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Cycloaddition Reactions: Overview

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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.

Cycloaddition Reactions: MO Requirements for Thermal Activation

Cycloaddition Reactions: MO Requirements for Thermal Activation

Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

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.

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

Pericyclic Reactions: Introduction

Pericyclic Reactions: Introduction

Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...

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Keeping the name clean: [2 + 2] photocycloaddition.

Jayaraman Sivaguru¹, Thorsten Bach², Vaidhyanathan Ramamurthy³

¹Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH, 43403, USA. sivagj@bgsu.edu.

Photochemical & Photobiological Sciences : Official Journal of the European Photochemistry Association and the European Society for Photobiology

|May 24, 2022

Summary

This perspective clarifies the terminology for crossed [2+2] photocycloaddition reactions. It aims to establish a standard for naming diverse photocycloaddition processes to aid researchers.

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Area of Science:

Organic Chemistry
Photochemistry
Chemical Nomenclature

Background:

The term "crossed [2+2] photocycloaddition" is often misused for intermolecular reactions.
This ambiguity can lead to confusion in scientific literature.

Purpose of the Study:

To clarify the correct terminology for various [2+2] photocycloaddition reactions.
To establish a standardized nomenclature for photocycloaddition processes.
To prevent confusion among researchers in the chemical community.

Main Methods:

Review of existing literature on photocycloaddition reactions.
Analysis of terminology used in scientific publications.
Proposal of a standardized naming convention.

Main Results:

Identification of the ambiguity in the term "crossed [2+2] photocycloaddition".
Distinction between intramolecular and intermolecular [2+2] photocycloaddition reactions.
A proposed framework for consistent terminology.

Conclusions:

Clearer terminology for [2+2] photocycloaddition reactions will enhance scientific communication.
Standardized nomenclature is crucial for accurate reporting and understanding of photochemical reactions.
Adoption of proposed terminology will benefit the broader chemical research community.