Related Concept Videos
01:26Photochemical 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
Selection Rules: Photochemical Activation
01:12Cycloaddition 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.
01:17Thermal 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.
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.
01:26Thermal 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.
01:16Cycloaddition 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.
01:11ortho–para-Directing Activators: –CH3, –OH, –NH2, –OCH3
All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Fe(III)/Pyridine N-oxide LMCT Photocatalysis for Unactivated C(sp3)-H Functionalizations.
Jun Luo1, Deepak Ranjan Pradhan2, Jujhar Singh3
1Department of Chemistry and Chemical Biology, Indiana University Indianapolis, Indianapolis, Indiana 46202, United States.
ACS Catalysis
|June 1, 2026
Summary
Selective C-H functionalization is achieved using a novel Fe(III)/pyridine N-oxide (PNO) system. This approach enables efficient hydrogen atom transfer (HAT) from unactivated C(sp3)-H bonds, particularly primary sites, via cationic N-oxy radicals.
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Area of Science:
- Organic Chemistry
- Catalysis
- Photochemistry
Background:
- Heteroatom-centered radical-mediated hydrogen atom transfer (HAT) is crucial for C-H functionalization.
- Selective activation of unactivated C(sp3)-H bonds, especially primary sites, remains a significant challenge.
Purpose of the Study:
- To develop a catalytic system for selective HAT-mediated functionalization of unactivated C(sp3)-H bonds.
- To enable the generation of highly electrophilic cationic N-oxy radicals inaccessible via conventional methods.
Main Methods:
- Utilized an Fe(III)/pyridine N-oxide (PNO) catalytic system.
- Harnessed ligand-to-metal charge transfer (LMCT) excitation.
- Employed computational and kinetic studies to elucidate reaction mechanisms.
Main Results:
- Developed a catalytic system enabling diverse HAT-mediated functionalization of unactivated C(sp3)-H bonds.
- Achieved high regioselectivity (up to 20:1) in the hydrazination of various substrates using pentachloropyridine N-oxide.
- Demonstrated facile and reversible HAT by cationic N-oxy radicals, leveling energy barriers for C-H bonds.
Conclusions:
- The Fe(III)/PNO LMCT system provides a powerful new method for selective C-H functionalization.
- Cationic N-oxy radicals generated via LMCT are effective HAT agents for primary C(sp3)-H bonds.
- The developed system offers broad synthetic applicability for functionalizing challenging C-H bonds.