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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

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.

Thermal and Photochemical Electrocyclic Reactions: Overview

Thermal and Photochemical Electrocyclic Reactions: Overview

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.

Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene

Electrophilic 1,2- and 1,4-Addition of X₂ to 1,3-Butadiene

Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.

Preparation and Reactions of Thiols

Preparation and Reactions of Thiols

Thiols are prepared using the hydrosulfide anion as a nucleophile in a nucleophilic substitution reaction with alkyl halides. For instance, bromobutane reacts with sodium hydrosulfide to give butanethiol.

Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.

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Related Experiment Video

Updated: Jun 27, 2026

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

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

Published on: July 27, 2022

Redox-regulated ethylene binding to a rhenium-thiolate complex.

Craig A Grapperhaus¹, Kagna Ouch, Mark S Mashuta

¹Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA.

Journal of the American Chemical Society

|December 17, 2008

Summary

The oxidation state of rhenium complexes dictates their ability to reversibly bind ethylene. This control over carbon-sulfur bond formation allows for tunable ethylene uptake and release, crucial for catalytic applications.

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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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The Synthesis, Characterization and Reactivity of a Series of Ruthenium N-triphosPh Complexes

The Synthesis, Characterization and Reactivity of a Series of Ruthenium N-triphos^Ph Complexes

Published on: April 10, 2015

Area of Science:

Organometallic Chemistry
Coordination Chemistry
Catalysis

Background:

Investigating the reactivity of metal complexes with unsaturated hydrocarbons.
Understanding the role of oxidation states in chemical transformations.
Exploring reversible carbon-sulfur bond formation for potential applications.

Purpose of the Study:

To determine the influence of rhenium complex oxidation state on ethylene binding.
To quantify the reversibility of carbon-sulfur bond formation reactions.
To explore potential for controlled ethylene release and uptake.

Main Methods:

Spectroscopic analysis of reactions between ethylene and rhenium complexes ([Re(DPPBT)(3)](n+)).
Electrochemical determination of binding and dissociation constants (k(f), k(r), K).
X-ray crystallography for structural characterization of products.

Main Results:

Neutral [Re(DPPBT)(3)] showed no detectable reaction with ethylene (K(1) = 1.9 x 10(-11) M(-1)).
Oxidation to [Re(DPPBT)(3)](+) enabled rapid, reversible ethylene binding (K(2) = 4.0 M(-1)).
Further oxidation to the dication stabilized the C-S bond (K(3) = 2.5 x 10(9) M(-1)), with the product characterized by X-ray crystallography.

Conclusions:

The oxidation state of the rhenium complex is a critical factor in controlling ethylene binding reversibility.
Tunable binding and release of ethylene is achievable by adjusting the complex's charge.
This system offers potential for applications requiring controlled gas uptake and release.