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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Aromatic Hydrocarbon Cations: Structural Overview01:18

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
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Photochemical Electrocyclic Reactions: Stereochemistry01:26

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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Stability of Substituted Cyclohexanes02:30

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This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Porfirinas ultrastables sostenidas por ciclófano

Wenqi Liu1, Chenjian Lin1, Jacob A Weber1

  • 1Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Journal of the American Chemical Society
|April 4, 2020
PubMed
Resumen

Este estudio detalla la encapsulación de porfirinas dentro de un receptor de ciclófano, logrando altas afinidades de unión y mejorando significativamente sus propiedades ópticas y estabilidad química para aplicaciones de materiales avanzados.

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Área de la Ciencia:

  • Química supramolecular
  • Ciencias de los materiales
  • La fotoquímica

Sus antecedentes:

  • Las porfirinas son cromóforos vitales con aplicaciones en catálisis y fotomedicina.
  • El control de la agregación y la estabilidad de la porfirina en medios acuosos sigue siendo un desafío.

Objetivo del estudio:

  • Desarrollar un receptor ciclófano para encapsular porfirinas en agua.
  • Investigar el impacto de la encapsulación en las propiedades fotofísicas y la estabilidad química de la porfirina.

Principales métodos:

  • Síntesis de un receptor ciclófano tricíclico.
  • Estudios de unión de las porfirinas de base libre y de zinc con el receptor en agua.
  • Análisis espectroscópico (absorción, emisión) de las porfirinas encapsuladas.
  • Evaluación de la estabilidad química en condiciones adversas (acidez, intercambio D/H).

Principales resultados:

  • Se obtienen afinidades de unión subnanomólicas para la encapsulación de porfirina en agua.
  • Descubrimiento de dos isómeros coconformacionales debido a la estructura tricíclica del receptor.
  • Mejora significativa del rendimiento cuántico de fluorescencia y las propiedades ópticas de desplazamiento al rojo.
  • Estabilidad química sin precedentes, bloqueando el intercambio D/H, la protonación y la sololisis.

Conclusiones:

  • El ciclófano tricíclico encapsula eficazmente las porfirinas, modulando sus propiedades.
  • La encapsulación conduce a una mejora de las características fotofísicas y a una excepcional inercia química.
  • Aplicaciones potenciales en materiales avanzados, fotosíntesis artificial y dispositivos biomédicos.