Abstract The potential energy surface of four stereoselective Diels-Alder reactions was studied, namely: cyclopentadiene-maleic anhydride, furan-maleic anhydride, the dimerization of cyclopentadiene, and cyclopentadiene-cyclopropene. For completeness, we also studied the reaction between ethylene and 2-hydroxy-6-methyl-1,4-benzoquinone, a [5+2] cycloaddition reaction. For all cases at least a stationary state of supramolecular nature a van der Waals complex, was determined. These stationary states are complexes formed by the interaction between the reagents, minima located in the paths between the non-interacting molecules and the transition states. The existence of these complexes makes it necessary to reconsider the role of Secondary Orbital Interactions in the selectivity of these reactions. As it is the case with other complexes, the stability of these supramolecular intermediates depends on electrostatic phenomena such as dispersion forces. The observation of [5+2] intramolecular complexes in solution is important since up to now, this kind of van der Waals complexes had only been described in the gas phase.

Resumen La superficie de energía potencial de cuatro reacciones estereoselectivas Diels-Alder fue estudiada, específicamente: ciclopentadieno-anhídrido maleico, furano-anhídrido maleico, la dimerización del ciclopentadieno y ciclopentadieno-ciclopropeno. Para complementar, también se estudió la reacción entre etileno y 2-hidroxil-6-metil1, 4-benzoquinona, una reacción de cicloadición [5+2]. Para todos los casos, fue identificado al menos un estado estacionario, un complejo de van der Waals de naturaleza supramolecular. Estos estados estacionarios de mínima energía son complejos formados por la interacción entre los reactivos y se localizan como mínimos en la trayectoria de las moléculas no interaccionantes y los estados de transición. La existencia de estos complejos hace necesario reconsiderar el papel de las Interacciones de Orbitales Secundarios en la selectividad de estas reacciones. De la misma forma que ocurre con otros complejos, la estabilidad de estos intermediarios supramoleculares depende de fenómenos electrostáticos como las fuerzas de dispersión. La existencia del complejo intramolecular [5+2] en solución es importante, ya que hasta ahora los complejos del tipo van der Waals sólo han sido descritos en fase gaseosa.

With the aim of evaluating the effect of substitution at the aryl moiety of the aroyloxy group of captodative olefins 1 on their reactivity in Diels-Alder and other reactions, the new series of olefins 1-acetylvinyl arenecarboxylates 1c-1k were prepared. No correlation was found between 13C NMR chemical shifts of the carbon atoms of the double bond and the electronic effects of the diverse substituents. However, an excellent correlation was observed between the energies of the FMOs (HF/6-31G*), or the corresponding atomic coefficients or the Mülliken charges at the carbon atoms of the double bond, and the Hammett σm and σp constants of the phenyl ring substituents of some members of the series, and of some other calculated analogues. These results strongly suggest that, in addition to the major effect of the electron-withdrawing group, the reactivity of captodative olefins 1 is also controlled by the long-range inductive effects of the substituent at the phenyl ring of the aroyloxy group.

Con el objeto de evaluar el efecto de la sustitución del fragmento arilo del grupo aroiloxi de las olefinas captodativas 1 sobre su reactividad en reacciones de Diels-Alder y otras reacciones, se preparó la nueva serie de olefinas 1-acetilvinil arencarboxilatos 1c-1k. No se encontró correlación alguna entre los desplazamientos químicos en RMN de 13C de los átomos de carbono del doble enlace y los efectos electrónicos de los diversos sustituyentes. Sin embargo, se encontró una excelente correlación entre las energías de los OMF (HF/6-31G*), o los coeficientes atómicos correspondientes o las cargas de Mülliken en los átomos de carbono del doble enlace, y las constantes de Hammett σm y σp de los sustituyents del grupo fenilo de algunos miembros de la serie, y para algunos otros compuestos análogos que fueron calculados. Estos resultados sugieren fuertemente que además del efecto dominante del grupo electroatractor, la reactividad de las olefinas captodativas 1 está también controlada por el efecto inductivo a larga distancia de los sustituyentes en el fenilo del grupo aroiloxi.

As olefinas captodativas 3a e 3b derivadas dos ácidos metacrílico e trans-crotônico foram preparadas. A presença de uma segunda ligação dupla na molécula, atuando como marcador interno, permitiu-nos comparar sua reatividade relativa em reações de Diesl-Alder e Friedel-Crafts. A reatividade foi avaliada com ciclopentadieno (6) atuando como dieno em cicloadição Diels-Alder, e com furano (9) e tiofeno (10) como substratos heteroaromáticos Friedel-Crafts. Em ambos os processos, a ligação dupla da enona captodativa mostrou-se mais reativa que a do sistema acrílico. A teoria FMO considera essa quimiosseletividade como consequência da maior contribuição da enona ao orbital LUMO dessas moléculas. A pequena seletividade exo observada na cicloadição com 6 concorda com a maior estabilidade do estado de transição, de acordo com os resultados de cálculos B3LYP/6-311G(d,p).

The captodative olefins 1-acetylvinyl esters of methacrylic and trans-crotonic acids, 3a and 3b, have been prepared. The presence of a second double bond in the molecule, acting as an internal probe, allowed us to compare their relative reactivity in Diels-Alder and Friedel-Crafts reactions. The reactivity was evaluated with cyclopentadiene (6) as diene in Diels-Alder cycloadditions, and with furan (9) and thiophene (10) as heteroaromatic Friedel-Crafts substrates. In both processes, the captodative enone double bond proved to be more reactive than that in the acrylic moiety. FMO theory accounted for this chemoselectivity as a consequence of the major p contribution of the enone to the LUMO of these molecules. The slight exo stereoselectivity observed in the cycloaddition to 6 parallels the higher stability of the corresponding transition state, according to the results of B3LYP/6-311G(d,p) calculations.

As olefinas captodativas 3a e 3b derivadas dos ácidos metacrílico e trans-crotônico foram preparadas. A presença de uma segunda ligação dupla na molécula, atuando como marcador interno, permitiu-nos comparar sua reatividade relativa em reações de Diesl-Alder e Friedel-Crafts. A reatividade foi avaliada com ciclopentadieno (6) atuando como dieno em cicloadição Diels-Alder, e com furano (9) e tiofeno (10) como substratos heteroaromáticos Friedel-Crafts. Em ambos os processos, a ligação dupla da enona captodativa mostrou-se mais reativa que a do sistema acrílico. A teoria FMO considera essa quimiosseletividade como consequência da maior contribuição da enona ao orbital LUMO dessas moléculas. A pequena seletividade exo observada na cicloadição com 6 concorda com a maior estabilidade do estado de transição, de acordo com os resultados de cálculos B3LYP/6-311G(d,p).

The captodative olefins 1-acetylvinyl esters of methacrylic and trans-crotonic acids, 3a and 3b, have been prepared. The presence of a second double bond in the molecule, acting as an internal probe, allowed us to compare their relative reactivity in Diels-Alder and Friedel-Crafts reactions. The reactivity was evaluated with cyclopentadiene (6) as diene in Diels-Alder cycloadditions, and with furan (9) and thiophene (10) as heteroaromatic Friedel-Crafts substrates. In both processes, the captodative enone double bond proved to be more reactive than that in the acrylic moiety. FMO theory accounted for this chemoselectivity as a consequence of the major n contribution of the enone to the LUMO of these molecules. The slight exo stereoselectivity observed in the cycloaddition to 6 parallels the higher stability of the corresponding transition state, according to the results of B3LYP/ 6-311G(d,p) calculations.

A new series of β-heterosubstituted captodative olefins 1-acetylvinyl arenecarboxylates (7a and 10a-10h) has been prepared, by introducing nitrogen or sulphur as heteroatoms substituted by alkyl and aryl groups. Three preparation methods were evaluated by modifying the leaving group of the starting material, as well as the nucleophilic character of the adding thiols. All of them were efficient and stereoselective, providing the desired alkenes in good yields and with the Z configuration of the double bond. Ab initio calculations (HF/6-31G*) of the FMOs, of some of the beta amino and bromo olefins, explained their experimental reactivity in Diels-Alder additions with respect to the unsubstituted olefin 1a. It also appears that the HOMO and LUMO energies of the beta sulphur analogues are governed by the particular electronic features of the sulphur atom, and that their very low reactivity before a diene is due to steric hindrance. A comparison between bond distances obtained by X-ray crystallography of different β-substituted and unsubstituted olefins seems to correlate with the delocalization effect of the heteroatom lone electron pair for the bromo and amino β-substituted olefins.

Se describe la preparación de una nueva serie de olefinas captodativas β-heterosustituidas 1-acetilvinil arencarboxilatos (7a and 10a-10h), donde el heteroátomo es nitrógeno o azufre, sustituido por grupos alquilo y arilo. Se evaluaron tres métodos para su preparación, modificando el grupo saliente en el sustrato, y el carácter nucleofílico de los tioles que se adicionaron. Los tres métodos fueron eficientes y estereoselectivos, proporcionando los alquenos deseados en buenos rendimientos y con la configuración Z del doble enlace. El cálculo de orbitales moleculares frontera (ab initio, HF / 6-31G*) de algunas de las olefinas preparadas permitió explicar sus diferencias en reactividad experimental con respecto a la olefina no sustituida 1a. Así, se sugiere que las energías de los orbitales HOMO y LUMO de los análogos azufrados están gobernadas por las propiedades electrónicas particulares del átomo de azufre, y que su baja reactividad ante un dieno depende también del efecto estérico. Una comparación de los datos de cristalografía de rayos X entre distancias de enlace de diferentes olefinas β-sustituidas y la no sustituida parece correlacionarse con el efecto de deslocalización del par de electrones no compartidos del heteroátomo para las olefinas β-bromo y β-amino sustituidas.