese, destabiliserende Pauli-afstoting- en stabiliserende orbitaal-interaksies. In Hoofstuk 3, met die doel om die onderliggende meganisme en neigings wat by die oksidatiewe byvoeging gevind word te verstaan, het ons ons kwantumchemiese verkenning van die palladium-gemedieerde aktivering van C(spn )–X bindings (n = 1–3; X = F, Cl, Br, I) in die argetipiese model-substrate H3C–CH2–X, H2C=CH–X, en HC≡C–X deur middel van 'n model kaal-palladium katalisator uiteengesit. Eerstens het ons die bindingsdissosiasie- 88 entalpieë (BDE's) van die bindings wat geaktiveer moet word, ondersoek. Ons het dus by die C(sp3 )–X begin en daarna na C(sp2 )–X en uiteindelik na die C(sp)–X bindings beweeg vir elkeen van die geselekteerde stel X-atome hierbo. Ons het gevind dat soos wat ons in groep 17 afbeweeg, word die C(spn )–X-binding swakker en sodanig makliker om te breek. Gebaseer op ons gevorderde ontledingsmetodes, het ons ontdek dat wanneer die substituent X verander word, deur in Groep 17 af te gaan vanaf X = F na Cl na Br na I met betrekking tot die C(spn )– X substraat, die oksidatiewe-addisie-aktiveringsenergie drasties verminder word. Hierdie gunstige stabilisering van die aktiveringsenergie spruit uit twee faktore: (i) 'n minder destabiliserende aktiveringsspanning; en merkwaardiglik (ii) 'n gunstiger elektrostatiese aantrekking tussen die katalisator en die substraat. Indien die substraat van C(spn )–F na C(spn )–I verander word, word die elektrostatiese interaksie tussen die katalisator en substraat gevolglik ook gunstiger. Jodium, as die grootste halogeen uit die substituente, besit 'n meer diffuse- en elektronryk-digtheid wat op hulle beurt gunstiger elektrostatiese aantrekkings met onderskeidelik die palladiumkern en elektrondigtheid vorm. Hierdie effek maak dat die oksidatiewe addisiereaksies van die C(spn )–X bindings met groter halogeenatome met meer stabiliserende interaksies en dus laer reaksieaktiveringsenergie ooreenstem. Vervolgens, in Hoofstuk 4 het ons die palladium-gemedieerde aktivering van HnA–AHn bindings (AHn = CH3, NH2, OH, F) deur katalisators PdLn met Ln = geen ligand, PH3 of (PH3)2 kwantum-chemies ondersoek. Hierin het ons gevind dat soos wat ons oor die reeks vanaf C tot F beweeg, d.w.s. van H3C–CH3 na H2N–NH2 na HO–OH na F–F, die aktiveringsenergie afneem en, meer interessant, die F–F binding kan totselfs sonder aktivering met die palladium reageer. Soos ons van C na F op die geselekteerde substrate beweeg, word die aantal substituente rondom die A–A binding minder en sodanig stel dit die katalisator in staat om die substraat met meer gemak te benader, wat lei tot dalende aktiveringsenergie. Die oorsaak van hierdie stabiliserings van die oorgangstoestande spruit uit: (i) 'n verminderde aktiveringsspanning as gevolg van 'n swakker HnA–AHn binding; (ii) 'n verminderde Pauliafstoting as gevolg van 'n verskil in steriese afskerming van die HnA–AHn binding; en (iii) 'n verbeterde terugdonasie vanaf die besette 4d atoomorbitale van die palladiumkatalisator na die *-akseptororbitaal van die substraat. Die bevindinge in hierdie tesis het die potensiaal om eksperimentele toe te rus met gedetailleerde meganistiese insig wat 'n diepgaande begrip van die neigings in reaktiwiteit van palladium-gemedieerde oksidatiewe addisiereaksies kan fasiliteeraf_ZA
dc.description.versionDoctorateen_ZA
dc.format.extent93 pages : illustrationsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/129048
dc.language.isoen_ZAen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshComputational chemistryen_ZA
dc.subject.lcshPalladium catalystsen_ZA
dc.subject.lcshDensity functionalsen_ZA
dc.subject.lcshQuantum chemistry -- Computer programsen_ZA
dc.titleFundamental Palladium Catalyzed Oxidative Addition Reactionsen_ZA
dc.typeThesisen_ZA
Files
Original bundle
Now showing 1 - 1 of 1
Thumbnail Image
Name:
moloto_fundamental_2023.pdf
Size:
4.23 MB
Format:
Adobe Portable Document Format
Description:
Download
Collections
Doctoral Degrees (Chemistry and Polymer Science)