Applications of density functional theory (DFT) in organic and inorganic reactions

The first chapter provides a brief introduction to the computational methods used in this thesis. The second and third chapters of this thesis, ‘’Amino-Cope Rearrangement and its anionic form: a mechanistic study from the DFT perspective’’ and ‘’ Development of a Novel Application of the Amino-Cope Rearrangement’’, outline the development of an original, unified synthetic strategy that will provide conspicuous and distinctive access to a range of structurally diverse lycopodium, cylindricine, and lepadiformine alkaloids and their derivatives. Specifically, this is achieved by establishing an unprecedented method for the efficient preparation of annulated medium-size N-heterocycles via the novel application of an anionic Amino-Cope Rearrangement (ACR). In addition to identifying and defining new synthetic strategies, the results in these chapters will improve the ability to construct complex biologically significant compounds, enhance fundamental understanding of organic molecules and explore key principles of chemical reactivity and mechanism. The Amino-Cope Rearrangement and its anionic form have been investigated mechanistically and experimentally in these chapters. Progress in synthesis is a few steps away from making the Amino-Cope Rearrangement substrate required to provide the product after the rearrangement. From a mechanistic point of view, different kinds of this rearrangement were explored: Neutral ACR, metallated ACR, and naked ACR, finding that while the neutral ACR proceeds via a concerted pathway, metallated and naked ACR goes through the step-wise mechanism. With the information from the DFT results, it is anticipated that the rearrangement on the substrate will be able to be performed in the future in order to investigate this area more deeply. In the fourth and fifth chapters of this thesis, an important organic reaction called Nazarov cyclization is explored. In the fourth chapter, DFT has been utilised to study the mechanism of Nazarov cyclisations initiated by oxidation of pentadienyl ethers by a benzoquinone derivative (DDQ), as recently reported experimentally by West et al. (Angew. Chem. Int. Ed., 2017, 56, 6335). It was found that the reaction is most likely initiated by a hydride transfer from the pentadienyl ether to an oxygen of DDQ through a concerted pathway and not a single electron transfer mechanism. Interestingly, an excellent correlation between the hydride transfer activation energy and the gap between the ether HOMO and the benzoquinone LUMO (R\(^2\)=0.99) was found. Based on this correlation, a formula for predicting the activation energy of the oxidation process mediated by DDQ is provided. In the fifth chapter, an efficient chiral BrØnsted acid-catalyzed enantioselective Dehydrative Nazarov-type Electrocyclization (DNE) of electron-rich aryl- and 2-thienyl-β-amino-2-en-1-ols is described. The 4π conrotatory electrocyclization reaction affords access to a wide variety of the corresponding 1H-indenes and 4H-cyclopenta[b]thiophenes in excellent yields of up to 99% and enantiomeric excess (ee) values of up to 99%. Computational studies based on a proposed intimate contact ion-pair species that is further assisted by hydrogen bonding between the amino group of the substrate cation and chiral catalyst anion provide insight into the observed product enantioselectivities. Since cancer remains among the most widespread and difficult to cure diseases, and often involves problems due to the use of cisplatin, the most common drug for cancer treatment currently, finding an alternative drug which shows the same properties of cisplatin without side effects is highly desirable. Recently, gold complexes, especially in the oxidation state of +III, have enjoyed renewed interest in this field, mainly because platinum(II) and gold(III) have the same isoelectronic configuration (d\(^8\)). As a result, the last chapter of this thesis, which talks about gold chemistry, provides a mechanistic exploration of the reduction of gold(III) complexes by the amino acid glycine. Interestingly, when the nitrogen atom of glycine coordinates to the gold(III) centre, its C\(^α\)-hydrogen atom becomes so acidic that it can be easily deprotonated. This deprotonation, that can be facilitated by a mild base, converts the amino acid into a potent reductant by which gold(III) is reduced to gold(I) with a moderate activation energy. Apparently, this is the first investigation suggesting that primary amines are oxidized to imines via direct α-carbon deprotonation. This work also provides a rationalization behind why gold(III) complexes with amine-based polydentate ligands are reluctant to undergo a redox process