Estudio computacional de reacciones de nitronas que no son lo que parecen

Nitrones are highly versatile compounds in organic synthesis due to their diverse reactivity, mainly as 1,3-dipoles and electrophiles (in the presence of activating agents). Reactions of nitrones allow a facile introduction of heteroatoms in organic compounds and have some advantages over similar functional groups such as oximes, hydrazones and imines. Therefore, understanding mechanisms involved in nitrone reactivity is an extremely important goal. Nitrone functional group consists of 3 atoms (C, N and O) that share 4 π electrons. Their typical reactivity includes electrophilic attacks to the O atom, nucleophilic attacks to the C atom (usually after electrophilic activation) and the most known 1,3 dipolar cycloadditions with unsaturated systems. Furthermore, nitrones are also attractive substrates for radical processes such as coupling reactions with unsaturated heteroatomic multiple bonds. Even though the most important nitrone reactivity is already known, there is still controversy regarding (i) concertedness of several cycloaddition reactions and (ii) involvement of diradical species in particular processes. The present Ph.D. Thesis summarizes the studies carried out on several different nitrone-related processes. In particular, four nitrone reactions, whose mechanisms were not fully established, have been studied through DFT and topological (ELF and NCI) methods. The results are collected in the indicated published scientific papers. The reactions are: A) The mechanism of cyclodimerization of nitrones, a process apparently disallowed by the orbital symmetry rules. Several open and closed shell mechanisms were studied and, ultimately, we were able to prove that the reaction takes place through a bispseudopericyclic concerted mechanism. (Org. Biomol. Chem. 2014, 12, 517). B) The thermal E/Z isomerization of nitrones, a process observed in several nitrones bearing electron-withdrawing groups at the carbon atom. After ruling out both unimolecular torsional and oxaziridine-mediated mechanisms, as well as bimolecular concerted and zwitterionic stepwise ones, we determined that the preferred mechanism for this type of isomerization involved a dirradical C C coupling between two nitrone molecules. (J. Org. Chem. 2014, 79, 8358). C) The reaction of nitrones with lithium ynolates and enolates. Some of these reactions take place through an only kinetic step that, actually, involves a two-stage, one-step process in which the formation of the two newly formed bonds is sequential. NCI and ELF analyses were also carried out to study the bond evolution along the reaction. (J. Org. Chem. 2015, 80, 4076, lythium ynolates; Eur. J. Org. Chem. 2015, 4143, lythium enolates)