Development and Mechanistic Investigation of Hydrogen Atom Transfer Reactions Catalyzed by Metal Clusters and Porphine Complexes

Emil Krogh Nielsen

Research output: Book/ReportPh.D. thesis

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Abstract

The work presented in this thesis investigates new methods within photocatalyzed direct hydrogen atom transfers (d-HAT) as well as transition metal catalyzed acceptorless alcohol dehydrogenations (AAD). The first project described revolves around the development of new d-HAT catalysts based on abundant transition metals and the tetradentate ligand mesotetraphenylporphine. To investigate the metal complexes, density functional theory (DFT) was applied and complexes were evaluated based on their excited state energies, hydrogen abstraction energies and Mulliken analysis. The computational results indicated that the Cr(IV) and Mo(V) complexes could function as d-HAT catalysts. Due to experimental limitations, complexes with Ti(IV), V(IV), Mo(V) and W(V) metal centers were the only to be synthesized. Experimental results showed no d-HAT activity, encouraging further computational evaluation of the orbital symmetries. The analysis indicated that none of the synthesized complexes exhibited the necessary orbital symmetry to facilitate an unpaired electron at the terminal oxide of the complexes, hence inhibiting the d-HAT activity.
The second study investigated the extended usage of furfural in organic synthesis. Furfural was investigated as a hydrogen donor substrate in a photocatalyzed d-HAT reaction with olefins, catalyzed by the established photocatalyst tetrabutylammonium decatungstate (TBADT). During the study, an optimization was performed on the model reaction with furfural and benzylidene malononitrile. This optimization investigated the influence of temperature, reaction time, light source, solvent, degassing and relative stoichiometries. The substrate scope was extended by using a variety of olefins indicating that electron withdrawing groups and sterics play a vital role in modulating the reactivity. Lastly, 5-hydroxymethyl furfural (5-HMF) and furfuryl alcohol were tested as hydrogen donors. Reactivity was established with 5-HMF resulting however in low yields and the promotion of sideproducts. Furfuryl alcohol showed no reactivity towards the desired product.
The third project undertook a computational investigation of the mechanism for the already established AAD reaction utilizing the vanadyl meso-tetraphenylporphine catalyst. Mechanisms investigated included metal ligand cooperation, internal hydride shifts and in situ reductions of the ligand. None of the investigated mechanisms showed suitable energy barriers, hence disproving the proposed pathways. Experiments conducted on the Cr(III) equivalent of the catalyst combined with the challenges of reproducing the vanadyl work experimentally, indicated that the catalyst performed the reaction via a different mechanistic pathway than initially theorized.
Original languageEnglish
PublisherDTU Chemistry
Number of pages266
Publication statusPublished - 2024

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