In this thesis, Density Functional Theory (DFT) methods have been applied to study the mechanisms of three different multicomponent organic reactions. Also, a new synthetic procedure for the preparation of quinolinium salts is presented, and its mechanism also studied by DFT calculations. The thesis summarizes the work realized in two universities, and is divided in the following way: The first part of the thesis concerns the development of an experimentally simple, but mechanistically complex, reaction for the formation of quaternary quinolinium salts catalyzed by palladium salts. This multicomponent process uses readily available propylamine and its derivatives as starting materials. Through DFT studies a mechanism through the activation of two aliphatic C-H bonds is proposed. The second part focuses on the mechanistic investigation of a three-components reaction, namely terminal alkynes, CO₂ and allylic chlorides, mediated by an N-heterocyclic carbene catalyst that yields propargylic esters. By DFT calculations, the rate-limiting step was identified to be the reaction between the carboxylated catalyst and the allylic chloride. Through DFT modelling, we were also able to understand the limitations of this reaction. The mechanism of a multicomponent reaction in which allylic alcohols are transformed into α-functionalized carbonyls was also investigated. The reaction relies on an umpolung strategy that enables to react enol intermediates with different nucleophiles. By DFT studies, a mechanism via enolonium intermediates is proposed, which provides an understanding of the selectivity of the reaction. The final chapter of the thesis deals with another multicomponent solvent-free reaction for synthesizing propargylamines catalyzed by manganese via a KA² coupling. DFT studies were undertaken and a mechanism via manganese phenylacetylide species is proposed.

Herein, we report a catalytic system based on the earth-abundant manganese for the ketone, amine, alkyne (KA²) reaction. The efficiency of manganese manifests at relatively high temperatures, combined with sustainable reaction conditions, and provides a tool for accessing propargylamines from structurally diverse starting materials, including synthetically relevant and bioactive molecules. Our efforts were also aimed at shedding light on the catalytic mode of action of manganese in this transformation, in order to explain its temperature-related behavior. The use of computational methods reveals mechanistic aspects of this reaction indicating important points regarding the reactivity of both manganese and ketones.

The selective synthesis of a-functionalized ketones with two similar enolizable positions can be accomplished using allylic alcohols and iridium(III) catalysts. A formal 1,3-hydrogen shift on allylic alcohols generates catalytic iridium-enolates in a stereospecific manner, which are able to react with electrophiles to yield alpha-functionalized ketones as single constitutional isomers. However, the employment of nucleophiles to react with the nucleophilic catalytic enolates in this chemistry is still unknown. Herein, we report an umpolung strategy for the selective synthesis of alpha-alkoxy carbonyl compounds by the reaction of iridium enolates and alcohols promoted by an iodine(III) reagent. Moreover, the protocol also works in an intra-molecular fashion to synthesize 3(2H)-furanones from gamma-keto allylic alcohols. Experimental and computational investigations have been carried out, and mechanisms are proposed for both the inter- and intramolecular reactions, explaining the key role of the iodine(III) reagent in this umpolung approach.

An efficient and straightforward organocatalytic method for the direct, multicomponent carboxylation of terminal alkynes with CO₂ and organochlorides, towards propargylic esters, is reported for the first time. 1,3-Di-tert-butyl-1H-imidazol-3-ium chloride, a simple, widely-available, stable, and cost-efficient N-heterocyclic carbene (NHC) precursor salt was used as the (pre)catalyst. A wide range of phenylacetylenes, bearing electron-withdrawing or electron-donating substituents, react with allyl-chlorides, benzyl chlorides, or 2-chloroacetates, providing the corresponding propargylic esters in low to excellent yields. DFT calculations on the mechanism of this transformation indicate that the reaction is initiated with the formation of an NHC-carboxylate, by addition of the carbene to a molecule of CO₂. Then, the nucleophilic addition of this species to the corresponding chlorides has been computed to be the rate limiting step of the process.