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Thermal Sigmatropic Reactions: Overview

Thermal Sigmatropic Reactions: Overview

Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in...

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

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.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

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

Atomic Nuclei: Nuclear Relaxation Processes

Atomic Nuclei: Nuclear Relaxation Processes

In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis, the precessing magnetic moments are randomly oriented around the z-axis.

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

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

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.

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...

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Related Experiment Video

Updated: May 23, 2025

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Published on: May 15, 2017

Excited-State Aromatization Drives Nonequilibrium Planarization Dynamics.

Yusuke Yoneda^1,2, Tomoaki Konishi³, Kensuke Suga^3,4

¹Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, National Institutes of Natural Sciences, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan.

Journal of the American Chemical Society

|March 10, 2025

Summary

Excited-state aromaticity drives conformational changes in cyclic systems. This study reveals that electronic structure changes precede structural planarization in cyclooctatetraene derivatives, clarifying photoinduced dynamics.

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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

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Published on: July 27, 2022

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Published on: May 29, 2018

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Last Updated: May 23, 2025

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Published on: May 15, 2017

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

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Published on: July 27, 2022

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Published on: May 29, 2018

Area of Science:

Photochemistry
Physical Chemistry
Spectroscopy

Background:

Excited-state aromaticity predicts photoinduced conformational changes in cyclic π-conjugated systems.
The precise relationship between electronic and structural dynamics during these changes remains poorly understood.

Purpose of the Study:

To investigate the nonequilibrium planarization dynamics of a cyclooctatetraene (COT) derivative.
To elucidate the temporal sequence of electronic structure changes and conformational shifts upon photoexcitation.

Main Methods:

Utilized femtosecond transient absorption spectroscopy.
Employed time-resolved time-domain Raman spectroscopy with 13C labeling.

Main Results:

Observed a continuous peak shift in time-resolved Raman data, indicating a bent-to-planar structural transition.
Demonstrated that electronic structure modification precedes the observed planarization.
Unambiguously identified the marker band shift with 13C labeling.

Conclusions:

Photoexcitation induces aromaticity in the COT derivative, followed by a conformational change to planarization.
Provides a dynamical perspective on excited-state aromaticity and photoinduced structural evolution.