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01:18Aromatic Hydrocarbon Cations: Structural Overview
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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...
Removing one hydrogen from the intervening CH2 group...
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01:13Five-Membered Heterocyclic Aromatic Compounds: Overview
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Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom,...
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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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.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
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01:18Aromatic Hydrocarbon Anions: Structural Overview
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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
Due to the absence of continuous...
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01:16Structure of Conjugated Dienes
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Introduction
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double...
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double...
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Un heterocíclico plano totalmente conjugado [9]Circuleno
Stephan K Pedersen1, Kristina Eriksen1, Hans Ågren2,3
1Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
Journal of the American Chemical Society
|August 14, 2020
Resumen
Los investigadores sintetizaron el primer circuleno, un nuevo compuesto aromático macrocíclico, ampliando los límites conocidos de las estructuras de circuleno. Este avance ofrece nuevas posibilidades en la ciencia de los materiales y la química orgánica.
Área de la Ciencia:
- Química orgánica
- Ciencias de los materiales
- Química supramolecular
Sus antecedentes:
- Los [n]circulenos totalmente aromáticos, hidrocarburos aromáticos macrocíclicos, se limitaban anteriormente a un máximo de ocho anillos aromáticos (n = 8).
- La síntesis de circulenos de orden superior (n>8) seguía siendo una frontera inexplorada en la química orgánica.
Objetivo del estudio:
- Para sintetizar y caracterizar nuevos circulenos de orden superior más allá del límite previamente conocido de n = 8.
- Investigar las propiedades estructurales, electrónicas y aromáticas de estos nuevos sistemas macrocíclicos.
Principales métodos:
- Se utilizó una reacción de condensación dimerizante de alto rendimiento entre 3,6-dihidroxicarbazol y glyoxal para la construcción del macrociclo.
- Se utilizó la difracción de rayos X de cristal único para determinar la estructura tridimensional precisa del circuleno sintetizado [9].
- Se realizaron cálculos de desplazamiento químico independiente del núcleo (NICS) y de anisotropía de la densidad de corriente inducida (ACID) para evaluar la aromaticidad y las propiedades electrónicas.
Principales resultados:
- El primer circuleno [9] (diazatrioxa [9] circuleno) y un análogo de circuleno [10] (tetrahidro-diazatetraoxa [10] circuleno) fueron sintetizados con éxito.
- El análisis de rayos X confirmó que el circuleno [9] es perfectamente plano con anillos de benceno alargados, lo que sugiere una planaridad inducida por la tensión.
- Las longitudes de enlace alternas y las corrientes de anillo paratrópicas en los anillos no aromáticos centrales indican una contribución significativa [9]de resonancia de radialeno y paratropicidad inducida.
Conclusiones:
- La síntesis exitosa del primer [9]circuleno representa un avance significativo en el campo de los compuestos aromáticos macrocíclicos.
- Las propiedades estructurales y electrónicas, incluida la planaridad y el carácter radialeno, proporcionan nuevos conocimientos sobre el comportamiento de los sistemas aromáticos tensados.
- La paratropicidad inducida observada en el núcleo no aromático abre caminos para explorar nuevas propiedades electrónicas y fotofísicas en materiales avanzados.