Estudio teórico de mecanismos de reacciones orgánicas.
In the present Doctoral Thesis, a theoretical mechanistic study of many organic reactions has been made using quantum methods defined within Density Functional Theory. The organic reactions analyzed can be divided in four different sections: Diels-Alder reactions, 1,3-dipolar reactions, [4+3] cycloadditions and one collaboration made with an Organic Chemistry experimental investigation group. The Diels-Alder reactions mediated by molecular recognition processess via hydrogen-bond interactions have high relevance, as they are the cause of the formation of molecular complexes that shift some of the entropic cost associated to the association of the reagents. In the 1,3-dipolar cycloadditions of alkynilboronates with benzonitrile N-oxides, the terminal substituent over the acetylenic skeleton has capital importance, as it can invert the regioselectivity of the final product through sterical interactions. The competitive formation of oximes and [3+2] cycloadducts in the reactions of benzonitrile N-oxides and ftalimides it is related to the substitution over the dipole: the presence of electron-acceptor groups over benzonitrile N-oxides makes possible the oximes formation. The symmetry or asymmetry in the substitution of electron-deficient dipolarophiles in the reactions with nitrones is a decisive factor in the extent of the acceleration caused by hydrogen-bond catalysis. The asymmetry in the substitution of electron-deficient dipolarophiles permits a higher electrophilic activation and, therefore, a larger acceleration of ther reaction than a symmetric substitution pattern. In absence of a Lewis acid, the reaction between 2-silyloxyacroleines and 1,3-dienes takes place through a concerted and asynchronous process that gives rise to [4+2] endo cycloadduct. In presence of Lewis acid, the mechanism changes to a polar process in three steps that leads to the [4+3] endo adduct. The synthesis of imidazo[1,2-c]pyrimidines through the reaction of 2-aminopyrimidines and 2-bromoacetamides takes place through the nucleophilic substitution of the pyrimidine over the haloacetamide. The 2-dihydropyrimidine obtained suffers a diastereoselective intramolecular Michel-type cyclization that gives rise to the imidazo[1,2-c]pyrimidines wanted. This last step requires a high electrophilic activation of the pyrimidine ring through an acid catalysis.