Halogenation Strategies in Organic Synthesis

Halogenation, the introduction of fluorine, chlorine, bromine or iodine into organic frameworks, underpins the synthesis of numerous pharmaceuticals, agrochemicals and functional materials. Traditional approaches employ molecular halogens or N-halosuccinimides to achieve electrophilic aromatic substitutions or radical chain processes. More recent advances exploit transition-metal catalysis to activate inert C–H bonds directly, enabling site-selective halogenation under mild conditions without prefunctionalisation. Photoredox methods have further expanded the toolkit by harnessing visible light to generate halogen radicals or electrophiles in situ, improving selectivity and reducing waste. Organocatalytic and heterogeneous platforms now allow cooperative activation of both halide sources and substrates, enhancing sustainability through improved recyclability and lower energy demands. Across these strategies, the choice of halogen source, catalyst and reaction medium determines regioselectivity, functional-group tolerance and environmental footprint. The ongoing challenge is to balance efficiency and scalability with green chemistry principles, driving the development of new catalysts, activation modes and benign reagents for next-generation halogenation processes.

Research from Nature Portfolio

Recent studies have demonstrated a heterogeneous organophotocatalytic system in which an Al₂O₃-supported perylene diimide photocatalyst couples HBr oxidation with oxygen reduction to generate Br₂ and H₂O₂ in situ. This dual-electron transfer mechanism produces active brominating species that enable efficient arene bromination under mild conditions, while the solid support enhances catalyst stability and recyclability. Mechanistic investigations highlight the role of oxygen vacancies in anchoring O₂ and facilitating cooperative catalysis between the dye and support.

Another breakthrough employs oxoammonium salts derived from TEMPO and its analogues as versatile catalysts for electrophilic halogenation of diverse substrates, including olefins, alkynes and aromatics. The catalyst activates halenium reagents such as Br⁺, I⁺ and Cl⁺ through a synergistic model that simultaneously polarises the halogen source and substrate. This organocatalytic approach achieves haloarylation, dibromination and other valuable transformations with broad scope and high selectivity.

Halogenation Strategies in Organic Synthesis publication trend

The graph below shows the total number of articles in halogenation strategies in organic synthesis across all publications each year (not limited to Nature Index journals).

Technical terms

Electrophilic halogenation: introduction of a halogen electrophile onto an electron-rich substrate, typically via an EAS mechanism.

Photoredox catalysis: use of visible light to drive redox reactions via photoexcited catalysts that generate radicals or ionic species.

N-halosuccinimide: a common electrophilic halogen source in which succinimide stabilises positively polarised halogen atoms.

C–H activation: direct functionalisation of a C–H bond, often by transition-metal catalysts, without prior substrate preactivation.

Oxoammonium salt: a positively charged nitrogen species derived from TEMPO that can activate halenium reagents for electrophilic halogenation.

Heterogeneous catalysis: catalysis in which the catalyst is in a different phase (usually solid) from the reactants, facilitating separation and reuse.

References

1. Heterogeneous organophotocatalytic HBr oxidation coupled with oxygen reduction for boosting bromination of arenes. Nature Communications (2024).
2. Oxoammonium salts are catalysing efficient and selective halogenation of olefins, alkynes and aromatics. Nature Communications (2021).
3. Thiol Chlorination with N‑Chlorosuccinimide: HCl-Catalyzed Release of Molecular Chlorine and the Dichotomous Effects of Water. ACS Catalysis (2023).
4. Room temperature C(sp 2 )–H oxidative chlorination via photoredox catalysis. Chemical Science (2017).
5. Base- and Additive-Free Ir-Catalyzed ortho-Iodination of Benzoic Acids: Scope and Mechanistic Investigations. ACS Catalysis (2018).