DABCO as a practical catalyst for aromatic halogenation with N-halosuccinimides

An organocatalytic domino annulation approach via C(sp2)-OMe cleavage to unlock the synthesis of pyranochromenones enabled by HFIP

Fused pyranochromenone derivatives have extensive applications in medicinal chemistry. Herein, we report the first HFIP/TsOH catalyzed, one-pot domino reaction by cleavage of the C(sp2)-OMe bond. Control experiments reveal that 1,3,5-trimethoxybenzene is rapidly protonated in the presence of HFIP to yield a dearomatized cationic diene intermediate. The gram-scale reaction and late-stage functionalization of natural products justified the practicality of this protocol.

Halogenation of aromatic compounds with N-halosuccinimides (NXS) catalysed by D-camphorsulfonic acid-BiCl3

Aromatic bromination catalysed by 0.5-10 mol% of D-camphorsulfonic acid-BiCl3 with N-bromosuccinimides (NBS) was carried out in MeCN under air conditions, and the procedure was extended to the reactions with N-chlorosuccinimides (NCS) and N-iodosuccinimides (NIS). The halogenation of some drugs and natural products was also attempted. One-pot bromination/Suzuki-Miyaura cross-coupling and bromination/Sonogashira coupling reactions were achieved without the removal of the solvent.

Elemental Sulfur-Mediated Aromatic Halogenation

A method for aromatic halogenation using a combination of elemental sulfur (S8) and N-halosuccinimide has been developed. A catalytic quantity of elemental sulfur (S8) with N-bromosuccinimide (NBS) and N-chlorosuccinimide (NCS) effectively halogenated less-reactive aromatic compounds, such as ester-, cyano-, and nitro-substituted anisole derivatives. No reaction occurred in the absence of S8, underscoring its crucial role in the catalytic activity. This catalytic system was also applicable to aromatic iodination with 1,3-diiodo-5,5-dimethylhydantoin.

A Porphyrin Iron(III) π-Dication Species and its Relevance in Catalyst Design for the Umpolung of Nucleophiles

Isoporphyrins have recently been identified as remarkable species capable of turning the nucleophile attached to the porphyrin ring into an electrophile, thereby providing umpolung of reactivity (Inorg. Chem. 2022, 61, 8105-8111). They are generated by nucleophilic attack on an iron(III) π-dication, a class of species that has received scant attention. Here, we explore the effect of the porphyrin meso-substituent and report a iron(III) π-dication bearing the meso-tetraphenylporphyrin (TPP) ligand. We provide an extensive study of the species by UV/Vis absorption, 2 H NMR, EPR, applied field Mössbauer, and resonance Raman spectroscopy. We further explore the system's highly dynamic and tunable properties and address the nature of the axial ligands as well as the conformation of the porphyrin ring. The insights presented are essential for the rational design of catalysts for the umpolung of nucleophiles. Such catalytic avenues could for example provide a novel method for electrophilic chlorinations. We further examine the importance of electronic tuning of the porphyrin by nature of the meso-substituent as a factor in catalyst design.

Solid-state mechanochemical cross-coupling of insoluble substrates into insoluble products by removable solubilizing silyl groups: uniform synthesis of nonsubstituted linear oligothiophenes

Conventional solution-based organic reactions that involve insoluble substrates are challenging and inefficient. Furthermore, even if the reaction is successful, the corresponding products are insoluble in most cases, making their isolation and subsequent transformations difficult. Hence, the conversion of insoluble compounds into insoluble products remains a challenge in practical synthetic chemistry. In this study, we showcase a potential solution to address these solubility issues by combining a mechanochemical cross-coupling approach with removable solubilizing silyl groups. Our strategy involves solid-state Suzuki-Miyaura cross-coupling reactions between organoboron nucleophiles bearing a silyl group with long alkyl chains and insoluble polyaromatic halides. The silyl group on the nucleophile can act as a solubilizing group that enables product isolation via silica gel column chromatography and can be easily removed by the addition of fluoride anions to form the desired insoluble coupling products with sufficient purity. Furthermore, we demonstrate that after aromatic electrophilic bromination of the desilylated products, sequential solid-state cross-coupling of the obtained insoluble brominated substrates, followed by desilylation, afforded further π-extended functional molecules. Using this conceptually new protocol, we achieved the first uniform synthesis of the longest nonsubstituted linear insoluble 9-mer oligothiophene.

Visible light-enabled regioselective chlorination of coumarins using CuCl2via LMCT excitation

An efficient, regioselective chlorination of coumarins using Earth-abundant and cost-effective CuCl₂ under visible light irradiation is reported. A key feature of this protocol is the photocatalytic dissociation of the copper(II) complex in acetonitrile through ligand-to-metal charge transfer (LMCT) to give the chlorine atom which then selectively chlorinates the coumarin. This method can chlorinate a broad scope of coumarins with either electron-withdrawing or electron-donating substituents to regioselectively afford 3-chlorocoumarins in good to excellent yields and can be further extended to other electron-deficient heterocycles and olefins such as flavones, 8-methoxypsoralen and naphthoquinones.

Automated grindstone chemistry: a simple and facile way for PEG-assisted stoichiometry-controlled halogenation of phenols and anilines using N-halosuccinimides

A simple electrical mortar–pestle was used for the development of a green and facile mechanochemical route for the catalyst-free halogenation of phenols and anilines via liquid-assisted grinding using PEG-400 as the grinding auxiliary. A series of mono-, di-, and tri-halogenated phenols and anilines was synthesized in good to excellent yields within 10–15 min in a chemoselective manner by controlling the stoichiometry of N-halosuccinimides (NXS, X = Br, I, and Cl). It was observed that PEG-400 plays a key role in controlling the reactivity of the substrates and to afford better regioselectivity. Almost exclusive para-selectivity was observed for the aromatic substrates with free o- and p-positions for mono- and dihalogenations. As known, the decarboxylation (or desulfonation) was observed in the case of salicylic acids and anthranilic acids (or sulfanilic acids) leading to 2,4,6-trihalogenated products when 3 equiv of NXS was used. Simple instrumentation, metal-free approach, cost-effectiveness, atom economy, short reaction time, and mild reaction conditions are a few noticeable merits of this environmentally sustainable mechanochemical protocol.