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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.

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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.

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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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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.

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Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

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From Organic Fragments to Photoswitchable Catalysts: The OFF-ON Structural Repository for Transferable Kernel-Based

Frédéric Célerse¹, Matthew D Wodrich^1,2, Sergi Vela¹

¹Laboratory for Computational Molecular Design (LCMD), Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland.

Journal of Chemical Information and Modeling

|February 6, 2024

Summary

A new database, OFF-ON, provides diverse molecular geometries for training AI models to predict the behavior of flexible organic molecules, especially photoswitchable organocatalysts. This enables computationally efficient exploration of complex chemical landscapes.

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Area of Science:

Computational chemistry
Materials science
Organic chemistry

Background:

Accurate computational models require diverse structural and conformational data.
Developing such databases for flexible organic molecules is challenging.
Existing databases often lack coverage for complex, non-modular compounds.

Purpose of the Study:

Introduce the OFF-ON database for conformationally flexible organic molecules.
Provide a resource for training machine learning models in computational chemistry.
Facilitate the study of photoswitchable organocatalysts.

Main Methods:

Curated a database of equilibrium and nonequilibrium geometries (OFF-ON).
Focused on organic compounds and dimers, emphasizing photoswitchable organocatalysts.
Trained a local kernel regression model using the database.

Main Results:

The OFF-ON database contains 7869 equilibrium and 67,457 nonequilibrium geometries.
A trained model demonstrated reliable predictions for simulating conformational behavior.
Free energy surfaces of photoswitchable organocatalysts were accurately reproduced.

Conclusions:

The OFF-ON database is a valuable resource for computational studies of organic molecules.
Enables computationally feasible exploration of complex free energy surfaces.
Facilitates rationalization and prediction of catalytic behavior for organocatalysts.