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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical 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
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[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement01:21

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
2.9K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.5K
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.
2.5K
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.2K
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
2.2K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.1K
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.
2.1K
IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations01:08

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1.3K
Identical bonds within a polyatomic group can stretch symmetrically (in-phase) or asymmetrically (out-of-phase). Similar to hydrogen bonding, these vibrations also influence the shape of the IR peak. Generally, asymmetric stretching frequencies are higher than symmetric stretching frequencies. For example, primary amines exhibit two distinct IR peaks between 3300–3500 cm−1 corresponding to the symmetric and asymmetric N-H stretching, while secondary amines exhibit a single...
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Helicity Modulation in NIR-Absorbing Porphyrin-Ryleneimides.

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Bulky substituents on π-extended porphyrins create curved, helical structures. Metal ion complexation, particularly with Cd(II) or Hg(II), induces chiral propeller configurations, offering stable, enantiopure compounds.

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ChiralityCoordination ChemistryMetal ComplexesPorphyrinsStereochemistry

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Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Porphyrins are versatile macrocyclic compounds with tunable electronic and optical properties.
  • Controlling the three-dimensional structure of porphyrins is crucial for developing advanced functional materials.
  • Achiral porphyrin precursors can be transformed into chiral architectures through specific chemical modifications.

Purpose of the Study:

  • To investigate the synthesis and stereochemical control of curved, π-extended porphyrins.
  • To explore the conversion of achiral porphyrin complexes into chiral propeller-like structures.
  • To demonstrate the stability and enantiopurity of these novel chiral porphyrin derivatives.

Main Methods:

  • Synthesis of π-extended porphyrins with bulky peripheral substituents.
  • Complexation of porphyrins with metal ions (Cd(II), Hg(II)).
  • Structural characterization using X-ray diffraction analysis.
  • Thermodynamic and kinetic control studies of stereochemical transformations.

Main Results:

  • Peripheral substitution leads to curved porphyrin chromophores with helical stereogenic units.
  • Coordination with Cd(II) or Hg(II) ions induces a conversion from saddle-shaped to chiral propeller configurations.
  • Apical water coordination can induce bowl-like distortion while maintaining chirality in cadmium complexes.
  • Thermodynamic control observed for phenyl- and tolyl-substituted derivatives; kinetic control (heating) for 4-(tert-butyl)phenyl derivatives.
  • Chiral mercury porphyrins can be converted back to free bases or other metal complexes without loss of configurational purity.

Conclusions:

  • The stereochemistry and curvature of π-extended porphyrins are controllable via substituents, metal ions, and ligands.
  • Metal-induced transformation provides a pathway to stable, enantiopure porphyrin propellers.
  • These chiral porphyrin architectures hold potential for applications in asymmetric catalysis and molecular recognition.