The [3+2] cycloaddition (32CA) reactions of N-benzylcyclohexylnitrone 1 with methyl-3-butenoate 2 have been studied within molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p)level of theory. These zwitterionic type 32CA reactions...

Abstract. The zwitterionic-type (zw-type) [3+2] cycloaddition (32CA) reactions of N-methyl-C-3-bromophenyl-nitrone 1 with dimethyl maleate 2 with increased electrophilicity were investigated using the Molecular Electron Density Theory (MEDT) at the MPWB95/6-311++G(d,p) computational level. Both reactivity and selectivity are rationalized in relation to the polarity of the reaction. The associated zw-type 32CA reactions are accelerated due to the high nucleophilic character of N-methyl-C-3-bromophenyl-nitrone 1 and the strong electrophilic character of dimethyl maleate 2, which also play a critical part in the mechanism of the reaction, influencing the stereoselectivity, with activation enthalpies in between 34.04 and 38.37 kJ.mol-1 in the gas phase. The CDFT indices are used to anticipate global electron density flux from the nucleophilic N-methyl-C-3-bromophenyl-nitrone 1 to the electrophilic dimethyl maleate 2. These exergonic 32CA reactions have negative Gibbs free energy along the endo and exo stereochemical pathways. The endo stereochemical process is preferred over the exo stereochemical pathway in this kinetically controlled 32CA reaction. The predictions of bonding evolution theory (BET) for the endo and exo pathways indicate a one-step process with early transition states, which is compatible with the ELF topological examination of transition states.   Resumen. Las reacciones de cicloadición de tipo zwitteriónico (tipo zw) [3+2] (32CA) de N-metil-C-3-bromofenil-nitrona 1 con maleato de dimetilo 2 con aumento de la electrofilicidad se investigaron utilizando la Teoría de la Densidad Electrónica Molecular (MEDT, por sus siglas en inglés) utilizando el nivel de teoría MPWB95/6-311++G(d,p). Tanto la reactividad como la selectividad se racionalizan en relación con la polaridad de la reacción. Las reacciones asociadas de tipo zw 32CA se aceleran debido al alto carácter nucleofílico de N-metil-C-3-bromofenil-nitrona 1 y al fuerte carácter electrofílico del maleato de dimetilo 2, que también juega un papel crítico en el mecanismo de la reacción, influyendo en la estereoselectividad, con entalpías de activación entre 34.04 y 38.37 kJ.mol-1, en fase gaseosa. Los índices de la Teoría de Funcionales de la Densidad Conceptual (CDFT, por sus siglas en inglés) se utilizan para anticipar el flujo global de densidad de electrones desde el nucleófilo N-metil-C-3-bromofenil-nitrona 1 hasta el maleato de dimetilo electrofílico 2. Estas reacciones exergónicas 32CA tienen energías libres de Gibbs negativa a lo largo de las vías estereoquímicas endo y exo. El proceso estereoquímico endo es preferido sobre la vía estereoquímica exo en esta reacción 32CA controlada cinéticamente. Las predicciones de la teoría de la evolución de enlaces (BET, por sus siglas en inglés) para las vías endo y exo indican un proceso de un paso con estados de transición temprana, que es compatible con el examen topológico de la Función de Localización Electrónica (ELF, por sus siglas en inglés) de los estados de transición.

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[3+2] cycloaddition reactions play a crucial role in synthesizing complex organic molecules and have significant applications in drug discovery and materials science. In this study, the [3+2] cycloaddition (32CA) reactions of N-methyl-C-4-methyl phenyl-nitrone 1 and 2-propynamide 2, which have not been extensively studied before, were investigated using molecular electron density theory (MEDT) at the B3LYP/6–311++G(d,p) level of theory. According to an electron localization function (ELF) study, N-methyl-C-4-methyl phenyl-nitrone 1 is a zwitterionic species with no pseudoradical or carbenoid centers. Conceptual density functional theory (CDFT) indices were used to predict the global electronic flux from the strong nucleophilic N-methyl-C-4-methyl phenylnitrone 1 to the electrophilic 2-propynamide 2 functions. The 32CA reactions proceeded through two pairs of stereo- and regioisomeric reaction pathways to generate four different products: 3, 4, 5, and 6. The reaction pathways were irreversible owing to their exothermic characters: −136.48, −130.08, −130.99, and −140.81 kJ mol−1, respectively. The enthalpy of the 32CA reaction leading to the formation of cycloadduct 6 was lower compared with the other path owing to a slight increase in its polar character, observed through the global electron density transfer (GEDT) during the transition states and along the reaction path. A bonding evolution theory (BET) analysis showed that these 32CA reactions proceed through the coupling of pseudoradical centers, and the formation of new C-C and C-O covalent bonds did not begin in the transition states.

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