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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 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.

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.

Deactivation Processes: Jablonski Diagram

Deactivation Processes: Jablonski Diagram

Luminescence, the emission of light by a substance that has absorbed energy, is a process that involves the interaction of molecules with light. The energy-level diagram, or Jablonski diagram, is a graphical representation of these interactions, illustrating the various states and transitions a molecule can undergo. In a typical Jablonski diagram, the lowest horizontal line represents the ground-state energy of the molecule, which is usually a singlet state. This state represents the energies...

Stereochemical Effects of Enolization

Stereochemical Effects of Enolization

The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.

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.

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Updated: Jan 5, 2026

Light-driven Enzymatic Decarboxylation

Light-driven Enzymatic Decarboxylation

Published on: May 22, 2016

Light-driven deracemization enabled by excited-state electron transfer.

Nick Y Shin¹, Jonathan M Ryss², Xin Zhang¹

¹Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.

Science (New York, N.Y.)

|October 19, 2019

Summary

This summary is machine-generated.

This study introduces a novel deracemization method for asymmetric synthesis using visible light and molecular catalysts. The process achieves spontaneous optical enrichment of amine derivatives through a unique catalytic cycle.

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Area of Science:

Organic Chemistry
Photochemistry
Catalysis

Background:

Deracemization is crucial for asymmetric synthesis but faces energetic limitations.
Developing efficient deracemization strategies remains a key challenge in synthetic chemistry.

Purpose of the Study:

To develop a novel visible-light-driven deracemization method for amine derivatives.
To overcome intrinsic energetic barriers in deracemization processes.
To achieve spontaneous optical enrichment using molecular catalysts.

Main Methods:

Utilizing visible light and three distinct molecular catalysts for deracemization.
Employing an excited-state iridium chromophore to initiate the reaction.
Leveraging sequential electron, proton, and hydrogen-atom transfer steps.
Breaking and reforming a stereogenic C-H bond within the catalytic cycle.

Main Results:

Achieved spontaneous optical enrichment of amine derivatives under visible light.
Demonstrated a catalytic cycle involving excited-state redox events.
Identified two independent stereoselective steps that combine for enhanced enantioselectivity.
Generated out-of-equilibrium product distributions between substrate enantiomers.

Conclusions:

The developed method offers a new approach to deracemization, overcoming previous energetic challenges.
The sequential nature of stereoselective steps leads to superior composite selectivity.
This work expands the toolkit for asymmetric synthesis using photoredox catalysis.